Dr. Lissy F. Jarvik hadn't been talking about her twins much lately. Until last week. She's been busy with her current research into the causes of Alzheimer's disease and other intellectual deficits of aging and a host of other projects associated with geriatrics, including two new books.

But it was her twins -- 134 pairs of them -- which thrust her into the forefront of aging research some 40 years ago.

And some of the findings from that 20-year-long study of the aging process are, in retrospect and in fact, as important to the field today as they were in the 1940s and '50s when they nudged "Webster's Universal Dictionary" to drop "old age" as a definition for "senility."

Jarvik showed that the normal aging process does not necessarily include a significant decline in mental abilities from age 60 to age 75. But as she reminded the Gerontological Society of America last week, the problems after that age begin to mount.

In her lecture marking the society's award to her for outstanding contributions to geriatric research, Jarvik charged that instead of trying to find reasons for these mental deficits and learning to slow or stop them, the medical profession is incorporating them into their picture of an aging adult.

She particularly criticized a test of adult mental ability -- the revised Wechsler Adult Intelligence Scale, published in 1981 -- as an exercise in "ageism one underground." In effect, she said, the test adds "bonus points for years to the scores of those past their prime," quietly diluting the effect of very real intellectual decline.

This kind of ageism, she said, "is more dangerous than ageism openly expressed . . . If today's attitude had prevailed 40 years ago, we would never have done the twin study, but more than that, if there are no intellectual declines in old age, then we don't have to look for any causes."

The twin study also showed that declining mental skills often precede premature death. If one identical twin showed a decline in intellectual capacity, known as "critical loss," that twin "rarely made it to the next testing period," while the other continued to do well. This suggests that there are many factors besides genetics determining how long someone lives.

Moreover, when there was an initial low score on these "critical loss" tests, there was a high likelihood that dementia would develop within the next two decades. Analysis continues, but, Jarvik noted, the findings suggest that with the use of standardized tests, individuals at risk for developing dementia may be identified long enough in advance to take some preventative action. ::

Lissy Jarvik decided to become a doctor at the age of 5. But when her family left their native Holland in 1940, a single day before Hitler's troops marched in, it appeared her dream was not to be. She worked as a Dutch-English translator in New York during the war, going to Hunter College at night. When she was initially unable to get into medical school, she took a doctorate in psychology at Columbia University. Then she got into medical school anyway, and eventually became a neuropsychiatrist, pediatrician, geriatrician and psychologist, as well as wife and mother.

She met her husband, psychopharmacologist Murray Jarvik, "in the basement of Mount Sinai Hospital one Sunday morning when I was an intern. I had blood from a patient and I couldn't find the lab and there was nobody around because it was Sunday, but he was working in his lab and he came with me to show me the way."

They have two sons, one a filmmaker and the other an intern at UC San Diego headed toward a career in radiology. (Dr. Robert Jarvik of the artificial heart fame is her husband's nephew.)

The day before she would deliver her award lecture at the Gerontological Society conference at the Washington Hilton, she talked about her life and her work with wit and determination.

As chief of the West Los Angeles Veterans Administration Medical Center Psychogeriatric Unit and chief of the UCLA Neuropsychiatric Institute's Section on Neuropsychogeriatrics, Jarvik is involved deeply in Alzheimer's research and has just become co-editor of the world's first International Journal on Alzheimer's disease.

She became involved in the twin studies in much the way she met her husband -- by chance. "I needed a job and Franz Kallman needed a research assistant."

But although serendipity may guide her life, her work is marked by meticulous attention to detail and a penchant for perfection. She is regarded as incisive and blunt by her colleagues, who admit that their respect for her brilliance is tinged with a little unease at her evenhanded but sometimes painful criticisms of the work of others. In his introduction of her at the award ceremony, Dr. Charles Shamoian, Alzheimer specialist from the Cornell Medical Center, noted that "everyone knows her little box of colored pencils . . . When a paper comes back from being reviewed by Lissy, it looks like a rainbow, although not so pleasant."

Alzheimer's research under her direction will include a protocol on THA, the experimental drug said to improve intellectual capacity in early Alzheimer's patients. THA experiments were temporarily halted last month by the Food and Drug Administration to investigate evidence of toxic reactions, but are expected to resume shortly. "It won't be a panacea," Jarvik says, "but it may help a few people a little bit for a little while."

Another research project involves the study of certain white-cell components that serve various transport functions in the brain. Under normal circumstances, these tend to move toward heat sources, part of the body's natural immune defense system. "If you think about it," she says, "heat is a vital ingredient of infections or inflammations."

"We think this system becomes damaged in Alzheimer's patients and therefore they become more vulnerable to whatever outside agent -- a virus or some toxin -- actually causes the disease. The story is still very complicated. There are a lot of clues scattered about, but nobody yet knows how to put them together."

Jarvik is also involved in promoting knowledge about geriatrics to members of other medical specialties. Until a few years ago, "geriatrics had been almost ignored by psychiatry except for a very small group. But by and large, the field as a whole shunned the elderly.

"And for psychopharmacologists it is extraordinarily tricky to treat elderly people because it is very complicated with other drugs, so they left it to the internists.

"But the internists were not psychologically sophisticated nor psychiatrically trained. The same applied to family practice. So we decided there should be some way to designate people who have competence in geriatrics, because everybody thinks they know about old people and 'old people are the same and you use the same drugs and there are no good treatments, anyhow . . .' "

So Jarvik and her colleagues have prepared a syllabus (to be published in the next few months) for the use of any medical specialists who deal with the elderly. ::

Another book, to be published early next year, is clearly dear to her heart and her first book aimed at the general public. It is tentatively titled "Parent Care: A Common-Sense Guide to Adult Children," and it is aimed at the children of the Baby Boomers "who will have to be aware that the health care system will be unable to take care of their parents. We are talking about the relationship between parents and children when parents are old and infirm and need help. Not old people with Alzheimer's disease, but just old and needing help.

"Most people don't plan on taking care of their parents as part of the ordinary life plan. But they have to know." Then she adds with her elfin grin, "They won't like it very much. But it's reality."