Several months ago, Matthew Wilson, a neuroscientist at the Massachusetts Institute of Technology, announced that he had figured out what the rats in his lab dream about. Wilson and his graduate assistant Kenway Louie had implanted tiny electrodes directly into the rats' hippocampi, the region responsible for memory and learning. Then they trained the rats to scurry around a circular track and stop periodically for food rewards. As the rats ran, the electrodes monitored the firing of a dozen or so neurons in each rat's brain. Wilson found that the neurons fired in a distinctive pattern that varied from rat to rat but remained the same for each individual animal.
Later, when the rats experienced rapid eye movement sleep, those neurons began to fire again. "The patterns are not exactly the same," Wilson says, "but we can definitely say that they are derived from those generated during the rats' awake experience on the track." Apparently the rats' nocturnal visions are constructed from the mundane events of their daily lives, replayed in detail. In some cases, Wilson and Louie could even tell where on the track the animal dreamed it was.