What Were the Split Brain Experiments?

Do you really have two minds, one from the left hemisphere and one from the right?. By Lightspring/Shutterstock

Are you right-brain dominant or left-brain dominant? Today these terms are ubiquitous, and a medley of pop-psychology quizzes abound that purport to determine all sorts of things about you based on which brain hemisphere you favor. The list of traits associated with being right-brained or left-brained are well known. If you're right-brained, you're more imaginative, creative, artistic, intuitive. If you're left-brained, you're more rational, logical, scientific.

Though many of these characterizations are overly simplified, there is a scientific basis for them. Back in the 1960s, a researcher by the name of Roger Sperry conducted a number of bizarre, but groundbreaking experiments on patients who had their corpus callosums — brain structures responsible for connecting the two hemispheres of the brain — severed. The experiments would later be ominously referred to as "the split brain experiments."

The results of the experiments would forever change how we understand the brain, and ultimately, the self. They would also later earn Sperry the Nobel Prize in Physiology or Medicine in 1981.

How do some individuals get their corpus callosums severed? Well, in the 1960s the surgical severing of the corpus callosum, a corpus callosotomy, was the best method known for treating some forms of severe epilepsy. Though it is an extreme procedure, patients with severed corpus callosums often exhibit totally normal behavior to the untrained eye, even though the corpus callosum is the only link that exists between the right and left hemisphere (severing it renders the hemispheres unable to communicate).

Sperry knew from previous studies on patients with brain injuries that the two hemispheres had some specialized functions. For instance, the left side of the brain is largely in control of language. So he set out to test individuals who had received corpus callosotomies with the aim of fleshing out exactly what makes the two hemispheres unique.

To comprehend how the experiments work, it is first important to understand that the brain and body have a backwards association: the right hemisphere is connected to the left side of the body, and the left hemisphere is connected to the right side of the body. Most people's brains cope with stimuli presented to one side of the body or the other by sharing information via the corpus callosum, but split-brain patients have no such link. So Sperry surmised that he could test how each hemisphere performs differently in these patients by restricting stimuli to one side of the body or the other.

His experiments came in three varieties: the visual test, the tactile test, and the visual and tactile test.

In the visual test, subjects were presented with a row of horizontal lights that could flash across the visual field. Fascinatingly, Sperry found that subjects were only capable of vocally reporting when the lights flashed on the right side (thus corresponding to the left brain hemisphere). But when the subjects were asked to instead point at the lights when they flashed, rather than make a vocal report, the subjects were able to accurately identify all the lights. This essentially proved that while both brain hemispheres are capable of perceiving the lights, only the left hemisphere is capable of translating this knowledge into a vocalized report. Sperry therefore proved that the left hemisphere contains the speech center.

The tactile test had similar results, only using tactile stimuli instead of visual stimuli, and the visual and tactile test combined the two, further establishing the link between speech and the left brain hemisphere. Later tests showed that the right brain hemisphere also has special abilities, particularly in regards to nonverbal and spatial tasks, facial and object recognition, and symbolic reasoning.

All in all, the left hemisphere is typically referred to as the "analytic" or "logical" side, while the right hemisphere is "holistic" or "intuitive." Sound familiar? This is essentially the source of our pop-psychology notions of what it means to be right-brained or left-brained, even though all healthy individuals are perfectly capable of performing tasks associated with either hemisphere.

The experiments have led to a lot of questions about our perception of self. For instance, some scientists have wondered whether split-brain patients are actually "of two minds," with each hemisphere individually perceiving and analyzing the world separately. The difficult question therefore is: In what sense do any of us truly have a single "mind's eye"? Might the perception of having a single conscious self be an illusion?

If you're interested in exploring more details about the split-brain experiments, an animated infographic can be found about them at Nobelprize.org.