The human brain has approximately 100 billion neurons, and each, on average, connects to about 1,000 other neurons. A quick multiplication reveals that there are 100 trillion synaptical connections. So how is all this input getting spliced and integrated into a coherent package? How do we get order out of this chaos of connections? Even though it may not always seem so, our consciousness is rather kicked back and relaxed when you think about all the input with which the brain is being bombarded and all the processing that is going on. In fact, it is as if our consciousness is out on the golf course like the CEO of a big company while all the underlings are working. It occasionally listens to some chatter, makes a decision and then is out sunning itself.

We have gotten some clues about how consciousness emerges from studying “split brain” patients. The surgical procedure to cut the corpus callosum is a last ditch treatment effort for patients with severe intractable epilepsy for whom no other treatments have worked. Very few patients have had this surgery, and it is done even more rarely now because of improved medications and other modes of treatment. In fact, there have only been 10 split-brain patients who have been well tested. William Van Wagenen, a Rochester, N.Y., neurosurgeon, performed the procedure for the first time in 1940, following the observation that one of his patients with severe seizures got relief after developing a tumor in his corpus callosum. Epileptic seizures are caused by abnormal electrical discharges that in some people spread from one hemisphere to the other. It was thought that if the connection between the two sides of the brain were cut, then the electrical impulses causing the seizures would not spread from one side of the brain to the other. The great fear was what the side effects of the surgery might be. Would it create a split personality with two brains in one head?

In fact, the treatment was a great success. Most patients’ seizure activity decreased 60 to 70 percent, and they felt just fine: no split personality, no split consciousness. Most seemed completely unaware of any changes in their mental processes. This was great, but puzzling nonetheless. Why don’t split-brain patients have dual consciousness? Why aren’t the two halves of the brain conflicting over which half is in charge? Is one half in charge? Are consciousness and the sense of self actually located in one half of the brain?

Split-brain patients will do subtle things to compensate for their loss of brain connectivity. They may move their heads to feed visual information to both hemispheres, or talk out loud for the same purpose, or make symbolic hand movements. Only under experimental conditions, when we eliminate cross cueing, does the disconnection between the two hemispheres become apparent. We are then able to demonstrate the different abilities of the two hemispheres.

Before we see what is separated after this surgery, we need to understand what continues to be shared. There are subcortical pathways that remain intact. Both hemispheres of the split-brain patient are still connected to a common brain stem, so both sides receive much of the same sensory and proprioceptive information automatically coding the body’s position in space. Both hemispheres can initiate eye movements, and the brain stem supports similar arousal levels, so both sides sleep and wake up at the same time.There also appears to be only one integrated spatial-attention system, which continues to be unifocal after the brain has been split. Attention cannot be distributed to two spatially disparate locations. The left brain is not attentive to the blackboard while the right brain is checking out the hot dude in the next row. Emotional stimuli presented to one hemisphere will still affect the judgment of the other hemisphere.

You may have been taught in anatomy lectures that the right hemisphere of the brain controls the left half of the body and that the left hemisphere controls the right half of the body. Of course, things are not quite that simple. For instance, both hemispheres can guide the facial and proximal muscles, such as those in the upper arms and legs, but the separate hemispheres have control over the distal muscles (those farthest from the center of the body), so that the left hemisphere controls the right hand. Although both hemispheres can generate spontaneous facial expressions, only the dominant left hemisphere can do so voluntarily. Because half the optic nerve crosses from one side of the brain to the other at the optic chiasm, the information from the parts of both eyes that attend to the right visual field is processed in the left hemisphere, and vice versa. This information does not cross over from one disconnected hemisphere to the other. If the left visual field sees something in isolation from the right, only the right side of the brain has access to that visual information.

It has also been known since the first studies by French neuroanatomist Paul Broca that our language areas are usually located in the left hemisphere (with exceptions in a few left-handed people). A split-brain patient’s left hemisphere and language center have no access to the information that is being fed to the right brain. Bearing these things in mind, we have designed ways of testing split-brain patients to better understand what is going on in the separate hemispheres and have verified and learned that the left hemisphere is specialized for language, speech and intelligent behavior, whereas the right is specialized for such tasks as recognizing upright faces, focusing attention and making perceptual distinctions.

Where attention is concerned, the hemispheres interact quite differently in their control of reflexive and voluntary attention processes. There is a limited amount of overall available attention. The evidence suggests that reflexive (bottom-up) attention-orienting happens independently in the two hemispheres, whereas voluntary attention-orienting involves hemispheric competition with control preferentially lateralized to the left hemisphere. The right hemisphere, however, attends to the entire visual field, whereas the left hemisphere attends only to the right field. When the right inferior parietal lobe is damaged, the left parietal lobe remains intact. Yet the left parietal lobe directs its visual attention only to the right side of the body. There is no brain area paying attention to what is going on in the left visual field. The question that is left is, Why doesn’t this bother the patient? I’m getting there....

