New evidence for distinct right and left brain systems for deductive versus probabilistic reasoning

Lawrence M. Parsons, Daniel Osherson

Research output: Contribution to journalArticlepeer-review

147 Scopus citations


Deductive and probabilistic reasoning are central to cognition but the functional neuroanatomy underlying them is poorly understood. The present study contrasted these two kinds of reasoning via positron emission tomography. Relying on changes in instruction and psychological 'set', deductive versus probabilistic reasoning was induced using identical stimuli. The stimuli were arguments in propositional calculus not readily solved via mental diagrams. Probabilistic reasoning activated mostly left brain areas whereas deductive activated mostly right. Deduction activated areas near right brain homologues of left language areas in middle temporal lobe, inferior frontal cortex and basal ganglia, as well as right amygdala, but not spatial-visual areas. Right hemisphere activations in the deduction task cannot be explained by spill-over from overtaxed, left language areas. Probabilistic reasoning was mostly associated with left hemispheric areas in inferior frontal, posterior cingulate, parahippocampal, medial temporal, and superior and medial prefrontal cortices. The foregoing regions are implicated in recalling and evaluating a range of world knowledge, operations required during probabilistic thought. The findings confirm that deduction and induction are distinct processes, consistent with psychological theories enforcing their partial separation. The results also suggest that, except for statement decoding, deduction is largely independent of language, and that some forms of logical thinking are non-diagrammatic.

Original languageEnglish (US)
Pages (from-to)954-965
Number of pages12
JournalCerebral Cortex
Issue number10
StatePublished - 2001

All Science Journal Classification (ASJC) codes

  • Cellular and Molecular Neuroscience
  • Cognitive Neuroscience


Dive into the research topics of 'New evidence for distinct right and left brain systems for deductive versus probabilistic reasoning'. Together they form a unique fingerprint.

Cite this