Dissociating Language and Thought in Human Reasoning

Does language shape human cognition? One aspect of this debate concerns whether the combinatorial operations of natural language also serve, supramodally, as a basis for the combinatorial operations of other domains of human thought such as mathematics, music, cognition, and theory of mind, among others. In the context of human reasoning, a growing body of research has generated two opposing views concerning the role of language in enabling deductive inference-making. Under the first, the structure-dependent operations required for deductive reasoning are linguistic in nature and thus based on the neural mechanisms of natural language. Under the second, deductive reasoning is supported by language independent processes principally implemented in frontomedial and left frontopolar cortices (in Brodmann areas [BA] 8 and 10, respectively). We tested the two competing views using continuous Theta Burst Stimulation (cTBS), an inhibitory neuromodulation approach. Here, we report a neural dissociation between the structure-dependent operations of natural language and those of deductive reasoning. Specifically, we found that transient cTBS inhibition of Brocas area, in the left inferior frontal gyrus (IFG), a region often associated with processing the hierarchical dependencies of natural language, impairs accuracy for linguistic problems but not for logic ones. Conversely, cTBS to frontomedial cortices produces the opposite pattern. Our results demonstrate that, in the adult brain, the structure-dependent operations of reasoning are not parasitic to the neural substrate of natural language.