Left Hemisphere and Intelligence
After the human cerebral hemispheres have been disconnected, the verbal IQ of a patient remains intact, and so does his problem-solving capacity. There may be some deficits in free-recall capacity and in other performance measures, but isolating essentially half of the cortex from the dominant left hemisphere causes no major change in cognitive functions. The left hemisphere remains unchanged from its preoperative capacity, yet the largely disconnected, same-size right hemisphere is seriously impoverished in cognitive tasks. Although the right hemisphere remains superior to the isolated left hemisphere for some perceptual and attentional skills, and perhaps also emotions, it is poor at problem solving and many other mental activities.

The difference between the two hemispheres in problem solving is captured in a probability-guessing experiment. We have subjects try to guess which of two events will happen next: Will it be a red light or a green light? Each event has a different probability of occurrence (for example, a red light appears 75 percent of the time, and a green 25 percent of the time), but the order of occurrence of the events is entirely random.

There are two possible strategies one can use: frequency matching or maximizing. Frequency matching would involve guessing red 75 percent of the time and guessing green 25 percent of the time. The problem with that strategy is that because the order of occurrence is entirely random it can result in a great deal of error—being correct only 50 percent of the time—although it could result in being correct 100 percent of the time as well. The second strategy, maximizing, involves simply guessing red every time. That ensures an accuracy rate of 75 percent because red appears 75 percent of the time. Animals such as rats and goldfish maximize. The “house” in Las Vegas maximizes. Humans, on the other hand, match. The result is that nonhuman animals perform better than humans in this task.

Use of this suboptimal strategy by people has been attributed to a propensity to try to find patterns in sequences of events even when they are told that the sequences are random. At Dartmouth College, psychologists George Wolford, Michael Miller and I tested the two hemispheres of split-brain patients to see if the different sides used the same or different strategies. We found that the left hemisphere used the frequency-matching strategy, whereas the right hemisphere maximized! Our interpretation was that the right hemisphere’s accuracy was higher than the left’s because the right hemisphere approaches the task in the simplest possible manner with no attempt to form complicated hypotheses about the task.

More recent tests have even more interesting findings. They have shown the right hemisphere does frequency-match when presented with stimuli for which it is specialized, such as in facial recognition, and the left hemisphere, which is not a specialist in this task, responds randomly. This division of labor suggests that one hemisphere cedes control of a task to the other hemisphere, if the other hemisphere specializes in that task. The left hemisphere, on the other hand, engages in the human tendency to find order in chaos and persists in forming hypotheses about the sequence of events even in the face of evidence that no pattern exists: slot machines, for instance. Why does the left hemisphere do this even when it can be nonadaptive?

Know-It-All Left Hemisphere
Several years ago we observed something about the left hemisphere that was very interesting: we had elicited from the disconnected right hemisphere how it deals with behaviors about which it had no information. We showed a split-brain patient two pictures: a chicken claw was shown to his right visual field, so only the left hemisphere saw that, and a snow scene was shown to the left visual field, so the only right hemisphere saw that. He was then asked to choose from an array of pictures placed in full view in front of him. Of the pictures placed in front of the subject, the shovel was chosen with the left hand and the chicken with the right. When asked why he chose these items, his left hemisphere speech center replied, “Oh, that’s simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” Here the left brain, observing the left hand’s response without the knowledge of why it has picked that item, has to explain it. It will not say, “I don’t know.” Instead it interprets that response in a context consistent with what it knows, and all it knows is “chicken claw.” It knows nothing about the snow scene, but it has got to explain that shovel in the left hand. It has to create order out of its behavior. We called this left-hemisphere process “the interpreter.”

We also tried the same type of test with mood shifts. We showed a command to the right hemisphere to laugh. The patient began to laugh. Then we asked the patient why she was laughing. The speech center in the left hemisphere had no knowledge of why its person was laughing, but out would come an answer anyway: “You guys are so funny!” When we triggered a negative mood in the right hemisphere by a visual stimulus, the patient denied seeing anything but suddenly said that she was upset and that it was the experimenter who was upsetting her. She felt the emotional response to the stimulus—all the autonomic results—but had no idea what caused it. Ah, lack of knowledge is of no importance, the left brain will find a solution. Order must be made. The first plausible explanation will do: the experimenter did it! The left-brain interpreter makes sense out of all the other processes. It takes all the input that is coming in and puts it together in a makes-sense story, even though it may be completely wrong.

The Interpreter and Consciousness
So here we are, back to the leading question of the article. Why do we feel unified when we are made up of a gazillion modules? Decades of split-brain research have revealed the specialized functions of the two hemispheres and have provided insights into specialization within each hemisphere. The answer may lie in the left-hemisphere interpreter and its drive to seek explanations for why events occur.

