The Confabulating Interpreter
Split-Brain Patients and Why the Narrative Self Is Written After the Act
Nothing feels more certain to ordinary introspection than the unity of the person sitting behind our eyes. We experience ourselves as a single chief executive who weighs evidence, forms an intention, and commands the body to act. In the early 1960s, that common-sense picture was dismantled in a basement laboratory at the California Institute of Technology.
To treat patients suffering from intractable grand mal epilepsy, Los Angeles neurosurgeons Philip Vogel and Joseph Bogen had severed the corpus callosum—the great white-matter bridge of two hundred million axons connecting the left and right cerebral hemispheres. Remarkably, once the patients recovered from surgery, casual observers noticed nothing unusual. Their intelligence scores, vocabulary, and everyday manners appeared completely unchanged. Then in 1962, Caltech neurobiologist Roger Sperry and his graduate student Michael Gazzaniga built a tachistoscopic screen that flashed images for a tenth of a second to either the left or right visual field alone.
Because the human optic chiasm routes the left visual field exclusively to the right hemisphere and the right visual field to the left hemisphere, Sperry and Gazzaniga could interrogate each half of the surgically divided brain in isolation. In almost all right-handed adults, speech and grammatical syntax reside in the left hemisphere, while the right hemisphere is largely mute but possesses its own independent perception, spatial comprehension, and control over the left hand.
When Gazzaniga flashed the word ring to the right side of the screen, the talking left hemisphere immediately read ring aloud. When he flashed the word key to the left side of the screen—reaching only the mute right hemisphere—the patient insisted he had seen nothing at all. Yet when asked to reach beneath a screen with his left hand and select what he had seen from a tray of objects by touch, his left hand bypassed the ring and picked up the key, while his speaking mouth insisted he had no idea what his left hand was doing. Inside a single skull lived two separate spheres of awareness, each blind to the visual world of the other.
Occasionally, the two hemispheres clashed in public: one patient found his left hand unbuttoning his shirt while his right hand buttoned it up; another had to sit on his left hand to stop it from pulling down his trousers. Using Scrabble tiles arranged by the left hand, Gazzaniga even interviewed the mute right hemisphere of a teenage patient known as P.S. When asked what career he wanted to pursue, his speaking left hemisphere politely told his family he wanted to be a draftsman, while his left hand spelled out automobile racer.
It was P.S. who revealed Gazzaniga's deepest discovery: the Left-Brain Interpreter. Gazzaniga flashed a winter snow scene to P.S.'s mute right hemisphere and a chicken claw to his speaking left hemisphere. Asked to select matching cards from a row of pictures, P.S.'s right hand pointed to a chicken while his left hand pointed to a snow shovel. Gazzaniga asked him why he had chosen those two items. Because the speaking left hemisphere had never seen the snow scene, it had zero access to the true reason the left hand was pointing at a shovel.
Instead of admitting ignorance, P.S.'s left hemisphere answered without a millisecond of hesitation: the chicken claw went with the chicken, and you needed a shovel to clean out the chicken shed. In healthy brains just as in split brains, corroborated two decades later by Benjamin Libet's 1983 readiness-potential recordings and Daniel Wegner's choice-blindness trials, the conscious narrator is rarely the author of our actions. It is a press secretary sitting at the end of the neural chain, observing what the brain's hidden sub-routines have already done and weaving a seamless story claiming it was in charge all along.