Have fun with strabismus!

Special Effects

The Pulfrich Effect occurs when you put a "neutral density (grey) filter" in front of one eye and watch a swinging pendulum - it will appear to describe an elliptical path, giving a sort of "3D" illusion, or described more accurately: "A pendulum swinging in a plane perpendicular to the direction of observation appears to follow an elliptical path when viewed binocularly with a filter in front of one eye. (Pulfrich illusion)"

(In: "Psychophysical determination of visual processing time by comparing depth seen in Pulfrich and Mach-Dvorak illusions.", by Mojon D; Zhang W; Oetliker M; Oetliker H - Am J Physiol, 267: 6 Pt 3, 1994 Dec, S54-64), and explained as:

"A significant proportion of the processing delays within the visual system are luminance dependent. Thus placing an attenuating filter over one eye causes a temporal delay between the eyes and thus an illusion of motion in depth for objects moving in the fronto-parallel plane, known as the Pulfrich effect" (In: "Retinal adaptation of visual processing time delays", by Wolpert DM; Miall RC; Cumming B; Boniface SJ - Vision Res., 33: 10, 1993 Jul, 1421-30).

"Using a device based on the Pulfrich effect, we examined 70 patients with various strabismus problems and 20 normal subjects for gross stereopsis. We found an excellent correlation between this dynamic stereopsis test and random dot stereograms. Additionally, in several instances the Pulfrich device was easier to use and interpret correctly than random dot stereograms.

The Pulfrich effect can be elicited reliably in patients as young as 3 1/2 years of age. Three patients with anisometropic amblyopia and stereopsis were found to possess a spontaneous Pulfrich effect." (In: "The Pulfrich effect in anisometropic amblyopia and strabismus." by Tredici TD; von Noorden GK - Am J Ophthalmol, 98: 4, 1984 Oct 15, 499-503) Unfortunately the usefulness of experiencing 3D through a grey filter seems limited to viewing a swinging pendulum, but there may be other moving objects (and some pictures) giving interesting illusions when viewed through a filter.

Some people notice a spontaneous Pulfrich effect from their amblyopic eye - especially when there is some diplopia, the second image (for instance a blinking white cursor on a black screen) appears to blink at different moments in time - its image is perceived slightly later in time. If this is due to the decreased quality or blurriness of the amblyopic eye, is not clear.

Now, what's the story on the above three images? When you click on them, a large version appears (use the "back" button on your browser to return to this page after viewing). If you view an image while holding one lens of a pair of sunglasses in front of one eye, a sensation of depth perception seems to be experienced.

This 'trick' is used in some video games. The images have been processed such that the contrast is exaggerated, and they should contain lots of other depth clues (see the 3D pages). The highlights and shadows provide the major part of the illusionary 3D experience in these images.
Note: If you have a severely lazy eye and/or large angle strabismus the trick will not work for you, as some, but not necessarily high-grade binocular vision is needed.
Second note: Translation of the text on the sign in the first image: "Save Nature". And I have nothing whatsoever to do with PTT Telecom or the feat of architecture displayed in image 2.

Someone sent me the following tip: Watching football (or soccer, for that matter) on TV using a 'special purpose' pair of glasses with a neutral gray filter in one of the lenses, gives a vivid 3D illusion. A pair of sunglasses with one lens removed works too.

View from a train: Asymmetric OKN

OKN is the abbreviation of OptoKinetic Nystagmus, sometimes nicknamed "railway nystagmus". People with plain boring normally developed binocular systems, notice no difference between the two eyes when they view the world from a moving train - but you will. Provided you got the infantile type of strabismus, that is, got it before you were 6 months old. Board the train. Take a window seat of the "riding forward" variety with the window on your right side. Close your right eye and focus on things not too far in the distance, like a fence along the railway or trees bordering it. Nothing special. Now close your left eye and don't forget to open the right one. It's like the train is going much faster now! (Warning: don't do this from a moving car if you're driving yourself!)

Nystagmus is a mechanism to keep track of moving objects. The eyes follow the object(s) you want to see, but obvisously have to go back to the starting position every time, do this very quickly, and then start following again. Electrically, the signals accomplishing this, look like saw-tooth waveforms. Follow-rapid return-follow.. etc. The difference between normals and people with infantile strabismus is that the oculomotor system of strabismics hasn't implemented the OKN mechanisms fully. The left eye is much better at following things going from left to right, and the right eye favors the other direction.

The implementation is asymmetrical. If you look with one eye, things look normal in one direction, and appear "smeared" in the other, giving a false impression of greater speed. This is why it can be more difficult to estimate speed when you are looking from extreme directions of gaze, i.e. when looking over your shoulder, and you're actually using only one eye for a moment. When you turn your head more, and the other eye kicks in, speed seems to increase or decrease. Be careful out there, especially when operating a moving vehicle.

