Orthographic precision refers to the specificity with which letters are assigned to positions within words. In some theoretical models of visual word processing, precision is the optimal end state of a lexical representation; the associations between letters and positions are initially approximate and noisy, but they become more precise as readers gain exposure to the word. In other models, flexible orthographic codes that allow for rapid access to semantics are the optimal end state, and precise representations are only relied upon under specific circumstances. I will introduce a series of event-related potential transposed-letter priming studies designed to test the predictions of these opposing models and determine the functionality of precision within the orthographic system. More specifically, I examine the lexical properties that modulate precision within readers and the skills that modulate precision across readers. In Chapter 2, I demonstrate that words from high-density orthographic neighborhoods that are easy to confuse are encoded more precisely than words from low-density orthographic neighborhoods. In Chapter 3, I demonstrate that precision differs across languages in two groups of bilinguals with different levels of proficiency. In Chapter 4, I compare orthographic precision across hearing and deaf readers to examine how access to phonology contributes to orthographic tuning. However, the results indicate that there were negligible differences between groups. Overall, the results are consistent with a dynamic orthographic system in which precision varies systematically but does not necessarily characterize more developed representations.

Iconicity refers to the structured mapping between a lexical form and the conceptual representation of what it means. In this dissertation, I present data from a set of three experiments using Event-Related Potentials (ERPs) that investigate the potential role of iconicity in sign production. In Study One, I present the findings from a picture-naming paradigm, which found the iconic signs were retrieved more quickly than non-iconic signs, particularly when there was a strong visual overlap between picture and sign. The production of iconic signs was additionally associated with increased N400 amplitude, indicating additional semantic processing, though this was reduced for the trials with the greatest visual overlap. In Study Two, I compared the effects of iconicity in a picture-naming task to that in an English-to-ASL translation task to explore whether the faciliatory effect of iconicity was general or task-specific. I found reduced response latencies and N400 amplitudes for iconic signs only in the picture-naming task, with no differences in the translation task, indicating that the effect of iconicity was task-specific. In Study Three, I extended the investigation of iconic signs in picture-naming paradigms by exploring the effects on two distinct manners of iconic mapping: perceptual vs. motoric iconicity. As in Study Two, reduced N400 amplitudes were observed compared to non-iconic signs, but there were no differences in response latencies. Through the use of Laplacian transformations, I compared the distribution of effects for perceptually-iconic signs, which map onto how the referent is perceived, and motorically-iconic signs, which map onto how the referent is handled. I found increased activation at frontal and central electrode sites for perceptually-iconic signs, and increased activation at parietal sites for motorically-iconic signs. These findings suggest that perceptually-iconic mappings engage the ventral stream, while motorically-iconic mappings engage the dorsal stream. Overall, this dissertation finds evidence that iconicity facilitates sign retrieval when features encoded in the sign form map to visual features of the picture stimulus, and that the type of iconicity impacts the neural regions involved in the mapping between sign form and picture stimulus.

Auditory sentence processing is astonishingly complex and involves the rapid processing and integration of many different forms of information. While this is seemingly effortless for neurologically unimpaired listeners, it is clear from the literature that brain damage can cause the normal language system to be disrupted in specific and testable ways. One major goal of this dissertation is to describe how the system is fractionated in aphasia by focusing on the time-course of using specific information types that appear to be involved in the unimpaired language system.

A series of sentence processing studies are presented exploring the impact of syntactic structure in neurologically unimpaired listeners and in listeners with Broca’s aphasia (Chapter 3), the impact of thematic fit and prosody in college-age adults (Chapter 4) and individuals with aphasia along with a group of age-matched healthy controls (Chapter 5).

Chapter 2 reviews research on sentence processing, and accounts of the sentence comprehension deficit in aphasia are also discussed. Chapter 3 provides evidence that similarity-based interference, which results from certain syntactic structures, contributes to the sentence comprehension deficit in aphasia. Chapter 4 examines how thematic fit/plausibility and prosody impacted syntactic structure building in college-age adults using event-related potentials (ERPs). Results revealed that the parser was able to use thematic fit/plausibility information to predict upcoming syntactic structure before the critical verb. Syntactic reanalysis was triggered at the critical verb in sentences with incongruent prosody and no plausibility cue. Chapter 5 examined how individuals with Broca’s aphasia and age-matched controls use plausibility and prosodic cues. The results from the age-matched controls were nearly identical to the college-age adults (Chapter 4). However, the group of individuals with aphasia with a less severe comprehension deficit could predict upcoming syntactic structure when provided with a plausibility cue, but without a plausibility cue had difficulty integrating prosody with syntactic structure. Those with a more severe comprehension deficit had difficulty integrating prosodic and lexical-semantic cues with syntactic structure. Thus, similarity-based interference, lexical-semantic processing and prosody are all implicated in the sentence comprehension deficit seen in individuals with Broca’s aphasia.