Playing computer games may increase the risk of dementia and schizophrenia, a study found.
Action games with on-screen maps to aid navigation did most harm.
They caused the most damage to the hippocampus — the part of the brain responsible for orientation and memory.
People who played most had a more depleted hippocampus and a higher risk of developing brain illnesses or diseases.
Researchers scanned the brains of 97 volunteers before and after 90 hours of gaming.
Many of those who played action games, such as Call of Duty or Killzone, saw their hippocampus shrink.
But those who played 3D platform games, such as Super Mario, boosted their brain power.
The Canadian researchers say games makers should consider removing maps from the screen as a result.
Professor Greg West, of the University of Montreal, said: “Video games have been shown to benefit certain cognitive systems in the brain.
“But there is also evidence that there might be a cost to that, in terms of the impact on the hippocampus.
“We saw less grey matter in the hippocampus of habitual players.
“We then followed that up with two studies to establish causality, and we found that it was indeed the gaming that led to changes in the brain.”