Separability of logic and language: a TMS study

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The nature of the relationship between language and abstract thought has long been a contentious issue. One theory is that the hierarchical and combinatorial operations that subserve natural language also serve as the basis for similar operations in a diverse range of abstract human thought, such as mathematics, music cognition, and action sequencing. With regard to deductive reasoning, there are two dominant views about the relationship between it and language. According to the first, deductive reasoning is parasitic on language, in the sense that the structure-dependent operations which underlie deduction make use of the same neural machinery that underlies the structure dependent operations of language, with that common machinery being found in the inferior frontal gyrus, in an area traditionally referred to as Broca's area. According to the second, the structure dependent operations that underlie deductive reasoning, and the neural machinery that make them possible, are independent of the structure dependent operations of language. In this second view, the operations that support language occur primarily in Broca's area, while the operations that support deduction occur primarily in frontomedial (Brodmann area 8) and frontopolar (Brodmann area 10) cortices. We tested these two views using continuous theta burst stimulation (cTBS), a form of patterned transcranial magnetic stimulation (TMS) which has been shown to demonstrate relatively long lived inhibition of neural activity in a localized area. Using this approach, we were able to demonstrate that inhibition of Broca's area impairs accuracy on a linguistic task but not on a matched logic task. Additionally, we found that inhibition of frontomedial cortex (medial BA 8) produced a pattern opposite to that found in Broca's area. These results support the view the structure dependent operations which underlie deductive reasoning are not parasitic on those that support language, but are in fact ontologically distinct from them.