Amblyopia (lazy eye) is the incomplete development of binocular or two-eyed vision in the brain. The most well-known symptoms of amblyopia are the lack of 3D vision (stereopsis) and reduced visual acuity. Notwithstanding, amblyopes also often have other visual anomalies besides poor stereopsis or reduced visual acuity. May also be affected:
• 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.
Deficiencies can also exist in color vision, visual span of recognition or the ability to discriminate visually.
• 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.
• abnormalities in accommodation and pupillary responses
Eye lens focusing and pupillary responses may be off because the eye is not properly integrated within the nervous system.
• slower neural transmission from the amblyopic eye
Errors of eye position may not be “registered” and corrected by the re-fixation reflex. Pursuits tend to be jerky and nystagmoid-like. There may be a delay between the presentation of a stimulus and a saccadic movement toward the stimulus (saccadic latency). Visual reaction time and eye-hand reaction time may be prolonged and inaccurate causing clumsiness.
• 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.
• eccentric fixation (EF)
Amblyopic 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. If Eccentric Fixation is present, the fovea no longer acts as the source of Principal Visual Direction and foveal sensitivity becomes gross and vague. Anomalous spatial localization is frequently seen. This means the patient’s location of an object does not match the true physical location of the object.
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 (AC)
Technically speaking, Anomalous Correspondence is a sensory adaptation in Strabismus (crossed eyes) and not in Amblyopia. However, a person who has Strabismic Amblyopia may have AC but that would be because of the Strabismus not the Amblyopia.
Check out the Squinty Josh post on this topic.
• In addition the non-amblyopic eye may also show irregularities
It is equally important to say that abnormal or incomplete visual brain development also impacts the development of other senses and cognitive skills.
• Amblyopia in early childhood may alter speech perception
Big thanks to Thomas O Salmon OD for his course on binocular vision