In spite of "recent, spectacular defections," an expert on the Soviet Union said yesterday, there is no sign that Soviet cultural policies are hardening.

Jack F. Matlock, deputy director of the Foreign Service Institute and former deputy chief of mission at the U.S. Embassy in Moscow, told a group of scholars and Kremlin-watchers at the Kennan Institute that the defections are not a sign that conditions are getting worse in Russia but that artists and intellectuals are losing hope that they may get better.

For the past 15 years, he said, the trend has been "toward stability -- not toward liberalization or hardening of policy."

The dominant factor, he said, is the Soviet cultural bureaucracy, which tends to favor unadventurous mediocrity in the arts and which has not changed significantly in half a century.

Matlock quoted a Soviet writer on the promotion of mediocrity: "Where else but in the Soviet Union could one live well for writing trash?"

The present situation is better than in the time of Stalin, when artists lived in terror, and perhaps better than the Khrushchev era when nobody knew what to expect next, Matlock said. "But in the early '60s, most intellectuals were more optimistic than they are today. It was not that there was much more freedom but that there seemed to be a prospect of more freedom. Now, the outlook seems to be more of the same."

"Soviet cultural controls can be seen as a sort of straitjacket," he said. "The victim occasionally manages to loosen it a little, and then the regime hastens to tighten it again. The jacket is getting a bit frayed in places, but it's still strong."

It may be a hopeful sign, he said, that being published abroad no abroad no longer means an automatic criminal presecution for a writer. Poets and fiction writers are allowed to be more direct and honest in writing about personal emotions and in psychological probing, and restrictions of form are loosening a little. But it is still impossible, for example, to engage in political satire or to question the value of Marxism as a political system.

Matlock said that the political leaders of the Soviet Union "do want a culture they can be proud of" and that they "do worry about their image abroad. They can sometimes be dissuaded from a course of action that will give them an international black eye.

"This may not be immediately obvious because they have a remarkable talent for shooting themselves in the foot," Matlock said.