Glossa Psycholinguistics

1.1 Subphonemic durational cues in production

Let us consider first the nature of subphonemic cues to word structure that are available to listeners. One of the most commonly studied set of features relates to duration. As a general rule, it is well known that adding more phonological material to a word or syllable tends to shorten the absolute duration of the individual components. This general phenomenon can be understood as breaking down into two patterns: polysyllabic shortening and segmental compression.

Under polysyllabic shortening, the identical set of segments tends to be produced with shorter duration when more syllables are added to a word. For example, sequences like [kæp] and [hæm] are shorter when they form the first syllable of a word like captain or hamster than when they form a single-syllable word like cap or ham (Davis et al., 2002; Salverda et al., 2003).¹ When words acquire additional morphological structure, they often increase their syllable count, and so become subject to polysyllabic shortening. Thus, the stem speed is longest when it is a free-standing word, shorter when it is the first syllable followed by a suffix, such as speedy, and shorter still when further suffixes add a third syllable, as in speediness or speedily (Lehiste, 1972; but see also Blazej & Cohen-Goldberg, 2015; Kemps, Ernestus, et al., 2005; Kemps, Wurm, et al., 2005).

Yet adding morphological structure does not always increase syllable count. Singular and plural nouns in English, which are related by morphological suffixation, typically suffixed with the single-segment suffix -s, escape the conditions for polysyllabic shortening, because the plural -s suffix does not always add additional syllables to the stem. Singular rock and plural rocks in English are both one syllable. Nevertheless, even one-syllable words contain durational cues to the presence of an upcoming suffix, in a pattern known as segmental compression.

Segmental compression in English has been reported for decades. Klatt (1975), for example, observed that consonants in syllable onsets were shorter when they were part of a consonant cluster than when they were a simple onset. Katz (2012) observed that vowels are shorter when they occur in syllables with a coda than syllables with no coda, and that vowels in syllables with simple codas shorten further when the coda becomes complex, although only for certain types of consonants. Munhall et al. (1992) also observed significant shortening of vowels when syllable codas became more complex – including cases in which syllable codas were made more complex by the addition of a final -s. And most recently, Cohen and Carlson (2024) observed in a corpus study that both noun stems and especially verb stems shorten when a suffixal -s is added. These findings lead us to expect that adding a plural suffix to a noun stem should shorten the stem, providing listeners with a durational cue signaling the upcoming suffix.

1.2 Subphonemic durational cues in perception

Are these durational cues actually used to guide perception? For polysyllabic shortening, a large body of research seems to indicate that listeners are robustly sensitive to them. In a series of gating and priming experiments, Davis et al. (2002) found that when English-speaking listeners were presented with shortened initial syllables (e.g., cap) they expected that the rest of the word would form a polysyllabic continuation (e.g., captain). Salverda et al. (2003) replicated these findings for Dutch listeners using visual-world eye-tracking. Listeners were presented with a screen containing an image of a single-syllable word (ham) and its corresponding polysyllabic counterpart (hamster). Listeners then heard sentences in which the polysyllabic target (hamster) contained a manipulated initial syllable. This syllable had been spliced in from other recorded tokens of either ham or hamster, carefully selected so that its duration was either long or short. Regardless of whether the spliced syllable came from ham or hamster, listeners looked more towards the target, hamster, when the initial syllable had shorter duration, signaling a longer polysyllabic word, than when it had longer duration, signaling a shorter free-standing word. In other words, listeners showed that they rely on the duration of the initial syllable to distinguish single-syllable nouns from polysyllables.

Shatzman & McQueen (2006) extended Salverda et al.’s (2003) study to show that Dutch listeners’ ability to use this polysyllabic shortening pattern in perception reflects a highly generalized knowledge of the phenomenon. In their study, all target words were novel words (e.g., bap and baptoe), which listeners had been trained to associate with novel shapes before the eye-tracking task. Crucially, the recordings used to teach listeners the word-shape associations during the training task had been manipulated to neutralize all polysyllabic shortening patterns. The duration of [bæp] was identical in the training recordings of both bap and baptoe. During the test phase of the experiment, the durations of the initial syllables were then lengthened or shortened, producing durational distinctions that the listeners had not encountered when learning the words. Yet despite their lack of experience with polysyllabic shortening in these particular words, listeners nevertheless looked more towards the bap image if the duration of the stem was long, and more towards the baptoe image if the duration was shortened. The results suggest that the polysyllabic shortening patterns that listeners have learned throughout a lifetime of exposure generalize strongly enough to overrule the durational neutrality of the novel training words.

This ability to generalize extends to morphologically related words. Kemps and colleagues (Kemps, Ernestus, et al., 2005; Kemps, Wurm, et al., 2005) used number decision and lexical decision tasks to study listeners’ perception of English and Dutch words with inflectional or derivational suffixes, all of which added an additional syllable to a word stem. They then manipulated these words, so that originally unsuffixed words were presented either unsuffixed, or with a suffix spliced on, while originally suffixed words were presented with the suffix removed, or with a suffix present. They observed that reaction times were slowed in both English and Dutch listeners when the duration of a stem did not match the constructed word’s structure. Finally, Blazej & Cohen-Goldberg (2015) directly manipulated the duration of the initial syllables of single-syllable English stems (e.g., clue) and morphologically complex derivations (e.g., clueless). Focusing on the single-syllable stems as their critical targets, they used acoustic analysis software to explicitly set the duration of single-syllable targets (clue), so that the duration ratio of shortened targets to lengthened targets mirrored the ratio of initial syllables in suffixed words to free-standing words in their naturally recorded stimuli. They observed that listeners’ mouse tracks veered more towards the suffixed competitor (clueless) when the free-standing target (clue) had a shortened syllable than when it had a lengthened syllable.

The findings summarized above all present a picture showing that, at least as far as polysyllabic shortening is concerned, listeners are certainly capable of perceiving and using durational cues to word structure to guide their speech perception. After long initial syllables, listeners expect word boundaries; and after shortened initial syllables, they expect more of the word to follow, whether that additional word structure is a suffix or a continuation of the same root.

What happens, then, when the addition of a suffix does not add an additional syllable? Can listeners draw on segmental compression patterns to more quickly distinguish a single-syllable, unsuffixed noun, like rock, from a plural, like rocks? If so, then listeners should perform better when distinguishing singular nouns from plural competitors when the singular nouns have lengthened duration, compared to when those same singular nouns have been shortened.

The first goal of this study, therefore, is to confirm that listeners can indeed draw on segmental compression patterns in the same way they draw on polysyllabic shortening. This question is particularly interesting because inflectionally related words, like singulars and plurals, differ from pairs like rock/rocket and clue/clueless in two key ways. First, they belong to the same lexeme, and therefore share highly similar sets of lexico-semantic features. The consequence of misinterpreting a singular for a plural would lead to less communicative chaos than misinterpreting a singular for an unrelated two-syllable word. Looking for a rock instead of some rocks would still lead eventually to an identification of a mostly-correct target. Looking for a rock instead of a rocket will not. Listeners may therefore have learned that attending to detail that distinguishes different lexemes (rock/rocket) avoids more miscommunication than attending to detail distinguishing different word forms of the same lexeme (rock/rocks). If that is the case, then segmental compression effects on perception may be weaker or entirely absent, compared to polysyllabic shortening effects.

The second goal of this study capitalizes on another key feature of inflectionally related pairs: their morphosyntactic connections to the rest of the sentence context. The number of a noun can be highly predictable in some contexts – e.g., following agreeing determiners – and entirely unpredictable in others. Therefore, if listeners do draw on segmental compression patterns to aid perception, the extent to which they use them may vary as a function of the predictability of a given noun’s number in its morphosyntactic context. There are three possible accounts for how listeners might integrate morphosyntax with subphonemic phonetic detail. I shall call these accounts the baseline ideal observer account; the phonetic predictor account; and the strategic listener account. All three accounts allow for listeners to draw on durational detail to inform their differentiation of singular, plural, and two-syllable words. They differ, however, in their predictions of how morphosyntactic context affects listeners’ use of those cues.

1.3 Baseline ideal observers

Ideal observer models propose that listeners incorporate all possible sources of information to estimate the most likely interpretation of a target utterance (Kleinschmidt & Jaeger, 2015; Norris & McQueen, 2008). Such models are probabilistic: Listeners do not fully settle on one predicted interpretation as the utterance unfolds (e.g., “singular noun”), but rather derive probability distributions over possible interpretations (e.g., “singular = 73%, plural = 27%”), updating them as new information becomes available in the incoming input. Whether this new input takes the form of long-term repeated, accumulated exposure or the short-term immediate speech context, the mechanism is the same: When the input is highly consistent with a particular interpretation (e.g., an unsuffixed noun stem), the probability of that interpretation is increased, while the probability of competing interpretations is down-weighted.

