The cerebral hemispheres have been shown to be differentially sensitive to sentence-level information; in particular, it has been suggested that only the left hemisphere (LH) makes predictions about upcoming items, while the right (RH) processes words in a more integrative fashion. The current study used event-related potentials to jointly examine the effects of expectancy and sentential constraint on word processing. Expected and unexpected but plausible words matched for contextual fit were inserted into strongly and weakly constraining sentence frames and presented to the left and right visual fields (LVF and RVF). Consistent with the prediction/integration view, the P2 was sensitive to constraint: words in strongly constraining contexts elicited larger P2s than those in less predictive contexts, for RVF/lh presentation only. N400 responses for both VFs departed from the typical pattern of amplitudes graded by cloze probability. Expected endings in strongly and weakly constraining contexts were facilitated to a similar degree with RVF/lh presentation, and expected endings in weakly constraining contexts were not facilitated compared to unexpected endings in those contexts for LVF/rh presentation. These data suggest that responses seen for central presentation reflect contributions from both hemispheres. Finally, a late positivity, larger for unexpected endings in strongly constraining contexts, observed for these stimuli with central presentation was not seen here for either VF. Thus, some phenomena observed with central presentation may be an emergent property of mechanisms that require interhemispheric cooperation. These data highlight the importance of understanding hemispheric asymmetries and their implications for normal language processing.
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