We humans think we make all our decisions to act consciously and willfully. We all feel we are wonderfully unified, coherent mental machines and that our underlying brain structure must reflect this overpowering sense. It doesn’t. No command center keeps all other brain systems hopping to the instructions of a five-star general. The brain has millions of local processors making important decisions. There is no one boss in the brain. You are certainly not the boss of your brain. Have you ever succeeded in telling your brain to shut up already and go to sleep?

Even though we know that the organization of the brain is made up of a gazillion decision centers, that neural activities going on at one level of organization are inexplicable at another level, and that there seems to be no boss, our conviction that we have a “self” making all the decisions is not dampened. It is a powerful illusion that is almost impossible to shake. In fact, there is little or no reason to shake it, for it has served us well as a species. There is, however, a reason to try to understand how it all comes about. If we understand why we feel in charge, we will understand why and how we make errors of thought and perception.

Without Thinking

When I was a kid, i spent a lot of time in the desert of Southern California—out in the desert scrub and dry bunchgrass, surrounded by purple mountains, creosote bush, coyotes, and rattlesnakes. The reason I am still here today is because I have nonconscious processes that were honed by evolution.

I jumped out of the way of many a rattlesnake, but that is not all. I also jumped out of the way of grass that rustled in the wind. I jumped, that is, before I was consciously aware that it was the wind that rustled the grass, rather than a rattler. If I had had only my conscious processes to depend on, I probably would have jumped less but been bitten on more than one occasion.

Conscious processes are slow, as are conscious decisions. As a person is walking, sensory inputs from the visual and auditory systems go to the thalamus, a type of brain relay station. Then the impulses are sent to the processing areas in the cortex, next relayed to the frontal cortex. There they are integrated with other higher mental processes, and perhaps the information makes it into the stream of consciousness, which is when a person becomes consciously aware of the information (there is a snake!). In the case of the rattler, memory then kicks in the information that rattlesnakes are poisonous and what the consequences of a rattlesnake bite are. I make a decision (I don’t want it to bite me), quickly calculate how close I am to the snake, and answer a question: Do I need to change my current direction and speed? Yes, I should move back. A command is sent to put the muscles into gear, and they then do it.

All this processing takes a long time, up to a second or two. Luckily, all that doesn’t have to occur. The brain also takes a nonconscious shortcut through the amygdala, which sits under the thalamus and keeps track of everything. If a pattern associated with danger in the past is recognized by the amygdala, it sends an impulse along a direct connection to the brain stem, which activates the fight-or-flight response and rings the alarm. I automatically jump back before I realize why.

If you were to have asked me why I had jumped, I would have replied that I thought I’d seen a snake. The reality, however, is that I jumped way before I was conscious of the snake. My explanation is from post hoc information I have in my conscious system. When I answered that question, I was, in a sense, confabulating—giving a fictitious account of a past event, believing it to be true.

I confabulated because our human brains are driven to infer causality. They are driven to make sense out of scattered facts. The facts that my conscious brain had to work with were that I saw a snake, and I jumped. It did not register that I jumped before I was consciously aware of it.

In truth, when we set out to explain our actions, they are all post hoc explanations using post hoc observations with no access to nonconscious processing. Not only that, our left brain fudges things a bit to fit into a makes-sense story. Explanations are all based on what makes it into our consciousness, but actions and the feelings happen before we are consciously aware of them—and most of them are the results of nonconscious processes, which will never make it into the explanations. The reality is, listening to people’s explanations of their actions is interesting—and in the case of politicians, entertaining—but often a waste of time.

Left-Brained Oreintation

With so many systems going on subconsciously, why do we feel unified? I believe the answer to this question resides in the left hemisphere and one of its modules that we happened upon during our years of research, particularly while studying split-brain patients.

Some people with intractable epilepsy undergo split-brain surgery. In this procedure, the large tract of nerves that connects the two hemispheres, the corpus callosum, is severed to prevent the spread of electrical impulses. Afterward, the patients appear completely normal and seem entirely unaware of any changes in their mental process. But we discovered that after the surgery, any visual, tactile, proprioceptive, auditory, or olfactory information that was presented to one hemisphere was processed in that half of the brain alone, without any awareness on the part of the other half. Because tracts carrying sensory information cross over the midline inside the brain, the right hemisphere processes data from the left half of the world, and the left hemisphere handles the right.

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.

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.

As We See It Today

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.

[This article originally appeared in print as "The Big Split."]