Qian Liu, 23, was found dead April 15, 2011 in her off-campus basement apartment, mostly naked and face down on the floor.
The Chinese student had been chatting with her boyfriend when he saw her open the door to a man who tried to hug her, then forced his way in and later turned off the computer while naked from the waist down, court has heard.
Dr. Jeff Tanguay couldn't arrive at a definitive cause of death for Liu, but he said the best overall explanation is mechanical asphyxiation, which could be her neck or chest being compressed or something interfering with the intake of air through her nose or mouth.
Brian Dickson, who was a tenant in the same building as Liu, has pleaded not guilty to first-degree murder, but his lawyer has told the jury he'll be urging a finding of manslaughter.
Liu did not have many external injuries, Tanguay said, but the autopsy revealed internal bruising on her right temple, in a muscle on her left shoulder blade and on her triceps.
She did, however have multiple petechial hemorrhages in her eyes, which appear as dots and are the results of blow flow backing up in the veins and a rise in pressure, Tanguay said.
Those hemorrhages could be caused by pressure being applied to the neck or chest, or if the upper airways are blocked, Tanguay said. He ruled out heart disease as a likely cause.
But defence lawyer Robert Nuttall suggested Liu's heart may have been a factor in her death. He asked Tanguay about commotio cordis, which is sometimes seen in youth athletes who drop dead after a blow to the chest because it interrupts the firing mechanism of the heart, Nuttall said.
The jury has heard that Liu's boyfriend saw the man who forced his way into her room shove her off camera.
Nuttall asked if that shove could be fatal because of commotio cordis, and Tanguay said he could not exclude it as a possible cause of death.
Since there were no external marks to go along with the bruising under the skin of Liu's temple, shoulder and triceps, Tanguay couldn't say how they were caused, though he did offer some possibilities.
The injury to the muscle in the back could have been caused either by a forceful impact, firm pressure applied by an object such as a knee or an elbow or, since it's part of the rotator cuff, it could have happened if the arm was moved outside of its normal range of motion, Tanguay said.
The tricep bruises could have been caused by impacts or by forceful grasping or squeezing, he said. There was no evidence the bruises had started to heal, which means they happened shortly before Liu's death, Tanguay said.
Bleeding that the pathologist found in Liu's neck either could have been the result of an injury or could be from pooling of the blood after death, he said.
Forensic experts also found semen on Liu's abdomen and groin area, but Tanguay said they found no evidence of injury to her genitals.
Nuttall raised the "white staining" on the front of Liu's body then asked the pathologist about chest compression and the force that would be created by a heavy person sitting on a smaller person's chest.
The defence lawyer did not make a link between the two, but suggested that Liu's shoulder injury could be consistent with someone sitting on her chest and an object such as a phone cord — which was found on the floor of her room — was pressing into her shoulder.
The jury watched the beginning of a videotaped police interview during which Dickson was informed he was a person of interest. The rest of the interview is expected to be played in court Thursday.
Dickson had been attending classes at York University on and off since 2000, he told police, but wasn't currently a student since he owed the university money.