Wednesday, February 22, 2012

Rivalry and Diplopia

A simulation of binocular rivalry and fusion with eye movements:

First, the input:

If you can cross-fuse, you want to fuse that white rectangle (and the matched noise background). It's hard to do, especially since there will be a strong urge to fuse the face, not the background. If you succeed, the girl's face will be diplopic (seen double). The video below is a simulation of what is happening in the parts of the visual field where the face is seen.

The photo of the girl is represented at two different ('disparate') locations for the two 'eyes' (just different filter streams in the simulation), while both eyes see the same background (noise with a little white block below the photos). At locations where the two eyes get different inputs (i.e. wherever the photo is seen), the two streams suppress one another and 'binocular rivalry' is induced. This rivalry is unstable, and results in periodic fluctuations where either one or the other eye's image is seen, but not both.

On the other hand, when both eyes get the same input, there is no suppression between streams (this isn't physiologically accurate, just convenient in this simulation). This results in 'fusion' of the two eyes images.

Every second, the filter streams - the eyes - shift to new, random coordinates (they are yoked together of course). You can see that by the shifts in position of the little black dot, which starts out near the white block.

(Both of these videos look a lot better if magnified, i.e. hit that little box in the lower-right corner and look at them full-screen.)

video 

To make a little clearer what's happening, here's a color-coded version:

video 
Here, locations where one eye's image gets through to be 'seen' are colored red or green (depending on which eye - geometrically it only makes sense that green-is-left and red-is-right, which would mean that the photo is between the viewer and the gray background), while regions where there is fusion are colored yellow or brown. The stream marker is now a blue dot (not really; the googlevideo encoder seems to favor dumping small blue dots against red/green backgrounds, go figure)!

Look at that mess of imbalanced fusion that builds up all over the scene. What a mess!

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