The capacity to comprehend language lies at the core of a person's ability to gain information from the environment, perform everyday tasks, and maintain normal social relations. The critical role of the left hemisphere (LH) in supporting these processes has served as a paradigmatic example of neural specialization for higher cognitive functions. However, it is increasingly apparent that the right hemisphere (RH) also makes important, distinctive contributions to language comprehension.
The aim of the proposed research is to delineate how processing resources distributed across the 2 cerebral hemispheres come together in real time to mediate language and how these processes and their underlying mechanisms change over the course of normal aging, with the long-term goal of elucidating factors that can protect against or compensate for age-, trauma-, or disease-related reductions in language capabilities. The proposal puts forward and tests a theoretical framework, based on recent neuropsychological, behavioral, and event-related brain potential (ERP) studies of language asymmetry, which asserts that LH and RH language comprehension differ because comprehension is cognitively and neurally integrated with language production only in the LH. 17 proposed experiments, which use visual half-field presentation techniques to preferentially stimulate 1 hemisphere and measure ERPs in order to examine asymmetries with temporal and functional specificity, test the hypotheses that (1) topdown connections have a greater impact on LH language comprehension, with concomitant implications for when and how difference information types are brought to bear and for the nature and timecourse of processing difficulties that may be encountered, (2) RH processing lead to more veridical representations of verbal stimuli, which can aid explicit and implicit retention over long time intervals and protect against certain types of memory errors, and (3) older adults show reduced functional hemispheric specialization, with attendant changes in both language comprehension and verbal memory. These experiments lay the foundation for an understanding of the computational and neurobiological roots of the complex and critical cognitive skill that is language.
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