Inside the Narrative System: Split-Brain Phenomena

 

The corpus callosum is a thick bundle of neural fibers that runs down the middle of the cortex, at the bottom of the longitudinal fissure (the long fold in the brain that divides its two hemispheres). Because of the longitudinal fissure, signal traffic between the left and right side of the cortex is limited. The corpus callosum is the superhighway through which most of that traffic flows. Signals can still get from one place to another using side routes, of course, again much like a road system. But it's a lot slower and less efficient to go the long way around, and fewer signals can make it through that way. Note that only the brain's cerebral cortex is divided in this way -- the "gray matter" that forms the outside of the brain, and where we like to think that our "higher thinking" resides. The "white matter" on the inside of the brain doesn't have the same left/right dichotomy, only the cortex does. 

You might recall that I argue for a very specific role of the cortex in human behavior: It produces the mental experiences associated with the Narrative System. In other words, it tells us stories about our own behavior (the Intuitive System, located mainly down in the white matter of the sub-cortical areas, is what actually produces the behavior). Most of the time, the Narrative System's stories are expressed in language, and language is one of the few highly-differentiated functions in most people's brains. It depends on two areas of the left temporal lobe of the neocortex, Wernicke's area (responsible for understanding words) and Broca's area (responsible for producing them). This does not, however, mean that the left brain is responsible for "logic and reason" while the right brain is responsible for "creativity and emotion" -- that's a common brain myth. The best evidence against that myth comes from some of the same studies that originally led to it -- research on patients with a "split brain," where the corpus callosum superhighway between the brain's two hemispheres has been removed.

Severing the corpus callosum is a surgical procedure that was first used in the 1940s in my hometown of Rochester NY. It's a last-ditch solution to treat intractable epilepsy, where signals reverberate across the brain and disrupt normal functioning. With the corpus callosum removed that process is interrupted, and the symptoms of severe epilepsy can resolve or be dramatically reduced. (In my blog post about neuroplasticity, I wrote about an even more dramatic surgical option, which involved fully removing one of the two cortical hemispheres to prevent severe seizures). Patients without a corpus callosum are described as having a "split brain" because their two cortical hemispheres become functionally independent of one another, without any fast lanes available for traffic between them.

When split-brain patients are studied after their surgery, they show some surprising abilities. In a classic research design, a barrier is placed in the middle of the patient's visual field, so that their left eye sees something different from what their right eye sees. For example, the word "face" could be shown to the right eye and not the left. That sends the word to the left brain (because neurons from the eyes cross to the opposite cortical hemisphere through an "X"-shaped structure called the optic chiasm), and the left brain can accurately report, using language, "I see the word 'face.'" When the same word is presented to the right hemisphere (via the left eye), on the other hand, the patient reports that they don't see anything at all. That's because their right brain has the information, but doesn't have the resources to say what it sees using words. The person can, however, draw a face using their left hand (hands are also controlled by the opposite hemisphere), because their right brain has other ways of expressing Narrative-level knowledge. The inability to say what was seen in words (verbal aphasia), combined with the ongoing ability to draw it, is one of the key findings that has led to the myth of a "logical" left brain versus a "creative" right brain. 

The actual relationship between hemispheres is more complex. Patients without a corpus callosum can still integrate information from different parts of the brain, they just can't necessarily describe what's happening. For example, a variation of the first study might involve flashing the word "face" to the left hemisphere (right eye) while simultaneously flashing the word "smile" to the right hemisphere (left eye). The patient tells the investigator that they saw the word "face," and they draw a smiling face. They cannot, however, say why they chose to make the face smile. In one example, a patient's response to the investigator's question was "what do you want, a sad face? Who wants that?" The left hemisphere constructs a story to make sense of what it was doing, even though it can't express the right-hemisphere inputs in language. The right hemisphere knew that a face was the drawing required, even though it only received the input "smile." The two hemispheres are not as specialized as people say. They just usually rely on a greater degree of integration to achieve coherence in their narratives. Even without that integration, the Narrative System's ability to weave a tale is impressive.

One other interesting example involved a case where a teenage boy was shown the word "girlfriend" via his left eye (right hemisphere) only. He was then asked to choose letters from a bag, which spelled out the word "LIZ," the name of a girl in his class. When the investigator asked why he had selected those letters, the boy blushed, but couldn't explain why he had chosen to spell the name of a girl he liked. Again, we see information from the right hemisphere making its way to the left hemisphere and expressed in language (in this case nonverbally, by spelling out a word using tiles). What's lacking is the integration and insight. Perhaps passing through the Intuitive-mind regions of the brain added some emotion or impulsiveness to the cue word that wouldn't have been there if it had been able to take the normal Narrative-system route from one hemisphere to the other. 

The key to all of these split-hemisphere stories is not strict differentiation between the two hemispheres, but rather a lack of awareness or metacognition. Some other research has shown that the (few) true cognitive problems experienced by split-brain patients are in exactly this domain. For example, they may be less able to engage in moral reasoning, or to verbally express a "theory of mind" that accounts for someone else's behavior based on what that person is probably thinking. Metacognition is a useful skill, and one that clearly depends on the abstract-reasoning capabilities of the Narrative System. It is not, however, the same thing as an inability to process logical information in the right brain or creative ideas in the left. The true divisions of the mind are between the gray-matter Narrative System of the cortex, and the white-matter Intuitive System of the deep brain.