How the Split Brain Undermines Our Understanding of Consciousness
Human higher-order cognition is not what it seems.
In my last post on Medium about the Problem with Consciousness, I outlined a bare-bone summary of the shortcomings and pitfalls of human consciousness; that our higher-order cognition that we perceive interprets our immediate environment and directs our moment-to-moment decisions and actions doesn’t actually do those things. Here I describe a series of studies from almost 50 years ago that blew open our understanding of consciousness.
An early indication that our consciousness is dispensable and even problematic comes from work to curb the deleterious effects of epilepsy. In epilepsy the seizure begins as a storm of neural activity localized in a specific brain region but can then spread throughout the brain leading to the acute seizure symptoms associated with the disease. Preventing the migration of this neural storm was achieved in the 1960s by a procedure called a commissurotomy that severed the corpus callosum, a thick band of nerves connecting the brain’s left and right hemispheres. (Drugs are now preferred to control epilepsy although brain ablation is sometimes still used in severe cases.) Disconnecting the two hemispheres isolated the neural storm to one side of the brain lessening the symptoms.
Roger Sperry and his students Brenda Milner and Michael Gazzaniga at Caltech set up experiments to test if these patients suffered any consequences from the commissurotomy surgery. They discovered a startling result in these split-brain people, who superficially looked and acted as normal as non-split-brain people. The researchers found that pictures or printed words presented to only the right visual field, which were processed in the left hemisphere where the language centers were, could be verbally represented — spoken. Visual cues to only the left visual field processed by the right hemisphere were cut off from the language centers in the left hemisphere and therefore were unable to be verbalized. For example, when the word or object hammer was presented to the left hemisphere (right eye), the subject said, “hammer,” but when presented to the right hemisphere (left eye), the subject couldn’t identify or report what was seen. The subjects would say they see nothing or they didn’t know.
As a check on their conclusions, the researchers put several items on a table next to the subject, but the objects were screened so the subject couldn’t see them. A picture or word of one of the objects was presented to the left visual field (the right hemisphere visual cortex), the one that would not result in speaking the name of the object. The subjects were asked to feel with their left hand (controlled by the right hemisphere sensorimotor cortex) and find the physical object that was displayed from among the several items on the table. The subjects performed this task successfully and consistently. They selected the object by hand that they could not acknowledge verbally.
To review these results: a picture or word presented to the left visual field could not be spoken by the subject, but the subject could tactilely select that same object from among several other objects. Observations by psychiatrist Fredric Schiffer, who did his own split-brain studies, showed that in some cases the right hand literally did not know what the left hand was doing. One hand might be pulling up the subject’s pants while the other hand was trying to pull them down. In another experiment, the split-brain subjects could not verbalize the names of Adolph Hitler and Johnny Carson from photos shown to the left visual field. However, with their left hands they signaled a vigorous “thumbs down” for the dictator and “thumbs up” for the comedian.
In a Brain Science Podcast episode, Milner described a test in which the experimenters showed a “sexy” picture to a split-brain woman’s left visual field. The woman claimed she saw nothing unusual, but showed the emotional signs of embarrassment. Her right brain registered an emotion, but her left brain could not understand it or at least indicate the proper integration of the experience. There was an internal misunderstanding between the visual experience and the signal to the language centers. Milner interpreted the woman’s awkwardness by saying
This young woman — NG — she blushed a little. Of course, emotion can get transmitted by lower centers that are not separated [by the commissurotomy surgery], you see. So, this patient probably felt a bit uncomfortable. And the left hemisphere said, “Oh, Dr. Sperry, what a strange machine you have there,” or made some comment about the machine, or what Sperry was doing. Because she obviously had no idea of what actually had happened — did not have access — but had access to the emotion and was rationalizing, trying to understand the way she was feeling, I suppose.
In a 1998 Scientific American article Gazzaniga described working with split brain patients by asking subjects to draw images of the words presented. The researchers showed the word bow to one hemisphere and arrow to the other, and then asked the split-brain subject to draw what was seen.
To our surprise, our patient drew a bow and arrow! It appeared as though he had internally integrated the information in one hemisphere; that hemisphere had, in turn, directed the drawn response.
We were wrong. We finally determined that integration had actually taken place on the paper, not in the brain. One hemisphere had drawn its item — the bow — and then the other hemisphere had gained control of the writing hand, drawing its stimulus — the arrow — on top of the bow. The image merely looked coordinated. We discovered this chimera by giving less easily integrated word pairs like “sky” and “scraper.” The subject did not draw a tall building; instead he drew the sky over a picture of a scraper […] Finally, we tested to see whether each hemisphere could, on its own, integrate words. We flashed “fire” and then “arm” to the right hemisphere. The left hand drew a rifle rather than an arm on fire, so it was clear that each hemisphere was capable of synthesis.
In split-brain subjects each brain hemisphere controls the graphic representation through its opposite hand, but only one hemisphere, the left, is capable of verbally expressing what is observed. Knowledge of the perceived written word or image existed intact through other means of expression regardless of the ability to communicate this knowledge through language.
The inability to verbalize a thought or feeling that is represented and actionable elsewhere in the brain of these subjects seemingly indicates that consciousness is not required to perform a task. A valid criticism is that this is an interruption of signals to the language centers, not necessarily consciousness. However, there’s an implicit assumption that our sense of consciousness is perceived internally through language. This raises the difficult question of what constitutes consciousness? Can there be consciousness without language? We are used to interpreting consciousness with language, but can we say communicating through a drawn image is an indication of consciousness? Alternatively, is the ability to draw a picture based on a visual cue something that can happen unconsciously, without the need for consciousness? Perhaps even language perception and vocalization happen primarily at an unconscious level despite our sense that it is intimately tied to consciousness.
This opens a neurophilosophical can of worms, but the split brain experiments are just the tip of the iceberg, the wedge that opens a crack into the window of consciousness. What constitutes consciousness is an enormous historical and philosophical problem. In many ways this issue is as unresolved today as it was hundreds of years ago. Much of this challenge is caught up in the cyclical and self-referential nature of trying to use our self-aware consciousness to evaluate our own consciousness. Does observer bias influence how we perceive ourselves? It could be reasonable for us to analyze our own consciousness if our consciousness actually performs as we believe it does, but what if it doesn’t? Most people have the sense that they and their consciousness are the director of their own movie. Whether or not this is true is an open question, but how do you handle the science when it contradicts your most fervently-held convictions?
I had an epiphany when I learned about the split-brain experiments. I was already predisposed to seeing humans as organic rather than elevated or divine. The human brain, I believed, was not so different and special from other animals. Human intelligence constituted a higher level of processing than other animals were capable of, but the more I studied, the more it became obvious that so much of the human experience made sense if interpreted through a biological lens. Today it is clearer than ever that animals are capable of far more cognitively than we ever knew narrowing the behavioral gap between us and them.
In my next article, I describe different examples of the discontinuities of consciousness. In the meantime, what do you do with this information?
Campbell, Virginia. (2008). Brain Science Podcast. Accessed 2018, Dec. 3. http://brainsciencepodcast.com/bsp/dr-brenda-milner-pioneer-in-memory-research-bsp-49.html
Gazzaniga, Michael S. (1998). The split brain revisited. Scientific American, 279(1), 50–55.
Gazzaniga, Michael S. (2000). Cerebral specialization and interhemispheric communication: Does the corpus callosum enable the human condition? Brain, 123(7) 1293–1326.
Schiffer, Fredric. (2000). Can the different cerebral hemispheres have distinct personalities? Evidence and its implications for theory and treatment of PTSD and other disorders. Journal of Trauma and Dissociation, 1(2), 79–88.