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Abstract:
Reasoning is central to human cognition and behavior, yet its
functional neuroanatomy is poorly understood. Research in the
cognitive sciences implies either that (a) deduction has a
linguistic basis and is processed in left hemispheric language
areas; (b) deduction is independent of language and is not
subserved by language brain areas; or (c) both deduction and
subjective probabilistic induction are based on mental models and
situated in right hemispheric areas representing visual-spatial
information. We tested these implications via positron emission
tomography. Ten healthy males and females, untrained in formal
logic, reasoned covertly about arguments derived from propositional
logic, for which no imagistic or spatial strategy is known to yield
correct judgments. Deductive and probabilistic tasks activated few
brain areas in common. When brain areas active in the two tasks
were directly contrasted, deduction activated mostly right brain
areas and a few left brain areas related to attention, and
probabilistic reasoning activated mostly left brain areas and a few
right brain areas related to attention. The amygdala, which has
been shown to register emotional significance, activated only
during deduction, consistent with the sudden insight reported by
subjects. These findings indicate a clear hemispheric dissociation
among brain areas activated for deduction and for probabilistic
induction, and suggest that deduction is largely independent of
language.
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