Your Brain, Region by Region

Chapter 3 The bridge between the hemispheres

The corpus callosum and the split brain

Two hemispheres, joined by one band of about 200 million nerve fibers. Cut it, and each half of the brain is left to itself.

7 min read · 4 interactive figures

The brain cut at the midline and seen from the left: the corpus callosum is the arch above the thalamus, cut across. Turn the model to Above to see it at the bottom of the groove between the hemispheres.

The corpus callosum is the thick band of nerve fibers that joins the left and right cerebral hemispheres: about 200 million axons (some estimates run to 300 million), carrying sensory, motor and other information from one side to the other so that the two halves of the brain work as one[1][2]. It is the largest white-matter structure in the brain, about 10 cm long at the midline, and only placental mammals have one[2]. What it does became clearest in the 1960s, when it was cut in a few people with severe epilepsy and each half of their brain could be tested on its own.

Where it is

Look down on a brain and you see a deep cleft running front to back, the longitudinal fissure, between the two hemispheres. At the bottom of the cleft, roughly in the middle of the brain, lies the corpus callosum[3]. Cut the brain down the midline, as in the figure at the top of the page, and it appears in section as an arch above the thalamus, thick at the front (the genu), thinner along the body, and thickest at the back (the splenium).

Figure 1 The brain from above and slightly behind. The hemispheres are pulled apart to open the longitudinal fissure; the corpus callosum is the band left spanning the gap, the only large connection between the two hemispheres in this model. The thalamus and brainstem below it belong to neither hemisphere. Slide Pull the hemispheres apart back to zero to close the gap.#

Its fibers are not all alike. Counting them under the microscope in twenty brains, Francisco Aboitiz and colleagues found thin fibers most dense at the front, in the genu, and fewer toward the middle of the body, where large fibers peak; the pattern repeats toward the back[4]. Different parts of the band carry fibers between different parts of the cortex, a regional specificity that later split-brain studies mapped in detail[5]. established

Each hemisphere gets half the world

Before the experiment, the wiring it depends on. The brain's control is crossed: the left hemisphere controls the right half of the body and the right hemisphere the left half[1]. Vision is crossed too, in a way that is easy to misremember. It is not split by eye but by visual field: everything to the left of the point you are looking at, seen by either eye, goes first to the right hemisphere, and everything to the right goes to the left hemisphere[1][5]. And in most people, language production depends on the left hemisphere, where Broca's area is[3].

With the corpus callosum intact, none of this shows. Whatever one hemisphere receives, the other gets within a fraction of a second.

The split-brain experiment

Cutting the corpus callosum stops seizures from spreading from one hemisphere to the other, and it is still used as a last-resort treatment for epilepsy that does not respond to anything else[2][1]. In the early 1960s, at the California Institute of Technology, the psychologist Roger Sperry, his student Michael Gazzaniga and the neurosurgeon Joseph Bogen began testing patients who had had the operation. Their first report appeared in 1962[6][5].

In everyday life the patients seemed little changed. The trick was to give information to one hemisphere only. A patient looked at a dot in the middle of a screen, and a word or picture was flashed to one side of it, too briefly for the eyes to move to it. Then the patient was asked about it: aloud, or by feeling for the object among others, out of sight, with one hand[7][1].

Question 1 Prediction: a word to the left#
Figure 2 The screen the patient faces (top) and the two hemispheres seen from above (below), the front of the head toward the screen. Choose where to flash the word and how the patient should answer; the drawing follows the word to the hemisphere that receives it and, if the corpus callosum is intact, across it. The answer appears under each hand or under speech.#

Work through the six combinations with the callosum cut and a rule emerges. The patient succeeds whenever the hemisphere that saw the word is the one that controls the answer: a word on the right can be read aloud and found with the right hand; a word on the left cannot be said but can be found with the left hand. With the callosum intact, every combination succeeds. OpenStax describes the same result with pictures: a split-brain patient cannot name a picture shown in the left visual field, but can draw it with the left hand, and once the speaking left hemisphere sees the drawing, can name it[1]. established

Sperry received a share of the 1981 Nobel Prize in Physiology or Medicine for this work on the functional specialization of the cerebral hemispheres; the other half went to David Hubel and Torsten Wiesel, whose visual-cortex recordings appear in Chapter 1[8].

The left hemisphere explains

The most famous split-brain test shows something more unsettling than a split. Gazzaniga flashed two different pictures at once: a chicken claw to the right of the dot, so that the left hemisphere saw it, and a snowy scene to the left, for the right hemisphere. Then the patient chose matching pictures from cards in front of him. His right hand picked a chicken; his left hand picked a shovel. Asked why, the speaking left hemisphere, which had never seen the snow, answered at once: the chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed[9].

Figure 3 The two-picture test. Each hemisphere sees one picture and guides the opposite hand to a matching card; then the patient is asked, aloud, why the left hand chose what it chose. Compare Cut with Intact.#

Gazzaniga called this left-hemisphere system "the interpreter": it produces a plausible explanation for whatever the person finds themself doing, whether or not it has access to the real cause[9][5]. The patient was not lying. The part of him that could speak had no way to know about the snow, and it filled the gap without noticing that it was filling one. documented

Left brain, right brain: what the split brain does not show

The split-brain studies are the origin of a popular idea: that each person is "left-brained" (logical, verbal) or "right-brained" (creative, intuitive). The studies show that some functions are lateralized, handled more by one hemisphere: in most people language production is on the left, and some aspects of spatial attention lean to the right[1][10]. They do not show that people differ in which hemisphere they use.

That second claim was tested directly. Jared Nielsen and colleagues at the University of Utah analyzed resting-state brain scans of 1,011 people aged 7 to 29 and found left-lateralized hubs (including language areas) and right-lateralized hubs (including attention areas), but no sign that some individuals had stronger left-sided networks across the board and others stronger right-sided ones. In their words, the data "are not consistent with a whole-brain phenotype of greater 'left-brained' or greater 'right-brained' network strength across individuals"[10]. OpenStax's anatomy text makes the same point from the other direction: neither hemisphere has a monopoly on functions such as language[3]. established

The better lesson of the split brain is about the corpus callosum itself. Two hemispheres that each perceive, remember and act, joined by a cable of 200 million fibers, behave as one mind. Cut the cable and, under the right conditions, you can see two.

This page describes anatomy and classic experiments for education. It is not medical advice. Surgery on the corpus callosum is a rare, specialist treatment for some forms of severe epilepsy; questions about epilepsy treatment belong with a neurologist.