Partially dissociative role of the left inferior frontal gyrus and left dorsolateral prefrontal cortex in reasoning

Reasoning is the ability to formulate inferences or conclusions from available information. The two major types, deductive and inductive, are thought to rely on distinct cognitive mechanisms and recruit separate brain areas. Neuroimaging studies yield mixed results; some found the left inferior frontal gyrus (IFG) activations for deductive reasoning and the left dorsolateral prefrontal cortex (DLPFC) for inductive reasoning. This assumption was put to the test in the present study. In two double-blinded, sham-controlled experiments, high-definition transcranial direct current stimulation (HD-tDCS) was used to systematically explore the left IFG’s and DLPFC’s causal role in deductive and inductive reasoning. Participants with no formal training in logic judged deductive and inductive arguments before and after 10 minutes of anodal, cathodal, or sham tDCS of the left IFG (Experiment 1, n = 20) or left DLPFC (Experiment 2, n = 21). Left IFG anodal tDCS impairs reaction times (RTs) for easy categorical ( p = < .001) and propositional ( p = .025) deductive arguments and the accuracy for easy inductive propositional arguments ( p = .003). Meanwhile, regardless of the active stimulation conditions, left DLPFC tDCS shortens RTs (anodal: p = < .001, cathodal: p = .014) and increases accuracy (anodal: p = .029, cathodal: p = .001) for difficult categorical inductive arguments, but decreases accuracy (anodal: p = .027, cathodal: p = < .001) for difficult propositional inductive arguments. The overall results showed a partial dissociation of the left frontal lobe areas subserving the two types of reasoning and argument difficulty-dependent stimulation effects. This study extends knowledge of the neural basis of reasoning and hopefully inspires interventions that could augment reasoning impairments associated with normal aging and brain lesions.