Michael Corballis Ph.D.

The Recursive Mind

Split-Brain Researchers Are Split

The classic view of the split brain has been challenged.

Posted Nov 20, 2017

In Nobel Prize research beginning in the 1960s, Roger W. Sperry and colleagues studied the effects of cutting the forebrain commissures in patients as a radical treatment for intractable epilepsy. Splitting the brain, they concluded, divided consciousness itself, and showed the two sides of the brain to have mental capacities that were in some ways complementary. The left brain was said to be linear, verbal, and logical; the right brain diffuse, nonverbal, emotional, intuitive — and creative.

This work has had a profound influence on contemporary culture in diverse areas, including art, education, literature, medicine — and even business. So-called right-brain thinking has even come to be seen as a corrective to the left-brain bias in the Western education systems. More emphasis on the right brain, it is suggested, should enhance empathy and creativity.

Recently, though, an article by the Dutch neuropsychologist Yair Pinto and colleagues in the prestigious journal Brain has challenged the view that consciousness is divided into the split brain. Consciousness, they suggest, is in fact integrated into the split brain, but visual perception is divided. But this, in turn, was rebutted by Lukas J. Volz and Michael S. Gazzaniga, who had worked closely with Sperry in the early work.

It is true that split-brained patients often seem more unified than one might expect. Each side of visual space projects to the opposite side of the brain, yet split-brained patients don’t seem to act in everyday life as though the visual world was divided down the middle. Pinto and colleagues review many studies in which these patients seemed to integrate information between the two sides of space and projected to opposite sides of the brain. But Volz and Gazzaniga suggest that this integration is illusory. Each disconnected side of the brain, they suggest, can send nuanced cues to the other side by external means, perhaps using their hands or subtle movements of the face, and give the appearance of unified perception.

All of these authors overlook the role of subcortical processes, which maintain connections between the two sides of the brain when the cortical connections are cut. A subcortical visual system, sometimes called the second visual system, goes far back in evolution and remains connected when the forebrain commissures are cut. It probably maintains a degree of visual unity in the split brain, and is critical to such relatively primitive activities as paying visual attention and detecting movement. It can explain much of what seems to be the integrated vision in the split brain.

SkyClickPhoto/Pexels
Source: SkyClickPhoto/Pexels

A more focused visual system involving circuitry in the cortex evolved later, dedicated mainly to identifying objects and patterns. It is mainly this system that is divided into the split brain, so that the patients can’t compare objects, words, faces, or colors across the two sides of visual space. At that level, too, each side of the brain may indeed be separately conscious. Typical is a split-brained patient who was unable to name a word or object projected to the right brain, and therefore separated from the speech centers in the left brain. He insisted that he saw nothing, yet was able to point to the correct word or object in an assemblage using his left hand.

In cortical vision, too, the two sides of the brain are indeed specialized somewhat differently, although the differences have been exaggerated well beyond the evidence. There is no convincing evidence that the right brain is more creative than the left, although it may be more involved than the left in nonverbal endeavors, such as art or music.

The split-brain era is now largely over; because the drug routines for epilepsy are now more effective and more widely used, the split-brain operation is largely restricted to more severe cases, and surgery often involves only a partial section of the commissures. Yet the split brain continues to raise deep questions about the nature of human consciousness, some of them still unresolved.

References

Pinto, Y., Neville, D.A., Otten, M., Corballis, P.M., Lamme, V.A.F. ... Fabri, M. (2017).  Split brain: divided perception but undivided consciousness. Brain (advance publication). doi:10.1093/brain/aww358.