It’s not a study, but it’s been hypothesized that radiation and cosmic rays could cause lasting damage of just this sort through particle showers within the brain. Truthfully though there is very little in the way of studies for the simple fact that it’s hard to set one up for astronauts. Given how radically different the environment in space is compared to Earth though, I’m not finding this surprising. I wouldn’t be surprised if it’s just the tip of a very dangerous iceberg too, and it turns out that it takes a lot of time and sacrifice to get us to live and thrive somewhere other than Earth’s surface.