This view of incremental, probabilistic speech processing has much in common with earlier perspectives, such as the polysystemic speech perception (Polysp) view introduced by Hawkins (2003). It differs, however, by focusing on listeners’ abilities to identify linguistic structures, rather than aiming purely at extracting meaning.² It also employs an explicitly Bayesian framing of its computational mechanics. According to this Bayesian architecture, listeners’ expectations form prior distributions, which, upon the arrival of new information, are updated to form posterior distributions over likely utterances. Regardless of the shape of the prior distribution, new information will always combine with it in the same way. Thus, even if previous portions of the utterance have provided sufficient information to effectively disambiguate an acoustic signal, a listener will continue to integrate new input to update the posterior distribution. In cases where the previous input identifies the correct interpretation to a high degree of certainty, the new input is informationally redundant. “Redundant” here does not mean irrelevant or useless, but rather refers to the amount of information that a given input can carry. In contexts where previous input can be used near-categorically to identify the intended interpretation, then further input pointing in the same direction carries no new information. Equivalently, from a Bayesian perspective we can say that this new input can do little to affect the shape of the listener’s posterior distribution. The distribution is already so heavily skewed towards the correct interpretation that skewing it further has very little practical effect on the listener’s decisions. Nevertheless, the listener does not disregard this input. Regardless of its informational redundancy, it is allowed to influence the shape of the posterior, and in the event that the speaker misspoke, or the listener misheard, this strategy allows the listener to recover as the conflicting new information alerts them to the error.

This view of speech perception is “greedy”: Listeners attend to all possible sources of information to ensure that their predicted probability distributions over upcoming material are as detailed as possible. Such use of cues is consistent with research showing that listeners form and store highly detailed representations of previously encountered linguistic structures (Goldinger, 1998; Hawkins, 2003; Johnson, 1997, 2007; Pierrehumbert, 2002), down to the individual patterns used by different speakers (Nygaard & Pisoni, 1998; Nygaard et al., 1994; L. B. Smith & Colunga, 2012; R. Smith, 2015).

To illustrate how this account applies to the test case here, of recognizing singular or plural nouns in different contexts, consider (1)–(2) below, with particular attention to the underlined noun phrase.

Both (1a) and (2a) feature a singular noun, rock, while (1b) and (2b) feature a plural noun, rocks. But the two pairs differ in the quantity and nature of cues pointing to the noun’s number up to the moment of the suffix onset. In (1a–1b), the cues are minimal and found in the domain of whichever coarticulatory processes go along with the the presence or absence of a final plural suffix. Such cues for (1b), for example, might comprise an adjustment to the place of articulation in the /k/ to accommodate the upcoming alveolar /s/, along with a slight shortening of the noun stem duration due to segmental compression (Katz, 2012; Klatt, 1975; Munhall et al., 1992). For (1a), a cue might be the corresponding lengthening of the segments that would result from the lack of any additional coda consonant. By contrast, (2a–2b) not only contain the same acoustic information in the noun stem as (1a–1b), but also add high-level morphosyntactic cues to upcoming plurality in the preceding demonstrative determiners this and these. Under an ideal observer model, the listener will therefore have a weaker probabilistic expectation about the number of the noun in (1a–1b), formed solely on the basis of the acoustic information in the noun stem, than in (2a–2b), where the prediction is supported by the combination of acoustic and morphosyntactic cues in the preceding words.

Crucially, although the ideal observer model might predict stronger overall expectations in the presence of multiple cues, as in (2a–2b), it does not predict different use of those cues. A listener who already has strong probabilistic expectations of a singular noun from the determiner in (2a) will still be able to strengthen those expectations further when they hear the patterns of lengthened stem duration in the noun stem. Likewise, a listener who has weaker expectations, due to the non-agreeing determiner the in (1a), will also be able to adjust those expectations when they encounter the same lengthening in the noun stem. Thus, ideal observer models predict an additive, rather than interactive, combination of cues: Regardless of the presence of morphosyntactic information, subphonemic detail will still be incorporated in the same way to the listener’s perceptual processing. Listeners may be slower to process the final noun in sentences like (1) than sentences like (2), because of a more diffuse probability distribution, but they will derive equivalent benefit from acoustic information that matches the intended interpretation. This idea – of additive rather than interactive benefits of additional cues to noun number – is the baseline ideal observer account.

1.4 Phonetic predictors

A variant account of ideal listener models, which I shall call the phonetic predictor account, argues that listeners not only use these durational cues even when they are redundant in context, but actually predict them from that same context. This view is built upon two sets of evidence. First, listeners are adaptable in their use of phonetic information not just in response to accumulated exposure (Mitterer & Reinisch, 2017; Norris et al., 2003; Nygaard & Pisoni, 1998; L. B. Smith & Colunga, 2012), but also in response to the immediate speech context. Second, listeners can use perceptual processing predictively, such that their ability to process incoming speech is assisted when the phonetic realization of that speech matches predictions formed from the preceding context.

Evidence regarding adaptability in context is plentiful. Dilley and Pitt (2010), for example, observed that listeners use speech rate in deciding whether highly reducible function words, such as or or are, are present in the speech stream. The identical acoustic stimulus is heard as, e.g., leisure or time when the preceding speech rate is fast, but as leisure time when the preceding speech is slowed down. Phoneme category boundaries are even more changeable, shifting as a function of the lexical context of the embedding word (Ganong, 1980); the semantic context of the embedding sentence (Borsky et al., 1998); the pragmatic context of the discourse actors (Rohde & Ettlinger, 2012); and, crucially, the syntactic context of the target phoneme. van Alphen and McQueen (2001), for example, reported that Dutch listeners shift their interpretation of an ambiguous plosive on a /t-d/ continuum as a function of the preceding syntactic context. When the context leads participants to expect a noun phrase, they are more likely to report the plosive to be /d/, resulting in the determiner de, which can appropriately precede nouns. When context leads participants to expect a verb phrase, they shift the category boundary and report more /t/s, resulting in the infinitival marker te. Fox & Blumstein (2016) found a similar pattern in English one word further downstream. They asked listeners to distinguish between an ambiguous bilabial plosive that could form a noun in one case and a verb in the other (e.g., [?ai] allows the noun pie if the plosive is interpreted as voiceless, and the verb buy if the plosive is interpreted as voiced). In contexts requiring a noun, participants shifted their boundary to produce more noun judgments (e.g., reporting more [p], to produce pie). In contexts requiring a verb, participants shifted their boundary in the other direction, (e.g., reporting more [b], resulting in buy).

This type of adaptation applies to morphological structures as well as phoneme category boundaries. Barden & Hawkins (2014) exposed English-speaking participants to an unfamiliar accent that produced the prefix re-, typically pronounced [ɹiː], instead as [ɹɪ]. After exposure, listeners completed a speech-in-noise task. Those who had been exposed to the novel accent more accurately identified new words containing non-standard [ɹɪ] than members of the control group, who had not previously heard the accent. Importantly, this perceptual learning was stronger when the non-standard [ɹɪ] formed a prefix, such as re-supply, than when it formed a non-morphological initial syllable, such as recent. Thus, the adaptation cannot be interpreted solely as a category boundary shift at the phoneme or syllable level, but also reflects listeners’ recognition of the prefixal status of the re- syllable used during training.

In sum, it is undeniable that listeners can flexibly use both syntactic and morphological context to inform their processing of phonetic detail. Yet the studies described above relied on tasks such as transcription and categorization of stimuli. They capture listeners’ offline judgments about the nature of the stimuli, rather than their real-time processing of what they hear. The second foundation of the phonetic predictor account is that listeners use perceptual processing predictively, such that they draw on their knowledge of fine phonetic detail to guide their real-time processing of incoming input in the speech stream.

For evidence of this ability, we must look to work on accent adaptation. Trude & Brown-Schmidt (2012), for example, used visual-world eye-tracking to examine listeners’ real-time use of talker identity to guide perception of key words, both in isolation (Experiments 1 and 2a) and in a sentence context (Experiment 2b). Listeners were exposed to one of two talkers, one male and one female, during a training phase. The male talker used a regional variety of American English that raises /ae/ to [eɪ] before /g/, while the female talker used a General American accent. Listeners were asked to distinguish target words like tack from competitors like tag. They were faster to look to the target when listening to the regional-accented male talker than the standard-accented female talker, and especially so when they had information about the talker’s identity – either through a visual cue of the talker’s face when the target was an isolated word (Experiment 2a) or through a preceding sentence preamble that made the talker’s identity clear (Experiment 2b). In other words, listeners learned that the regional-accented talker would have raised the /æ/ vowel to [eɪ] if the target ended in /g/. With the standard-accented talker, listeners knew not to expect such raising, and so had to wait longer for the disambiguating final consonant to identify the target. Thus, this study shows that listeners can form highly specific predictions about the phonetic form that upcoming words might take, and use those predictions in real time to guide perception.

Yet there are limits on this type of phonetically detailed prediction. Trude et al. (2013), for example, failed to replicate this pattern when listeners were exposed to a French-accented speaker who shifted /i/ toward [ɪ] before voiceless plosives, while maintaining the [i] vowel quality before fricatives. When asked to distinguish a target word like wheat from a competitor like wheeze, listeners had more difficulty with the French-accented talker – even though that talker distinguished between the vowel qualities in these two words – than with the English-accented talker, who used the same [iː] in both target and competitor. In this case, listeners could not use this information predictively.

