The Interpreter

The answer appears to be that we have a specialized left hemisphere system that my colleagues and I call the "interpreter." This interpreter is a device (or system or mechanism) that seeks explanations for why events occur. The advantage of having such a system is obvious. By going beyond simply observing contiguous events to asking why they happened, a brain can cope with such events more effectively should they happen again.

We revealed the interpreter in an experiment using a "simultaneous concept test." The split-brain patient is shown two pictures, one presented exclusively to his left hemisphere, one exclusively to his right. He is then asked to choose from an array of pictures the ones he associates with the pictures that were presented (or "lateralized") to his left brain and his right brain. In one example of this, a picture of a chicken claw was flashed to the left hemisphere and a picture of a snow scene to the right. Of the array of pictures then placed in front of the subject, the obviously correct association was a chicken for the chicken claw and a shovel for the snow scene. Split-brain subject case 1 did respond by choosing the shovel with his left hand and the chicken with his right. Thus each hemisphere picked the correct answer.

Now the experimenter asked the left-speaking hemisphere why those objects were picked. (Remember, it would only know why the left hemisphere had picked the shovel; it would not know why the disconnected right brain had picked the shovel.) His left hemisphere replied, "Oh, that's simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed." In other words, the left brain, observing the left hand's response, interprets the response in a context consistent with its own sphere of knowledge —one that does not include information about the snow scene presented to the other side of the brain.

One can influence the left-brain interpreter in many ways. As I mentioned, we wanted to know whether the emotional response to stimuli presented to half of the brain would influence the emotional tone of the other half. Using an optical computer system that detects the slightest eye movement, we projected an emotionladen movie to the right hemisphere. (If the patient tried to cheat and move the eye toward the movie, it was electronically shut off.)

When we did this experiment with case 2, the movie that her right hemisphere saw was about a vicious man pushing another off a balcony and then throwing a firebomb on top of him. The movie then showed other men trying to put the fire out. When V. P. was first tested on this problem, she could not access speech from her right hemisphere. She was able to speak only out of her left brain. When asked what she had seen, her left brain (the half brain that had not actually seen the movie) replied, "I don't really know what I saw. I think just a white flash.'' When I asked, "Were there people in it?'' case 2, replied, "I don't think so. Maybe just some trees, red trees like in the fall.'' I asked, "Did it make you feel any emotion?'' and V. P. answered, "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; you're getting me nervous.'' She 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.''

This kind of effect is common to all of us. A mental system that is operating outside the conscious realm of the left hemisphere's interpreter generates a mood that alters the general physiology of the brain. Because the alteration in brain physiology is general, the interpreter is able to note the mood and immediately attributes some cause to it. This is a powerful mechanism; once clearly seen, it makes one wonder how often we are victims of spurious emotional/ cognitive correlations.

Our recent investigations have looked further at the properties of the interpreter and how it influences mental skills. For example, there are hemisphere-specific changes in the accuracy of memory processes. Specifically, the predilection of the left hemisphere to interpret events has an impact on the accuracy of memory. When subjects are presented with pictures representing common events (e.g., getting up in the morning or making cookies) and several hours later asked to say if pictures in another series appeared in the first, both hemispheres are equally accurate in recognizing the previously viewed pictures and rejecting the unrelated ones. Only the right hemisphere, however, correctly rejects pictures in the second set that were not previously viewed but were related to pictures previously viewed. The left hemisphere incorrectly "recalls" significantly more of these related pictures as having occurred in the first set, presumably because they fit into the schema it has constructed. This finding is consistent with the hypothesis that a left hemisphere interpreter constructs theories to assimilate perceived information into a comprehensible whole. In doing so, however, the process of elaborating (story making) has a deleterious effect on the accuracy of perceptual recognition. This result has been shown with verbal as well as visual material.

A recent example of the interpreter can be found in studies of case 3, a split-brain patient who can speak out of his right hemisphere as well as his left. His naming of stimuli in the left field seems to be increasing at a rapid rate. Although there is no convincing evidence of any genuine visual transfer between the hemispheres, during trials when J. W. was certain of the name of the stimulus, he maintained that he saw it well. On trials when he was not certain of the name of the stimulus, he maintained that he did not see it well. This is consistent with the view that the left hemisphere interpreter actively constructs a mental portrait of past experience, even though that experience did not directly occur in that hemisphere. This experience was probably caused by the left hemisphere's interpreter giving meaning to right hemisphere spoken responses, possibly by activating the left hemisphere mental imagery systems.

The left hemisphere's capacity for continual interpretation may mean that it is always looking for order and reason, even where there is none. This came out dramatically in a study by George Wolford and me. On a simple test that requires one to guess if a light is going to appear on the top or the bottom of a computer screen, we humans perform in an inventive way. The experiment manipulates the stimulus to appear on the top 80% of the time. While it quickly becomes evident that the top button is being illuminated more often, we keep trying to figure out the whole sequence—and deeply believe that we can. We persist even if, by adopting this strategy, we are rewarded only 68% of the time (whereas if we guessed "top" repeatedly, by rote, we would be rewarded 80% of the time). Rats and other animals are more likely to learn to maximize their score by pressing only the top button! Our right hemisphere behaves more like the rats. It does not try to interpret its experience to find the deeper meaning; it lives only in the thin moment of the present. But when the left brain is asked to explain why it is attempting to psych out the whole sequence, it always comes up with a theory, however spurious.



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