Practice, practice, practice. The umbrella term for this is "perceptual learning" and it turns out that training can tweak the visual system: baseball players

Practice, practice, practice. The umbrella term for this is "perceptual learning" and it turns out that training can tweak the visual system: baseball players learn to read where an incoming pitch will go, radiologists can find subtle clues that indicate a tumor, and--as someone pointed out above--chicken sexers can tell the sex of a baby chicken somehow. These are fairly "high-level" phenomenon. Surprisingly, training also works on low-level visual phenomena, which you might think are limited by the eye itself or 'hard-wired' neural circuits. It's mostly just practice. One of the classic experiments looks at vernier (hyper-)acuity. You're shown pairs of lines that are slightly offset, like: | or | | | You report whether the top is shifted rightwards or leftwards. The computer gives you feedback, and shows you another pair (varying the distance to make it easier or harder). If you keep doing this, your threshold (i.e., the smallest offset you can reliably report) will about decrease 5-fold. The same thing happens for many other phenomena too--reporting the direction that some dots are moving, the tilt of a line, etc. In some cases, the improvements continue for days or even weeks of training. The wild thing is that they're often very specific to the training. For example, if you trained with the vertical stimuli above, the improvement does not transfer over to stimuli like: _ or _ _ _ There are "tricks" to designing a curriculum that generalizes and is relatively efficient, but there's still a lot to learn.