"I would be surprised if we could do similar experiments in humans and not get the same results," Stickgold says.

Sleep and Memories

In a study published in an October issue of Science, Stickgold had a group of people with normal memory function and a group of amnesiacs, who have damage to their hippocampi, play Tetris over a period of days.

Although the amnesiacs could not remember having played the game later that day, both groups reported thinking of falling blocks as they fell asleep at night. These results suggest that some forms of dreaming may not rely solely on the hippocampus.

According to Stickgold, the neocortex, a part of the brain that is normally associated with the high intelligence of primates, takes part in the dreams of amnesiacs.

During rapid eye movement (REM) sleep, the part of the sleep cycle most closely associated with dreaming, Stickgold says "there is a lot of communication between the neocortex and the hippocampus."

The precise nature of that communication and whether it is necessary for dreaming is still unknown.

Amnesiacs may have been able to later remember the Tetris game because their neocortexes, which analyze short-term memory and establish longer-term memory, were undamaged.