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optical illusions

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sudu | 22:01 Fri 25th Nov 2005 | Science
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take a look at, this is so clever and slightly weird but how does it work?



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The retinal cells at the back of your eye become fatigued, and put your brain under the illusion that they are seeing something because they are not tuned ever to be staring at a high contrast object for so long, and the ganglion behind the retina output a signal as if an object was present.

Well, I think this is how they work, and please correct me if i'm wrong, but you know how if you look into a light for a few minutes then looks away you see different shapes and spots, then they fade?

I think the people who have created this have done the opposite and shown you the outcome of what it would look like first and then you brain comes up with the original.

Sorry if it's confusing, it's just how i imagined this worked.

It's not particularly special - if you look at the same thing for a while then your eyes get used to seeing what they're looking at. So the part of your eye which has been looking at a black area gets used to it, and as soon as you close your eyes, the back of the eye compensates for what it is used to and becomes over-sensitive to white instead. (And the same thing the other way round for looking at white areas). It doesn't have to be black and white - if it were a yellow face on a red background, then your eyes would "see" a blue face on a green background afterwards. Also, you don't have to close your eyes to make it work. It would work almost as well, as long as you look at a blank surface (e.g. an empty sheet of white paper).

A more extreme example of this phenomenon is if you accidentally catch a glimpse of the sun - for a few seconds afterwards the same part of your eye will be "blind" and will only see a black spot while it recovers. Obviously you should never look directly at the sun deliberately.

P.S. You will also notice that the white background of the screen will appear black, and the black edge of the screen at the edge will be lighter.

To be honest, that's not strictly correct. Colour and contast opponency are present in early stages of visual processing (at the eyeball and where it meets the optic nerve (retina and ganglia), however, it is not in fact a delay in normalization of this opponency that causes the after effect. It can best be explained as follows:

-the visual system never normally has to view high contrast images, statically, for a long period. It is not tuned to do this.

-when this does occur, however, the retinal rods in this case (or cones for colour, since this can be a colour after effect too) become fatigued and the summed output of the local groups of retinal cells becomes inverted...what should say 'black' now says white, etc.

It can take about 10 minutes for these neurons to overcome this fatigue and begin discharging properly anc communicating the right signal with the other cells in the ganglion.

ps...the effect will be slightly stronger after one glass of wine (and actually your eyesight will improve also). Your vision works by movement...your eyes are constantly trembling slightly, but they actually tremble just a bit more than is necessary, reducing visual acuity. A glass of wine/beer brings this trembling down to just the right level. This more static state will also cause faster fatigue of specific retinal cells, and thus a greater illusion effect.
Thank you, SteveD! Can you recommend anything for , the headache and nausea please?

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