Blindsight is a condition superficially resembling blindness. In blindsight, people with damage to the visual cortex have residual visual sensitivity in a subjectively blind part of the visual field.
The difference is that a person experiencing blindsight, although not consciously able to see, is still influenced by 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.
Another manifestation of blindsight occurs when an affected individual is shown a dot on a screen and asked whether it was on the left or right side. The subject may be unable to answer; but if asked to guess, will guess with an accuracy significantly above chance level.
Blindsight may also be experienced by fully sighted people when attention is elsewhere or the image happens too fast for them to become aware of it. Many believe that this effect can be exploited by advertisers in the form of subliminal messaging, although its effectiveness is highly questionable.
A person experiencing blindsight can be said to be lacking visual qualia.
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