There is no perception without action

"Try the following simple experiment: Close or cover one eye. Then look up and down. You might notice that the scene appears to move down when you move your eyes up and vice versa. Yet, your sense of the world is that it is fixed and stable.

Now, take your index finger and push it gently against the eyeball of your open eye. You may notice that the whole world appears to move. Indeed, you might find this slightly nauseating.

In both situations, images moved across your retina, yet your perceptual experience was very different. This difference intrigued some of the greatest psychophysicists of the last century, such as von Holst, Mittelstaedt, and von Helmholtz. One explanation involves a concept called efference copy or corollary discharge. Prior to activating your eye muscles and thus moving your eyes, a command was given by motor areas in your brain that specified the eye movements. This command was sent not just to the eye muscles but to other regions of your brain that then compared the command to the actual consequences. If the command matched the sensory changes, that is, images moved across the retina to the extent predicted by the eye movements, then you saw the world as stable. When you pushed on your eyeball, however, no commands were sent to the eye muscles to move the eyes, so you interpreted the changing images on your retina as an indication that the whole world was moving. Thus, corollary discharge or a copy of the motor command greatly influences your interpretation of what you see."

For the full article by Susan Barry go to: http://www.psychologytoday.com/blog/eyes-the-brain/201104/there-is-no-perception-without-action