QUESTION: Why aren't people with surgically severed hemispheres of the brain considered to be two separate human beings?

ANSWER: Because the right side ain't human.Now don't scoff at this. It's a serious question. As far back as the 19th century, before such surgery existed, German psychologist Gustav Fechner proposed that by splitting the hemispheres you could create two separate human beings.

As it turned out, such surgery - called commissurotomy, because the "commissures" between the hemispheres are cut - was first conducted on humans in the early 1940s. It was intended to treat epilepsy. In some cases it worked. Amazingly, it didn't alter personality. Some patients had some memory loss or perceptual problems, but otherwise they were normal. The surgery is still done in rare cases and at the moment there are about 300 commissurotomy patients in the United States, says Mike Gazzaniga, a professor of neuroscience at Dartmouth Medical School.

In split-brain patients, the two hemispheres have no way of communicating with each other. In one experiment, a patient, N.G., was shown a picture of a spoon in her left field of vision. Because the brain and body are crosswired, only the right half of the brain could see what was on the patient's left. N.G. was asked what she saw. She said, "Nothing." She wasn't lying: The language center of the brain, the part that allows us to translate what we see into words, is in the left hemisphere - and that part couldn't see the spoon. The left brain answered for itself; the right brain had no way to communicate.

Then, N.G. was shown a picture of a naked woman in her left field of vision. She giggled and blushed. She was asked what she saw. "Nothing," she said again. She was asked why she was giggling. She said merely, "Oh, doctor, you have some machine."

An analysis of this experiment is in "Left Brain, Right Brain," by Sally Springer and Georg Deutsch. They said, "Her right hemisphere saw the picture and processed it sufficiently to evoke a general, nonverbal reaction - the giggling and blushing. The left hemisphere, meanwhile, did not `know' what the right had seen, although its comment about `some machine' seems to be a sign that it was aware of the bodily reactions induced by the right hemisphere."

Now then: You've got two hemispheres, they don't communicate, they are essentially autonomous. Why isn't that two "human beings"? Because, in most cases, the right side cannot truly think. There's a distinction, says Australian physiologist Sir John Eccles, between "mere consciousness," which your average dog or cat has, and the world of language, thought and culture that denote human consciousness. And that world is almost entirely in the left brain. (Though 1 percent of right handers and about 40 percent of left-handers are an exception, with the right brain being dominant. Another 20 percent of left-handers are of mixed dominance.)

"The right side is a very stupid hemisphere. It's a very minor cognitive system. I don't even know you could assign it the attribute of having an attitude. It might have some conditioned responses to things," says Gazzaniga.

That said, he had one unusual split-brain patient who was able to manipulate Scrabble letters with his left hand, and thus communicate from his right brain. When the left brain was asked what job he would most like, it said, "Draftsman." When the right brain was asked the same question, it said, "Automobile race."

QUESTION: Why do rap singers always use pseudonyms?

ANSWER: You know: Tone-Loc. M.C. Hammer. LL Cool J. Ice Cube. Chill Rob G. Sir Mix-A-Lot. Sinister Minister. DJ Skinny Bones. You can check the hip-hop bins and never see a name that in any way resembles "Bruce Springsteen."

We spoke to Harry Allen, who calls himself a "hip-hop activist & media assassin" - yes, he specifies the ampersand - and who is also the publicist for the rap group Public Enemy (his formal title is Director of Enemy Relations). Allen, citing a book called "The United Independent Compensatory CodeSystemConcept," said that rap artists often reject their "government names" and take on a new name that more adequately addresses their aspirations, and which is "compensatory" for societal racism. For example, a member of Main Source calls himself Large Professor - that's not just a name, it's an identity.

"The reason they do it is the same reason that black people march - it's compensatory . . . it's making up for racism," says Allen.

There's a simpler explanation, too: Whether involved with music or not, young people in the inner city have often used street names. Tone Loc was born Tony Smith but was known by his homeboy handle of Tony Loco. When he became a rapper he changed that to Tone-Loc.

1991, Washington Post Writers Group