When you move your head or shift your gaze, the image on your retina continually changes, but this is not perceived as a change in the objects around you. You continually compensate for your own eye movements, so when something does move in the environment, you register it. This requires that you keep track of your own decisions to act. With an efference copy mechanism, as you decide to act, sending a “command” of some sort to your muscles, you also send a faint image of the same command (a “copy” of it, in a rough sense of that term) to the part of the brain that deals with visual input. This
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