Michael Gazzaniga

The Mind's Past

What system ties the vast output of our thousands upon thousands of automatic systems into our subjectivity to render a personal story for each of us?

A special system carries out this interpretive synthesis. Located only in the brain's left hemisphere, the interpreter seeks explanations for internal and external events. It is tied to our general capacity to see how contiguous events relate to one another. The interpreter, a built-in specialization in its own right, operates on the activities of other adaptations built into our brain. These adaptations are most likely cortically based, but they work largely outside of conscious awareness, as do most of our mental activities.

The left-hemisphere interpreter was revealed during a simultaneous concept test in which split-brain patients were presented with two pictures. One picture was shown exclusively to the left hemisphere and the other exclusively to the right. The patient was asked to choose from an array of pictures ones that were lateralized to the left and right sides of the brain. In one example, a picture of a chicken claw was flashed to the left hemisphere and a picture of a snow scene to the right hemisphere. Of the array of pictures placed in front of the subject, the obviously correct association was a chicken for the chicken claw and a shovel for the snow scene. One of the patients responded by choosing the shovel with his left hand and the chicken with his right. When asked why he chose these items, 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 this case the left brain, observing the left hand's response, interpreted that response in a context consistent with its sphere of knowledge—one that does not include information about the snow scene.

What is amazing here is that the left hemisphere is perfectly capable of saying something like, 'Look, I have no idea why I picked the shovel—I had my brain split, don't you remember? You probably presented something to the half of my brain that can't talk; this happens to me all the time. You know I can't tell you why I picked the shovel. Quit asking me this stupid question.' But it doesn't say this. The left brain weaves its story in order to convince itself and you that it is in full control.

The interpreter influences other mental capacities, such as our ability to accurately recall past events. We are poor at doing that, and it is the interpreter's fault. We know this because of neuropsychologists' research on the problem. My favorite comes from studies of the two half brains of split-brain patients. The memory's accuracy is influenced by which hemisphere is used. Only the left brain has an interpreter, so the left hemisphere has a predilection to interpret events that affect the accuracy of memory. The interpreterless right hemisphere does not. Consider the following.

When pictures that represent common events—getting up in the morning or making cookies—were shown to a split-brain patient who was later asked to identify whether pictures in another series had appeared in the first, both hemispheres were equally accurate in recognizing the previously viewed pictures and rejecting unseen ones. But when the subject was shown related pictures that had not been shown, only the right brain performed well. The left hemisphere incorrectly recalled more of these pictures, presumably because they fit into the schema it had constructed regarding the event. This finding is consistent with the idea of a left-hemisphere interpreter that constructs theories to assimilate perceived information into a comprehensible whole. In so doing, however, the elaborative processing has a deleterious effect on the accuracy of reconstructing the past.