The corpus callosum is a bundle of hundreds of millions of myelinated nerve fibers linking the brain’s two hemispheres. Surgeons first cut it partially in the 1930s, when Walter Dandy sought a path to deep pineal tumors, and later as a last resort for violent grand mal seizures that spread from one hemisphere to the other. This episode explains why split-brain patients seem normal in daily life and how, in the 1960s, Roger Sperry and Michael Gazzaniga exposed the divided mind by flashing images to one visual field at a time. A patient shown an apple only on the left side would say he saw nothing, while his left hand pulled the apple from a hidden box.

In the chicken and snow experiment, the right hand chose a picture of a chicken and the left hand a snow shovel, and the speaking left hemisphere instantly explained that the shovel was for cleaning out the chicken coop. Gazzaniga called this the left brain interpreter. The episode follows patient W.J., a World War II paratrooper who had the first complete corpus callosotomy in 1962; patient J.W., whose right hemisphere could add but not subtract, multiply or divide; patient V.P., whose small surviving sliver of fibers carried words but not images; and Kim Peek, who was born without a corpus callosum.

• Sperry’s 1981 Nobel Prize and the crossed wiring that ties the left hand to the right hemisphere
• J.W.’s 150 millisecond math tests: about 95 percent accuracy on the left side, 58 percent on right-side addition
• Face-morphing tests showing that recognizing your own face relies on the left hemisphere
• V.P.’s 1984 MRI revealing about 1.8 percent of the corpus callosum intact, enough to merge head and stone into tombstone
• Kim Peek, inspiration for Rain Man, reading two pages at once with separate eyes and memorizing over 9,000 books