When lawyers raise the insanity defense, they usually rely heavily on the testimony of psychiatrists who have looked at the defendant's upbringing and life experiences to try to explain who he is.

But in the case of John W. Hinckley Jr., the man accused of attempting to assassinate President Reagan, the choice of witnesses suggests his lawyers have reached out further, to experts at the frontiers of research on changes in behavior that relate to diseases of the brain.

The list of defense experts for Hinckley, who contends he was insane when he fired on the president, includes two doctors who study brain disease and a third who specializes in schizophrenia--a mental disorder typified by a distorted view of reality.

The prosecution, which must prove beyond a reasonable doubt that Hinckley was sane, will rely on a team of psychiatrists whose specialty is court testimony about mental illness and criminal responsibility--the key question in the Hinckley case.

What those defense--and prosecution--experts have found out about John Hinckley, and what they will say at his trial, is not yet known. The final analysis about Hinckley's state of mind on March 30, 1981, will be left to the 12 jurors who are expected to be selected today in U.S. District Court.

The key research-oriented witness for the defense is expected to be Dr. David Michael Bear, 39, a psychiatrist with special training in neurology, the branch of medicine that deals with diseases of the brain, spinal cord and central nervous system. Bear, an associate professor of psychiatry at the Harvard Medical School, studies the physical aspects of mental disease.

Bear is an internationally known contributor to the growing body of knowledge about the relationship of the brain to changes in behavior, according to Dr. Norman Geschwind, the James Jackson Putnam professor of neurology at Harvard Medical School.

The possibility that some alterations in behavior may be related to the condition of the brain cannot be ignored or just called coincidence, said Geschwind, who was a teacher of Bear's at Harvard.

For example, Geschwind noted, an autopsy in 1966 revealed that sniper Charles J. Whitman had a malignant brain tumor the size of a golf ball. Whitman was killed by police after he murdered his wife and mother and then killed 13 persons and wounded 31 others in a shooting spree from the University of Texas observation tower.

A panel of experts said that while they could not link the tumor to Whitman's actions, it "conceivably could have contributed to his inability to control his emotions and actions." No evidence has been disclosed to indicate that Hinckley suffers from any such organic disorder.

Dr. Ernst Rodin, a Detroit neurologist trained in psychiatry who examined Hinckley for the prosecution, would be the government's counterpoint, if necessary, to Bear's testimony. Rodin, who studied at the University of Vienna and the Mayo Clinic, is the medical director of the Epilepsy Center of Michigan.

The defense witness list also includes Dr. Marjorie LeMay, a radiologist at Massachusetts General Hospital--where David Bear has been chief of the neuropsychiatry section.

LeMay, whose recent writings and lectures have focused on degenerative brain disease, is now an associate professor of radiology at the Harvard Medical School and was a visiting professor at the American University of Beirut. LeMay said she works regularly with what is known as a CAT scan, a sophisticated X-ray machine, enhanced by computer technology, originally designed for use on the brain but since developed for use over the entire body.

Hinckley had his latest CAT scan at Johns Hopkins Hospital in Baltimore on April 23, just four days before jury selection in his trial began.

Also on the defense witness list is Dr. David J. Greenblatt, chief of the clinical pharmacology unit at the New England Medical Center hospital, a teaching hospital affiliated with Tufts University. He specializes in the study of drugs and their effects on humans.

Defense expert Dr. William Carpenter, the director of the Maryland Psychiatric Research Center, is best known for his work on diagnosing schizophrenia, a separation of the thought processes from emotions, commonly described as a withdrawal from reality. Carpenter is also a professor of psychiatry at the University of Maryland Medical School.

Yale University psychologist Ernst Prelinger is also expected to testify for the defense. Prelinger, who specializes in psychological evaluation of young adults, previously has examined persons charged with crimes on the question of mental illness and its relationship to criminal responsibility.

The defense is also expected to call as a witness Dr. Thomas C. Goldman, a widely regarded Washington psychiatrist who has worked closely with the city's Public Defender Service on criminal cases.

Psychiatrists who will testify for the prosecution include Dr. Park Elliott Dietz, who runs the Institute for Law and Psychiatry at the Harvard Medical School; Dr. James Cavanaugh, who tested Chicago mass murderer John Wayne Gacy; Dr. Jonas Rappeport, who examined Sara Jane Moore, the assailant of President Ford, and Arthur Bremer, convicted of shooting former Alabama governor George C. Wallace; and Dr. Sally Johnson, who saw Hinckley 57 times while he was held at a federal prison in North Carolina.