These studies suggest that listeners are capable of adapting their processing of phonetic detail across a wide variety of contexts, and that they can develop quite detailed and specific phonetic predictions in at least some situations. Nevertheless, such findings have only been demonstrated in relation to the phonetic realization of a specific (morpho)-phonological contrast, such as the difference between two realizations of a prefix (Barden & Hawkins, 2014) or a vowel (Trude & Brown-Schmidt, 2012; Trude et al., 2013); a place of articulation contrast in fricatives (Rohde & Ettlinger, 2012); or a voicing contrast in plosives (Borsky et al., 1998; Fox & Blumstein, 2016; Ganong, 1980; van Alphen & McQueen, 2001). Lengthening or shortening a stem in the absence or presence of a suffix is not a phonological process of the type these perceptual learning studies have examined. There is no categorical contrast to be realized. It is a much more gradient phonetic process, and so it remains an open question whether listeners include that degree of detail in their predictions.

If they do, then we can expect a different set of results from the baseline ideal observer account summarized in 1.3. Consider once again the contrast between (2a) and (1a). In the case of (2a), the presence of an agreeing determiner signals that the upcoming noun is singular, and hence likely to be unsuffixed. Unsuffixed nouns tend to have longer stem durations. Thus, the determiner could allow the listener to adjust their probabilistic distribution over likely stem realizations to favor longer duration. When the listener encounters a noun stem that is, in fact, lengthened, it will land in a high-density region of this probability distribution, allowing the stem to be more quickly processed. If the noun stem is shortened, it will land in the lower-density tail of that distribution, and so take longer to be processed. As a result, after a singular determiner, a lengthened singular stem should be processed more easily than a shortened singular stem.

By contrast, after non-agreeing determiners, as in (1a), no predictions about noun number, suffix status, and hence stem duration, can be formed. The probabilistic distribution over likely stem durations will need to remain flatter, to encompass a wider range of durations. The probability density of the distribution where a lengthened stem lands will be more similar to the density in the region where the shortened stem lands. The listener will therefore process the two stem durations more similarly after non-agreeing determiners than after agreeing determiners. They should also take slightly longer to make a decision, because they will need to wait until they reach the end of the word to incorporate morphosyntactic information into their distribution of likely interpretations.

In sum, ideal observer models predict two possible sets of outcomes. The phonetic predictor account says that listeners make phonetically detailed predictions about stem durations, and so the benefits of listening to nouns whose stem durations match their number should be stronger after agreeing determiners, which allow those durations to be predicted, than after non-agreeing determiners. In other words, it predicts an interactive use of cues. This contrasts, recall, with the baseline ideal observer account, which predicts an additive use of cues. Since listeners’ predictions under the baseline ideal observer account do not include as high a level of phonetic detail, the benefits of listening to nouns with expected stem durations will not differ across determiner types.

Note that both of these accounts agree in granting listeners equal ability to detect patterns of fine phonetic detail in online speech perception. They differ only in the extent to which listeners make syntactically informed predictions about whether to expect that pattern in upcoming speech.

1.5 Strategic listeners

A third perspective derives from the fact that cognitive capacity is limited. Monitoring, predicting, and processing incoming information across many different types of linguistic representation is cognitively expensive, and it is not obvious that listeners are always willing to expend that effort in all circumstances. This final view, which I shall call the strategic listener perspective, holds that listeners’ attention to different types of information can change dynamically, adapting to circumstances. Kim et al. (2020), for example, demonstrated this adaptability in a perceptual study focusing on the difference between the vowels [ɛ] and [æ]. When the stimuli were manipulated to reduce spectral cues to vowel identity, listeners shifted their attention to durational cues. Intriguingly, exploratory analyses suggested that this flexible use of cues was larger for listeners with weaker inhibitory control. The authors propose that this pattern reflects a tendency for weak inhibitory control to be associated with the ability to keep attentional focus broad, across a larger variety of information. This breadth of attentional focus could have facilitated the shift in relative cue-weighting for listeners with low inhibitory control. Rather than disregarding less important cues to vowel identity, such as duration, they were more likely to attend to it, and so were well-placed to up-weight its importance when spectral cues became unreliable.

When cognitive resources are taxed, attention to acoustic detail is reduced. Mattys and colleagues showed that increasing cognitive load interferes with a listener’s ability to process low-level acoustic detail, shifting focus instead to lexico-semantic information (Mattys & Wiget, 2011; Mattys et al., 2009, 2014). Data from Clayards et al. (2021) further suggests that this flexible attention to cues may be deployed strategically: Over the course of an experiment, listeners became less responsive to fine phonetic detail distinguishing the initial syllable in words like discolor from its counterpart in words like discover. The authors propose that this reduced use of phonetic cues reflects rapid, strategic adaptation to the task: Listeners learned that the rest of the sentence would disambiguate the target words, thus obviating the need to attend to phonetic detail so closely.

Christiansen & Chater (2016) describe the need to keep up with the ever-changing flood of input in spoken language processing as the ‘Now-or-Never Bottleneck’, which listeners navigate through strategic compression. One of the core principles of the Now-or-Never Bottleneck is that listeners process input according to a hierarchical organization of linguistic representations from lower-level, less abstract units, such as the phonetic speech stream, into higher-level, more abstract units, such as words and discourse. Incoming ‘chunks’ of perceptual information are compressed rapidly into abstract, higher-level units, and passed up to other levels of representation to free up low-level processing capacity for new input. Low-level information lost during compression is lost for good: If it’s not processed now, it is never processed at all. Under this view, dynamic changes in perceptual processing reflect changes in which information gets lost during the initial compression. When processing demands increase, but the real-time flow of new input does not abate, listeners must compress more of the low-level information to keep up, which means that more acoustic detail is lost during compression. It is this trade-off that is responsible for the observed patterns, such as reduced use of acoustic-phonetic detail under high cognitive load conditions (Mattys & Wiget, 2011; Mattys et al., 2009, 2014).

Kuperberg & Jaeger (2016) propose that the trade-off between cognitive capacity limitations and information processing might be understood in terms of a utility function. This utility function weighs the benefits of accurately predicting upcoming information from previous cues against the ‘metabolic costs’ associated with forming those predictions (p. 44). Thus, listeners may disregard certain types of cues that are not immediately relevant to the listening task at hand, such as semantic information in a phoneme monitoring task. They may further disregard or down-weight those cues which are not as reliable as other cues at hand, or carry less new information in a given context. The benefit of using these cues is not worth the effort of making the predictions. Importantly, optimizing these metabolic costs can influence the types of cues listeners attend to in real-time speech processing. Since acoustic input changes quickly, listeners making predictions about upcoming phonetic information must make many more predictions for a given stretch of speech than they would need to make if they were predicting syntactic structures. A rational listener, according to Kuperberg & Jaeger (2016), would therefore hesitate to predict upcoming phonetic information unless the benefits to perceptual processing are sufficient to justify the metabolic cost associated with making those predictions.

Consider now how strategic listeners would handle sentences like (1–2), repeated below for ease of reference.

In sentences like (1a–b), the lack of morphosyntactic information in the determiner means that listeners would need to wait until the very end of the noun to determine whether it was unsuffixed or suffixed, singular or plural. In such a context, they might well find it worth their while to attend to the rapidly-changing phonetic information in the stem. Even though they must use it to form an additional prediction quite rapidly, incurring an additional metabolic cost (Kuperberg & Jaeger, 2016), that prediction still gives them a head start on building their expectations about the grammatical number of the noun in question. By contrast, in sentences like (2a–b), the demonstrative already provides ample information about the number of the noun, and the suffix or lack thereof will offer further confirmation. In these sentences, there is little benefit to using this phonetic information to make further predictions about the form or number of the noun. Unlike phonetic predictors, a strategic listener would not bother expending the effort to predict the lengthening associated with an unsuffixed singular stem, or the shortening associated with a suffixed plural stem. And unlike baseline ideal observers, they would not bother attending to that detail to form predictions about the likely presence or absence of the upcoming suffix. In sum, a strategic listener may be more attentive to segmental compression patterns after non-agreeing determiners – where they carry more information – than after agreeing determiners.

1.6 The current study

The current study was designed to test whether listeners use their knowledge of segmental compression to the same extent as polysyllabic shortening, and, if so, whether that use depends upon morphosyntactic context. The baseline ideal observer account predicts that listeners should use phonetic compression similarly, regardless of morphosyntactic context. The phonetic predictor account holds that listeners should use phonetic compression more after agreeing determiners than non-agreeing determiners; and the strategic listener account predicts that listeners should use phonetic compression more after non-agreeing determiners than after agreeing determiners.

To test these three accounts, I constructed a visual-world eye-tracking study, and asked listeners to click on a target picture whose name was embedded in sentences like (3–4). Note that the (a) sentences will serve as the critical stimuli, while the (b–c) sentences will serve as fillers.

In such sentences, lengthening the duration of a singular target stem in (3a) and (4a) should facilitate listeners in distinguishing it from plural and two-syllable competitors, because the longer duration matches listener expectations about one-syllable, unsuffixed noun stem duration. By contrast, shortening the duration of the stem should cause more difficulty in disambiguating a singular target from competitors, because the shortening should lead listeners to expect a plural suffix or further syllables – a prediction which does not match the eventual speech signal. This pattern of superior identification of singular targets when their stems are lengthened rather than shortened shall be called the Match effect. By comparing the Match effect when listeners must distinguish between singular targets and plural competitors to the Match effect when they must distinguish between singular targets and two-syllable competitors, we will be able to compare listeners’ use of segmental compression to polysyllabic shortening in their online speech perception. Then, by comparing the Match effect after agreeing determiners, as in (3), relative to non-agreeing determiners, as in (4), we will be able to distinguish between the baseline ideal observer, the phonetic predictor, and the strategic listener accounts.

