Hemispherectomy vs commissurotomy in the monkey: one hemisphere can be better than two.

Split-brain cats using a single hemisphere show impaired performance on a variety of behavioral tasks. This has been attributed to a loss of cortical mass action. To test this proposal in primates, performance on a nested match-to-sample task was compared in split-brain, hemispherectomized, and normal monkeys. As expected, split-brain monkeys using a single hemisphere performed worse than normal monkeys. In contrast, hemispherectomized monkeys were unimpaired, indicating that the deficit in split-brain animals results from interference by the idle hemisphere rather than from a loss of cortical mass. The results also suggest that the processing ability of a single hemisphere is equivalent to that of the normal brain.