Split brain
A brain whose corpus callosum and other connections between the halves have been cut, so that the right and left halves work separately. Animal studies in the 1950s and patient studies from 1962 showed that what one half saw or learned, the other did not know.
In the 1950s Roger Sperry's laboratory cut the optic chiasm and the corpus callosum in cats and monkeys and showed that a task learned through one eye was unknown to the other half of the brain. In a 1961 review Sperry wrote that such a brain behaves in many respects like two separate brains[1][3].
In 1962 Michael Gazzaniga, Joseph Bogen and Sperry reported that W.J., whose hemispheres had been disconnected, said he had seen nothing when a shape was flashed to the left of center, while his left hand could point to it correctly[2][3]. Sperry argued that each disconnected hemisphere seemed to have its own perceptions, learning and memories[4]. In 1977 researchers reported that in P.S., whose right hemisphere also had rich language, the right hemisphere spelled out his name and the job it wanted with Scrabble letters[5].
Whether consciousness itself is divided is still not settled. In 2017 a report described two patients whose corpus callosum had been cut completely and who could show where something had appeared, on either side, by speaking or with either hand. Those researchers argued that what is divided is perception, not consciousness[6].
Related terms
- Corpus callosumThe thick bundle of nerve fibers that joins the right and left halves of the cerebrum. Surgeons began cutting it in 1940 to treat epilepsy, and in the 1960s researchers began testing the two halves of such patients separately.
- Optic chiasmThe place at the base of the brain where some of the fibers from each eye cross to the other side. In split-brain animal studies it was cut down the middle, so that what one eye saw reached only the hemisphere on the same side.
- Left-brain interpreterThe work of the left hemisphere that explains one's own actions and ties them into a single story. Michael Gazzaniga and Joseph LeDoux named it from observations of split-brain patients. It supplies a sensible reason even when it does not know the real one.
Sources
- R. W. Sperry, “Cerebral organization and behavior: the split brain behaves in many respects like two separate brains, providing new research possibilities,” Science 133(3466), 1749–1757 (1961).
- M. S. Gazzaniga, J. E. Bogen and R. W. Sperry, “Some functional effects of sectioning the cerebral commissures in man,” Proceedings of the National Academy of Sciences 48(10), 1765–1769 (1962).
- M. S. Gazzaniga, “The split-brain: rooting consciousness in biology,” Proceedings of the National Academy of Sciences 111(51), 18093–18094 (2014).
- R. W. Sperry, “Hemisphere deconnection and unity in conscious awareness,” American Psychologist 23(10), 723–733 (1968).
- J. E. LeDoux, D. H. Wilson and M. S. Gazzaniga, “A divided mind: observations on the conscious properties of the separated hemispheres,” Annals of Neurology 2, 417–421 (1977).
- Y. Pinto, D. A. Neville, M. Otten, P. M. Corballis, V. A. F. Lamme, E. H. F. de Haan, N. Foschi and M. Fabri, “Split brain: divided perception but undivided consciousness,” Brain 140(5), 1231–1237 (2017).