By Christian Jarrett

Back in the 1960s, Nobel-prize winning research shook our understanding of what it means to be a conscious entity. Epilepsy patients who’d had the thick bundle of nerves connecting their two brain hemispheres either severed or removed (as a drastic treatment for their epilepsy) responded in laboratory tasks as if they had two separate minds.

It’s an unsettling idea that has appeared in psychology textbooks for decades. But dig into the original studies and you’ll find the evidence for split brains leading to split minds was mostly descriptive. Now a team of researchers led by Yair Pinto at the University of Amsterdam has conducted systematic testing of two split-brain patients over several years, specifically to find out whether the division of their brains has also separated their consciousness. In fact, the results, published recently in the journal Brain, suggest their consciousness remains unified. It may be time to rewrite the textbooks.

To understand the original observations and the new results, it helps to revisit a little basic neurobiology. There’s a lot of cross-wiring in the brain (the formal term is decussation), which means that the left side of space is processed by the brain’s right hemisphere and the right side of space is processed by the left hemisphere, which in most people is also the hemisphere that controls speech. Similarly, the left arm is controlled by the right hemisphere, and the right arm by the left hemisphere.

In a healthy person, the two hemispheres communicate extensively across a thick bundle of neurons called the corpus callosum (there are other lesser connections) meaning information from either side of space is shared and available to your unified consciousness. Consequently, if your right hemisphere sees something in the left side of space, your left hemisphere (controlling speech) can still name it or point to it with your right arm.

But in split brain patients with a severed or removed corpus callosum, the original reports suggested that they were unable to verbally describe anything appearing on the left side of space, nor could their right hand point to it, or point to a picture matching it, even though their right hemisphere knew what was there (it could use the left hand to point to the target or to a picture that matched it). It’s as if the patients had two separate minds: one half knew what was on one side of space even while the other half was ignorant. It wasn’t only their brains that were split, their consciousness was too.

Roger Sperry, the scientist who won the Nobel prize for this work, hinted that there were exceptions to this split-mind phenomenon, but before now no one seems to have tested the idea systematically. For the new research, Pinto and his colleagues conducted five thorough experiments with two split-brain patients. Each experiment was really a variation on the same theme. Either verbally or by pointing, the patient had to assess the position of, or identify, pictures or shapes that appeared on one side or other of a computer screen, or they had to compare shapes or pictures on either side of space.

The patients were impaired: they were unable to make comparisons between shapes or pictures that appeared simultaneously on either side of space. And when identifying or localising a picture or shape on just one side of space, their accuracy varied depending on which side of the screen it appeared on – suggesting the two sides were processed separately at a perceptual level. But here’s the critical finding: regardless of whether the target shapes or pictures were located on the left or right side of space, the patients were able to report on them by any modality – by speech, with their right hand or left hand.

In other words, despite their brain hemispheres being largely separated, information processed by one side or other did at some stage become available at a unified conscious level, which the patients could report on verbally or act on with either arm. Pinto and his colleagues pointed out that not only does this contradict established textbook accounts of split-brain patients, but also raises problems for contemporary theories of consciousness, such as the Global Workspace theory, which is predicated on the idea that information reaches consciousness when it is broadcast across the brain hemispheres.

These tests were conducted many years after the patients underwent their radical brain surgery so it remains possible that the original split minds phenomenon is real, but only transient. If so, the compensatory mechanisms that overcome the phenomenon remain unknown. Changes to neurosurgical practices mean that split-brain patients are rare today and few of those tested in the original research are still alive. This means time is running out to explore these fascinating questions further.

“We have shown that severing the cortical connections between the two brain hemispheres does not seem to lead to two independent conscious agents within one brain,” the researchers said. “This raises the intriguing possibility that even without massive communication between the cerebral hemispheres, and thus increases in modularity, unity in consciousness and responding is largely preserved.”

—Split brain: divided perception but undivided consciousness

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest