It seems that when a ball is seen in peripheral vision as spinning it appears to be moving in the direction of the spin more than it actually is. Combine this illusory motion with the normal gravitational acceleration and baseball’s famously hard-to-hit curveball is the result.
This effect must also occur in other sports, eg soccer, when a goalkeeper has to cope with spinning freekicks which appear to dip much more than aerodynamics suggests. (This, rather than any drag related phenomena, may be the main reason why professional bowlers in cricket sometimes scratch the ball on one side).
Balls used in sports all seem to have some kind of surface texture which shows up the direction of their spin. Today’s invention is therefore sports contact lenses which blur the periphery just enough to make marks or seams on the ball in their particular event so diffuse as to be just undetectable. Peripheral vision already provides low levels of detail perception.
The keeper or batter can still locate the ball very accurately in space but can’t see its (rotating) internal surface texture and is thus not fooled by the apparent motion illusion.
