Research on one of the strangest and rarest brain disorders in medical literature has yielded a clue that helps explain one of life's most common experiences -- recognizing a familiar face.

The clue also shows it is possible for the brain to react to people and places by triggering psychosomatic reactions, from sweaty palms perhaps even to ulcers, with no conscious perception.

The new evidence comes from a study of people who have lost the ability to recognize familiar faces.

These people are normal in all ways except that when they see the face of someone familiar, even someone they have known for years such as a spouse, parent or child, they are unable to identify the person.

Victims of the brain disorder -- all afflicted after sustaining brain damage from infections or strokes -- cannot recognize their own faces in a mirror or photograph.

A sufferer of the disorder shown a photograph of himself and of a famous actor or politician, say President Reagan, could not say which was which.

Victims say they have learned to recognize people important in their lives from other clues such as body build, clothing or voice, and by memorizing facts such as that "Dad is bald" or "the boss always wears a bow tie."

Those with the disorder have normal vision and reading ability and have no trouble describing facial features or pointing out differences between faces. They can even tell when certain people look alike. Their defect is in connecting their perceptions with a stored memory of the same face and producing a sense of familiarity, or recognition.

The disease is called prosopagnosia (from the Greek words proso, meaning "forward," and agnosia, meaning "inability to perceive") and, in this week's issue of the journal Science, a University of Iowa neuroscientist reports that he has found something in victims that suggests how the normal brain works.

Antonio R. Damasio said he wondered whether prosopagnosics might actually be recognizing familiar faces at a subconscious level.

Using instruments similar to those of a lie detector, Damasio tried to detect emotional responses that altered the body in subtle ways without producing a conscious reaction. He used electrodes attached to the hand to measure changes in the ability of skin to conduct imperceptible electric currents.

Damasio said he found that even though experimental subjects could not recognize photographs of faces that should have been familiar, the electrodes picked up a definite change in skin conductance.

When the faces were of people that Damasio knew the subject had never seen, the conscious reaction was no different, but the electrodes picked up no skin change.

Damasio concluded that the brains of prosopagnostics were carrying out part of the process of recognizing a face, but were blocked at a key stage in the sequence of brain events.

The first step in this series is that nerve endings in the eye gather the facial image and send signals to the optic region of the brain. There the pattern of incoming signals, which is known to correspond in a rather maplike way with the features of the face, is matched against stored "templates" representing all the familiar faces that have been placed in memory.

If a match is found, associated memories, such as the face's name and history, are retrieved from storage and, in the final step, the whole set of associated memories becomes conscious.

Damasio's explanation of the sequence of events, like many in brain research, refers to presumed functions rather than known structures within the brain.

Nobody knows exactly what parts of the brain might account for these processes. But because prosopagnosics have a skin response, Damasio said he believes they must be carrying out all the necessary steps up to the point when the associated memories would normally reach the conscious brain.

If Damasio's scenario of brain events is correct, it provides a model that may apply to other forms of memory. If so, he suggests that subtle forms of brain damage -- too small to cause diagnosable problems -- may be depriving people of conscious reactions to the world around them but still permitting side effects of those reactions to alter not just skin conductance but perhaps a variety of other physiological reactions.

Subconscious activities in the brain are known to alter not just skin conductance but heart and breathing rates, blood pressure, digestion, and a host of other bodily processes that play roles in psychosomatic disease.