MONTREAL -- Luka Rocco Magnotta was suffering from schizophrenia, was mired in a psychosis and could not tell right from wrong when he killed Jun Lin, a forensic psychiatrist testified Thursday.

Marie-Frederique Allard is an expert witness hired by the defence to assess Magnotta's criminal responsiblity at the time of the slaying.

The argument is key to the defence case at Magnotta's first-degree murder trial because although he has admitted to killing and dismembering Lin in 2012, he has pleaded not guilty by way of mental disorder.

Allard told the jury Thursday she consulted police evidence and medical records and saw Magnotta personally for about 25 hours over several meetings, beginning in December 2013.

She said she believes Magnotta was suffering from schizophrenia, that his perception of reality was skewed and that while he knew what he was doing, he didn't realize it was wrong.

"That illness was present on May 25, 2012," Allard said. "It's responsible for the five charges against him."

Magnotta, 32, faces four other charges in addition to premeditated murder: criminally harassing Prime Minister Stephen Harper and other members of Parliament; mailing obscene and indecent material; committing an indignity to a body; and publishing obscene materials.

The Crown contends Lin's slaying was planned and deliberate.

According to Allard, Magnotta was "experiencing a loss of reality" on the day Lin was killed.

"He was capable of judging the nature and quality of his actions," Allard said, adding he was not able to rationally apply the notion of right and wrong to those actions.

Allard testified that Magnotta was one of her most difficult cases because, she says, he suffered from schizophrenia while at the same time minimizing his own mental disorder.

Allard said Magnotta's case is interesting from a medical point of view -- his father was schizophrenic and the stigma linked to mental disorder in part fuelled Magnotta to dissociate himself from his own illness.

The jury heard that by August 2001, Magnotta had been diagnosed as a paranoid schizophrenic at age 19.

That diagnosis was found in medical documents from Ross Memorial Hospital in Lindsay, Ont. A psychiatrist who saw him in April 2001 referred to Magnotta as "a bizarre presentation" and couldn't tell if he was faking.

"I am not sure what we are dealing with," Allard quoted the doctor's notes as saying. "I have a feeling he is looking for some secondary gain but what I don't know what exactly he is intending to get."

When he was diagnosed as a paranoid schizophrenic, Magnotta was living at a home for schizophrenics and had overdosed on anti-psychotic pills in a suicide attempt, Allard said.

Magnotta didn't like living there and felt isolated, she said. The risk of suicide is very high among schizophrenics -- in particular those who are coping with the illness at an early stage and are battling depression.

Magnotta's medical history is peppered with moments where he outright denied symptoms or dependency problems.

"There is a negation of the symptoms, of the illness, over the years," Allard said.

The trial resumes Monday when she continues her testimony.

Earlier on Thursday, the jury heard Magnotta fell in love with a male nurse in April 2013, misreading the man complimenting him on his clothing as an opening to a sexual relationship.

Magnotta's treating psychiatrist, Dr. Renee Roy, testified that he penned an explicit letter to the nurse and even dimmed the lights to his cell in an attempt to seduce the unidentified man.

The sexual interest came from Magnotta alone, Roy said.

"The only thing that happened ... the nurse in question was kind and complimented him on his clothing," Roy said, adding there was ultimately no contact between the two.

Magnotta later expressed disappointment at misreading the signs. Roy said he was saddened when the detention centre reassigned the nurse.

Crown prosecutor Louis Bouthillier asked Roy whether it was uncommon for someone with a serious mental health issue to have such romantic attachments.

Roy, who has treated Magnotta since November 2012, replied that it was not, although medications can sometimes have an impact on libido in patients.

The jury also heard that the psychiatrist who initially treated Magnotta in a Quebec detention decided in August 2012 he didn't need constant psychiatric follow-up and discharged him once his medications had been regulated.

Dr. Jacques Talbot saw Magnotta six times between June and November 2012, mainly because he was displaying side-effects from his medication.

Roy said Magnotta, through his lawyer, eventually asked to be seen by a different psychiatrist.

Magnotta expressed concerns about Talbot's lack of English, but he also wanted to get back on Risperidone, an anti-psychotic medication Talbot had phased out.

The medication has secondary effects that are not unlike symptoms associated with Parkinson's disease, Roy said.

She advised Magnotta not to resume taking it when she replaced Talbot.