In the hours before his respirator was disconnected last March, comatose patient Harry Gessner showed no signs of spontaneous brain function, a brain specialist testified yesterday at the murder trial of former nurse Mary Rose Robaczynski.

Dr. Harry A. Spalt, chief of neurology at the hospital where Robaczynski worked here, thus bolstered the defense contention that, under Maryland law, Gessner legally was dead at the time Robaczynski allegedly disconnected his life-support system.

Later, under cross-examination by prosecutors in the case, Spalt conceded that he would not have made the decision to disconnect Gessner's respirator based on the medical information then available to him.

"I didn't have enough data for that," Spalt said when asked if, based on Gessner's medical chart, he would have disconnected the respirator.

Later, when the prosecution asked if a nurse with 2 1/2 years of experience -- as Robaczynski had -- has the right to disconnect a patient's respirator, Spalt asserted: "No nurse, however old, however much experienced, has that right."

The prosecution has introduced testimony that Robaczynski had spoken in favor of mercy-killing in the cases of comatose patients who had little or no hope of recovery. She is charged in separate indictments with the murders of Gessner and three other comatose patients whose respirators were unhooked.

The defense has contended that under Maryland law Gessner legally was dead hours before his respirator was unhooked, and, therefore, 24-year-old Robaczynski cannot be held responsible for his death.

In making this argument, defense attorneys have focused on a provision of the Maryland law that states, in part, that a physician may legally declare a patient dead when "spontaneous brain function" is absent.

Spalt testified that signs of spontaneous brain function include wakefulness or spontaneous breathing. According to Gessner's medical chart, the patient was neither awake nor breathing spontaneously while in the special care unit of Maryland General Hospital, he testified.

The 48-year-old Gessner, according to testimony, was found in his hospital room with his heart stopped at 2:20 a.m. on March 7. Gessner's heart beat was restored and he was hooked to a respirator in the hospital's special care unit, where Robaczynski was on the night shift.

She is accused of disconnecting the respirator between 5 and 5:30 a.m. March 8.

In its ninth day, the trial developed into a sparring match between defense attorney Joseph Murphy Jr. and prosecutor Howard Gersh as each tried to pin down neurologist Spalt on precisely what constitutes brain death.

Spalt testified that Gessner's medical chart showed that the pupils of his eyes reacted to bright light, a response that would indicate some function in a portion of Gessner's brain. Spalt added that a response to such a stimulus is not considered a sign of spontaneous brain function.

A 1972 law leaves to doctors the determination of precisely what criteria will be used to determine the absence of brain function.

Today, Gersh introduced into evidence the most widely accepted criteria, those set out by a Harvard Medical School committee in 1968.

In his testimony, Spalt acknowledged that under those criteria Gessner's brain was "not totally dead," and added that he himself uses the Harvard criteria in determining what constitutes brain death.

Under questioning by the defense, he said other doctors at other hospitals may apply their own criteria, and that in March 1978 there were no written criteria in Maryland law to offer doctors guidance.