Have fun with strabismus!

Perception

For those members of the general public who derive some strange sense of pleasure out of viewing the meaningless patterns in Magic Eye books, it must be hard to understand there are people who cannot, never, no way, experience the illusion of depth from these Magic Pictures. Now be a sport and don't harass your cross-eyed nice, neighbour or colleague with "You just need more practice, and by the way, I don't understand you don't see anything in them - I need to cross my eyes to see it, and yours are already crossed!" There are lots of random dot stereograms and other Magic Pictures on the Web already, so it's high time somebody sets the record straight with a at least one example of how the world looks like according to somebody with a somewhat screwed up binocular system.
There are other visual illusions and Fun Things In Vision to enjoy monocular people, too!

First, a standard view	Then a "diplopia version"

Search the differences! There aren't all that many. Some objects or pieces of objects that contrast highly to their surroundings are duplicated, sometimes things appear slighlty undefinable. These things are easily ignored. If there is a problem at all with perceiving things like this, it arises from ignoring too much of the visual input, like glass doors, bikes parked on the sidewalk, and not noticing a friend (or worse, your boss) waving to you from across the street.

I bet you expected this spectacular psychedelic show instead, though!

But this kind of overall diplopia is reserved for those with a strabismus of recent onset; when no compensatory mechanisms (amblyopia, suppression..) have developed.

It gets even more interesting (when your browser supports displaying "animated .GIF's") and when suppression fails to do a nice full-time job of avoiding diplopia (see the page about amblyopia, diplopia and confusion):

Here you are warned for a bump in the road.. or maybe even two. (Click on image to see a larger version)

Does programming computers do weird things to your mind.. or are these merely a blinking text cursor and bright white mouse pointer, contrasting with the black background?

Something equally intriguing is suddenly becoming aware of a wandering eye, because something doubles up, and then "remembering to fuse". This can happen, as far as I know, with some types of "latent" squint when you're tired (or drunk) - in which last case it is really sensible to not drive your car but to take a train:

Note: This image does not display well in some versions of Netscape.

There is almost nothing written about the differences in perception between "normal" people and those with (congenital) strabismus. (If you know of any literature references about this, please e-mail me now!) So.. Are there any differences at all? It has been established strabismus induces a different development of Visual Area 1 (a drastically reduced number of binocularly driven cells), and as V1 provides input for all other visual areas, it makes me wonder what the neurological differences are further along the path, and what the impact of these changes would be. Is the obvious difference (absence of stereopsis) the only one? What about motion perception? "Difficulty judging the speed of oncoming cars" is the one reference I've found. As mentioned in the "Disclaimer" part, much emphasis has been placed on ignoring diplopia. But how is it done? How to selectively ignore a piece of abberant input without ignoring all the rest in its vicinity? My theory (yellow alert, amateur scientist decloaking) is this takes place in the areas doing the "higher" level processing of visual input (object recognition) and requires a more or less conscious guidance - when distracted, this "filtering process" suddenly becomes a whole lot less effective. But even at moments when one doesn't notice any doubled up things, closing one eye makes it all "easier". There are two possible causes for this: the absence of peripheral depth perception is perceived as "easier", or, now the "filtering process" isn't needed anymore, one has a whole lot of object recognition brain power left to do more challenging things like perceiving the relations between different objects (the "bigger picture"). BTW the very limited amount of "data" on which I base this, is derived from personal communications with just a small population of strabismics.