How absurd that human beings who want to understand the world should study history, sociology and even psychology, yet possess no information at all about the regulator of all human abilities, the brain, writes Richard M. Restak, a Washington neurologist.

To remedy this lack, he has produced a densely packed and conscientious compendium of everything you ever wanted to know about the human brain and its workings. It has something for everyone.

Members of the armed forces may like Restak's foolproof way of finding out whether a mute prisoner actually understands English or not. All you need to do is place a helmet conatining an array of electrodes on the suspects head and record his brain waves while you speak to him. Within seconds a telltale brain wave (the "P300 response") will reveal whether the man is faking or not, since the P300 response to anything understood differs significantly from the response to words that are incomprehensible. This is just one example of the many ways our brain waves, when monitored by a computer, can now be made to mirror our mental states.

For joggers and other health enthusiasts, Restak describes a Yale University experiment with professional actors, half of whom were told to work themselves into an angry state by imagining frustrating situations, while the other half concentrated on happy thoughts. Then both groups did some strenuous exercises. Only the group that had happy thoughts actually benefited from the exercise. Because of their angry mood, the others developed undesirably rapid heart rates and high blood pressure. In their case, the exercise actually did more harm than good. Mental imagery has physical consequences, it seems and the context in which exercise is carried out makes all the difference.

Parents and educators, as well as philosophers, may be interested in Restak's evidence that the infant brain "does not so much develop as respond to tens of thousands of environmental variables," and that each person's "reality" depends largely on the kind of experiences he has had in his earliest years. This new understanding of early experience has already produced enormous improvements in the care of premature infants, Restak notes. Eventually it may lead to a revolution in the way we treat all babies.

Occasionally Restak exaggerates, as when he says that research on the older, emotional parts of the brain "offers a more rational basis for informed judgment than anything suggested by the empty speculations of political scientists." He offers Nixon's paranoia as a case in point, but never explains how such research could have helped the nation to deal with it.

At times he also talks down to his readers, particularly in such contrived chapter headings as "Dr. Punishberg and Dr. Rewardnik," which are not usefully related to anything in the text.

All in all, however, "THE BRAIN: The Last Frontier" is a noble effort to cover the entire range of developments in brain research during the past 20 years, showing its contributions as well as some of its dangers. As Restak points out, this immense body of research has barely begun to penetrate the brain's essential mystery. It awaits major discoveries, which could come from an entirely different field, such as physics. In facts, when two former Nobel Prize laureates in physics were recently asked to guess what area of research would win the prize for physics in the year 2000, both men said without hesitation that it would be brain research.