2.1 Materials

Critical stimuli were built around a set of 84 stems, which appeared in one of three forms: a singular (e.g., rock), a plural (rocks), and a two-syllable word (rocket). The singular served as the target in critical trials, as in (3a) and (4a), while the plurals and two-syllable words were the targets of filler trials, as in (3b–c) and (4b–c). These triplets were selected to satisfy to the following four priorities:

• The singular had to end in a non-sibilant consonant, to avoid the /-әz/ allomorph of the suffix, which would increase the syllable count.

• All three members of the triplet needed to be nouns, given the use of the determiner to manipulate syntactic context.

• All three members of the triplet needed to be sufficiently imageable for use in a visual-world eye-tracking paradigm.

• The two-syllable word needed to lexically embed the singular stem in its first syllable, without using morphological derivation to achieve the embedding. This allowed the materials to fully dissociate the morphological distinction between singulars and plurals from the syllable-count distinction between singulars and two-syllable words.³

Given these constraints, the 84 stems selected represented approximately the full range of such triplets available in the English vocabulary.⁴ For this reason, it was not possible to further control for properties such as frequency or prosodic structure.

In Agreeing contexts, each target word appeared after an agreeing determiner (e.g., this/that for singular and two-syllable targets, and these/those for plural targets), while in Non-Agreeing contexts, they were uniformly preceded by the. In all sentences, the word immediately following the singular target began with the same initial phonemes as the continuation of the two-syllable word. For example, the portion of the sentence following the singular target rock in both the Agreeing sentence frame (3a) and the Non-Agreeing sentence frame (4a) begins with [әt]. This preserves a segmental ambiguity between the competitor rocket ([ˈɹakәt]) and the sequence in the stimulus sentence rock attracts ([ˈɹakәˈʧɹakts]).⁵ In this particular example, the following context fully matched the second syllable of the two-syllable word, but in most cases it was not possible to match more than the onset and nucleus of the second syllable, or occasionally just the initial onset. Forcing a more complete overlap in the following phonological environment would have required torturing the sentences beyond any hope of producing naturalistic stimuli. A full list of sentences is provided in the Appendix, in Table 7.

Matching the following phonological context with the second syllable of the two-syllable word served to minimize any coarticulation bleeding into the noun stem from the following word. This removed a disambiguating cue that listeners might otherwise use to exclude the two-syllable competitor from consideration. It also served to maximize the period in which listeners would have to draw solely on durational cues to identify the target before later phonemic information became available. One consequence of this manipulation is that singulars were disambiguated from plural competitors earlier, at the end of the stem, than from two-syllable competitors. I return to this point in the Section 4.

The raw recordings, therefore, comprised a set of six sentences for each stem, crossing the three word forms (singular, plural, two-syllable) with the two sentence contexts (Agreeing, Non-Agreeing) to produce a set of the sort shown in (3–4). All recordings were produced in a sound-attenuated booth by a native male speaker of Scottish Standard English, who was not aware of the experimental focus on polysyllabic shortening and segmental compression. He was instructed to produce all sentences in such a way as to make the six versions of each item as similar as possible. He read each set of sentences twice, and the recordings with the most similar prosodic structure across the three word types were selected for further analysis and stimulus-creation.

2.2 Acoustic analysis

Since Scottish Standard English has not previously been used in studies of polysyllabic shortening and segmental compression, an initial analysis of the selected raw recordings was conducted to confirm that the speaker’s natural, unmanipulated speech still contained the durational patterns of interest. Accordingly, the raw stem durations (see Table 1) were extracted, log-transformed, and analyzed with linear mixed effects regression in Julia (version 1.10.2; Bezanson et al., 2017), using the package MixedModels (version 4.24.0; Bates et al., 2024).

Independent variables included Word Type, Context, and Voicing. The values for Word Type were Singular, Plural, and Two-Syllable, as described in 2.1. The values for Context were Agreeing, which coded sentences containing agreeing demonstrative determiners, and Non-Agreeing, which coded sentences using the non-agreeing determiner the. Finally, Voicing coded for whether the final obstruent of the stem was Voiced or Voiceless, to account for the well-known fact that the voicing of a coda consonant or consonant cluster produces substantial, highly perceptible durational effects on the preceding vowel (Raphael, 1972).

All variables were contrast-coded, to ensure that coefficient estimates for simple effects reflected proper main effects (Brehm & Alday, 2022). Context and Voicing were effects-coded, with Agreeing and Voiced set at 1, and Non-Agreeing and Voiceless set at –1. Thus, positive model coefficients indicate longer stem duration for Context compared to No-Context, and for Voiced compared to Voiceless, while negative model coefficients indicate the reverse. Word-type was Helmert-coded. The first contrast compared Two-Syllable stem duration with the combination of Singular and Plural stem duration, to confirm that the polysyllabic shortening pattern was present. The second contrast compared Plural stem duration with Singular stem duration, to confirm that the expected segmental compression pattern was present.

The model’s fixed effects included all three variables, plus their higher-order interactions, while the random effects included intercepts by stem, along with random slopes for the interaction Word Type and Context. As Voicing did not vary within stems, it was not included as a random slope.

The final model summary is provided in Table 2, with its effects visualized in Figure 1. The model revealed that, as expected, Two-Syllable durations were shorter than Plural and Singular durations (β = –0.038, p = .004), reflecting a polysyllabic shortening pattern. Plural durations were also quite substantially shorter than Singular durations (β = –0.123, p < .001), reflecting a robust segmental compression pattern – which, as Figure 1 shows, seemed to be even stronger than the polysyllabic shortening pattern for stems with voiced final sounds. Whereas voiceless stem durations (right panel, Figure 1) were shortest in Two-Syllable words, voiced stem durations (left panel) were shortest in Plurals. The negative coefficient for Context indicates that stems were shorter after Agreeing determiners than after Non-Agreeing the (β = –0.048, p < .001), while the positive coefficient for Voicing confirms that, as expected, stems were longer when they ended in voiced sounds than voiceless obstruents (β = 0.081, p < .001).

The interactions are visualized in Figure 1. The positive interaction terms for Word Type by Context indicate that the effect of Word Type was larger for Non-Agreeing than Agreeing conditions. This was a non-significant tendency for the difference between Two-Syllable against other Word Types (β = 0.014, p = .064), but significant for the difference between Plural and Singular (β = 0.032, p < .001). The positive interaction term between Voicing and the contrast between Two-Syllable against other Word Types corresponds to a stronger polysyllabic shortening effect with Voiceless stems compared to Voiced stems (β = 0.127, p < .001). The negative interaction term for Plural against Singular indicates that the segmental compression effect was weaker in Voiceless stems relative to Voiced stems (β = –0.089, p < .001). The positive interaction term between Context and Voicing suggests that the effect of Context may be slightly larger with Voiceless stems than with Voiced stems. Finally, the negative three-way interaction (β = –0.02, p = .031) reveals that the difference between Plural and Singular stems, which is exaggerated in Non-Agreeing compared to Agreeing contexts, is not quite so exaggerated with Voiced stems as it is with Voiceless stems.

Figure 1: Model effects plot for the interactions between Word Type, Context and Voicing in the recorded stimuli. Error bars indicate 95% confidence intervals.

With this confirmation in hand that the raw speech of the speaker contains both the polysyllabic shortening and segmental compression patterns of interest in its natural form – albeit in varying degrees across Context and Voicing – the next step was to manipulate the speech to produce the desired experimental conditions.

2.3 Acoustic manipulation

For each item, each of the six raw sentences was segmented in Praat (Boersma & Weenink, 2015) into the following component elements: The preamble consisted of everything up to the onset of the determiner. The determiner followed, with the stem next, followed by the suffix in plural sentences and the rest of the word in sentences containing two-syllable targets. The postamble consisted of everything after the offset of the word (Figure 2, step 1). The stem was further subdivided into onsets, vowels, sonorant codas, and obstruent codas. Segmentation criteria generally followed recommendations from Turk et al. (2006), and are described in more detail in the supplemental materials on the Open Science Framework (OSF) archive for this project.

Figure 2: Sample algorithm for creating critical stimuli containing singular targets designed to be presented with a plural competitor. (1) Raw sentences are first segmented into preambles, determiners, stems, and postambles. (2) The target durations for Match and Mismatch stems are then calculated in order to achieve an 80 ms difference between the Match and Mismatch stems, with the Match stems 40 ms longer than the average duration of the singular and plural raw durations, and the Mismatch stems 40 ms shorter. (3) The duration of the rime of the extracted singular stem is adjusted so that the full stem yields the target Match and Mismatch durations. (4) The sentence components are spliced together, forming Match and Mismatch stimuli. In this case, the preamble is used from sentences recorded with two-syllable targets, followed by the original determiner from the singular raw recording, followed by the adjusted stem, and finishing up with the original postamble from the singular raw recording.

Next, the duration of each critical target – which was always the singular stem – had to be set. Within each sentence, the critical singular target needed to be adjusted to appear in a Match or a Mismatch condition. In the Match condition, the singular target was lengthened, so that it contained duration cues consistent with a word boundary immediately following the stem. In other words, Match targets were designed to signal that the word was singular. In the Mismatch condition, the stem was shortened, so that its acoustic cues signaled that something would follow the stem – either an additional segment on the syllable coda, such as a plural suffix, or else an additional full syllable. In other words, Mismatch targets were designed to mislead the listener into expecting a plural or two-syllable noun.

