The key to the phenomenon, says Shapiro, is understanding how the human visual system works. One of its two components, the fovea, or central visual area, can track motion very well. The region around the fovea provides our peripheral vision; it can detect only motion and can't track it very well. "We often confuse different signals in peripheral vision," adds neuroscientist and co-author Zhong-Lin Lu of the University of Southern California (USC) in Los Angeles. "For example," he says, "if we see a moving car with our peripheral vision, we may confuse [its movement] with any movement in its immediate background."
Regarding baseballs, the problem is that the fovea can focus on only a very small area—only about 2 degrees of the visual field (or an area smaller than your thumbnail held at arm's length)—so as a pitched ball moves closer it can easily slip into your peripheral vision as it becomes larger. When that happens, Shapiro explains, the movement and spin of the ball combine in the hitter's mind and create the perception that the ball is veering off track. Hence, curveballs seem to curve more, fastballs seem to break, and the best hitters in baseball succeed in getting a hit only about three times in every 10 at bats.
The work is available online, and you can even try the visual test done in the study. TRY IT! It is astounding!
Relying on our eyes alone as accurate description of something is not sufficient. It is another reason why anecdotal evidence can't be trusted as valid.