What Roger Sperry's Split-Brain Experiment Revealed

In the early 1950s, neuroscientist Roger Sperry, then at the University of Chicago, and his doctoral student Ronald Myers made a discovery that would change how we understand the brain.

Working with cats, they severed both the corpus callosum (the bridge of fibers connecting the two hemispheres) and the optic chiasm, so that visual information from each eye reached only the hemisphere on its same side.

The result was surprising: each hemisphere functioned completely independently, as if it were an entire brain on its own.

A cat could learn to solve a problem using one eye, but if that eye was then covered and the same problem shown to the other, it didn’t recognize it: it had to learn it from scratch. Each half of the brain was entirely unaware of what the other had learned.

From cats to human patients

In the 1960s, by then at the California Institute of Technology (Caltech), Sperry gained access to a small group of people with severe epilepsy that didn’t respond to other treatments. Neurosurgeons Philip Vogel and Joseph Bogen severed their corpus callosum to reduce the frequency and intensity of their seizures; the first patient in the series, known in the literature as W. J., was operated on in 1962.

Sperry and his team, with a young doctoral student named Michael Gazzaniga, then designed a series of psychological tests to study these patients, and one of the first observations was, in itself, remarkable: the operation produced no perceptible change in temperament, personality, or general intelligence in those who’d had it. In everyday life, everything seemed normal.

The differences only came to light with very specific lab tests. Words were projected to one half of the visual field, so that the information reached only the opposite hemisphere.

When the word was sent to the left hemisphere, the patient could read it and say it out loud with no problem. When the same word was sent to the right hemisphere, the patient was unable to name it out loud, even though in many cases they could point to the correct object with their left hand (controlled, precisely, by the right hemisphere).

One half of their brain knew something the other half couldn’t put into words. Gazzaniga, Bogen, and Sperry published the first observations of W. J. that same year, 1962.

Two brains under one skull

Sperry’s findings, which earned him the Nobel Prize in Physiology or Medicine in 1981, confirmed and refined, in living patients, what lesion studies had suggested: in most people, the left hemisphere controls mechanisms tied to speech, language, and the ability to name things, while the right, more “mute” in that sense, is especially skilled at visuospatial and nonverbal tasks, such as copying drawings, assembling shapes, or recognizing faces.

Over the years, the original idea of one hemisphere being “dominant” over the other has given way to a more nuanced concept, hemispheric specialization: each hemisphere is relatively better at certain tasks, and the difference is one of degree, not all or nothing.

What the operated patients showed is something else, though related: with the corpus callosum severed, each hemisphere could perceive, learn, and remember on its own without the other finding out. Sperry described this as two separate spheres of consciousness under the same skull, a reading still debated in the philosophy of mind.

What the corpus callosum is for

Even before Sperry, it was known that severing the corpus callosum produced no obvious behavioral changes. So little was known about its function that a joke circulated among neuroscientists, attributed to Karl Lashley or to Warren McCulloch depending on the source, that perhaps its only job was to hold the two hemispheres together.

Sperry’s achievement was designing the precise lab tests that showed it did have a real, very specific function: keeping the two hemispheres working as a coordinated team, not as two brains that simply share a skull without communicating.

The interpreter: what Gazzaniga added

Gazzaniga went on studying split-brain patients for decades and described a phenomenon that is now cited as often as the original findings. In a classic experiment, a chicken claw was shown to the left hemisphere and a snowy scene to the right at the same time, and the patient chose a related picture with each hand: a chicken with the right and a shovel with the left.

Asked why they had chosen the shovel, the left hemisphere, which could speak but hadn’t seen the snow, didn’t say “I don’t know”: it improvised a coherent explanation, such as that the shovel was for cleaning out the chicken shed. Gazzaniga called this system the left hemisphere’s interpreter, the tendency to build a plausible account of one’s own behavior even when the real information is missing (Gazzaniga, 2005).

Why this finding changed how the brain is studied

Sperry’s work opened the door to studying each hemisphere separately and asking, far more precisely, which function depends on which side.

That question remains open in many areas, and it doesn’t support the idea that each input channel (visual, auditory, verbal) depends on just one side of the brain: in a healthy brain, both sides work in an integrated way.

This biological foundation is what would later inspire consultant Ned Herrmann to design his brain dominance model, crossing Sperry’s hemispheric specialization with the so-called “triune brain” to describe different styles of thinking and learning.

A specific sensory channel, like the visual or auditory one, cannot, however, be assigned to a single hemisphere: under normal conditions, the brain’s architecture works as an integrated whole, not split in two.

You can find out which channel predominates for you with our learning style quiz.

Sources

• Herrmann, N. (1996). The Whole Brain Business Book. McGraw-Hill.
• Gazzaniga, M. S. (2005). Forty-five years of split-brain research and still going strong. Nature Reviews Neuroscience, 6(8), 653-659. doi:10.1038/nrn1723
• Gazzaniga, M. S., Bogen, J. E. & Sperry, R. W. (1962). Some functional effects of sectioning the cerebral commissures in man. Proceedings of the National Academy of Sciences, 48(10), 1765-1769. doi:10.1073/pnas.48.10.1765
• Myers, R. E. & Sperry, R. W. (1953). Interocular transfer of a visual form discrimination habit in cats after section of the optic chiasma and corpus callosum. Anatomical Record, 115.
• Sperry, R. W. (1968). Hemisphere deconnection and unity in conscious awareness. American Psychologist, 23(10), 723-733.
• Sperry, R. W. (1982). Some effects of disconnecting the cerebral hemispheres. Science, 217(4566), 1223-1226.