The raw durational differences between singular and two-syllable stems were not consistently higher or lower than the raw durational differences between singular and plural stems. When stems were voiceless, two-syllable stems were shorter than singulars to a greater degree than plural stems, while when stems were voiced, it was plural stems that showed the greatest degree of shortening. For this reason, the same durational manipulation was used to distinguish Match and Mismatch targets, regardless of the nature of the competitor object. I settled on an 80 ms difference, which was large enough to be detectable, but not so extreme as to sound unnatural to my ear. Previous pilot work had determined that Match effects were elicited by durational differences of 60 ms and 90 ms, so the final difference of 80 ms used here fell within that range.

The procedure for manipulating the duration of the rimes is visualized in Figure 2, steps 2–3. This was done twice for each stem, depending on the intended competitor type. In a trial with a singular target and a plural competitor, the raw durations of the singular and plural recordings were averaged together; to this was added 40 ms to arrive at the Match duration for the singular, and from this was subtracted 40 ms to arrive at the Mismatch duration for the singular. In a trial with a singular target and two-syllable competitor, the singular and two-syllable raw stem durations were averaged together, after which everything proceeded in the same way as before (Figure 2, steps 2–3).

The durations of all stem rimes were then adjusted to make the stem match the target Match or Mismatch duration, using the overlap-add implementation of PSOLA in Praat. I applied the durational manipulation only to the rime of the singular stem – i.e., vowel + coda, treated as a single unit – because previous work on polysyllabic shortening patterns in Dutch has suggested that the bulk of the durational effects are carried by the rime, while the onset is invariant across forms (Kemps, Ernestus, et al., 2005). This invariant onset possibly serves to provide listeners with a baseline against which the rest of the stem durational patterns can be evaluated. English is not, of course, Dutch, but the two languages show a great deal of similarity in listeners’ use of polysyllabic shortening patterns (for Dutch, see Salverda et al., 2003; Shatzman & McQueen, 2006; for English, Blazej & Cohen-Goldberg, 2015; Davis et al., 2002; for both together, see Kemps, Wurm, et al., 2005). More practically, adjusting the duration of the entire stem resulted in some unnatural-sounding tokens, particularly those beginning with voiceless stops, while limiting the duration to the rime alone produced better-sounding results.

Finally, it was necessary to ensure that the preamble contained no cues biasing the listener to expect either the singular target (rock) or its competitor. If the competitor was plural, then neither the preamble from the raw singular recording (as in (3a) or (4a)) nor the preamble from the raw plural recording ((3b) or (4b)) could be used. Therefore, for singular targets with plural competitors, the final sentence was constructed by splicing the determiner, manipulated stem and following postamble onto the preamble of the raw two-syllable recording ((3c or 4c)). This ensured that any cues in the preamble to the identity of the upcoming noun would point to the two-syllable word (rocket), which is not present on the screen, rather than to the target (rock) or the competitor (rocks). By the same logic, when the competitor was a two-syllable word (rocket), the determiner, manipulated stem, and postamble were spliced onto the preamble from the plural recording ((3b or 4b)), so that any unintentional cues in the preamble would point to a plural noun, absent from the screen, rather than a singular target or two-syllable competitor (Figure 2, step 4). This splicing also had the effect of ensuring that all possible speech rate cues were fully controlled, as the same preamble was used across different Match conditions. In this way, speech rate has no opportunity to affect how listeners process durational cues (Dilley & Pitt, 2010; Pitt et al., 2016).

Selections of these manipulated recordings were evaluated by members of the Glasgow University Laboratory of Phonetics at a lab meeting, and a full experimental list was further evaluated by a trained phonetician in the context of an experimental run. Both sets of evaluators were fully informed about the logic and design of the manipulations, and the lab group was further informed about the identity of each recording – whether it had been lengthened or shortened. The lab group were instructed to attend to the length contrast between Match and Mismatch conditions, while the trained phonetician was instructed to listen carefully for acoustic artifacts as the experiment unfolded. Both groups reported that the manipulations in question (durational contrast for the lab group; acoustic artifacts for the trained phonetician) sounded natural. Experimental participants were all asked about the quality of the audio, and although they volunteered comments about the structure of the sentences, the soothing quality of the speaker’s voice, and their subjective experience of being temporarily confused between singular and two-syllable stems, none reported noticing a durational manipulation, and all agreed that the quality of the audio was excellent.

Filler sentences – those containing plural or two-syllable targets – were created according to the same procedure, differing only in that the stems for the plural and two-syllable fillers appeared only in the shortened form. This ensured that the noun target in all sentences was manipulated, avoiding any systematic auditory difference between the critical and filler targets. Further, this manipulation provided a crucial experimental control. As Clayards et al. (2021) observed, listeners might reduce their use of durational cues across the course of an experiment if they learn that those cues are unreliable. No matter how uniformly, predictively, or strategically listeners can deploy their knowledge of polysyllabic shortening and segmental compression patterns, they may cease to do so if they repeatedly encounter critical stimuli whose Match manipulations render such patterns useless. Presenting all filler targets with a shortened stem mitigates this problem, because it reinforces the fundamental experimental assumption that there is a roughly 80 ms difference between a singular stem and a plural or two-syllable stem. In filler trials, with plural or two-syllable targets, this pattern was uniformly true. In critical trials, with singular targets, this pattern was true half the time. With a 50/50 split between critical and filler trials, the entire experimental list, therefore, tested participants’ ability to make use of a pattern that was 75% reliable. Thus, whatever listeners do with polysyllabic shortening and segmental compression, they are unlikely to stop doing it on the basis of the durational manipulation in the critical stimuli.

Plural fillers were spliced onto two-syllable preambles, parallel to their singular counterparts, and two-syllable fillers were spliced onto plural preambles. Thus, not only were all speech rate cues controlled across Match and Mismatch critical stimuli, they were also controlled across critical and filler stimuli.

2.4 Design

For each of the 84 stems, twelve critical sentences were created. Four of these contained a singular target whose duration was manipulated against its two-syllable competitor. These four sentences represented four conditions, formed by crossing the factors of sentence Context (Agreeing/Non-Agreeing) with duration Match (Match/Mismatch). Another four sentences all contained a singular target whose duration was manipulated against the plural competitor. They likewise represented the four conditions formed by crossing sentence Context and duration Match. These eight sentences comprised the audio stimuli for each item in a critical trial.

The final four sentences constructed for each stem formed the audio stimuli for filler trials. These contained the plural or two-syllable word as the target, and they varied across the two Context conditions. However, they only contained the Match duration, so that filler trials served to reinforce the expected polysyllabic shortening or segmental compression patterns. Each experimental list contained 42 critical and 42 filler trials. Stems were rotated across experimental lists in a Latin Square design, to ensure that all stems appeared in all conditions for critical trials, while each participant saw each stem only once.

The visual world for each trial contained four images: the target, its competitor, and two distractors. On critical trials, the target was always singular, and its competitor could be either plural or two-syllable. On filler trials, the target was either plural or two-syllable, and its competitor could be singular, two-syllable (if the target was plural), or plural (if the target was two-syllable).

Distractor pictures were selected so that in the critical trials, the relationship between the target and competitor was mirrored in the distractors. Thus, if a critical trial had a singular target and plural competitor, the distractor pictures also contained a singular and plural version of another stem. If a critical trial had a singular target and a two-syllable competitor, the distractor pictures were themselves a singular word and its two-syllable counterpart. In filler trials, the relationship between target and competitor was mirrored in the distractors half the time, but not the other half of the time. These fillers ensured that participants would not be able to reliably recognize that some trials were about singular vs. plural, while others were about singular vs. two-syllable, because a full quarter of all trials mixed word types.

For the rest of this article, singular targets with plural competitors will be called num-targets, as the key feature distinguishing target and competitor is grammatical number. Singular targets with two-syllable competitors will be called syll-targets, because the feature distinguishing these targets from their competitors is syllable count.

2.5 Participants

The experimental protocol was approved by the College of Arts Ethics Committee at the University of Glasgow. Fifty-one participants (28 female, 18 male, 3 other; mean age = 27, ranging from 18 to 67) were recruited from the University of Glasgow community, as well as through Glasgow-specific social media advertisements (specifically, the r/Glasgow and r/GlasgowMarket subreddits). Data from two subjects was discarded, one for not being a native Scottish English speaker, and another for a voluntarily disclosed early-onset pseudo-dementia diagnosis. All remaining participants were native Scottish English speakers, with normal or corrected-to-normal vision and hearing. Participants were compensated with £12 for their time.

2.6 Procedure

The experimental procedure began with the collection of informed consent from participants, after which they filled out a brief language history questionnaire, and completed the visual-world eye-tracking experiment.

Eye-tracking data was collected from the right eye at a 1000 Hz sampling rate with an Eyelink 1000+ camera; stimuli were presented using the Experiment Builder software, with audio stimuli playing binaurally over headphones at a comfortable volume. Participants were informed that they would see four images appear on the screen, and hear a sentence mentioning one of the images. They were instructed to use the mouse to select the picture on the screen that was named in the sentence. Each experiment began with a nine-point calibration and validation, which was repeated at the beginning of each block. The experiment began with seven practice trials before the 84 trials of the experiment itself, which were split into three blocks of 25, 25, and 34. Fillers and critical trials were randomly presented in each list, with the restriction that no particular combination of target type (singular, plural, two-syllable) and competitor type was presented more than twice in a row.

