Exposing the Sinister Shadows of the Mind’s Recesses : WHISPERS / The Voices of Paranoia <i> by Ronald K. Siegel</i> ; Crown $23, 336 pages

Somehow I wasn’t surprised to learn in “Whispers,” Ronald Siegel’s third book about the world of mental illness, that the UCLA Center for Health Sciences is the third-largest building in the world. I have been lost in its labyrinthine corridors and appreciate the author’s courage in descending to its basement, at night, on the advice of a virtual stranger who had promised him an opportunity to interview Hitler’s brain (safely inside a computer program).

What better subject than Hitler for a researcher interested in paranoia? Hitler, like all paranoids, saw himself at the center of a vast conspiracy organized, in his case, by communists and Jews. He saw conspirators all around him, in the intellectual and artistic circles from which he was excluded, and later inside the very bunker where he held out against his enemies.

Siegel uses his interview with Hitler’s silicon brain, the handiwork of the graduate student who had programmed it based on definitive biographical information, to reflect on the real Hitler’s mental state.

This was characterized by unpleasant obsessive tirades until 1937, when, Siegel tells us, Hitler began having full-blown psychotic episodes. Siegel attributes this shift to the regimen begun by Hitler’s physician, who had begun giving the dictator intravenous injections of the new drug methamphetamine.

The drug did not cause Hitler’s rages, but it did exacerbate them. Paranoia, Siegel suggests, is unlike other mental illnesses. It does not usually show up as an aberration on a brain scan, because the unfounded fears that feed paranoia are, in a sense, natural.

They arise from deep inside the limbic system, a pool of neurons, hormones and electric pulses that lies in the mammalian brain beneath the cerebral cortex and controls our primal urges. Hard-wired into all of us, a touch of paranoia may have helped us survive in the evolutionary past because it kept us on guard.

But like our appendix, paranoia is not useful to modern humans. The condition seems to erupt when confused signals from the limbic system trigger unwarranted fear and panic in the conscious mind. This may result from biologic stress, or from a flammable mixture of stress and drugs.

Siegel observes that there is a continuum of paranoia--much of it harmless--but always characterized by suspiciousness and hostility toward other people.

It is often the other face of narcissism--paranoids know without doubt that they are very important, the focus of a vast conspiracy orchestrated by people jealous of their tremendous talents and accomplishments.

Although the potential for paranoia lurks in all of us, the obsessive fears usually lie dormant but may be awakened, Siegel explains, by chemicals. A mixture of ordinary prescription medications as well as so-called recreational drugs can trigger murder and mayhem.

Truly a man for this season, Siegel is neither psychiatrist nor clinical psychologist, but as a research associate in UCLA’s Department of Psychiatry and Biobehavioral Sciences, he explores the effects of chemicals on the brain.

He does not, however, exclude psychological explanations from his research. In the description of Hitler’s behavior, for example, he mentions that the autopsy performed by the Russians (who found his body) revealed that Hitler was monorchid--he had a single testicle.

A psychoanalyst might point to feelings of sexual inadequacy arising from this condition as the origin of Hitler’s rage. Siegel discusses this with his friendly Freudian psychiatrist, Joel Morgan. Here, and elsewhere in the narrative, Siegel bounces ideas back and forth with Morgan, offering psychoanalytical interpretations of paranoia in tandem with, or in opposition to, biopharmaceutical explanations.

There is not much Siegel could do to have hands-on experience with Hitler.

He could, however, pursue what he calls the “demon of paranoia” in people still living. In the most remarkable episodes in this remarkable book, he describes his adventures slipping into the shoes of two of these people.

He took the same medications, watched the same movies, and re-experienced the same states of mind and body that led a loving mother to murder her 9-year-old daughter and a cocaine addict to kill his sister and infant nephew.

Always open to contradictions and paradox, Siegel does not omit occasions when apparent paranoia turned out to be a rational reaction to actual facts.

The late Martha Mitchell’s suspicion that her husband was engaged in illegal activities at the White House was diagnosed as delusional, until she was vindicated by Watergate. Mitchell has been posthumously honored with the term, “The Martha Mitchell Effect,” used to describe people who were misjudged for reporting improbable, but true, events.

More than a string of case histories, “Whispers” is a well-crafted tapestry in which individuals who are the focus of one episode become the reference point of another. Horrifying and utterly fascinating, “Whispers” is a hard book to put down.