The Hillsborough retrial jury has been told to ignore the lack of emotion shown by accused David Duckenfield as he suffers from post traumatic stress disorder (PTSD).
A judge gave the direction before Dolores Steel testified about the death of her 15-year-old son Philip.
Mr Duckenfield, 75, was match commander at the 1989 Liverpool and Nottingham Forest FA Cup Semi-Final.
He denies the gross negligence manslaughter of 95 fans.
Judge Sir Peter Openshaw told the panel of eight women and four men at Preston Crown Court Ms Steel's evidence was "likely to have an emotional impact and some of you will find it stressful.
"My prediction is Mr Duckenfield will show no emotion because he has shown no reaction throughout the trial. You may have noticed that and you may think it odd."
He added expert medical opinion recognised that because he suffers PTSD "his appearance and demeanour in court should not be taken as an indication of his state of mind".
"He has a resilient, passive and expressionless external presentation which gives no indication of his state of mind so don't draw an adverse inference against him," the jury was told.
Mrs Steele said she was in the stadium's West Stand with her husband Les while Philip and his brother Brian, 13, were on the Leppings Lane terraces below on 15 April 1989.
She said knew something was "wrong" and later saw fans being carried onto the pitch and an ambulance arriving.
"To my mind, that pitch just became a battlefield," she said.
The couple found Brian on the concourse of the ground but did not find Philip until they identified his body in the stadium's gym at about 21:30 BST.
Giving evidence, Liverpool fan Ian McDermott told the court how he struggled to breathe and lost consciousness when he was caught in the crush in pen three of the terraces.
"When I knew I was going, obviously that was extremely terrifying. I can still visualise that today, that's never left," he said.
"I've been told by doctors it was pre-death terror."
The trial continues.