The College of Optometrists

I was sent this link to view an illusion. Bit of fun!

http://www.patmedia.net/marklevinson/cool/cool_illusion.html

Amazing !

Excellent!

Come on then who can explain that in simple terms? Is it someting to do with after images??

Ok here is my attempt at an explanation although I think there maybe more to it:

I think this related to the on/off receptive field pathway, receptive field size, temporal frequency sensitivity and lateral inhibition the s-cone (blue) pathway.

It is known that the s-cone pathway has an "on" signal that is mediated by the bistraisfied (s-cone) ganglion cell. There is no evidence for an "off" signal and it is thought that the L-cone and M-cone pathways are responsible for this signal. The S-cone pathway has a fairly slow sustained response and so a low temporal resolution (ie it can't see flicker above 30Hz). I suspect the background colour, luminance and frequency of the pattern is such that the S-cone pathway does not perceive a change in colour or luminance and so there is no on signal but there will be a large luminance change for the M-cone pathway and so there will be a large "off" signal giving the greenish dot.

When you stare at the centre the purple dots disappear. This is in part due to the lower temporal resolution of the S-cone pathway in that it isn't sufficient to detect the spot disappearing, and that the receptive field size of the S-cone ganglion cell is relatively large (as there aren't many s-cone photoreceptors in the retina etc) so that the small saccades that your eye make will not move the target out of the receptive field. I suspect there is some lateral inhibition occurring that causes the s-can pathway response to be ignored until the target (spot) moves.

Any other ideas?

Yes... here goes............it's an illusion! lol. Jonesal2 you need to go out more!

what jonesal2 said.

I have nothing to add ;D

Try this link,

http://www.michaelbach.de/ot/col_lilacChaser/index.html

Although I like jonesal2's explanation, it can be explained more simply - see link. In summary, the green dot is an after-image, which occurs due to colour opponeny. Proof that its not only due to the absence of the off signal of the S-cones is that it can be observed for blue dots where the illusiory colour is yellow (try it at teh above link).

The rest is explained by the Gesalt effect and Troxler phenomenon.

Incidently, I just wasted almost a whole afternoon at the above site - fantastic.

Ok nearly there! I can vouch for Mikes time wasting!!!