The left hemisphere specializes in speech, language, and intelligent behavior, and a split-brain patient’s left hemisphere and language center has no access to sensory information if it is fed only to the right brain. In the case of vision, the optic nerves leading from each eye meet inside the brain at what is called the optic chiasm. Here, each nerve splits in half; the medial half (the inside track) of each crosses the optic chiasm into the opposite side of the brain, and the lateral half (that on the outside) stays on the same side. The parts of both eyes that attend to
the right visual field send information to the left hemisphere and information from the left visual field goes to and is processed by the right hemisphere.
More than a few years into our experiments, we were working with a group of split-brain patients on the East Coast. We wondered what they would do if we sneaked information into their right hemisphere and told the left hand to do something [pdf].
We showed a split-brain patient two pictures: To his right visual field, a chicken claw, so the left hemisphere saw only the claw picture, and to the left visual field, a snow scene, so the right hemisphere saw only that. He was then asked to choose a picture from an array placed in full view in front of him, which both hemispheres could see. His left hand pointed to a shovel (which was the most appropriate answer for the snow scene) and his right hand pointed to a chicken (the most appropriate answer for the chicken claw).
We asked why he chose those items. His left-hemisphere speech center replied, “Oh, that’s simple. The chicken
claw goes with the chicken,” easily explaining what it knew. It had seen the chicken claw. Then, looking down at his left hand pointing to the shovel, without missing a beat, he said, “And you need a shovel to clean out the chicken shed.” Immediately, the left brain, observing the left hand’s response without the knowledge of why it had picked that item, put it into a context that would explain it. It knew nothing about the snow scene, but it had to explain the shovel in front of his left hand. Well, chickens do make a mess, and you have to clean it up. Ah, that’s it! Makes sense.
What was interesting was that the left hemisphere did not say, “I don’t know,” which was the correct answer. It made up a post hoc answer that fit the situation. It confabulated, taking cues from what it knew and putting them together in an answer that made sense.
We called this left-hemisphere process the interpreter. It is the left hemisphere that engages in the human tendency to find order in chaos, that tries to fit everything into a story and put it into a context. It seems driven to hypothesize about the structure of the world even in the face of evidence that no pattern exists.
Our interpreter does this not only with objects but with events as well. In one experiment, we showed a series of about 40 pictures that told a story of a man waking up in the morning, putting on his clothes, eating breakfast, and going to work. Then, after a bit of time, we tested each viewer. He was presented with another series of pictures. Some of them were the originals, interspersed with some that were new but could easily fit the same story. We also included some distracter pictures that had nothing to do with the story, such as the same man out playing golf or at the zoo. What you and I would do is incorporate both the actual pictures and the new, related pictures and reject the distracter pictures. In split-brain patients, this is also how the left hemisphere responds. It gets the gist of the story and accepts anything that fits in.
The right hemisphere, however, does not do this. It is totally veridical and identifies only the original pictures. The right brain is very literal and doesn’t include anything that wasn’t there originally. And this is why your three-year-old, embarrassingly, will contradict you as you embellish a story. The child’s left-hemisphere interpreter, which is satisfied with the gist, is not yet fully in gear.
The interpreter is an extremely busy system. We found that it is even active in the emotional sphere, trying to explain mood shifts. In one of our patients, we triggered a negative mood in her right hemisphere by showing a scary fire safety video about a guy getting pushed into a fire. When asked what she had seen, she said, “I don’t really know what I saw. I think just a white flash.” But when asked if it made her feel any emotion, she said, “I don’t really know why, but I’m kind of scared. I feel jumpy, I think maybe I don’t like this room, or maybe it’s you.” She then turned to one of the research assistants and said, “I know I like Dr. Gazzaniga, but right now I’m scared of him for some reason.” She felt the emotional response to the video but had no idea what caused it.
The left-brain interpreter had to explain why she felt scared. The information it received from the environment was that I was in the room asking questions and that nothing else was wrong. The first makes-sense explanation it arrived at was that I was scaring her. We tried again with another emotion and another patient. We flashed a picture of a pinup girl to her right hemisphere, and she snickered. She said that she saw nothing, but when we asked her why she was laughing, she told us we had a funny machine. This is what our brain does all day long. It takes input from other areas of our brain and from the environment and synthesizes it into a story. Facts are great but not necessary. The left brain ad-libs the rest.
The view in neuroscience today is that consciousness does not constitute a single, generalized process. It involves a multitude of widely distributed specialized systems and disunited processes, the products of which are integrated by the interpreter module. Consciousness is an emergent property. From moment to moment, different modules or systems compete for attention, and the winner emerges as the neural system underlying that moment’s conscious experience. Our conscious experience is assembled on the fly as our brains respond to constantly changing inputs, calculate potential courses of action, and execute responses like a streetwise kid.
But we do not experience a thousand chattering voices. Consciousness flows easily and naturally from one moment to the next with a single, unified, coherent narrative. The action of an interpretive system becomes observable only when the system can be tricked into making obvious errors by forcing it to work with an impoverished set of inputs, most obviously in the split-brain patients.
Our subjective awareness arises out of our dominant left hemisphere’s unrelenting quest to explain the bits and pieces that pop into consciousness.
What does it mean that we build our theories about ourselves after the fact? How much of the time are we confabulating, giving a fictitious account of a past event, believing it to be true? When thinking about these big questions, one must always remember that all these modules are mental systems selected for over the course of evolution. The individuals who possessed them made choices that resulted in survival and reproduction. They became our ancestors.
From Who’s in Charge? Free Will and the Science of the Brain by Michael S. Gazzaniga, copyright 2011 by Michael S. Gazzaniga. Reprinted by arrangement with Ecco, an imprint of HarperCollins.