If a brain transplant were possible, would the recipient take on the personality of the donor? —Cleona Vassell

Stop right there, Cleona. In a brain transplant, who's the recipient and who's the donor?

Here's one way to think about it. Although a brain transplant at the moment is impossible, no doubt that won't always be so. What will probably become feasible first isn't a brain transplant but a head transplant.

This simplifies matters in two respects. First, on a practical level, it sidesteps the fantastically complicated project of reconnecting the brain to the multitude of sensory organs and blood vessels in the head. Second, it goes a long way toward answering your question.

We're not talking about grafting a new brain or head onto someone's body; we're talking about grafting a new body onto someone's head. The self that lives in that head remains the boss.

As for personality . . . well, that's a broader question, which we'll get to.

Currently the dealbreaker here is the spinal cord—as yet there's no way to reattach a severed cord to a brain. Some think stem cell research may yield a way to splice the two together. A more exotic possibility is severing the brain at midpoint and connecting the upper lobes—and thus, presumably, the higher functions and consciousness—of one individual to the brain stem, spinal cord, and body of someone else. The rationale seems to be that we keep all the control circuitry needed to operate the body intact and simply put someone new in the driver's seat. However you slice it, it won't be easy.

The practical science of brain transplants has been slow to evolve, and often grotesque. In 1954 Russian scientists transplanted the head and upper thorax of a puppy onto a larger dog, creating a two-headed dog. In 1965 one of the pioneers in the field, Robert White, topped this by transplanting the brain of a donor dog into the neck of another, thus briefly creating a two-brained dog. In 1970 White and his colleagues transplanted the head of a monkey onto another's headless body. The resulting monkey lived for eight days. Not only could it use its senses, it tried to bite the hand of a researcher.

In all three cases, the host body simply provided life support for the transplanted head or brain. There was no neurological connection between the two, and the newly added brain wasn't in any sense the master of the body.

But give it time. Current schemes for head transplants involve keeping bodies of donor and recipient in deep hypothermia and using ultra-sharp knives to cleanly cut each patient's spinal cord at the neck in hopes that the nerve cells will fuse when the brain end of one is joined to the body end of the other. A special glue promoting such fusion would be applied to severed ends.

Purely as a thought experiment, consider: Jane and John crash their motorcycles into each other. Helmetless Jane is left brain-dead but otherwise intact; John's brain is fine, but his body is mangled beyond repair. With death imminent, genius surgeons successfully implant John's brain in Jane's body.

Who wakes up, Jane or John?

The memories and consciousness clearly will be John's. But personality to an unknown but surely significant degree is formed by the interaction between brain and body.

More generally, John's brain must map itself to Jane's body. Maybe you'd just get one of those comical scenarios beloved of screenwriters: a woman's body with a man at the controls. Then again, maybe John becomes psychotic due to the brain/body disconnect.

But there's a third possibility. John wakes up thinking he's male, but after his body imprints itself decides: please, call me Jane.