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Date:         Wed, 19 Nov 1997 22:28:37 +0000
Reply-To:     "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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Sender:       "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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From:         Aaron Sloman <[log in to unmask]>
Subject:      Re: Illusions

Date: Tue, 18 Nov 1997 09:11:10 -0800 From: Stanley Klein <[log in to unmask]> > The disappearing face illusion is one of many examples of binocular rivalry. > One easy demonstration of it is to roll a sheet of paper into a tube, hold it > with your left hand and look through it with your left eye. Simultaneously > hold your right hand in front of your right eye. You will see a dramatic hole > in your hand. Depends on what you are looking at. From my simple experiments if the tube is aimed at something very bright (e.g. a sunny view outside) then the "hole in the hand" effect is very strong. If the tube is aimed at a very much duller background there may be no hole: the hand looks normal. In between various kinds of fusion or superimposition can occur: the hand is visible but partly transparent: e.g. as you move it up and down you can see BOTH the moving texture on the skin and the stationary background. A kind of fusion. But different from the integrative fusion (coherence) of normal stereo vision which creates a new unified information structure containing 3-D information. (How many theories of vision can explain how we see both motion and non-motion simultaneously in the same part of the visual field? Compare looking through a dirty window at trees swaying in the breeze, or through a moving transparent but dirty or textured sheet of glass at static objects. You can also have both moving in different directions at once.) > Binocular rivalry is especially effective if one image is > blurred (like the close hand in the tube example) or is moving (like the face > disappearing because of the dominant moving hand in the earlier example). In the hand and tube case the motion can sometimes UNDO the binocular rivalry and cause superimposition. > The neural mechanisms of binocular rivalry are especially interesting for > the study of the neural correlates of consciousness (NCC) because binocular > rivalry also occurs when the two eyes view two different static, in-focus > scenes. In that case the eye correlated with the perception switches back > and forth. Not always. A common counter example is seeing two fingers and the background at the same time when you hold up a single finger some distance from your eyes and look at the background (especially if you move the finger against a bright background??) Quite a lot of people seem to think that the ONLY possibilities are complete coherent fusion or binocular rivalry. E.g. Baars claims this unequivocally on page 89 of "In the Theater of Consciousness" (OUP 1996) This reminds me of the frequently made claim in text books that if you look at the famous face/vases ambiguous figure (Rubins?) you can see it either as two faces with the middle bit as background or as one vase with background on either side, but never as faces and vase simultaneously, as if this were a fact to be explained by brain science. But it's NOT a fact: many people can easily come to see both face and vase at once simply by interpreting the picture as representing a vase of the right shape wedged between two faces. It takes nothing more than the thought of the possibility to make it happen. (Except for people who have learnt that it is impossible...) Moral: beware of alleged facts about how the brain works -- for many can be refuted by considering different cultures, a new experimental situation, or simply doing a bit of creative thinking. E.g. some alleged facts about memory limits are refuted by learning simple tricks for remembering things. (Compare the alleged fact that mirrors reflect things with left and right sides swapped but never top and bottom swapped: often quoted by eminent people, who then propose elaborate explanatory theories. But it's not a fact. Put a mirror on the floor and look down into it. ) (The tendency of scientists to make claims that are not false, but lacking in any clear meaning, is another thing to be avoided: like the claim that a neural event precedes consciousness of something by a few hundred milliseconds. Compare: it's noon on the moon exactly 3 hours after it's noon in New York. Or Clinton's popularity first exceeded that of Bush exactly 0.005 microseconds after a certain sub-atomic event occurred in a cloud chamber in this lab.) > Many neurons in visual cortex don't switch and many do. The ones > that do switch in synch with perception are candidates for belonging to the > NCC. The vast number of neurons that don't switch are not part of the visual > NCC. Er... does that mean that when I see inconsistent left and right views superimposed something has gone wrong with my brain? Maybe I am just a mutant. Cheers Aaron ===== Aaron Sloman, ( ) School of Computer Science, The University of Birmingham, B15 2TT, UK EMAIL [log in to unmask] Phone: +44-121-414-4775 (Sec 3711) Fax: +44-121-414-4281

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