In fact, the learning curve for the game--measured by total points earned--was quite different for the three groups. Whereas both the experts and the novices showed considerable improvement, the amnesiacs did not. And this progress was somewhat reflected in the dream reports. The nine novices who were initially worst at the game were the very same who reported seeing falling pieces during sleep onset--suggesting again that the more a subject needed to learn, the more his or her brain reviewed the material. Only five experts saw the imagery. Two of them, however, described Tetris images associated not with the version they played in the lab but with the version they had played on Nintendo machines--a twist that Stickgold attributes to the integrative process.

Perhaps most surprising, three of the five amnesiacs described having the same kinds of hypnagogic dreams as the normal subjects. The researchers had assumed that the amnesiacs� dreams--especially those during the hypnagogic phase--would have nothing to do with recent events, if they occurred at all, due to the damage to their short-term memory centers. �We thought that if there�s one part of sleep that depends on episodic memories, which amnesiacs lack, it's sleep onset,� Stickgold says.

But even for these individuals, most of whom did not remember the game from one day to the next and had to be taught all over again, the Tetris dreams seemed to affect their waking behavior. Co-author David Roddenberry, an undergraduate at Harvard, noticed that one of the amnesiacs who didn�t remember the game nevertheless placed her fingers on the computer keys used in playing at the start of a session. �She did not quite know what she was doing and yet she did know what she was doing,� Stickgold comments. �In a way, this is Freud�s unconscious--things activated in our brain that are in fact memories that guide our behavior but are not conscious.�

To try to understand this barrier between waking and sleep, the researchers also compared the differences in reports of images or thoughts of Tetris both before sleep onset and right after. Curiously, thoughts about Tetris not associated with seeing falling pieces were more prevalent before sleep, whereas reports of images were more common during sleep. �What was most striking about the data,� the researchers write in the Science paper, �was the strong similarity in reports from different individuals.� All the subjects dreamed of pieces falling and sometimes rotating or fitting into empty spaces--and none reported seeing the picture surrounding the pieces, the scoreboard or the keyboard.

"What we�re really looking at here is the age-old mind-body problem: the mind-brain connection,� Stickgold notes. �We think of our mind as being ours. But there are real ways in which the brain has a set of rules of its own. We�re getting an idea of what the brain uses as its rules for picking out cortical memory traces to reactivate and bring into our conscious mind, and we�re trying to see across wake-sleep cycles how that process happens.� That game is far from over.