Reasoning & the Brain

There has been some research investigating brain areas involved in reasoning, and it suggests that people can bring different systems to bear on different reasoning problems. Consider an fMRI experiment by Goel, Buchel, Frith, and Dolan (2000). They had participants solve logical syllogisms, arguments consisting of two premises and a conclusion. Participants were presented with congruent problems such as
All poodles are pets. All pets have names. ∴ All poodles have names.
Most of the participants (84%) correctly judged that the third statement logi- cally followed from the first two. The content of this example is more or less consistent with what people believe about pets and poodles. Goel et al. con- trasted this type of problem with incongruent problems whose premises and conclusions violated standard beliefs such as
All pets are poodles. All poodles are vicious. ∴ All pets are vicious.
Fewer participants (74%) judged that the third statement was true if the first two were. Finally, Goel et al. contrasted both of these types with reasoning about abstract concepts, such as
All P are B. All B are C. ∴ All P are C.
77% of the participants judged this as correct. Logicians would call all three kinds of syllogism valid.
The reader might wonder about the sensibility of judging a participant as making a mistake in rejecting an incongruent conclusion such as “All pets are vicious”; we will return to this matter in the second section of the chapter. For now, of greater interest are the brain regions that were active when par- ticipants were judging material with content (like the first two syllogisms) and when they were judging material without content (like the last syllogism);  When participants were judging content-free material, parietal regions that have been found to have roles in solving algebraic equations were active. When they were judging meaningful content, left prefrontal and temporal-parietal areas that are asso- ciated with language processing were active. This indicates that people do not process all syllogisms in the same way but invoke different brain regions when the syllogisms are based on content than when they are content-free.
■ Faced with logical problems, people can engage either brain regions associated with the processing of meaningful content or regions associated with the processing of more abstract information.