It began as a quiet cocktail party at the home of U.S. charge d'affaires Mark Garrison in Moscow.

The guests were visiting congressmen and the Kremlin officials who were acting as their tour guides. The clinking of champagne glasses and exchange of diplomatic pleasantries were proceeding amiably -- until the conversation turned to the subject of Soviet dissidents.

Suddenly, the tone grew strident and angry. One visitng congressman became embroiled in a heated argument with a top-ranking Soviet official, Vitaliy Ruben. Their discussion centered on Anatoly Scharansky, the mathematician who is languishing in prison for allegedly passing anti-soviet information to the United States.

The American lawmakers threw down the gauntlet to Ruben, saying: Give us solid evidence against Scharansky and we'll present it on the floor of the House of Representatives.

The challenge could not be ignored; the Americans had called the Russians' bluff. And the next day, at a crowded session, a Soviet official named Lev Smirnov played the Kremlin's hand.

It turned out to be a diplomatic four-flush. All Smirnov had to show was a tired old claim that Scharansky hadn't talked to the right people before he issued a denunciation of the Russians' use of Sychiatric wards as detention centers for dissidents. Scharansky, Smirnov sniffed, had interviewed only drug addicts.

This was the accusation the congressional delegation had been waiting for, because it showed the emptiness of the official Soviet charges against Scharansky. The congressmen, led by Rep. Lester Wolff (D-N.Y.), had been doing a little investigating on their own that belied the official Kremlin line.

A confidential congressional briefing paper obtained by our associate Jack Mitchell lists the results of their findings and shows the cynical deceit of the Soviet attempt to put down Scharansky's charges.

The picture of Soviet mental hospitals in the congressmen's study makes it chillingly clear that they are every bit as nightmarish as the prison camps described by Alexander Solzhenitsyn in "The Gulag Archipelago." In fact, by officially stripping its victims of sanity, the psychiatric confinement is in many ways crueler than a labor camp sentence.

One of the crucial ingredients of the Soviet recipe for dissident director of the Institution of Psychiatry in Moscow, schizophrenia is attributed to genetic . . . deficits and can be present in persons displaying few or none of the classical symptoms."

Snezhnevsky's distortion of tentative research findings would be bad enough. But there is a Catch 22: even a conscientious psychiatrist, knowing the harrowing consequences of disagreement with the party line, would have little trouble diagnosing a Soviet dissident as a schizophrenic. No Soviet citizen in his right mind would speak out against the communist regime -- therefore, anyone who does so must be certifiably insane.

In any event, once committed to a psychiatric hospital, the Soviet dissident is quite likely to develop, as a result of physical and mental torture, the very symptoms of fear, depression and paranoid suspicion that justify the doctors' originally spurious diagnosis.

Powerful drugs are used in the treatment of such dissidents," the congressional briefing notes. "Three of these -- aminazin, triftazin and haloperidol -- are know as major tranquilizers and are used in the United States solely for the treatment of extreme cases of schizophrenia."

Other patients are given insulin, which can induce shock in non-diabetic cases, or a drug called sulfozin, which can cause raging fevers, painful joint inflammation and high blood pressure. Physical and psychological brutality are also common practice.

Despite condemnation by the World Psychiatric Association in 1977, and the heroic work of a Russian group called the Working Commission to Investigate the Use of Psychiatry for Political Purposes, the Kremlin shows no signs of stopping its distortion of the healing arts.

In the Orwellian world of Soviet tyranny, official designation as a mental case can become a self-fulfilling prophecy when applied to the sanest of political dissidents.