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Drawing Inferences During Discourse Comprehension

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Drawing Inferences During Discourse Comprehension: An ERP StudyMartin Paczynski1, Tali Ditman1, Kana Okano1, & Gina R. Kuperberg1,21Tufts University, Medford, MA; 2Massachusetts General Hospital, Charlestown, MAIntroductionUnderstanding discourse involves going beyond what is explicitly stated in text. Readers draw inferences to establish causal, referential, spatial, and temporal coherence. Causal coherence involves establishing cause and effect relationships between events. Electrophysiologically, the cost of establishing coherence during online comprehension is reflected by a larger N400 to words that are unrelated (compared to related) to their preceding discourse context.In healthy adults, sentences that are related through the generation of a causal inference are better recalled than those that do not involve a causal inference (e.g., Keenan, Baillet, & Brown, 1987; Myers, Shinjo, & Duffy, 1987; Trabasso & van den Broek, 1985). We hypothesized that the process of inference generation would result in an online processing cost to intermediately-related (inference-requiring) scenarios relative to highly-related scenarios. To test this hypothesis, we employed ERPs to examine the time-course of generating causal bridging inferences during online discourse comprehension. Three-sentence scenarios were created and rotated through three conditions that varied in their causal relatedness – highly related, intermediately related, and unrelated conditions. Importantly, because the N400 is modulated by lexico-semantic association between content words, all three conditions were matched on strength of lexico-semantic associations across words using a latent semantic analysis. This enabled us to examine purely discourse-level processing.PredictionsAttempting to link an unrelated (compared to a highly-related) sentence to a preceding discourse will incur a processing cost, reflected by a larger N400, regardless of similar lexico-semantic associations between conditions.Attempting to link an intermediately-related sentence (involving a causal inference) to preceding discourse context will also incur a processing cost, reflected by an N400 amplitude that is smaller than that observed for unrelated sentences but larger than that observed for highly-related sentences. This effect will be observed despite matching for lexico-semantic associations between these conditions. Table 1 - Behavioral DataFigure 4 – ERP for CWs69 (14.5)%25 (12.4)%6 (4.3)%2.63 (0.18)Unrelated10 (7)%61 (16)%29 (16.1)%1.81 (0.19)Intermediately related1 (1.3)%11 (5.9)%88 (6.1)%1.13 (0.07)Highly relatedRating 3(Difficult to integrate)Rating 2(Intermediately difficulty in integrating)Rating 1 (Easy to integrate)Percentage of ratings MeanratingScenario typeMean ratings and percentages are shown with standard deviations in brackets.Figure 2 –TrialWhen ERPs were categorized by participants’ ratings, critical words in scenarios that were rated as 3 (unrelated), relative to 1 (highly related) again evoked a significant effect that lasted for the whole N400 time window (325-475msec). Critical words in scenarios rated as 2 (intermediately related), relative to 1 (highly related), also evoked an effect that again lasted over the whole N400 time window (325-475msec) and was just as large as to the unrelated critical words.Summary and Discussion•The N400 effect to causally unrelated scenarios, relative to highly causally related scenarios, may reflect the cost in processing as readers attempt (but fail) to integrate the final sentence into their preceding discourse context. Of note, this integration difficulty cannot be attributed to any differences in lexico-semantic relationships between individual content words but must have reflected discourse-level processes.•The early N400 effect to the intermediately related, inference-requiring scenarios, relative to the highly related scenarios, may reflect the immediate, early perception of an incongruity in semantic coherence and/or the increased difficulty in generating and integrating a bridging inference. The integration of such bridging inferences as a whole, however, occurred very quickly such that participants incurred no further cost in semantic integration of the intermediately related scenarios by 325-400msec. This interpretation is supported by the observation that the N400 effect was more prolonged to those intermediately-related scenarios where participants were later more aware of incongruities (and perhaps failed to immediately generate bridging inferences).ReferencesHolcomb, P.J. (1993). Semantic priming and stimulus degradation: Implications for the role of the N400 in language processing. Psychophysiology, 30, 47-6.Keenan, J.M., Baillet, S.D., & Brown, P. (1984). The effects of causal cohesionon comprehension and memory. Journal of Verbal Learning and Verbal Behavior, 23,115–26.Myers, J.L., Shinjo, M., & Duffy, S.A. (1987). Degree of causal relatedness and memory. Journal of Memory and Language, 26, 453–65 Trabasso, T., & van den Broek, P. (1985). Causal thinking and the representation of narrative events. Journal of Memory and Language, 24, 612-30. MethodsWhen ERPs were categorized by the three predetermined scenario types, critical words in the unrelated scenarios, relative to the highly related scenarios, evoked an N400 effect (325-475msec). However, critical words in the intermediately related scenarios, relative to the highly related scenarios, evoked only a short-lived early N400 effect, significant only between 325-400msec. These effects were distributed evenly across the scalp.Figure 3 – ERPs for CWsJill’s skin always tanned well.She always put on sunscreen.She had sunburn on Monday.[There is no clear inference that can be made since the second sentence establishes that she always puts on sunscreen and the first sentence establishes that, even if she forgot to put on sunscreen, her skin would not burn.]The final sentence is unrelated in that it does not logically follow from the preceding information. Sentence 3 is the same in all three conditions.Unrelated (53 scenarios)Jill had very fair skin.She usually remembered to wear sunscreen.She had sunburn on Monday.[The


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