The most important information conveyed by language is often contained not in the utterance itself, but in the interaction between the utterance and the comprehender's knowledge of the world and the current situation. This dissertation uses psycholinguistic methods to explore the effects of a common type of inference - causal inference - on language comprehension. In 8 experiments, I investigate the effects of causal inference on the neuro-cognitive processes that occur during word processing (Experiments 1 -5) and the hemispheric basis of these processing effects (Experiments 6-8). The goal of Experiments 1-3 was to compare competing theoretical frameworks of language processing with respect to the ordering of "high-level" (causal inferential) and "low-level" (lexical association) context effects on word processing. To that end, participants listened to two-sentence short stories encouraging a causal inference, each followed by a probe word related to some aspect of the context story. ERP results showed that causal information affected word processing earlier than lexical associative information, and that lexical association effects were suppressed in discourse contexts. These results supported dynamic processing theories of the kind inspired by simple recurrent networks. Experiments 4 and 5 tested the impact of causal relatedness on multiple, semi-redundant discourse cues embedded in sentences. This study investigated whether causal inferences build up over time across several words, or if a full-fledged inference can be activated in response to a single critical word. Results indicated that different participants activated inferences in qualitatively different ways. Some showed evidence of predictive inference, while others showed evidence of a drawn-out inference activation process covering several cues to discourse implausibility. These results reflect individual differences in inference activation that are unrelated to common metrics of processing ability. Experiments 6-8 tested the hypothesis of a right hemisphere (RH) advantage for activating causal inferences. Results indicated that neither hemisphere had a processing advantage for causal related information, although left hemisphere (LH) experienced facilitated processing for strong lexical associations. This finding suggests causal inference processing is balanced between the two hemispheres, and that causal inference deficits in RH lesion patients are related to a dominant LH tendency to focus on local semantic relationships

One key question in neurolinguistics is the extent to which the neural processing system for language requires linguistic experience during early life to develop fully. We conducted a longitudinal anatomically constrained magnetoencephalography (aMEG) analysis of lexico-semantic processing in 2 deaf adolescents who had no sustained language input until 14 years of age, when they became fully immersed in American Sign Language. After 2 to 3 years of language, the adolescents' neural responses to signed words were highly atypical, localizing mainly to right dorsal frontoparietal regions and often responding more strongly to semantically primed words (Ferjan Ramirez N, Leonard MK, Torres C, Hatrak M, Halgren E, Mayberry RI. 2014. Neural language processing in adolescent first-language learners. Cereb Cortex. 24 (10): 2772-2783). Here, we show that after an additional 15 months of language experience, the adolescents' neural responses remained atypical in terms of polarity. While their responses to less familiar signed words still showed atypical localization patterns, the localization of responses to highly familiar signed words became more concentrated in the left perisylvian language network. Our findings suggest that the timing of language experience affects the organization of neural language processing; however, even in adolescence, language representation in the human brain continues to evolve with experience.

The developmental relationship between linguistic semantic processing and non-linguistic semantic processing (interpreting pictures) is investigated in a longitudinal neuroimaging study of two deaf individuals who did not begin acquiring their first language until the age of 14. 1-2 years after they began learning language, the two case studies performed a picture-sign priming task. Magnetoencephalography was localized to the cortical surface, showing that picture processing was initially bilateral or focused in the canonical left hemisphere language network, while single sign processing was initially focused in the right hemisphere. After 15 months of additional language experience, the neural responses too both pirctures and single signs reversed in lateralization, becoming more similar to those observed in a control group of native signers. The results shed new light on the interdependence between linguistic and non-linguistic semantics in cognitive development, as well as the neural underpinnings of semantic processing.