''Multiple personality offers a special window into psychosomatics,'' said Frank Putnam, a psychiatrist at the National Institute of Mental Health and a leading researcher in the field. ''Multiples exhibit some remarkable medical phenomena.'' Dr. Putnam said. He gives the example of one patient who reacted normally to a sedative drug in one personality, but was totally unaffected by it in another. ''Some multiples carry several different eyeglasses, because their vision changes with each personality,'' said Bennett Braun, who directs a unit devoted to treating multiple personalities at the Rush-Presbyterian-St. Luke's Medical Center in Chicago. Another woman, admitted to a hospital for diabetes, baffled her physicians by showing no symptoms of the disorder at times when one personality, who was not diabetic, was dominant. A young man was allergic to citrus fruit in some personalities, but not in others. The key sign of the disorder is a person seeming to have developed at least two distinctive personalities that alternate in control of the body. Another important symptom is partial amnesia: some but not necessarily all of the multiple personalities are unaware of the others. Circumstances that lead to the condition are usually brutal, typically extreme neglect or abuse. The disorder seems to represent a psychological adaptation to an otherwise unbearable situation. On average there are from 8 to 13 personalities in a typical patient, although there can be more than 60, according to Richard Kluft, a psychiatrist at the University of Pennsylvania who has studied more than a hundred cases.