Catalyst on ABC ran a story on study of illusionists by neuro-scientists. It appears that there is more to illusions than just misdirection - the art of attracting attention to something else while you engage in trickery. What the story "Magic Lab" revealed was how an illusionist can exploit a saccade - that time when, during movement of the eye, sight is deliberately suppressed. In this period (about 20% of the period in which your eyes are moving), you are effectively blind.
Saccades have been known about for at least a century and can be tested by looking in a mirror and moving your attention from one eye to another - you cannot see your eyes moving but others can. During scan of the horizon or the landscape outside a moving car, saccades provide stability in the image.
By observation and practice, illusionists have learnt to move their hands in paths that are less "predictable". Movement in a straight line from one point of interest to another affords the suggestion to the eye of returning to the original point. A less predictable path, such as a curve, does not provide the same affordance (the suggestion to the mind of the next place to look), causing the mind to have to guess where the next point of interest will be, slowing tracking. If the illusonist speaks, appropiate social behaviour suggests the speaker's eyes as the next point of interest.
In this way, saccades, misdirection, "poor tracking" and social expectations of where to look conspire to provide lengthy periods when the eye is either blind or resolving elsewhere - plenty of time for practiced movements to go unnoticed.
Which leads to the question. What can possibly be the evolutionary function of blindness? A clue may lie in animals which do not experience saccades - notably some frogs. They cannot see still objects. Movement, however, triggers a reaction that appears out of proportion to both their demeanor and size. Their prey never see them coming, so fast is their attack.
We can speculate that unnecessary response to stimulus is wasteful. If the frog sees everything, the movement of prey may be less noticeable and thus their attack less effective. So why do we see everything by using saccades, but have this 20% blindness?
Anticipation may be the answer. Humans seem to have the best ability of all species to guess the future. Experiments show that, even at a young age, we can anticipate where something will end up if it is moving. We automatically create a trajectory. It is possible, that once we have this trajectory, we do not need frequent visual updates to know where something is going. The sheer waste of processing images to know where a fast moving object will be could be a basis for simply shutting it off, thus allowing selective updates only.
So, maybe there is a good evolutionary reason for blindness.
