Asking people with amnesia to play Tetris has revealed clues to why dreams are so illogical 鈥� and perhaps to why we dream at all.
Most sleep scientists think that in our first dreams of the night we access 鈥渄eclarative鈥� memories of recent events 鈥� those we can consciously recall. People with amnesia cannot form this kind of memory because of damage to the hippocampus, a part of the brain.
But Robert Stickgold of Harvard Medical School found that people with amnesia who had no recollection of having played Tetris still 鈥渟aw鈥� the computer game鈥檚 falling blocks in their first hour of sleep.
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He thinks this shows that dreams during so-called Stage One sleep rely on abstract, subconscious memories, which amnesiacs are still able to access. And he thinks this explains why dreams often seem disordered and bizarre.
鈥淲hen I saw the results I almost fell of my chair,鈥� Stickgold says. 鈥淲e thought that if there鈥檚 one part of sleep that depends on declarative memories, which amnesiacs lack, it鈥檚 sleep onset.鈥�
Declarative memories 鈥渁re ones you can declare you know: 鈥業 had eggs for breakfast鈥� or 鈥楳y brother鈥檚 name is Ed鈥�,鈥� Stickgold says. 鈥淚mplicit memories are ones that are in your brain but you can鈥檛 access consciously.鈥�
Stickgold thinks the access to implicit memories that dreams give may be essential for learning. 鈥淥ne of the most difficult problems the brain faces is how to put together information from different sources, to see how things fit together.
鈥淏y blocking declarative memories and forcing the system to work with these weak associations, the brain is coerced into looking for unexpected, novel and potentially highly creative and useful connections that otherwise we would not notice.鈥�
鈥淭he study has shown that the mental activity at sleep onset is clearly related to the learning situation,鈥� agrees Carlyle Smith, a sleep scientist at Trent University in Peterborough, Ontario.
Stickgold thinks declarative memories are even more likely to be blocked during REM dream sleep. 鈥淲e鈥檙e going to look for Tetris images in REM sleep next,鈥� he says.
Stickgold asked three groups of people to play Tetris for a total of seven hours over a three-day period. People with amnesia, expert players and novices played the game every morning and evening. Stickgold monitored their dreams as they were drifting into sleep on the first and second evenings.
He found that 17 of his 27 players reported dreaming at least once in the hour after they fell asleep, and all 17 reported seeing falling Tetris pieces. But overall, there were more dream reports on the second night.
This time lag is important, Stickgold says. 鈥淚t鈥檚 as if the brain needs more time or more play before it decides 鈥極K, this is something that I really need to deal with at sleep onset鈥�.鈥�
Furthermore, he says, Tetris novices who saw falling blocks had not performed as well in their initial Tetris session compared to the novices who did dream about the blocks. 鈥淚t鈥檚 as if the more work you have to do, the more likely you are to get the imagery,鈥� he says.
鈥淭his is particularly interesting,鈥� says Richard Haier of the University of California in Irvine. 鈥淚t suggests that people who learn the best may use the early dreaming the least for learning.鈥�
Source: Science (vol 290, p350)