My latest column in the Wall Street Journal is on the purpose of
dreams:
Chancing last week on a study about the calming effect of dreams
on people with post-traumatic stress disorder, I decided to read
recent research on dreams. When I looked at this topic about 20
years ago, it was clear that our ignorance of the purpose of
dreaming was almost total, notwithstanding the efforts of Sigmund
Freud, Francis Crick and other fine minds. Is that still true?
To my delight, the answer seems to be no. Some ingenious
experiments have replaced general ignorance with specific and
intriguing ignorance (as is science's habit). We now know enough to
know what it is we do not know about dreams.
In the past, people often had one explanation for sleep and
another for dreams. That now seems wrong. One of the chief
functions of sleep seems to be achieved during dreaming: the
consolidation of memory. Sleep certainly improves memory
performance of several different kinds: emotional, spatial,
procedural and verbal.
But the new thinking is that, during sleep, the brain reprocesses
or transforms fragile new memories into more permanent forms, sets
them in mental context and extracts their meaning. And dreaming is
a symptom that this is going on.
In one experiment, subjects were asked to remember a novel word
that sounds like a real word: "cathedruke," for example. Much later
they were asked to recall the real word "cathedral," which it
resembles. People who had spent the intervening time awake recalled
the word more quickly than those who had spent the time asleep. To
cut a long experimental story short, it seems that the sleepers,
probably by dreaming, had refiled cathedruke in "lexical" memory in
the higher brain region known as the cortex rather than in the
hippocampus, where short-term memory resides. There it interferes
briefly with recall of cathedral.
In another experiment, people were asked to remember a series of
words, such as nurse, ill, patient, etc. Some hours later, they
were asked to recall if certain words were in the list. They
correctly rejected most of the wrong words-except the word
"doctor," which was not in the list but sounds as if it should have
been. Here's the surprise: The people who made this mistake most
were the ones who had been to sleep in the meantime. Dreaming sleep
had extracted the "gist" of the list.
According to this theory, dreaming is a symptom of such memory
processing. Contrary to popular belief, dreaming occurs throughout
sleep, not just in rapid-eye-movement sleep. But the dreams
reported by people woken from non-REM sleep tend to be literal and
straightforward recitations of recent experiences stored in the
hippocampus. The later dreams of REM sleep incorporate more distant
memories, becoming more fantastic and more emotional as the new
memories get mixed with old ones in the cortex.
Even the neurophysiology is becoming clearer. The same brain
circuits active when a rat learns a maze are reactivated during
sleep, though in faster and more fragmented bursts, and seem
indirectly linked with bursts of synchronized activity in neurons
in the cortex known as sleep spindles. People who experience more
spindles retain memories better.
In the laboratory of Erin Wamsley and Robert Stickgold at Harvard
Medical School, people were asked to play a game that required
navigating a virtual 3-D maze. During the next 90 minutes, some
napped while others stayed awake. Over that period, they were asked
several times what they were thinking (if awake) or dreaming (if
asleep). Those who were dreaming about the game were 10 times
better at it the next time they played. Those who were just
thinking about the game improved hardly at all.
When we replay a recent experience in a dream, we enhance our
memory.
