Before a big exam, a sound night's sleep will do you more good than poring over textbooks. That, at least, is the folk wisdom. And science, in the form. of behavioral psychology, supports that wisdom. But such behavioral studies cannot distinguish between two competing theories of why sleep is good for the memory. One says that sleep is when permanent memories form. The other says that they are actually formed during the day, but then 'edited'' at night, to flush away what is superfluous. To tell the difference, it is necessary to look into the brain of a sleeping person, and that is hard. But after a decade of painstaking work, a team led by Pierre Maquet at Liege University in Belgium has managed to do it. The particular stage of sleep in which the Belgian group is interested in is rapid eye movement (REM) sleep, when brain and body are active, heart rate and blood pressure increase, the eyes move back and forth behind the eyelids as ff watching a movie, and brainwave traces resemble those of wakefulness. It is during this period of sleep that people are most likely to relive events of the previous day in dreams. Dr. Maquet used an electronic device called PET to study the brains of people as they practiced a task during the day, and as they slept during the following night. The task required them to press a button as fast as possible, in response to a light coming on in one of six positions. As they learnt how to do this, their response times got faster. What they did not know was that the appearance of the lights sometimes followed a pattern--what is referred to as 'artificial grammar'. Yet the reductions in response time showed that they learnt faster when the pattern was present than when there was not. What is more, those with more to learn (i. e., the 'grammar', as well as the mechanical task of pushing the button) have more active brains. The 'editing' theory would not predict that, since the number of irrelevant stimuli would be the same in each case. And to eliminate any doubts that the experimental subjects were learning as opposed to unlearning, their response times when they woke up were even quicker than when they went to sleep. The team, therefore, concluded that the nerve connections involved in memory are reinforced through reactivation during REM sleep, particularly if the brain detects an inherent structure in the material being learnt. So now, on the eve of that crucial test, maths students can sleep soundly in the knowledge that what they will remember the next day are the basic rules of algebra and not the incoherent talk from the radio next door. Researchers in behavioral psychology are divided with regard to ______ .