Psychology Degree Guide: Split-Brain Consciousness

We think of the brain as a unified entity. Each part works together to create a force of consciousness greater than any individual neuron. But what happens when the brain is split into its left and right halves? In this guide to the split brain for the non-expert, we examine this fascinating medical state and what it tells us about the brain, the mind, and how humans use language.

The Two Halves of the Brain

Neurologists refer to the division of brain activity along the lines of the two hemispheres as lateralization. Most people are familiar with the two halves of the brain: a left brain that is more factual, mathematical, and logical, and a right brain that is more contextual, emotional, and intuitive. Notice that we say “more.” Many activities that are associated in the popular imagination with one hemisphere or the other are, in actuality, distributed across both. For instance, while math is generally thought of as a “left brain” activity, the ability to estimate is as prominent in the right hemisphere as the left. And emotion, while thought of as “right brain,” is distributed across both hemispheres as well. Some activities are limited to one hemisphere. Intonation and accentuation in language is based in the right brain, while specific numerical calculations are based in the left. However, broad categories like “emotion,” “language,” and “logic” are not restricted to one area.

To a certain degree, the popular imagination’s beliefs about the left versus right brain are probably dictated by the traditional divide in Western culture between a classicist, logical, rationalist worldview and an emotional, romantic worldview. It doesn’t necessarily reflect the actual, neurological differences between left and right brain. This is an important distinction to make when discussing the split brain.

Types of Split Brain

The two hemispheres of the brain are connected by a network of fibers known as the corpus callosum. Unlike the main body of the cerebrum, the corpus callosum is made of white matter rather than gray matter. This network carries communication between the two hemispheres. A wide range of tasks, even very simple ones, require the work of both hemispheres.

Split brain occurs in two categories. Acute disconnection is the term for a partially split brain, when the corpus callosum is only severed in parts. This results in difficulties in speech, coordination, and is frequently accompanied by a feeling that the left hand is independent of the rest of the body. In chronic disconnection, the corpus callosum is permanently severed. This can result in memory problems, and often leads to difficulty reading.

The Split Brain in Experiments on Language

In the 1950s, two researchers at the California Institute of Technology, Roger Sperry and his graduate student Michael Gazzaniga, performed experiments on cats and monkeys in which the corpus callosum was severed. In 1961, Sperry and his team moved on to experiments on humans; at the time, the partial severing of the corpus callosum was being explored as a potential cure for epilepsy, a procedure that is performed to this day.

Patients who had their corpus callosum severed experienced limited hemispheric interaction. Consequently, there was a frequent disconnect between their language and the world around them. For example, a patient might be shown a picture of a piece of cake. He wouldn’t be able to identify it. However, if asked “is it food or a tool?” the patient would identify it as food. And if asked “is it a dessert or a breakfast?” he would identify it as a dessert.

Another experiment involved showing a word to a patient, such as “cat.” The patient would interpret that with the right hemisphere only, and be able to write “CAT” with her left hand. But when asked what she wrote, she would have no idea. She would be aware that she was writing and interpreting, but unaware of what she was writing and interpreting. And the right hemisphere could only communicate with the left hand.

In a third experiment, a split-brain patient was shown word pairs. These word pairs came in four types: words that rhyme, but don’t look like they should (hair, wear); words that rhyme and look like they should (art, cart); words that don’t rhyme and look like they shouldn’t (mouse, tree); and words that don’t rhyme, but look like they should (tough, dough). The patient, interpreting the situation with her right hemisphere alone, was able to identify which words did and did not rhyme, regardless of whether or not they appeared to.

Experiments like this have confirmed the place of language in two regions of the left hemisphere: Broca’s Area and Wernicke’s Area. While the right hemisphere hears accent, prosody, and rhyme, the left hemisphere organizes these sounds into cogent language.

Further Reading

We’ve linked to some academic resources on the split brain, research involving split brain, and how these experiments have contributed to our knowledge of neurology and psychology.

• A rubric, hosted by Caltech– where the first split brain experiments were performed– on split brain in a medical context.
• Sperry was awarded the 1981 Nobel Prize for Medicine for his work. The Nobel Foundation has put together an educational page on his work.
• A flash demonstration of how split brain works, hosted by the University of Wisconsin, Milwaukee.
• Teaching about the brain in a K-12 classroom? The American Psychological Association hosts a fun lesson plan on the split brain for K-12 students.
• The Journal of Vision discusses the effects of split brain on vision.
• How does the brain divide functions between its left and right hemispheres? Read about the two halves in this page from Middle Tennessee State University.
• Video of a split-brain patient discussing his conundrum.
• An online biography of Roger Sperry
• Search for academic papers using the Neuroscience Information Framework.
• The Neuroscience Database Gateway is a great starting point for further neuroscience research on the web.

IMAGE: A cross-section of the human brain.  The white, U-shaped structure at the center is the corpus callosum, which is severed in split brain cases (Source: Wikimedia Commons)