Divergent thinking styles of the hemispheres: How syllogisms are solved during transitory hemisphere suppression

Psychiatric patients solved syllogisms while recovering from transitory ictal suppression of one hemisphere by electroconvulsive therapy (ECT). The premises were familiar or unfamiliar, true or false. While the right hemisphere was suppressed, syllogisms were usually solved by theoretical, deductive reasoning even when the factual answer was known a priori, the premises were obviously false and the conclusions were absurd. While their left hemisphere was suppressed, the same subjects applied their prior knowledge; if the syllogism content was unfamiliar or false, they refused to answer. We postulate a left-hemisphere mechanism capable of decontextualized mental operations and a right-hemisphere mechanism, the operation of which is context-bound and incapable of abstraction. We show that each hemisphere tends to overextend its perspective on the problem and that in the intact brain they both contribute to an extent that depends on the characteristics of the problem at hand. (C) 1996 Academic Press, Inc.