The Dark Half

From this week's NYT Magazine:

It starts out like it has a point, but essentially ends up just being "Isn't the brain weird? It's just crazy, man, crazy!"

But the weirdness works on its own terms, at least for me - like photos from Voyager or the Hubble, or the deep sea tubeworms, or the Cambrian critters from the Burgess Shale (I enthusiastically recommend Stephen Jay Gould's Wonderful Life), or the works of Escher, Magritte, Dali, Bosch, and Gaudi.

A few years ago, brain scans of London cabbies showed that the detailed mental maps they had built up in the course of navigating their city's complicated streets were apparent in their brains. Not only was the posterior hippocampus -- one area of the brain where spatial representations are stored -- larger in the drivers; the increase in size was proportional to the number of years they had been on the job.

(snip)

Patients with severe epilepsy sometimes used to undergo an operation in which the corpus callosum was severed.... After the operation, the two hemispheres of the brain could no longer directly communicate.... under careful observation, they exhibited some very peculiar behavior. When, for example, the word "hat" was flashed to the left half of the visual field -- and hence to the right (speechless) side of the brain -- the left hand would pick out a hat from a group of concealed objects, even as the patient insisted that he had seen no word. If a picture of a naked woman was flashed to the left visual field of a male patient, he would smile, or maybe blush, without being able to say what he was reacting to -- although he might make a comment like, "That's some machine you've got there." In another case, a female patient's right hemisphere was flashed a scene of one person throwing another into a fire. "I don't know why, but I feel kind of scared," she told the researcher. "I don't like this room, or maybe it's you getting me nervous." The left side of her brain, noticing the negative emotional reaction issuing from theright side, was making a guess about its cause, much the way one person might make a guess about the emotions of another.

Each side of the brain seemed to have its own awareness, as if there were two selves occupying the same head. (One patient's left hand seemed somewhat hostile to the patient's wife, suggesting that the right hemisphere was not fond of her.) Ordinarily, the two selves got along admirably.... Nevertheless... they lived in ever so slightly different sensory worlds. And even though both understood language, one monopolized speech, while the other was mute. That's why the patient seemed normal to family and friends.

Pondering such split-brain cases, some scientists and philosophers have raised a disquieting possibility: perhaps each of us really consists of two minds running in harness. In an intact brain, of course, the corpus callosum acts as a constant two-way internal-communications channel between the two hemispheres. So our everyday behavior does not betray the existence of two independent streams of consciousness flowing along within our skulls. It may be, the philosopher Thomas Nagel has written, that "the ordinary, simple idea of a single person will come to seem quaint some day, when the complexities of the human control system become clearer and we become less certain that there is anything very important that we are one of."

Consciousness, and normal brain function in general, is something that we take for granted as simple and uncomplicated, but it has tricksy biological underpinnings that we are still striving tounderstand, and which can be thrown alarmingly off-kilter by disease, physical damage or chemical imbalance.

The relationship between the two hemispheres is especially fascinating to me. I had heard of such experiments before, but had never really contemplated the idea of the less-dominant (and creative) right hemisphere as a mute prisoner within the mind, unable to even express itself without its link to the dominant, rational left hemisphere.

In a sense, we are all siamese twins.