A dreamy warm-up

In a paper published last month in the journal Nature Reviews Neuroscience,Dr. J. Allan Hobson,a psychiatrist and longtime sleep researcher at Harvard,argues that the main function of rapid-eye-movement sleep,or REM,when most dreaming occurs,is physiological. The brain is warming its circuits,anticipating the sights and sounds and emotions of waking. It helps explain a lot of things,like why people forget so many dreams, Hobson said in an interview. Its like jogging; the body doesnt remember every step,but it knows it has exercised.

Drawing on work of his own and others,Hobson argues that dreaming is a parallel state of consciousness that is continually running but normally suppressed during waking. The idea is a prominent example of how neuroscience is altering assumptions about everyday or every-night brain functions.

Most people who have studied dreams start out with some predetermined psychological ideas and try to make dreaming fit those, said Dr Mark Mahowald,a neurologist who is director of the sleep disorders programme at Hennepin County Medical Center,in Minneapolis. What I like about this new paper is that he doesnt make any assumptions about what dreaming is doing.

The paper has already stirred controversy and discussion among Freudians,therapists and other researchers,including neuroscientists. Dr Rodolfo Llinas,a neurologist and physiologist at New York University,called Hobsons reasoning impressive but said it was not the only physiological interpretation of dreams.

I argue that dreaming is not a parallel state but that it is consciousness itself, said Llinas,who makes the case in the book I of the Vortex: From Neurons to Self MIT,2001. Once people are awake,he argued,their brain essentially revises its dream images to match what it sees,hears and feelsthe dreams are corrected by the senses. This is not to say that dreams are devoid of meaning. Anyone who can remember a vivid dream knows that at times the strange nighttime scenes reflect real hopes and anxieties: the young teacher who finds himself naked at the lectern; the new mother in front of an empty crib.

But people can read almost anything into the dreams that they remember,and they do exactly that. In a recent study of more than 1,000 people,researchers at Carnegie Mellon University and Harvard found strong biases in the interpretations of dreams. For instance,the participants tended to attach more significance to a negative dream if it was about someone they disliked,and more to a positive dream if it was about a friend. In fact,research suggests that only about 20 per cent of dreams contain people or places that the dreamer has encountered. Scientists know this because some people have the ability to watch their own dreams as observers,without waking up. This state of consciousness,called lucid dreaming,is itself something of a mystery. But it is a real phenomenon.

In study published in September in the journal Sleep,Ursula Voss of J.W. Goethe-University in Frankfurt led a team that analysed brain waves during REM sleep,waking and lucid dreaming. It found that lucid dreaming had elements of REM and of wakingmost notably in the frontal areas of the brain,which are quiet during normal dreaming. Hobson was a co-author on the paper. You are seeing this split brain in action, he said. This tells me that there are these two systems,and that in fact they can be running at the same time.