When it comes to the artistic "brain drain" from the Soviet Union, a congressional commission was informed yesterday, it's quality -- not quantity -- that really matters.

The exodus of artistic talent to the West from the Soviet Union in recent years has reached many hundreds and probably is well into the thousands, said Soviet expert Michael Scammell at a hearing of the Commission on Security and Cooperation in Europe. The commission is an umbrella group of both houses meant to monitor compliance -- or lack thereof -- with the Helsinki Accords, especially its human rights provisions.

But the most "grievous" impact on so artistically advanced a culture, said Scammell, the founding editor of Index on Censorship and the author of "Solzhenitsyn: A Biography," comes from "the extremely high quality" of many artists who defect, are expelled or emigrate. He cited names like Nureyev, Makarova, Baryshnikov, Rostropovich, Ashkenazy, Solzhenitsyn and Brodsky, among many others, to help make his point.

Numbers are hard to come by, but Scammell mentioned a conference on "third wave" literature in Los Angeles in 1983 at which organizers listed 101 ex-Soviet writers alone who now live and work in the West.

To dramatize the point, three former Soviet artists were among the witnesses: actor Oleg Vidov, who came west in May; novelist Vasily Aksyonov; and celebrated conductor Maxim Shostakovich, son of the great composer.

Vidov, who is also a film director, told of a career that started with promise ("I had already played a leading role by the time I was 21") but had begun to shrink in recent years in the face of increasing hostility by the authorities toward his projects -- especially his directing.

"The straw that broke the camel's back," he said, "was the film ministry's decision to cancel my first full-length feature film after I had worked for a year on the script and was already searching for locations in the Caucasus Mountains. There was nothing wrong with the script -- four years later it was filmed by another director for another studio.

"The highest levels of government had pressured the ministry to end my career as a director. They had also ordered other directors not to cast me in lead roles. The Filmmakers Union, as usual, had no power to help me."

His crime, Vidov said, "was inability to conform, my attempt to win more artistic freedom, and, incredibly enough, the political machinations of an ex-wife. Probably without pressure from my ex-wife, the government would have ignored me -- there are many more difficult filmmakers in the Soviet Union. But my ex-wife's closest friend is Galina Brezhnev, daughter of the then secretary general of the Communist Party . . .

"As a proud Russian, with a deep love for his country and people, I am ashamed that I had to stand before you today and recite this little story, which not only documents that there is little or no artistic freedom in the Soviet Union, but shows how easily political power is abused by our leaders."

Shostakovich, who is here directing the Washington Opera version of Tchaikovsky's "Eugene Onegin," was outspoken about the character of Soviet leadership. "It is a power structure that fears the arts because it is so unsure of itself," he testified. "The reason is that culture forms public opinion."

Compared with literature, the conductor said, music has a bit of an advantage because it is less understandable in a literal sense. But when there are words, that is another thing. "Any music that is religious or spiritual is in trouble. You may hear some Bach. But the spiritual works of Rachmaninoff are not performed."

Later, Shostakovich pointed to the "irony that even the works of Tolstoy or Turgenev are rewritten because it is not allowed that the word 'God' have a capital 'G.' " Asked about the availability of the Bible in the Soviet Union, he replied, "it may be smuggled in from Poland."