Nasal-to-temporal moving scene
Temporal-to-nasal moving scene

First of all there's still a lot of stupid ideas circulating out there, and not just in rural areas. Having deviating eyes really sets you apart in todays perfectionist society where people get liposuctions, silicone boobs and facelifts to confirm to the standard look - and strabimus is mostly considered as a handicap that makes you look dumb. Esotropia is often considered as being "funny". See also the page about strabismus in the rich & famous.

"The psychologic effects of strabismus on the patient, and, in the case of a child, on the parents should not be underestimated. Superstition and folklore that label a squinter as being "shifty-eyed", "evil-eyed" and "not to be trusted or apt to lie" are still strong in our alledgedly enlightened world, especially in rural areas." (From: book reference von Noorden )

It is contended people derive other's emotions mostly from the left side of their faces. Has got something to do with the asymmetry between the two brain hemispheres (see for example: book reference Left brain, right brain). And as folklore states, eyes are the Windows to the Soul, so one could infer from this the left eye is the most looked-at feature of the face. Note: there are no studies confirming this! In general, people tend to get confused by face-to-face conversation with someone who's eyes are looking in two different directions. Avoiding straight-opposite seating arrangements helps. The 90 degree angle is the best for establishing a "cooperative atmosphere" anyway.

For some reason bright sunlight makes those with "wall-eyes" squint, and dimly lit surroundings make those with "crossed eyes" squint. Exotropes sometimes present with dark sunglasses on overcast days. So - do you prefer sitting in a dimly lit room behind your PC, or playing beach volleyball in the sun, and does this teach you something about your personality or type of strabismus?

Game software for VR helmets exaggerates the depth effect a great deal, because the player has got to get this really convincing feeling (s)he's totally immersed into the game. It's great for experiencing a sense of 3D you thought you never had! Your mileage may vary, though.

The reason a VR helmet can give you 3D illusions while you thought you didn't have any depth perception (catching a ball, threading a needle - impossible tasks) is that, mostly, not all depth perception is absent - it may be very much absent in the center of vision, but working OK in the periphery.

An anecdote about "wall-eyes" (from: book reference von Noorden ) - I've translated the medical gobbledygook a bit:
"A 46-year-old mailman servicing a rural mail route came for surgical correction of an exotropia that had been present since childhood. He was concerned about his appearance, but had no visual complaints. His uncorrected visual acuity was normal, and he had a constant very large exotropia (outward deviation) at near and distance fixation. He strongly preferred his left eye for fixation. [..] After surgical alignment the patient regained peripheral fusion without stereopsis.

However, he was most displeased with the result. Before surgery he had been able to keep his left eye on the road when driving his truck while scanning the mailboxes with his right eye. After surgery he found his field of vision substantially decreased, and it took several months of adjustment before he was able to resume his profession."

The moral of the story is exotropes can have panoramic vision - some are said to have "eyes in the back of their head" because of their uncanny ability to detect things going on almost behind their backs. In certain professions this can come in handy; the mailman is an example, teachers who still keep an eye on the class while writing on the blackboard, and everyone attending very boring meetings can keep an eye on the clock on the wall while still politely looking at the chairman.

There are these wallcharts for eye tests containing "tumbling E's". They are mostly used for testing vision in children. Somewhere along the line an expert decided the Tumbling E's chart could be improved by surrounding the E's by "crowding bars" like these:

A normal "E"	A "crowded E"

The crowded E test is much more difficult than the normal E test - imagine a wall chart full of tumbling E's with these bars around them. If you have nystagmus, like many strabismics do, you're likely to perform much worse on the crowded E test. For most people who have this kind of (latent) nystagmus it is only a problem when one eye is closed - they won't notice it in daily life. For some, a manifest component is always there.

And for them, the graphical user interface can be one big pain in the eyes. A typical desktop in Windows presents a forest of crowding phenomenon in the form of window borders, colored bars, icons and drop-down menus, all of which used to be mostly absent in the text-only screen era. Even if you don't have any nystagmus but (intermittent) diplopia and/or so-called "asthenopic complaints" (headache, 'tired eyes') these problems can get quite a bit worse if your desktop presents you with a cacophony of colors and lots of crowding stuff.

Two simple adjustments make the screen a whole lot easier to look at: use color only when it is useful, keep the rest of the color scheme in grey tones, avoid a lot of contrast, and use small fonts. Eh what?! Yup - small fonts surrounded by enough blank space are much easier to read than a large font when the lines are almost running into eachother.