How Does the Brain Reason?
New Clues from Brain Imaging

One of the most impressive human talents is the ability to reason. However, the brain anatomy of this function is little understood and has only recently undergone investigation through brain imaging. Some evidence from studies of brain imaging suggests that
deductive reasoning primarily activates the right hemisphere, while
probabilistic reasoning primarily activates the left.

Princeton researcher Daniel Osherson and psychologist Lawrence Parsons, of the University of Texas, present their findings in, “New Evidence for Distinct Right and Left Brain Systems for Deductive Versus Probabilistic Reasoning,” published in Cerebral Cortex, October, 2001 (Vol. 11, No. 10).

Deductive reasoning is characterized by necessity, such as the example, “No cat is both lazy and smart, and therefore every cat is either not lazy or not smart.” Probabilistic reasoning is characterized by likelihood. For example, many people conclude that the polar ice caps will likely melt by 2050, based on rising oil consumption.

The researchers measured blood flow in the brain using positron emission topography (PET) while participants were given various reasoning exercises. Based on the data, they hypothesize that the right hemisphere houses a logic-specific network comparable to the left hemisphere’s language specific network. Deductive rules are encoded in the right hemisphere.

“On this theory, deductive reasoning about an argument proceeds in three steps,” Osherson notes. “First, language areas in the left hemisphere retrieve the formal structure of the argument; next, this structure is transmitted via the corpus callosum to right hemisphere regions; finally, the latter regions carry out deduction on the formal structure they receive."

The researchers further postulate that probabilistic reasoning is processed in non-linguistic left hemisphere regions that are involved in recalling and evaluating a wide range of world knowledge.

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