Each trial began with a drift correction fixation point on the screen. After participants fixated upon the point, four images appeared on the screen, each labeled underneath the image. Participants had 3.5 seconds to look freely at this screen to familiarize themselves with the pictures’ names and locations on the screen before the audio began to play. Participants could not select an image before the audio began to play, but they were free to click on an image before the audio was completed. If they selected the correct image, a green frame appeared around the selected image; if they selected the wrong image, a red frame appeared around the selected image. Regardless of their selection, the entire audio file played to completion before the screen reset to the drift correction fixation point.

2.7 Analysis

2.7.2 Gaze traces

To analyze the gaze data, participant gazes were first binned into time windows. Since the duration of the determiners varied, with the being substantially shorter than demonstrative determiners, time windows during the determiner were simply set to cover the first half and second half of the determiner duration. Thus, the offset of the determiner occurred at the end of time window 2, regardless of the actual duration. Target word durations also varied across items, and within items, too, depending on whether they were in the Match or Mismatch condition. Therefore, time windows during the target word were also defined proportionally – this time as quintiles of the word’s duration. Thus, the third time window covered the first fifth of the target word’s duration, regardless of whether it was a Match or Mismatch word, the fourth time window covered the second fifth, and so on. The end of the seventh time window thus corresponded to the offset of the target word.

Time windows after the offset of the target were then set at 50 ms, to reduce the inherent dependencies holding between gaze positions separated by a single millisecond (Barr, 2008). Visual inspection of the overall averaged gaze trajectories revealed that the proportion of looks to syll-targets reached their peak by the 30th time window, or about 1100 ms after target offset, and for num-targets by the 25th window, or 850 ms after target offset. Accordingly, only the first 30 time windows were retained for statistical analysis for syll-targets, and only the first 25 time windows were retained for num-targets.

Within each time window, the proportion of looks to target and looks to competitor was calculated by taking the number of samples in which the gaze position rested on the target or competitor interest area, and dividing by the number of readable samples within the time window. This calculation had the effect of removing blinks or other unreadable samples. Samples registered during saccades were retained, as many saccades moved from one location to another within the same interest area – e.g., shifting from the label at the bottom of the image to the image itself. All proportions were then converted into empirical logits (elogs) (Barr, 2008). Finally, the target advantage was calculated by subtracting the elog-converted looks to competitor from the elog-converted looks to the target. This difference – henceforth termed target advantage – provided the dependent variable in the gaze-trace analysis.

Even with the time-window binning strategy, the distribution of the dependent variable was far from normal. Instead, it had a trimodal distribution, with peaks near –10, 10, and 0 (Figure 3). These peaks correspond to a preponderance of time windows in which listeners are looking, respectively, only at the competitor, only at the target, or at something else entirely, such as a distractor or the central fixation cross. Only time windows containing saccades with either target or competitor as the endpoint would produce values outside these three peaks. Yet regression models which estimate means of the dependent variable assume the normality of the dependent variable; and that assumption is not met here. Therefore, the data were analyzed with empirical Bayesian non-parametric quantile regression modeling (QGAMs; Baayen et al., 2022; Fasiolo et al., 2021). QGAMs are an extension of generalized additive mixed modeling methods (GAMMs; Sóskuthy, 2017; Wood, 2011, 2017), with the advantage that they do not rely on assumptions about the distribution of the dependent variable.

Figure 3: Histogram of target advantage values in empirical logits in the binned gaze data, separated by target type (Num-targets on top; Syll-targets on bottom) and Match (lengthened Match targets on left, shortened Mismatch targets on right). All binned gaze data is profoundly trimodal, with two peaks at either end of the distribution, and one at 0.

QGAMs estimate not mean values of the dependent variable, but a given quantile. Here, I report the results for the median, although the supplemental materials on the OSF archive for this project provide additional analyses examining the effects of Match and Context across different quantiles of target advantage. All analyses were conducted in the qgam package (version 1.3.4; Fasiolo et al., 2021). Syll-targets were analyzed with separate models from num-targets, but the analysis structure for each target type was comparable. In each QGAM, two types of predictor variables were included: parametric terms and smooth terms. The parametric terms operate similarly to terms in a linear model. Their coefficients capture the overall (median) height of the curve across the entire time period of interest (here, the first 25 or 30 time windows). Context and Match were both effects-coded, with the values Agreeing and Match set at 0.5, and Non-Agreeing and Mismatch set at –0.5. Thus, positive parametric coefficients for Context indicate higher overall values of target advantage for Agreeing sentences (in which the stem is preceded by this/that) than for Non-Agreeing sentences (in which the stem is preceded by the), and positive parametric coefficients for Match indicate higher overall target advantage for lengthened Match stimuli than for shortened Mismatch stimuli.

The smooth terms capture the dynamic changes of gaze trajectories across that time period, allowing us to observe differences in the curve shapes – e.g., steeper or shallower increases that begin or peak at different time points – even if there is no overall difference in the gaze proportions through the period of interest. Thus, QGAMs not only allow us to avoid violating assumptions about the distribution of the dependent variable; they also allow us to understand the nature of the gaze traces in a more nuanced way than traditional ANOVAs, and are more flexible in capturing different types of curve shapes than the strictly polynomial terms used in growth-curve analysis (Mirman, 2014).

The smooth terms for key variables were specified with thin plate regression splines. Both models further included random factor smooths by subject, to control for individual variation across gaze trajectories, and random factor smooths by item, to control for item-specific effects springing from variation across prosody, phoneme overlap, lexical frequency, and so on.

Because QGAMs do not allow for the straightforward interaction between factors in the smooth terms, interactions in the smooths were examined by crossing the Context and Match terms to create a four-level factor. All factors in the smooth terms were treatment-coded. Because the three accounts described in 1.3 through 1.5 differ specifically in their predictions regarding the interaction between Match and Context, both models contained Match, Context, and their interactions in both parametric and smooth terms.

For num-targets, the baseline ideal observer account predicts no interaction between Match and Context; the phonetic predictor account predicts larger effects of Match in Agreeing conditions than Non-Agreeing conditions; and the strategic listener account predicts a larger effect of Match in Non-Agreeing conditions than Agreeing conditions. All three accounts, however, concur that there should be no interaction between Match and Context for syll-targets, because the presence or absence of a determiner like that is irrelevant to distinguishing a word like cart from a word like carton.

3.1 Behavioral results

3.1.1 Accuracy

The model summary for the accuracy analysis is provided in Table 3, and a visualization of the partial effects is provided in Figure 4.

Table 3: Summary of the logistic regression model of accuracy. All variables are effects coded, with Mismatch, Non-Agreeing Context, and Syll-target Target Type set to –1, while Match, Agreeing Context, and Num-target Target Types set to 1. Positive coefficients reflect higher accuracy for Match, Agreeing Context, and Num-targets.

Dependent variable: accuracy
Variable	Estimate	(SE)	z	p
Intercept	5.187	(0.506)	10.26	<.001
Context	–0.065	(0.208)	–0.31	.753
Target Type	2.34	(0.360)	6.49	<.001
Match	–0.203	(0.231)	–0.88	.380
Context × Target Type	0.130	(0.219)	0.60	.552
Match × Context	–0.109	(0.268)	–0.40	.686
Match × Target Type	–0.465	(0.236)	–1.97	.048
Match × Context × Target Type	0.024	(0.257)	0.09	.925
Observations	2,058
Log Likelihood	–452.011
Akaike Inf. Crit.	1064.021
Bayesian Inf. Crit.	1514.380

Figure 4: Partial effects plot for the accuracy model, showing interactions between Match and Target Type. Error bars indicate 95% confidence intervals.

The effect of Target Type reflected higher accuracy with Num-targets compared to Syll-targets (β = 2.34, p < .001). There was no significant main effect of Match, but the interaction between Match and Target Type reveals that Syll-targets did have higher accuracy with Match than with Mismatch targets (β = –0.465, p = .048). As Figure 4 illustrates, it is unsurprising that Num-targets showed little effect of Match, as their accuracy was already at ceiling.

3.1.2 Reaction time

The model summary for the reaction time analysis is provided in Table 4, and a visualization of the partial effects is provided in Figure 5.

Table 4: Summary of the linear regression model of reaction time. All variables are effects coded, with Mismatch, Non-Agreeing Context, and Syll-target Target Type set to –1, while Match, Agreeing Context, and Num-target Target Types set to 1. Positive coefficients reflect slower RT accuracy for Match, Agreeing Context, and Num-targets.

