Both ideas have been well received by brain lateralization authorities eager to see a revival of their specialty.

''It's nice to see the left and right hemispheres are back,'' said Dr. Brenda Milner, a cognitive neuroscientist at the Montreal Neurological Institute in Quebec. The notion that the human brain has two halves and that the left side is associated with logical, analytical thinking while the right side is more intuitive, emotional and creative was popularized about 20 years ago, she said, and soon became received wisdom about how the brain works. ''This idea fell from fashion not because people didn't like it but because they got interested in other things,'' she said.

Dr. Marcel Kinsbourne, a cognitive scientist at the New School for Social Research in New York City and early pioneer in brain lateralization studies, believes that left and right brain ideas also fell from fashion because they were oversold. People looked for universal dichotomies -- the left brain is a whiz at legal briefs but the right brain is deft at poetry -- that carried things too far. But the new theories are ''intriguing,'' Dr. Kinsbourne said, although they have a long way to go before they can be accepted as valid. ''We are in half-baked land here,'' he said.

The new theories are also appealing to many experts because they take on a question that has divided researchers for decades. Do people have one overarching mind that spans the two hemispheres? Or are they born with two separate minds -- one on the left and one on the right -- which operate so seamlessly that the person simply does not notice that there are two?

The subjective sense of having just one mind is overwhelming and unmistakable, said Dr. Joseph Bogen, a neurosurgeon at the University of Southern California in Los Angeles. But if the thick band of fibers connecting the two hemispheres is severed, he explained, humans seem to end up with two separate minds that show different abilities. In one dramatic disparity, the left hemisphere does all the talking while the mute right hemisphere has better access to emotions. For example, when the right brain is shown a photograph, the talkative left brain will say that it does not see anything and cannot comment. But the left hand, which is connected to the right brain, can raise a thumb up or down in response to the question, ''Do you like the picture?''