11- Abnormal Visual Processing Flashcards
Which abnormal visual processing disorder do these characteristics fall under?
“Lazy eye”
Poor (‘blunt’) vision in one eye
Difference in visual acuity between the two eyes
Developmental disorder that affects 2-4% of the population
Associated with abnormal visual experience during the critical periods
No pathology in the eye itself (inside of retina completely fine, its the cortex that has damage)
“Lazy brain”
amblyopia
What are the 2 criteria for diagnosing amblyopia?
1- At least a two-line difference in acuity between the two eyes on a letter chart, despite full optical correction
2- No pathology or damage to the eye itself (optic nerve head/surface)
Amblyopia is commonly associated with which 2 disorders?
Strabismus - misalignment of visual axes in the two eyes
if eye is pointing inwards: esotropia
if eye is pointing outwards: exotropia
Anisometropia – unequal refractive error in the two eyes
one eye is more myopic: eyesomyopia
one eye is more longsighted: eyesoohyperopia
One-third have strabismus one-third have anisometropia; one-third have mixture of the two.
Where does the deficit lie?
Deficit in primary visual cortex
Deficit in binocular and stereoscopic vision
Leads to inability to combine visual input from the two eyes
Inter-ocular suppression and amblyopia:
In normal vision, object of interest is imaged on corresponding points on the two retinas and fused for binocular vision
What happens in amblyopic vision?
In amblyopic vision, suppression ’switches off’ vision in the deviating eye preventing signals reaching conscious awareness
In amblyopia (with strabismus), object of interest is imaged on non-corresponding points on the retinas in the two eyes
Confusion: Something other than object of interest is imaged on fovea of deviating eye
Diplopia: Object of interest imaged on non-foveal part of retina in deviating eye
Images on foveas in deviating and non-deviating eyes are dissimilar and cannot be fused
describe Amblyopia with Strabismus:
Strabismus is a condition where the eyes are misaligned (crossed eyes or wandering eyes).
When strabismus leads to amblyopia, the misalignment causes the brain to ignore input from the misaligned eye, leading to reduced vision in that eye.
Which condition involves one eye haing reduced vision because the brain favors the other eye. This can occur even if the eye itself is healthy?
The cause results from refractive errors (like nearsightedness or farsightedness) or deprivation (such as cataracts).
Amblyopia
Individuals who have Amblyopia with strabismus:
Deviations of the eye may cause rearrangements of connections between retina and cortex
This sometimes creates what?
A new point of fixation is created away from the fovea and new connections are formed from new point of fixation to cortex
These different rearrangements of the connections between retinas and cortex can occur to compensate for strabismus
Rearrangement of connections between retina and cortex can lead to complex array of visual (acuity, sensitivity, stereo, perceived distortions) deficits
Explain this diagram
1- Monocular deprivation:
no spatial input to amblyopic eye
leading to contrast deprivation
2- anisometropic amblyopia:
loss of high spatial frequency information, object becomes blurred
3- strabismic amblyopia:
deviated image in other eye which results in surpression of confusion
Amblyopia is more than an acuity deficit.
The perceptual consequences of amblyopia can be manifest as changes to different combinations of fill in below
Limits of central vision (at threshold):
Supra-threshold vision:
Limits of central vision (at threshold):
-Visual acuity (and crowding)
-Contrast sensitivity
-Stereo acuity
Supra-threshold vision:
-Perceived distortions
The particular combinations of perceptual consequences appears to depend upon the abnormal visual experiences during visual development (e.g. strabismus, anisometropia, cataract)
Acuity deficit in amblyopia: 66
In Anisometropia (A), the letter acuity, grating acuity and Vernier acuity deficits are … to each other in the amblyopic eye of an individual with anisometropia
proportional
Acuity deficit in amblyopia: 66
in Strabismus (S): Vernier acuity deficits are proportionally worse than … in the amblyopic eye of an individual with strabismus
letter acuity and grating acuity
What is the cowding deficit in peripheral vision in those with amblyopia?
a loss in visual acuity or visual recognition threshold by nearby contours (bringing other objects close makes it harder to discriminate objects).
Crowding: results:
Flanked letter acuity (other letters around R) is poorer than isolated acuity in amblyopia
Critical letter spacing for reading equals the critical spacing for crowding
Central amblyopic vision is crowded
Contrast sensitivity deficit in amblyopia (aniso):
Contrast sensitivity curve measured in the amblyopic eye shows substantial loss of sensitivity at high spatial frequencies, but not much loss at low spatial frequencies
There is a loss in high spatial frequency at 4 degrees (but at low ranges its fine)
Stereo acuity deficit in amblyopia:
Stereo acuity task
Is the central disk in front or behind the surrounding disk?
What is stereo acuity?
the smallest detectable depth difference (disparity signal) that can be seen in binocular vision
Amblyopia produces a deficit in stereo acuity, the severity of which depends upon the nature of the visual disruption during development
70% of people with anisometropia retain some stereopsis
But the vast majority of strabismic anisometropic patients…
retain no vision at all leading to stereoblindness
Amblyopia (with small angle strabismus) is like anisometropia:
When the angle of deviation in the amblyopic eye is small (less than 5 deg), the acuity and contrast sensitivity deficits are similar to anisometropic amblyopia
A particular form of retinal correspondence (harmonious ARC) is usually present
info
Perceived visual distortions:
Researcher asked patients to draw what they saw through they amblyopic eye
Perceived distortions are most apparent when you ask amblyopes with strabismus to reproduce a stimulus
Heterogeneous types of perceptual distortions of the position, orientation, and spatial frequency of visual patterns
These perceived distortions do not correlate with the loss of visual acuity and contrast sensitivity deficits in amblyopia
This is a subjective estimate of the distortions experienced by amblyopic observers
Supar-threshold forms of loss
-as spatial frequency is increased vision gets worse as does contrast sensitivity function
Perceived visual distortions:
Researchers developed an objective estimate of these perceptual distortions in individuals with forms of amblyopia
Task was to position the response probe in the diagonally opposite position to the target
All probed locations produces a coarse map of visual space
Stimuli was flickering constantly which stopped suppression (contrast stimulus)
Found:
Technique produces these maps
Red dots represent the location in space
Grey crosses are subject individual estimates
Errors are made at 7 degrees for normal people
But
Errors are made more prominent (distortion) for those with Strabismic amblyope
What is this description?
A significant difference in the refractive power (prescription) between the two eyes (anisometropia). One eye may be more nearsighted, farsighted, or have more astigmatism than the other. The brain relies more on the eye with the clearer image, leading to reduced vision in the other eye
Anisomotrpoic Amblyope
Pattern of visual deficits in amblyopia:
Black-High acuity and moderate sensitivity (visually normal)
Red-Moderate acuity and high sensitivity (strabismics)
Green- Moderate acuity and low sensitivity (anisometropes)
Blue- Low acuity and moderate sensitivity (strabismic anisometropes)
Which type of amblyopia has the poorest visual acuity?
Strabismic anisometropes
Which type of amblyopia has the poorest contrast sensitivity?
Anisometropes