Dependent variable: log reaction time
Variable	Estimate	(SE)	z	p
Intercept	7.312	(0.037)	197.47	<.001
Context	–0.001	(0.007)	–0.13	.898
Target Type	–0.083	(0.009)	–9.23	<.001
Match	–0.032	(0.007)	–4.53	<.001
Context × Target Type	0.018	(0.0068)	–2.66	.008
Match × Context	–0.012	(0.008)	–1.58	0.114
Match × Target Type	0.014	(0.006)	2.15	0.021
Match × Context × Target Type	–0.002	(0.007)	–0.40	.689
Observations	1,820
Log Likelihood	–198.067
Akaike Inf. Crit.	558.139
Bayesian Inf. Crit.	1004.173

The effect of Target Type revealed faster RTs to Num-targets than Syll-targets, (β = –0.083, p < .001), while an effect of Match revealed faster RTs to Match targets relative to Mismatch targets (β = –0.032, p < .001). These effects were qualified by two interactions. The first, between Target Type and Context (β = .018, p = .008), reflects a larger effect of Target Type in Agreeing contexts (filled triangles, left side of Figure 5) compared to Non-Agreeing contexts (grey diamonds). The second interaction, between Match and Target Type (β = .014, p = .021), reflects a larger Match effect for Syll-targets compared to Num-targets (right-hand panel of Figure 5).

Figure 5: Partial effects plot for RT model, showing the interactions between Context and Target Type (right), and Match and Target Type (left). Error bars indicate 95% confidence intervals.

In summary, then, the behavioural results reveal higher accuracy and faster RTs for num-targets compared to syll-targets, and a larger Match effect for syll-targets compared to num-targets. There is also evidence from the interaction between Context and Target Type that participants are alert to the agreement information, as the presence of an agreeing determiner facilitates RTs to num-targets more than to syll-targets. However, there was no three-way interaction in the behavioural data to support the prediction that sensitivity to agreement modulates the Match effect, as predicted by the strategic listener and phonetic predictor accounts.

3.2 Gaze traces

3.2.1 Syll-targets

Table 5 shows the model summary of the analysis for syll-targets, which is visualized in the right-hand side of Figure 6.⁶ The parametric effect of Match reveals that participants looked more towards the target in the Match condition than in the Mismatch condition (β = 1.812, p < .001). This can be observed in the right-hand panels of Figure 6, which show a clear target advantage for the gaze traces in the lengthened Match condition (dark ribbons) relative to the shortened Mismatch conditions (gray ribbons). The parametric effect of Context indicates that the median curve for targets after Agreeing determiners was lower than after Non-Agreeing determiners (β = –0.814, p < .001). In other words, participants looked more towards the target in Non-Agreeing sentences like (4), which used the determiner the (bottom right), than in Agreeing sentences like (3), where the target was preceded by an agreeing determiner (top right). Finally, the interaction between Match and Context indicates that the Match effect was smaller in Agreeing sentences than in Non-Agreeing sentences (β = –0.565, p < .001).

Table 5: Model summary of target advantage for looks to singular syll-targets. Parametric intercept represents the overall median. The ‘baseline’ smooth represents the curve with the Match value of Match (stem duration is lengthened to match expected unsuffixed duration), and the Context value of Agreeing (preceded by an agreeing determiner). Following smooths reflect differences between the baseline smooth and the other combinations of Match and Context.

Parametric terms
Term	Estimate	Std. Error	z	p
(Intercept)	1.526	0.282	5.405	<.001
Match	1.812	0.073	25.877	<.001
Context	–0.814	0.070	–11.587	<.001
Match × Context	–0.565	0.139	–4.057	<.001
Smooth terms:
Term	Effective df	Ref.df	χ 2	p
Baseline	4.98	5.56	49.35	<.001
Match=Mismatch, Context=Agreeing	4.98	6.02	89.16	<.001
Match=Match, Context=Non-Agreeing	4.05	4.96	25.15	<.001
Match=Mismatch, Context=Non-Agreeing	4.37	5.33	76.21	<.001
(by-subject smooths)	267.55	509.00	2070.96	<.001
(by-item smooths)	531.31	909.00	6854.40	<.001

Figure 6: Model estimates of median gaze trajectories to num-targets (left) and syll-targets (right) across Agreeing (top) and Non-Agreeing (bottom) contexts. Lines show model estimates for each time window, while ribbons represent 95% confidence intervals of model estimates. Targets in the Match condition (solid white lines with black ribbons) showed higher median target advantage than targets in the Mismatch condition (dotted black line with gray ribbons).

The smooth terms indicate that the curve shapes for the default condition – Matching duration and Agreeing Context – were significantly different from the other three combinations of conditions. As Figure 6 illustrates, the curve for the default condition (top right, black ribbon) rises slowly until time window 17 or so, before rising more steeply thereafter. In the Non-Agreeing condition, by contrast, the Match curve (bottom right, black ribbon) rises much more steadily from the target offset. The two Mismatch curves (right panels, gray ribbons) have quite decidedly distinct shapes compared to the Match-Agreeing reference curve. After the offset of the target, both curves show a distinct dip in target advantage, reflecting the period when participants would be misled into looking toward the competitor. At about time window 18, the Mismatch curves rise steeply, as participants recover and find the target. However, in the Non-Agreeing condition, the confusion from the Mismatch curve seems reduced relative to the Agreeing condition: The dip after target offset is less extreme, and the recovery happens sooner, crossing the 0-line into positive target advantage two time windows – roughly 100 ms – earlier.

4.1 Num-targets: Evidence for phonetic prediction

The gaze traces for num-targets showed a significant Match effect – the first demonstration that listeners can attend to segmental compression as well as polysyllabic shortening in processing morphologically complex words. The magnitude of the effect, however, was weaker in num-targets than in syll-targets, despite the identical durational manipulation. This could reflect the fact that the contrast between a singular and a plural, which both belong to the same lexeme, is less important for communication than the contrast between singular and two-syllable words, which belong to entirely separate lexemes. Alternatively, it may reflect the fact that contrasts between singulars and plurals can be reinforced by other morphosyntactic cues in a given sentence. By contrast, the difference between a two-syllable word and a one-syllable word lexically embedded within it may rest entirely on the phonetic information contained within the words themselves, leading listeners to attend more diligently. Or, perhaps, they simply benefit from the greater amount of time affforded by the extra syllable to process the phonetic detail more fully. Regardless, the presence of the Match effect in num-targets confirms that listeners’ sensitivity to morphologically-conditioned phonetic detail operates at very fine scales, within the space of a single syllable.

This finding complements and expands upon previous work showing listeners’ use of morphologically-conditioned durational variation – in particular, with affixes. Schmitz (2022), for example, examined listeners’ attentiveness to the duration of word-final /s/, which tends to be longer when it is part of a monomorphemic word, such as mix, than when it is a plural suffix, as in books. (See also Plag et al., 2017; Zimmermann, 2016, although cf. Seyfarth et al., 2018.) Using mouse-tracking in a number-decision task, Schmitz (2022) observed that listeners moved the cursor more directly across the screen toward the correct answer when the duration of word-final /-s/ matched these tendencies – longer for monomorphemic words and shorter for suffixed words – than when it reversed them.

Research on prefixes, too, has shown similar listener sensitivity to this sort of fine phonetic detail, in the context of words like discolor and mistime. According to R. Smith & Hawkins (2012), the meaning of these words is the composition of the meaning of the stem and the negating meaning of the prefix, and so these words can be characterized as containing a true prefix before the stem. Words like discover and mistake cannot be so decomposed; and so even though they contain the same initial triphones dis- and mis-, they are better described as pseudo-prefixed. The pronunciation of true prefixes and pseudo-prefixes varies systematically across a number of phonetic parameters, including duration. Vowels in true prefixes are longer than in pseudo-prefixes, while final consonants are shorter (R. Smith & Hawkins, 2012). Clayards et al. (2021) examined perception of these prefixes, along with two more – re- (as in re-strings/restricts), and ex- (as in expletive/ex-policemen) – and reported that, in addition to the other phonetic patterns reported by R. Smith & Hawkins (2012), true prefixes are longer than pseudo-prefixes overall, and that listeners can use these phonetic patterns to guide online speech perception of pseudo-prefixed and true-prefixed words.

The current study builds on these findings in two ways. First, the results reported in Clayards et al. (2021) are subject to an unavoidable constraint imposed by the vocabulary of English: There are only so many true/pseudo-prefix pairs in the language, and it is difficult to come up with many word pairs while respecting the strict controls the authors placed on the phonemic overlap and prosodic structure of the paired stimuli. By contrast, the current study used noun stems, which are far more numerous than syllables with a dual identity as true prefixes and pseudo-prefixes. As a result, the findings reported here provide evidence for listeners’ use of these durational patterns across scores of words in real-time speech perception.

Second, both Schmitz (2022) and Clayards et al. (2021) created their matching and mismatching stimuli by cross-splicing affixes. This approach preserves all components of fine phonetic detail associated with the donor words, faithfully capturing the richness of pronunciation variation that listeners have at their disposal. Yet such an approach makes it difficult to determine how abstract listeners’ knowledge of these pronunciation patterns might be. Do listeners need the full range of subphonemic cues to guide their perception of morphological structure, or can one cue in isolation do the trick? The current study did not cross-splice the stimuli, but instead manipulated duration alone, and so reveals that listeners do not require the full set of fine phonetic detail to discriminate morphological structure. Although cross-splicing may well produce a larger Match effect, the current study shows that duration of the stem by itself allows listeners to better distinguish unsuffixed from suffixed words, suggesting a degree of abstract generalization in their use of phonetic cues during speech perception.

