Stressful situations can cause false memories to form, a British-based psychologist said during the trial for three Calgary police officers charged with assault.

Dr. Julia Shaw, an expert on false memories and memory formation who works as a researcher at University College in London testified that false memories feel real.

Shaw was called by the defence.

James Othen, Kevin Humfrey, and Michael Sandalack are each charged with assault causing bodily harm for a violent arrest on July 30, 2016, which caused Clayton Prince to suffer broken ribs and a collapsed lung.

Othen and Humfrey also face a charge of mischief, accused of making false statements after the arrest. Their account of what happened contradicted the dash-cam video from a police cruiser that eventually surfaced.

Video of Calgary police charged with assault0:07

Although the video shows Prince clearly surrendering on the ground with his hands on his head, Othen then jumps on top of him and begins delivering punches. Humfrey joins in.

Humfrey and Sandalack each testified they delivered one blow to Prince despite evidence from the video and witnesses that contradicts their accounts.

Shaw said false memories happen to about half of the population and that "physical exertion" can further hinder a person's ability to recall an event.

Officers say Prince resisted arrest

All three officers testified they believed Prince was resisting arrest when they struck him. Othen and Humfrey acknowledged their accounts, which were written as official police notes after the arrest, differed from the video.

Othen testified earlier in the week that his stress and anxiety at the time of the arrest were "at an all-time high."

"Serious mistakes can be woven into important emotional life events," said Shaw.

Clayton prince injuries

Photos show some of the injuries Clayton Prince sustained when he was arrested by Calgary police officers who now face criminal charges. (Clayton Prince)

Prosecutor Jim Stewart suggested to Othen he'd "cooked up a story" about Prince resisting arrest and then tailored his notes to reflect that story because the officers knew the Alberta Serious Incident Response Team would be looking into the violent arrest. 

Shaw spoke hypothetically about how the officers may have been impacted by the situation.

"Not all details may have been reliably recorded by these individuals at the time and that can lead to filling in the blanks and creating a false memory, which might feel as part of original memory," Shaw said.

Closing arguments Friday

During cross-examination, Stewart focused on the fact that the officers' notes were made less than two hours after the arrest.

"There is no scientific study you can point to where someone has intended to implant a rich false memory into subjects an hour and 45 minutes ago, correct?" 

Shaw agreed.

The psychologist's evidence was held in a voir dire — a hearing to determine the admissibility of the evidence. Provincial court Judge Margaret Keelaghan will rule Thursday on whether the evidence can be admitted.

The judge will hear closing arguments on Friday.