A forensic psychiatrist who says Christopher Garnier may have suffered "automatism" following Catherine Campbell's death acknowledged Thursday it's possible the man's assertions of memory loss could be from a reluctance to tell family and friends about his involvement.

Dr. Stephen Hucker, who was hired by the defence, has testified in Nova Scotia Supreme Court the trauma of Campbell's death brought on "acute stress disorder" in Garnier, which may have led him to act without full awareness or control.

Garnier, 30, is on trial for second-degree murder in the off-duty Truro, N.S., police officer's death and interfering with human remains. He has pleaded not guilty to both counts.

The defence has argued Campbell's death at a Halifax apartment in the early hours of Sept. 11, 2015, was accidental. Garnier has testified the 36-year-old woman died after she asked him to dominate, choke and slap her.

Garnier has also testified he remembers little of what happened after, including loading Campbell's body into a green compost bin and dumping her body near Halifax's Macdonald Bridge.

Automatism is a relatively rare psychological condition and rare legal defence, said Hucker, who has also testified that acute stress disorder could also explain why Garnier says he can't recall much from the hours after Campbell died.

Catherine Campbell

Catherine Campbell, an off-duty police officer, was strangled and her body dumped in a green bin on Sept. 11, 2015. (CBC)

On cross-examination Thursday, Hucker acknowledged he never asked for Garnier's medical records or spoke with any of his former employers. He did interview Garnier for six hours and reviewed some of the evidence in the case.

He also acknowledged not all cases of acute stress disorder lead to memory loss.

Hucker has testified he does not believe Garnier was "malingering" by mimicking the symptoms of what he says was wrong with him. But Crown attorney Carla Ball suggested Hucker hadn't been privy to certain evidence that could have be used to test his claims of what happened.

Report by Dr. Stephen J. Hucker

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The psychiatrist is also an expert in erotic asphyxiation, the practice of choking for a sexual "high" the defence has said led to Campbell's death.

The practice is dangerous, said Hucker, who has testified unconsciousness can occur in a matter of seconds and often without warning.

Hucker also said of the 172 fatal cases in Ontario he has studied, all the victims were alone at the time of death and only two were women.

Campbell's colleague testifies

Truro police Const. Justin Russell also testified Thursday. He told the court that he and Campbell were trained in applying a vascular neck restraint when they attended the Atlantic Police Academy in Prince Edward Island.

The technique involves putting pressure on the carotid artery in the neck, reducing the flow of oxygenated blood to the brain and causing unconsciousness.

Russell said the practice could be lethal if not done properly. He testified he has never had to apply the neck restraint and as someone who worked with Campbell, never saw her do it either.

The constable also told the court they were trained how to break a choke hold if one were applied to them.

The trial continues Friday.

The CBC's Blair Rhodes was live blogging from court. Mobile users can read the blog here.​