Edward Alan Rowen, 84, appeared via video link from custody at the Supreme Court of Victoria on Monday for the investigation into his fitness to stand trial.

Rowen is charged with one count of murder over the death of his 79-year-old wife, Rosalie Rowen, in Creswick.

Police found Rowen's wife with life threatening injuries at a home on Melbourne Road after 10pm on December 2,5 2019.

She was taken to hospital where she died the following morning.

Two psychologists told the court on Monday Rowen had a mental impairment, likely to be Alzheimer's.

Clinical psychologist Christopher Drake assessed Rowen's fitness to stand trial in September 2020 and assessed him again on March 12.

Giving evidence to the court, he said Rowen's account of the alleged offence was 'incoherent' and his explanations of what happened were 'nonsensical' and 'changed over time'.

Dr Drake said Rowen told him their car was 'diseased', the 'car blew up when she was in it' and another man allegedly killed his wife.

He said Rowen would be unable to understand trial proceedings and would be unable to communicate with his lawyer.

Dr Drake said Rowen's presentation when they met last week was worse than during the assessment in September, showing increasingly disordered thinking.

"He is unfit on at least five or six (fitness to stand trial) criteria. There is no prospect he would become fit in the next 12 months," he said.

"If he does have Alzheimer's... The likely course is one of further deterioration."

Clinical neuropsychologist Martin Jackson assessed Rowen in April 2020 and said it was clear from the beginning he was 'profoundly impaired'.

He said Rowen received the lowest score possible on a number of cognitive tests.

"He denies killing his wife, but when asked about the plea he was saying 'other people said I killed her'," Dr Jackson said.

"He was saying his wife died naturally. I couldn't get from him he understood he was charged with murder but he did understand other people said he had killed his wife."

Dr Jackson said Rowen did not seem to understand the concept of guilty and not guilty.

"He kept going back to the question of when he will get out of here and said all he wants is a small plot of land and a vegetable garden and he will be happy," he said.

"He has a profound cognitive impairment. There would be no possibility of him knowing what is going on in a trial."

Both the prosecution and defence submitted Rowen was unfit for trial.

"It is a very clear cut case," defence barrister Tim Marsh said.

Justice Lesley Taylor said she found Rowen not fit to stand trial as he failed to meet at least five of the criteria.

The case will proceed as a judge alone special hearing on Tuesday.

"The rate of progression of Mr Rowen's illness is something of concern," Justice Taylor said.

The prosecution will call seven witnesses to give evidence, including Rowen's daughter, two neighbours and three police who attended the scene and a psychologist.

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