Severed Corpus Callosum

Summary

Joe underwent surgery to sever his corpus callosum—the bundle of nerve fibers connecting the brain’s two hemispheres—to limit the spread of electrical activity causing frequent epileptic seizures. His seizures came under control, and he functioned well in daily life. The operation also meant that information received by one hemisphere could no longer readily pass to the other.

Experiments with Joe illustrate how the hemispheres can process information independently. In general, each hemisphere controls the opposite side of the body, and information presented in one side of the visual field is initially processed by the opposite hemisphere. Because speech is largely supported by the left hemisphere, Joe could sometimes act on information presented to his right hemisphere without being able to say what he had seen.

The experiments also highlighted different abilities associated with the hemispheres. The left hemisphere could speak and often produced explanations for Joe’s choices, even when it had not received the relevant information. Gazzaniga described this tendency to construct plausible cause-and-effect explanations as a notable feature of the left hemisphere.

In a separate test, Joe’s right hemisphere was better at recognizing faces in Giuseppe Arcimboldo’s paintings, while his left hemisphere focused more on the objects making up the faces. The video presents these as examples of hemisphere specialization, not as evidence that either hemisphere works entirely alone in ordinary life.

Experiments and Findings

• Independent hand drawing: Joe’s hands could respond to different instructions because each hemisphere controlled a different hand and the hemispheres could not share the instructions.
• Words shown to one visual field: With Joe fixating on a central cross, researchers presented words to one side of his vision at a time. A word processed by the language-dominant left hemisphere could be reported aloud; information received by the right hemisphere could be unavailable to speech but still guide his left hand.
• Drawing information the participant could not name: Joe could draw an object prompted by information his right hemisphere had received, then identify it after seeing the drawing.
• Two words presented at once: When each hemisphere received a different word, Joe could point to an image matching one word while his speaking hemisphere gave an explanation based on the other word.
• Arcimboldo paintings: When images of faces made from objects were presented to different visual fields, Joe’s right hemisphere tended to select the face, while his left hemisphere tended to identify the component objects.

Researchers and Sources Featured

• Mike Gazzaniga — brain scientist who conducted split-brain research with Joe.
• Joe — the participant and patient whose corpus callosum had been severed; no surname is given in the subtitles.
• Giuseppe Arcimboldo — 16th-century painter whose paintings were used in the face-recognition experiment.
• Dartmouth College — the setting for the meeting and demonstrations described in the video.