In 1962 Stanley Schachter of Columbia University and Jerome E. Singer of Pennsylvania State University injected epinephrine into subjects participating in a research experiment. Epinephrine activates the sympathetic nervous system, and the result is an increased heart rate, hand tremors and facial flushing. The subjects were then put into contact with a confederate who behaved in either a euphoric or an angry manner. The subjects who were informed about the effects of the epinephrine attributed symptoms such as a racing heart to the drug. The subjects who were not informed, however, attributed their autonomic arousal to the environment. Those who were paired with the euphoric confederate reported being elated and those with the angry confederate reported being angry. This finding illustrates the human tendency to generate explanations for events. When aroused, we are driven to explain why. If there is an obvious explanation, we accept it, as did the group informed about the effects of epinephrine. When there is not an obvious explanation, the left brain generates one. This is a powerful mechanism; once seen, it makes one wonder how often we are victims of spurious emotional-cognitive correlations. (“I am feeling good! I must really like this guy!” As he is thinking: “Ah, the chocolate is working!”)

Although the left hemisphere seems driven to interpret events, the right hemisphere shows no such tendency. A reconsideration of hemispheric-memory differences suggests why this dichotomy might be adaptive. When a person is asked to decide whether a series of items appeared in a study set or not, his or her right hemisphere is able to identify correctly items that have been seen previously and to reject new items. “Yes, there was the plastic fork, the pencil, the can opener and the orange.” The left hemisphere, however, tends to falsely recognize new items when they are similar to previously presented items, presumably because they fit into the schema it has constructed. “Yes, the fork (but it is a silver one and not plastic), the pencil (although this one is mechanical, and the other was not), the can opener and the orange.” This finding is consistent with the hypothesis that the left-hemisphere interpreter constructs theories to assimilate perceived information into a comprehensible whole.

By going beyond simply observing events to asking why they happened, a brain can cope with such events more effectively should they happen again. In doing so, however, the process of elaborating (story-making) has a deleterious effect on the accuracy of perceptual recognition, as it does with verbal and visual material. Accuracy remains high in the right hemisphere, however, because it does not engage in these interpretive processes. The advantage of having such a dual system is obvious. The right hemisphere maintains an accurate record of events, leaving the left hemisphere free to elaborate and make inferences about the material presented. In an intact brain, the two systems complement each other, allowing elaborative processing without sacrificing veracity.

The probability-guessing paradigm also demonstrates why an interpreter in one hemisphere and not the other would be adaptive. The two hemispheres approach problem-solving situations in two different ways. The right hemisphere bases its judgments on simple frequency information, whereas the left relies on the formation of elaborate hypotheses. Sometimes it is just a random coincidence. In the case of random events, the right hemisphere’s strategy is clearly advantageous, and the left hemisphere’s tendency to create nonsensical theories about random sequences is detrimental to performance. This is what happens when you build a theory on a single anecdotal situation: “I vomited all night. It must have been the food was bad at that new restaurant where I ate dinner.” This hypothesis would be good if everyone who ate what you ate became ill, but not if it happened to just one person. It may have been the flu or your lunch. In many situations, however, there is an underlying pattern, and in these situations the left hemisphere’s drive to create order from apparent chaos would be the best strategy. Coincidences do happen, but sometimes there really is a conspiracy. In an intact brain, both these cognitive styles are available and can be implemented depending on the situation.

The difference in the way the two hemispheres approach the world might also provide some clues about the nature of human consciousness. In the media, split-brain patients have been described as having two brains. The patients themselves, however, claim that they do not feel any different after the surgery than they did before. They do not have any sense of the dual consciousness implied by the notion of having two brains. How is it that two isolated hemispheres give rise to a single consciousness? The left-hemisphere interpreter may be the answer. The interpreter is driven to generate explanations and hypotheses regardless of circumstances. The left hemisphere of split-brain patients does not hesitate to offer explanations for behaviors that are generated by the right hemisphere. In neurologically intact individuals, the interpreter does not hesitate to generate spurious explanations for sympathetic nervous system arousal. In these ways, the left-hemisphere interpreter may generate a feeling in all of us that we are integrated and unified.

A split-brain patient, a human who has had the two halves of his or her brain disconnected from each other, does not find one side of the brain missing the other. The left brain has lost all consciousness about the mental processes managed by the right brain, and vice versa. We don’t miss what we no longer have access to. The emergent conscious state arises out of each side’s capacity and probably through neural circuits local to the capacity in question. If they are disconnected or damaged, there is no underlying circuitry from which the emergent property arises.

Each of the thousands if not millions of conscious moments that we have reflects one of our networks being “up for duty.” These networks are all over the place, not in one specific location. When one finishes, the next one pops up, and the pipe organ–like device plays its tune all day long. What makes emergent human consciousness so vibrant is that our pipe organ has lots of tunes to play, whereas the rat’s (for instance) has few. And the more we know, the richer the concert.