Dissociating language and thought in human reasoning

Abstract

What is the relationship between natural language and complex thought? In the context of complex reasoning, there are two main views on this question. Under the first, language sits at the center of the ability to process the syntax-like combinatorial operations necessary for various forms of complex reasoning, such as deductive reasoning. Under the second, these operations are independent of the mechanisms of natural language. We used noninvasive brain stimulation to assess the effects of transient inhibition of neural activity in targeted neural systems. If language and deductive reasoning can be shown to be dissociable with this approach, then the hypothesis that language is crucial to deductive reasoning can be ruled out. We inhibited Broca's area, a region associated in prior research with parsing the syntactic relations of natural language, and dorsomesial frontal cortex, a region previously described as core for logic reasoning. We tested the effects of perturbing activity in these areas on processing the syntactic operations of natural language and the syntax-like operations of deductive logic. The dissociative hypothesis of language and deductive reasoning predicts an interaction between stimulated areas and tested functions, which we observed. This interaction demonstrates that the effects of brain perturbation are reliably different at the two stimulated sites (Broca's area and dorsomesial prefrontal cortex) and for the two functional processes (language and thought). Transient inhibition of Broca's area disrupted linguistic processing without affecting deductive reasoning, whereas transient inhibition of dorsomesial frontal cortex exhibited the reverse pattern, albeit to a lesser degree. These results are evidence for the independence of abstract complex reasoning from natural language, at least in the adult brain. (236 words) Whether complex cognition is enabled by or founded upon the mechanisms of natural language has long been debated in many fields, including philosophy, psychology, and, more recently, neuroscience. In the context of human reasoning, some view language as central to inference-making, while others view this ability as independent of the mechanisms of natural language. Using a neuromodulatory approach, we show that it is possible to disrupt the neural mechanisms of natural language without affecting reasoning and vice versa. This result provides the first causal evidence that in the adult brain logic reasoning is independent of the mechanisms of natural language.Introduction 1 Does language shape human cognition [1][2][3][4][5]? This question is generally framed within 2 two opposite positions: the communicative conception of language, in which language is 3 viewed primarily as an inert means of communicating preexisting (i.e., non-linguistic) 4 mental representations from one mind to another through a mutually intelligible code, 5 and the cognitive conception of language, in which language is viewed as constitutively 6 involved in human cognition and the medium of thought [6]. 7A u...