HALIFAX -- It's possible Christopher Garnier was suffering from a rare condition in which he was not fully aware of his actions immediately following the death of off-duty police officer Catherine Campbell, a psychiatrist told the Halifax man's murder trial Thursday.

Dr. Stephen Hucker was asked by the defence to prepare a report on the high-profile case, and he was later qualified as an expert witness in Nova Scotia Supreme Court.

Defence lawyer Joel Pink asked Hucker about "automatism," and the psychiatrist said it is defined as behaviour that a person is not consciously aware of.

Garnier has told the jury he has fragmented memories about what happened in the early hours of Sept. 11, 2015, and he has said he does not remember dumping Campbell's body near the Macdonald Bridge in downtown Halifax.

Hucker said it's possible the 30-year-old accused was in a state of automatism after Campbell's death.

"I think a good case can be made for that," he told the jury.

But under cross-examination by Crown attorney Carla Ball, Hucker agreed he never mentioned automatism in his report, and he acknowledged that it is a rare psychological condition.

Hucker previously testified that Garnier suffered from acute stress disorder following Campbell's death, which could explain his memory loss. However, Hucker added that not all cases of acute stress disorder lead to memory loss.

He said he based his conclusions on two interviews conducted with Garnier over roughly six hours, on evidence he was provided by the defence, and on interviews with Garnier's girlfriend and friend.

Hucker acknowledged he never looked at Garnier's medical records or spoke with any of his former employers.

He also agreed that it's possible Garnier's claims of memory loss in the hours after Campbell's death could be because he's reluctant to tell his family and friends about his actions on that night.

Garnier has pleaded not guilty to second-degree murder and interfering with a dead body.

The Crown alleges Garnier punched and strangled the 36-year-old Truro, N.S., police constable after they met at a Halifax bar, and used a compost bin to dispose of her body near the bridge.

In his opening statement, Pink told the 14-member jury that Campbell died accidentally during "rough sex" that she initiated.

Also on Thursday, Pink called a former colleague of Campbell's to the stand.

Const. Justin Russell said he attended the Atlantic Police Academy in Charlottetown with Campbell and also worked with her at the Truro Police Service.

Russell testified that they were trained in self-defence at the academy, including how to break unwanted neck restraints.

He said they also learned how to apply a vascular neck restraint. The restraint involves putting pressure on a certain artery in the neck, reducing the flow of oxygenated blood to the brain and causing unconsciousness, he said.

The trial continues Friday.y.