What did split-brain experiments reveal about the self?

Split-brain experiments showed that when the corpus callosum is cut, the two hemispheres can process information separately, each without awareness on the part of the other. That raised the possibility that one brain can host more than one stream of experience. Yet patients behave and feel like a single person, and a 2020 collective review concludes that the evidence is not sufficient to say whether consciousness is split or unified. [1] [2]

The surgery is used for some people with intractable epilepsy. The large tract connecting the hemispheres is severed to stop electrical activity from spreading. Afterward the patients appear normal. Michael Gazzaniga's team found that sensory information presented to one hemisphere was processed in that half alone. Because of the way sensory pathways cross the midline, the right hemisphere handles the left half of the world and the left hemisphere the right. [1]

Roger Sperry and Gazzaniga concluded that each half could process information independently, make its own decisions and control its own behaviours. In early tests, the right brain could point at or draw what was shown on the left side of the field of view, even though it could not talk about it. These experiments showed for the first time that each hemisphere has specialized tasks. [3] [4]

In one experiment a patient saw a chicken claw with the left hemisphere and a snow scene with the right. His right hand pointed to a chicken and his left hand to a shovel. Asked why, his speaking left hemisphere said the shovel was needed to clean out the chicken shed. It did not say it did not know. Gazzaniga calls this left-hemisphere process the interpreter, a tendency to fit events into a story, and it suggests that our sense of a coherent self is partly a constructed explanation. [1]

Laboratory tests suggested split consciousness, but everyday life points the other way. Patients seem socially ordinary and report feeling unchanged, although their behavior outside experiments has not been systematically observed. Self-face recognition seems available to both hemispheres. A further argument holds that if two consciousnesses emerge after surgery, both must have existed before it, which casts doubt on the two-consciousness view. [2]

Newer work complicates the classic picture. Perception appears largely split, but response selection and action control seem unified under some conditions. In one study, the side of the visual field affected matching performance, while the responding hand or speech had little or no effect. Patients also differ in their ability to compare stimuli across the midline. The reviewers note two readings: two agents who synchronize their output, or one agent experiencing an unintegrated stream, like a film with sight and sound out of sync. They also say the classic story, in which a patient says they saw nothing yet the left hand draws the object, still appears in textbooks but may depend on the type and complexity of the response. Disagreement exists about what mechanism keeps action unified, and answers may fall between a simple yes and no. [2]