The major developmental binocular anomalies are amblyopia and strabismus. Amblyopes have other visual anomalies besides poor stereopsis or reduced visual acuity. They also have:
• poorer contrast sensitivity (sometimes just high spatial frequencies, central vision)
Most patients with amblyopia have reduced contrast sensitivity, in addition to poor visual acuity. The
visual anomaly can differ, however if the amblyopia is caused by strabismus or optical defocus.
Strabismic amblyopes tend to have poor contrast sensitivity in the central visual field only, but normal
peripheral contrast sensitivity. Optical-deprivation amblyopes, on the other hand, tend to have poor
contrast sensitivity across a larger portion of the visual field.
• reduced vernier acuity
Vernier acuity is the ability by a person to detect the proper alignment of two line segments. In most people, Vernier acuity is particularly high, enabling accurate differentiation between aligned and misaligned marks on a vernier scale.
• experience the crowding effect
People with amblyopia experience the crowding effect, which is described as difficulty reading a letter
when it is surrounded by other letters or contours. For this reason, when testing visual acuity on
amblyopes, you may measure a better visual acuity using isolated letters than if you use a single line or
the entire chart. Also, when reading a row of letters, amblyopes can usually read the first and last letters
more easily than the middle letters. The crowding effect is more pronounced in strabismic amblyopes
than anisometropic amblyopes. Don't fail to read the recent Vision Help post on the crowding effect. Also check out Strabby's experiences with the crowding effect.
• reduced binocular luminance summation
In individuals with normal binocularity, pupils are about 30% smaller with binocular viewing than with
monocular viewing. This is an indication of summation of the input from the two eyes. In amblyopia, this
effect is less pronounced, indicating a less complete summation of luminance input from the two eyes.
Persons with poorer stereopsis appear to have less effective summation. Photophobia is not uncommon as a consequence. The reduced summation is
thought to be at the cortical level and not in the retina.
• slower neural transmission from the amblyopic eye
• abnormal space, direction and motion perception
Strabismic amblyopes also tend to have distorted space perception. This causes objects to appear distorted and causes an abnormal sense of visual direction. This may
underlie the poorer-than-normal vernier acuity seen in some amblyopes.
Two other anomalies sometimes seen in strabismic and amblyopic patients are:
• Eccentric fixation
Amblyopic and strabismic eyes sometimes do not fixate normally. Fixation may be unstable, or the eye may
have nystagmus. It may also be anomalous in the sense that some point other than the fovea is used for
fixation. This is known as eccentric fixation.
Eccentric fixation can be present during both monocular and binocular viewing conditions, but it is best
diagnosed under monocular viewing conditions. This is important to keep in mind to avoid confusion with
anomalous correspondence, which we will consider later. Anomalous correspondence is relevant only during
binocular fusion and must be measured in those conditions.
• Anomalous correspondence
Check out the Squinty Josh post on this topic.
Big thanks to Thomas O Salmon OD for his course on binocular vision
For more info:
- Deprivation and Binocular Vision Anomalies
- Developmental strabismus