Blindsight

Blindsight is a condition superficially resembling blindness. The difference is that a person experiencing blindsight, although not consciously able to see, is still aware of objects in his field of vision (see subconscious mind). This can be tested--when told to pick up an object in front of him, the person will hold his hand differently while reaching for a coffee cup and reaching for a tennis ball.

Blindsight may also be experienced by healthy people when attention is elsewhere or the image happens too fast for them to become aware of it. The fact that a person will see it without being aware of having seen it can be exploited by advertisers as a form of subliminal messaging.

A person experiencing blindsight can be said to be lacking visual qualia.

A java applet about blindsight is available on the web here.