Crucially, the Match effect for num-targets interacted with Context, in a direction that exactly matches the phonetic predictor account. As visualized on the left side of Figure 6, num-targets enjoyed a larger Match effect (top) after Agreeing determiners than after Non-Agreeing determiners (bottom) This is incompatible with the strategic listener account, which predicts the opposite pattern. It is also incompatible with the baseline ideal observer account, which predicts no difference across contexts. Even the timing of the separation between Match and Mismatch follows the mechanism of the phonetic predictor account: the separation occurs earlier after Agreeing determiners than Non-Agreeing determiners, which is expected if listeners are able to make predictions earlier based on the preceding morphosyntactic context.

If listeners’ ability to predict the number of the num-target is responsible for the strengthening of the Match effect, then it raises the intriguing possibility that being able to predict the identity of the syll-target might also increase the magnitude of the Match effect, by the same mechanism. Such a manipulation has not, to my knowledge, been investigated. In the existing studies that demonstrated robust responsiveness to polysyllabic shortening patterns in sentence contexts (Blazej & Cohen-Goldberg, 2015; Davis et al., 2002; Salverda et al., 2003; Shatzman & McQueen, 2006), the experimenters took great care to ensure that the sentences up to the target provided only neutral semantic context. Listeners could not use that preamble to predict whether the free-standing word (cap) or the two-syllable word (captain) was more likely. Suppose, though, that the sentences did provide adequate semantic context to allow listeners to predict the identity of the target word. If so, then part of that prediction would include the polysyllabic shortening pattern associated with a two-syllable word, or the lengthening pattern associated with the single-syllable stem. We might therefore expect that listeners would show larger effects of polysyllabic shortening in contexts where the syll-target’s identity can be predicted, by the same mechanism in the current findings that produced larger effects of segmental compression in contexts where the num-target’s number could be predicted.

4.2 Syll-targets: Difficult demonstratives?

The results for syll-targets (Figure 6, right-hand side) robustly replicated previous findings showing listener sensitivity to polysyllabic shortening (Blazej & Cohen-Goldberg, 2015; Davis et al., 2002; Kemps, Ernestus, et al., 2005; Kemps, Wurm, et al., 2005; Salverda et al., 2003; Shatzman & McQueen, 2006). Mismatch gaze traces hover around a target advantage of 0 until after the target offset. Then they dip, reflecting the temporary advantage of the competitor, which is, at that moment, the best match to the shortened stem in its intentionally ambiguous phonetic context. In other words, when listeners heard rock attracts, the shortening of the stem, combined with the initial phones of attract, pulled their gaze more toward the competitor rocket than towards the target rock. In the Match condition, the lengthening of the stem shielded listeners from that initial misparse, and target advantage rises consistently. The rise is more gradual with syll-targets than with num-targets, but that is unsurprising, as syll-targets were embedded in sentences whose following contexts were intentionally ambiguous, while num-targets were not.

A complication in the syll-target results is that, although the Match effect interacts with Context as it did with num-targets, it interacts in the opposite direction. Match targets have a higher target advantage over Mismatch targets in the Non-Agreeing sentences than in the Agreeing sentences. Yet this cannot be interpreted as evidence against the phonetic predictor account, or indeed evidence for the strategic listener account, because with syll-targets, both target and competitor were singular. Any agreeing determiner will be singular, and hence be equally compatible with both target and competitor, in exactly the same way a non-agreeing determiner is. The difference in the Match effect, therefore, cannot stem from any difference in suitability between determiner and target or determiner and competitor. What is responsible for it?

One possibility has to do with the nature of the determiner itself. Recall that, in addition to the weakening of the Match effect, syll-targets in the Agreeing sentences also showed a lower overall target advantage than their counterparts in the Non-Agreeing sentences. This suggests that demonstrative determiners might be, overall, more difficult for listeners to process than the definite determiner. Indeed, one pilot participant reported feeling that sentences with demonstrative determiners felt subjectively harder to understand than those with only definite determiners.

What is it, then, that makes demonstratives more difficult? Perhaps it is the contextual licensing that governs the form they take. A definite determiner has only one form in English – the. By contrast, even leaving aside the number agreement, demonstrative determiners can take a proximal form (this/these) or a distal form (that/those). The proximal/distal contrast carries with it substantial discourse-pragmatic information, which can reflect a variety of factors, including the location of the referent involved; its discourse status, and whether that status is common ground between speaker and hearer, or privileged only to the speaker; and sometimes even physical properties of the referent, such as size and harmfulness (Peeters et al., 2021). However, the sentences in this study were presented in isolation, lacking any of the real-world or discourse-pragmatic context that could license the choice of demonstrative. As a result, in sentences where any morphosyntactic agreement information was valueless, listeners may have struggled to process the demonstratives more than the definite determiner the. If recovering from this struggle spilled over into the following noun, then that could account for the weaker target advantage for syll-targets in the Agreeing sentences, where they followed demonstratives, relative to the Non-Agreeing sentences, where they followed the.

It is this recovery period that could also be responsible for weakening the Match effect. Recall that cognitive load reduces listeners’ ability to process phonetic detail (Christiansen & Chater, 2016; Mattys & Wiget, 2011; Mattys et al., 2009, 2014). Although the work by Mattys and colleagues imposed a cognitive load by virtue of a secondary, non-linguistic task, it is possible that the demands imposed by processing unlicensed demonstrative determiners had a similar impact. With their cognitive capacity otherwise occupied, listeners could not spare the same resources to handle phonetic detail the way they could following straightforward, easy-to-process the. As a result, the Match effect was reduced.

This interpretation has some conceptual similarities to the strategic listener account, because it suggests that there are indeed contexts where limits on cognitive capacity reduce listeners’ attention to phonetic detail. The key difference, however, is that it is not a strategic response to the informativity or redundancy of that detail. The Match manipulation is equally useful in distinguishing syll-target from competitor regardless of the nature of the preceding determiner. Rather, the reduced use of phonetic detail seems to be an automatic consequence of the increased processing load. To the extent that listeners are dynamically adjusting their use of phonetic detail, they may not be doing it out of strategy, but out of necessity.

4.3 Agreement vs. discourse-pragmatic licensing

If unlicensed demonstrative determiners are indeed more difficult to process than the definite determiner, then comparing their effect on syll-targets and num-targets affords an opportunity to evaluate the strength of agreement cues against discourse-pragmatic cues. In languages whose determiners regularly mark agreement, such as French, Spanish, and German, listeners show an advantage in identifying targets whose determiners disambiguate them from competitors (Berends et al., 2016; Dahan et al., 2000; Hopp, 2013; Lew-Williams & Fernald, 2007). Dahan et al. (2000), for example, asked listeners to distinguish a target like le bouton ‘the.masc button’ from a cohort competitor that began with the same few segments, but differed in gender, as in la bouteille ‘the.fem bottle’. In French, definite determiners agree in gender before singular nouns, but before plural nouns take the same form – les – regardless of gender. Listeners were able to distinguish the target from the competitor more effectively when it was preceded by a singular definite determiner, which provided gender information, than when it was preceded by a plural definite determiner, which did not. Lew-Williams & Fernald (2007) showed a similar pattern in Spanish, which was present in children as young as 34 months. Listeners distinguished targets from competitors more quickly when they differed in gender than when they shared a gender, because the agreement information on the determiner allowed listeners to exclude the competitor from consideration immediately. Hopp (2013) extended this pattern to German, demonstrating that not only can adults use gender-agreeing determiners to distinguish targets from competitors, but so can L2 speakers.

Even in English, which lacks grammatical gender, verb agreement in questions can be used to predict the identity of following subject nouns. Lukyanenko & Fisher (2016) showed adults and three-year-old children pictures of a singular or plural target, and asked them to respond to sentences whose verbs were either informative about target number (Where is/are the good cookie(s)?) or uninformative (Can you find the good cookie(s)?). They found that both adults and children began to look to the target picture even before the noun onset, as long as they had the informative agreement about target number in the verb. With uninformative instructions, they had to wait until the target noun was produced before they could look to the correct picture. Brown et al. (2022) replicated this finding in adults, and further showed that, in addition to directing listeners’ attention to the target object, verb agreement information also reduced looks to similar-sounding competitors that did not match the target’s number.

Clearly, then, morphosyntactic agreement is a robust and useful cue in online sentence processing. Is it strong enough to overcome the disadvantage posed by processing pragmatically complex demonstrative determiners? Although the design of this study was not constructed to directly test this question, it does accidentally offer some evidence that the answer is no. The main parametric effect of Context for num-targets was negative, indicating an overall disadvantage for num-targets in the Agreeing sentences (with demonstrative determiners) relative to the Non-Agreeing sentences (with the). In other words, whatever benefit was conferred by the agreement information in the demonstratives for distinguishing num-targets from plural competitors, it was not enough to overcome the difficulty associated with demonstratives relative to the. Nevertheless, the benefits of that agreement information were still detectable. The size of the Context effect – i.e., the disadvantage associated with demonstratives – was substantially smaller with num-targets, coming in at –0.205 elog units of target advantage, than with syll-targets, whose disadvantage, at –0.814 elog units, was nearly four times as large.

Figure 7 visualizes this result, collapsing across the Match and Mismatch conditions. The disadvantage brought by demonstrative determiners in the Agreeing context is clearly visible in the right-hand pane, which shows gaze traces for syll-targets. In the left-hand pane of num-targets, however, where the agreement information in the demonstrative determiners has the opportunity to offset the disadvantage, that difference is greatly reduced.

Figure 7: Modeled gaze-traces aggregated across Match and Mismatch, comparing the effect of Context for num-targets (left) and syll-targets (right).