Three American-based researchers whose pioneering work helped penetrate the complex workings of the brain were awarded the 1981 Nobel Prize in medicine yesterday.

Half of the $180,000 prize went to Roger W. Sperry, a prominent 68-year-old researcher at the California Institute of Technology who was cited by Sweden's Karolinska Institute for "extracting the secrets" of the two hemispheres of the brain.

A Harvard University team -- Drs. David H. Hubel and Torsten N. Wiesel -- received the other $90,000 for "breakthrough" discoveries of the manner in which visual images are processed in the brain.

The findings of all three have had not only theoretical but practical applications -- in the medical treatment of patients with various disorders involving the brain.

An official at the National Institute of Mental Health, which has funded Sperry's research for 23 years, called him a "bridge between the biology and psychology of the brain." The official, Dr. Lyle Bivens, added that Sperry's findings also have had application in the treatment and rehabilitation of patients with brain damage, such as presidential press secretary James Brady.

The work of Hubel and Wiesel has proven valuable in the treatment of such vision problems as strabismus, crossed eyes, in the young.

American dominance of the world's most prestigious medical prize thus continued. Scientists from the United States have captured 57 of the 132 citations for medicine since the prize was instituted in 190l. In the past 10 years, 17 Americans have won or shared the award, the formal title of which is the Nobel Prize in Physiology or Medicine.

Sperry is a native American and Canadian-born Hubel, 55, holds U.S. citizenship. Wiesel, 57, is a Swede who has been working in this country for more than 20 years.

When the news came early yesterday, Sperry was camping in Baja California. Hubel was in the shower at his Massachusetts home and a colleague said that Wiesel first heard of the honor and then went off for a game of tennis, which he lost.

Hubel, reached by telephone, said he was "surprised," despite the fact that he and his colleague had been considered likely candidates for a number of years. All three of the new Nobel laureates have long been recognized as leaders in the emerging area of brain research.

Sperry has been at Caltech in Pasadena since 1954, when he began attracting attention with his famous "split-brain" experiments. These involved animals in which the connective fibers between the two cerebral hemispheres had been surgically separated.

In these and succeeding studies with epileptic patients who had undergone similiar surgery as part of their treatment, he was able to demonstrate that the two sides of the brain were entirely independent with regard to learning and retention.

Sperry then began to map the various portions of the brain and their highly specialized functions. He showed that the left hemisphere, which controlled speech, was more involved with abstract thinking and logical reasoning, while the so-called "mute" right side focused on spacial and visualizing abilities.

The Nobel citation credited "his discoveries of the specialization of both cerebral hemispheres" with the development of an "entirely new dimension in our comprehension of the higher functions of the brain."

The work of Hubel and Wiesel provided a different insight into brain functioning, as their experiments with cats and monkeys unraveled the steps by which the brain processes visual information.

It had previously been thought that messages reaching the brain from the eyes were transmitted point by point, projecting the image on the cerebral cortex much as a movie is projected on a screen.

Their experiments demonstrated, however, that a complicated hierarchy of cells is involved in reading the image. It proceeds, noted the Karolinska Institute, as if "certain cells read the simple letters in the message and compile them into syllables that are subsequently read by other cells, which in turn compile the syllables into words and these are finally read by other cells that compile words into sentences that are sent to the higher centers in the brain," where the impression is stored as memory.

The team also found that normal functioning of these cells requires stimulation in the early stages of development, demonstrating the importance of correcting visual problems early in life.

Hubel noted that the work went beyond the vision area to general understanding of the brain, which he called the "most complicated thing in the universe."

Hubel and Wiesel went to Harvard Medical School in 1959 after working at the Wilmer Eye Institute of Johns Hopkins in Baltimore. Hubel earlier was a fellow at Walter Reed Army Institute of Research in Washington. Wiesel received his medical degree from the Karolinska Institute that administers the prizes.

Although their scientific partnership spanned more than two decades, Hubel noted that they are now conducting separate studies. He has gone into research on sleep, with Wiesel continuing in vision. Their research has long been funded by the federal National Institutes of Health.

A member of the Karolinska Institute was quoted yesterday as saying "there is no question that the money at the disposal of American scientists accounts for the large number of prizes they win." But the researchers and their colleagues used the occasion to express concern about the future of federal support.

Hubel said he could not "help but be a bit worried about budget cuts wiping out all kinds of things." Dr. Daniel X. Freedman, a University of Chicago scientist who is president of the American Psychiatric Association, praised the longterm government support of reseachers like Sperry but added that such projects were now threatened by "draconian budget cuts."