Gamasutra reports that a study has been made at the University of Rochester (NY) which concludes that playing Unreal Tournament improves overall visual acuity.

Wikipedia says: Visual acuity (VA) is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain. VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. The VA represents the smallest size that can be reliably identified. VA is the most common clinical measurement of visual function.

One group was allowed to play UT for one hour, while the other one played Tetris for the same duration. The group that played UT had a 20% increase in their VA ratings. The researchers are hopeful that this proves that action FPS games may be used to help people with vision disabilities.
Call your mother and tell her THIS proves she’s wrong.