What Dreams May Come Come Not From Waking Memory
Oct. 12, 2000 -- The stuff dreams are made of does not come from one's conscious memories. A clever study showing that people with severe amnesia can have the same dreams as normal people sheds new light on how we dream -- and why.
"What we're really looking at here is the age-old mind/body problem," says Harvard researcher Robert Stickgold, PhD. "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."
Stickgold and co-workers set out to explore the role played by conscious memory systems in the birth of dream images. To do so, they had people play a silent, black-and-white computer version of the game Tetris, which calls for a player to rotate blocks of various shapes and to fit them into place at an ever-faster pace. Each person played the game two hours a day for three days. Each night -- just as the players were falling asleep -- a researcher woke them and asked what they were seeing.
Three groups of players participated in the study. The first two groups were healthy people -- 12 who were new to the game and 10 who had played it many times before. Most interesting, however, was the third group: five people with amnesia. These patients have extensive damage to the parts of the brain responsible for conscious memory. Experts call this episodic or declarative memory -- the kind of memory that allows one to remember whether one saw a bicycle the day before. The amnesia patients, however, have intact procedural memory -- the kind of unconscious memory that allows one to remember how to ride a bicycle.
The players with normal memories got much better at playing the game. The people with amnesia did not. This suggests that Tetris skill involves conscious memory but not procedural memory. But when waked, the amnesia patients were just as likely to report seeing falling Tetris blocks as the healthy novice players -- even though they had no conscious memory of ever having played the game. What makes this particularly striking is that the players were waked just as they were falling asleep. During this very early period of sleep, people feel as though they are awake. Indeed, dreams that occur during this time are called hypnagogic hallucinations to distinguish them from the generally more surreal dreams that occur in later sleep. "We thought if there's one part of sleep that depends on episodic memories -- which amnesiacs lack -- it is sleep onset," Stickgold says.
These results suggest that the images one sees during dreaming have nothing to do with conscious memory. Another finding supports this conclusion -- most of the Tetris players who saw falling Tetris blocks in their dreams did so not on the first day of play, but on the second day. Moreover, some of the expert Tetris players saw falling blocks in color with music playing -- just as they did when they played a different version of the game years before.