I decided to turn to Dr Brock in search of an answer to my main question about attainment of stereo vision for an amblyope. (Thank you Sue Barry for making his lecture notes available.) These lecture notes are not easy reading, but I feel I have gained some interesting insights into my condition.
Unfortunately, Dr Brock's definition of amblyopia leaves the amblyope with little hope of attaining good stereovision. On page 35 under the heading 'Ambyopia-A Permanent Visual Disability', he defines ambyopia as 'the result of destruction (or congenital lack) of foveal nerve bundles in one or both eyes'. He does advocate 'Visual Training' to correct the dominancy of the better eye that is often seen in the amblyope, believing that in a binocular posture, the eyes will be aligned to give the 'stereoscopic clues that are most effective' under the existing condition.The individual is then capable of the most sufficient response which are possible to a partially disabled organism.' He repeats this disappointing prognosis on page 42: 'When it is found that adequate posture is maintained in amblyopia visual training can accomplish little to heighten the quality of the existing binocular vision response.'
This is in contrast to someone who has been suppressing as an adaptation to strabismus. On page 28 he states 'Foveal suppressions are almost always due to lack of adequate binocular posture. They usually disappear when adequate binocular posture has been regained', and on the next page he goes on to say '... the establishment of binocular posture must bring about a better ability to fixate directly with the subnormal eye and, perforce, bring about a heightened visual acuity. When a central scotoma is not the cause of low visual acuity, such training always produces increased acuity of that eye.'
In the last paragraph on page 29 he discusses a means of testing whether amblyopia 'is due to destruction of foveal fibers rather than non-use.'
I would be interested in understanding the test procedure better if any reader of this blog can explain what he is saying. Such a test would seem to be able to offer someone like myself a good indication of how much VT can help improve their vision.
It is amazing in vision how a person can live with 'problems' and spend a lifetime visiting eye doctors and still remain unaware of what the problems he/she has. It was in my late 20's after failing the vision test on my first US driving test that I learned I was an amblyope. Now in my 50's, I learn that I probably have something called a central scotoma. It is my understanding from reading Dr Brock's notes that a central scotoma is an area of diminished vision surrounded by normal vision. This seems to be a different phenomena than suppression scotomas that are sometimes discussed in VT blogs. This would seem to be an area of the retina where images are suppressed, but the surrounding areas are not. This is a strabismic adaptation to avoid diplopia.
The reason I believe I have a central scotoma is: when I occlude my right eye and analyze my left eye vision (with my glasses on) on a page of computer text, the left and center are blurred and the right, top, and bottom are clear, but because the clear areas are outside my foveal focus the text is not easy to read. Knowing this helps me to understand the difficulties I have during vision exams when reading with my left eye, particularly when trying to read the letters in the the middle of a line.
Dr Brock mentions scotomas a number of times in his notes:
1) In his discussion of Retinal Correspondence on page 9 he discusses how to deal with a central scotoma when measuring phorias and tropias.
2) On pages 28 & 29 under Note E where he states that 'foveal suppressions are almost always due to lack of binocular posture'., he goes on to say '... the establishment of binocular posture must bring about a better ability to fixate directly with the subnormal eye and, perforce, bring about a heightened visual acuity. When a central scotoma is not the cause of low visual acuity, such training always produces increased acuity of that eye.'
3) On page 37 where he discusses the difference between 'stereoscopic accuracy' and 'stereoscopic perception', he states that 'If a binocular field is lacking in the direct line of gaze, because of a central scotoma, this does not affect the peripheral stereoscopic ability unless the binocular field percept has also been abandoned. Such an individual remains permanently incapable of stereoscopic accuracy even though his peripheral depth clues may be normal.'
4) On page 59 In answer to the question 'Why is strabismus prevalent in certain families and not in others? his first statement is that 'The prevalence of central scotomas in certain families seems to be one of the reasons.'
In my family my three siblings are also amblyopes.
I would be interested to hear from fellow Sovoto members if they know of any more modern research into the topic of 'Central Scotomas', including anything about their diagnosis and about vision therapy procedures that help make the most of the peripheral vision in the affected eye.