[photo-3d] Re: 3d vision again

["Abram Klooswyk" <abram.klooswyk@xxxxxx>]

Rogers  [ka2hsu] wrote 7 Oct 2000 on his daughter's
stereoviewings problems.

> (...) she has never been able to obtain enough muscular 
>control of the affected eyeball to enable her to bring her 
>eyes to focus simultaneously on an object.

However, as Lynda Nygren and Jim Crowell have indicated
(more or less), fusion is required even before stereopsis 
eventually can arise.
Ophthalmologists and optometrists, and most opticians
(depending on national rules) have tests, not only for
stereopsis, but also for testing fusion.

The classical test for the latter is the Maddox cross. In a 
dark room a small luminous disk is shown, and the subject 
wears glasses with cylindrical rods in it, at the left 
horizontally and at the right vertically orientated, and 
mostly colored red, to prevent prismatic dispersion.
A person with binocular vision, although not necessarily 
having stereopsis, will see a luminous cross (the cylinders 
spread the disk's light  to a bar). But persons with a "lazy 
eye" (amblyopia) typically see only one bar, suppressing the 
other. There are several types of amblyopia.

Rogers said that his daughter has an acuity of 20/20
in both eyes (one after correction). This is not typical
of amblyopia, where the lazy eye tends to have a
lower acuity. But she might have alternating suppression,
using both eyes, but one at the time, so again no fusion
failing on the Maddox test, and no stereopsis.

Another test, which also can be tried at home, is with 
afterimages. In a dark room, let a person look with one eye at 
a vertical neon light bar. Then immediately after with the 
other eye at the light bar, but rotated  horizontally.
Next let him/her close both eyes and report what they
see. With binocular vision again the afterimages will
form a cross.
(Wheatstone did this afterimage test with _stereo_
drawings, an early proof that stereopsis depended
on retinal images, and not on changing of convergence.)

Since Rogers' daughter had the eye muscle condition from 
birth, it seems unlikely that she can acquire stereopsis now.
My references indicate that eye muscle correction
must be before the age of 3 years in these cases. (Jim has 
given the figure of 5-7 years, but I believe that applies 
only for children who have had stereopsis as babies or as 
infants, but developed strabismus at about 3 years of age.)

I'm sorry to disagree with Linda that Rogers' daughter
should "try it out regardless".
There are conditions in which fusion and stereopsis are 
_impossible_, and there are reliable tests to establish 
whether such a condition exists. It is very discouraging to 
spend efforts on an impossible task, and it might even harm 
father-daughter relationship :-)

I rather would advice Rogers to consult a specialist, and when 
indeed stereopsis is impossible, to develop a "wobble" viewer, 
or practice wobble projection with his many stereo slides. 
(The wobble issue has been discussed on the list a few 
months ago.)

Abram Klooswyk