Dissociation of mechanisms underlying syllogistic reasoning

A key question for cognitive theories of reasoning is whether logical reaso ning is inherently a sentential linguistic process or a process requiring s patial manipulation and search. We addressed this question in an event-rela ted fMRI study of syllogistic reasoning, using sentences with and without s emantic content Our findings indicate involvement of two dissociable networ ks in deductive reasoning. During content-based reasoning a left hemisphere temporal system was recruited. By contrast, a formally identical reasoning task, which lacked semantic content, activated a parietal system. The two systems share common components in bilateral basal ganglia nuclei, right ce rebellum, bilateral fusiform gyri, and left prefrontal cortex. We conclude that syllogistic reasoning is implemented in two distinct systems whose eng agement is primarily a function of the presence or absence of semantic cont ent. Furthermore, when a logical argument results in a belief-logic conflic t, the nature of the reasoning process is changed by:recruitment of the rig ht prefrontal cortex. (C) 2000 Academic Press.