Common and distinct neural substrates of forward-chaining and backward-chaining syllogistic reasoning

Syllogistic reasoning is a form of deductive reasoning in which a logical conclusion is drawn from premises. Forward-chaining syllogistic reasoning refers to response pattern of figure 1 syllogism (M-P, S-M, S-P with response chain of S-M-P) and backward-chaining syllogistic reasoning refers to response pattern of figure 4 syllogism (P-M, S-M, S-P with response chain of P-M-S). Previous psychological studies show that forward-chaining is significantly higher accuracy and less response time, and backward-chaining is significantly lower accuracy and more response time. However, little is known about the underlying neural mechanisms. We carried out a study using event-related fMRI to clarify their neural substrates. Conjunction analysis showed common areas of activations were the bilateral caudate, the left ventral lateral prefrontal cortex (VLPFC), the left superior parietal lobule, and the right lingual gyrus. Direct comparisons showed distinct area of activation for forward-chaining in contrast to backward-chaining involved more activation in the left inferior parietal lobule, whereas backward-chaining in contrast to forward-chaining recruited the left dorsal lateral prefrontal cortex (DLPFC), the left medial frontal gyrus, and the right cerebellum. The results suggested that forward-chaining and backward-chaining syllogistic reasoning engaged distinct neural substrates based on common brain areas.