The Pulfrich Effect: stereopsis without stereopsis?
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.
(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
"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
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
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
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
Squinters get a bad rap in rural folklore
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
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
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?
Depth perception - it doesn't have to be a 100%
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.
The power of exotropia: an expanded horizon
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.
The crowding phenomenon and a (Windows) desktop for nystagmus
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.
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
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