A court-appointed neuropsychologist and clinical psychologist found the most likely form of Rowen's brain degeneration was permanent and deteriorating dementia, which Justice Taylor said was "plainly correct".
The married couple went to a family Christmas dinner where one of the grandchildren had made "strong" vodka jelly shots.
Rowen ate about three of the vodka jelly shots despite his family's entreaties to stop, telling them he didn't believe them when they said they contained alcohol.
The couple were dropped at their home in Creswick near Ballarat by a grandson about 7.45pm.
He later told police he went into a "rage" after his wife disagreed with him while he was drunk on the jelly shots.
"At some time between 8.20pm and 9.45pm the accused attacked the deceased from behind with a solid wooden elephant statue," Justice Taylor said.
"He struck multiple blows to the head.
"At the time the deceased was seated in her 'usual' chair in the lounge room."
Rosalie Rowen had more than 20 lacerations to the face and head after the savage beating, an autopsy later found.
After the murder Rowen went into the street and tried to flag down passing cars for help, hugging a neighbour and begging her for assistance.
Police were called and Rowen told them when they arrived: "'I've killed her, you can take me in, I'll do 20 years."
He had blood on his hands and clothes.
His daughter had told the court Rowen, who had been drinking beer, refused to believe his relatives when they told him the jelly had strong alcohol in it.
"Her father had a sweet tooth and was motivated by desserts," Justice Taylor said.
"She said that she believed that her father did not understand that the jelly was not merely dessert, but also had alcohol in it.
"She did not think that her father would have been familiar with the concept of a jelly shot."
Rowen will be committed to be supervised in a facility such as a psychogeriatric unit that specialises in caring for people with dementia, with Justice Taylor to decide at a date to be fixed.
