Johnson, M. A. | Search bioRxiv

bioRxiv2018

Dissociating Language and Thought in Human Reasoning

What is the relationship between language and complex thought? In the context of deductive reasoning there are two main views. Under the first, which we label here the language-centric view, language is central to the syntax-like combinatorial operations of complex reasoning. Under the second, which we label here the language-independent view, these operations are dissociable from the mechanisms of natural language. We applied continuous theta burst stimulation (cTBS), a form of noninvasive neuromodulation, to healthy adult participants to transiently inhibit a subregion of Brocas area (BA44) associated in prior work with parsing the syntactic relations of natural language. We similarly inhibited a subregion of dorsomedial frontal cortex (BA8) which has been associated with core features of logical reasoning. There was a significant interaction between task and stimulation site. Post hoc tests revealed that performance on a linguistic reasoning task, but not deductive reasoning task, was significantly impaired after inhibition of BA44, and performance on a deductive reasoning task, but not linguistic reasoning task, was decreased after inhibition of BA8 (however not significantly). Subsequent linear contrasts supported this pattern. These novel results suggest that deductive reasoning may be dissociable from linguistic processes in the adult human brain, consistent with the language-independent view.

neuroscience↗

bioRxiv2017

Age-Related Differences In The Influence Of Task-Irrelevant Information On The Neural Bases Of Phonological And Semantic Processes

Although aging is often associated with cognitive decline, there is considerable variability among individuals and across domains of cognition. Within language, several indicators of semantic processing show stability throughout the lifespan. However, older adults have increased difficulty with phonological aspects of language, especially in language production. While these behavioral patterns are established, the neurobiology associated with these behaviors are less clear. Previously we have shown that, older adults were slower and less accurate in phonological compared to semantic decisions, and that older adults didnt exhibit brain-behavior relationships. In the present study, we examined phonological and semantic processes in the presence of task-irrelevant information. Older and younger adults made phonological and semantic decisions about pictures in the presence of either phonologically-related or semantically-related words, which were unrelated to the task. Behavioral results indicated that overall older adults had slower reaction times and lower accuracy compared to younger adults, and that all adults were less efficient when making phonological compared to semantic decisions. Patterns of brain activation for the semantic condition showed that all adults engaged typical left-hemisphere language regions, and that this activation was positively correlated with efficiency. In contrast, the phonological condition elicited activation in bilateral precuneus and cingulate, but only younger adults showed a significant relationship between activation and efficiency. Our results suggest that the relationship between behavior and neural activation when processing phonological information declines with age, but that the core semantic system continues to be engaged throughout the lifespan, even in the presence of task-irrelevant information.

neuroscience↗