Just when you thought most series were the TV equivalent of pulp-fiction -- material to occupy the mind but not challenge it -- along comes the Public Broadcasting System with one that will stimulate the brain at the same time it attempts to explain it.

The series, running in eight one-hour segments, starting Wednesday -- and appropriately titled "The Brain" -- is an attempt to pull together and explain the state-of-the-art scientific theory and research on the body's most complex organ.

"We've timed the series (to air at a point) where we're on a threshold" in our understanding of the brain, said Richard Hutton, science editor for the series and producer of the final installment. It comes at a time when a number of basic things about the brain have been discovered, opening the way, perhaps, for even more important findings in the near future.

"In the last 10 to 15 years, man has learned more about the brain than in his history on the planet," said George Page, the series host. "It's the last frontier in medical research."

The lack of inquiry into the very source of our curiosity seems odd. Hutton noted that what goes into the brain and what comes out has been studied. But some think, "Why bother with what's in the middle?"

What's in the middle is the three- pound organ that is responsible for our knowledge, serves as the storehouse of memory, is the supervisor of human activity and the source of emotion.

PBS will begin its series with "The Enlightened Machine." It includes a brief history of brain sience and discusses how the organ works.

Subsequent installments: "Vision and Movement" (Oct. 17) traces the pathways of the brain that allow people to see and move and discusses how the two functions relate to each other; "Rhythms and Drives" (Oct. 24) looks at the "animal brain," which mediates such things as depression, euphoria, aggression and sex; "Stress and Emotion" (Nov. 7 -- as Oct. 31 has been pre-empted) examines personality and emotions; "Learning and Memory (Nov. 14) studies changes that take place in the brain while we're learning; "The Two Brains" (Nov. 21) deals with the cerebral cortex, where thought, language and planning originate; "Madness" (Nov. 28) deals with a form of mental illness, schizophrenia, once thought of as an emotional phenomenon and now known to be a disorder of the brain; and "States of Mind" (Dec. 5) looks beyond emerging tools of brain science and into the future.

Such topics are not among the world's simplest. Indeed, making the series understandable to a general audience "was our biggest challenge," said Page. "What we try to do is interweave human stories. In dealing with vision and movement, we use Greg Louganis, the Olympic diver; in discussing epilepsy, we have a little boy who suffers from seizures. We attempted to tell it in terms of human stories and to make a scientific point with each one."

The series also presents a portrait of a victim of Alzheimer's disease, and introduces actress Kitty Carlisle Hart, author George Plimpton and conductor Michael Tilson Thomas, who discuss their early memories and the sensory associations they still carry with them. British actor Terry-Thomas talks about his battle with Parkinson's disease, following upon the discussion elsewhere of Muhammad Ali's recent hospitalization for Parkinson's syndrome.

"We also use graphics to show what happens," said Page, "down to the level of the cell . . . One thing that meant most to me is that so much of what wee traditionally thought of as manifestation of mental activity actually has a physcial basis in the brain. Not all mental problems have been traced to physical impairment, but so much of what we thought of as mental illness has been traced to physical impairment of the brain. That's the main point of the series -- much of what we know about human behavior has a physiological basis."

Funds for the series were provided by The Annenberg/CPB Project, the James S. McDonell Foundation, the National Science Foundation, the National Institute of Neurological and Communicative Disorders and Stroke, the National Institute on Aging, the National Institute of Mental Health, and CIBA-GEIGY Corp. It was produced by WNET/New York and Antenne 2 TV France in association with NHK, Japan; the Societe de Radio- Television du Quebec, and Kastel Enterprises, Ltd. College course credit is available through five area institutions; for information call WETA's education department at 998- 2709.

"The Brain" is indeed an ambitious project. "Carl Sagan has done outer space," said Page. "Now we're doing inner space."