Amblyopia is a developmental disorder of spatial vision that affects approximately 3% of the population worldwide.
1 2 It occurs at an early age as a result of abnormal visual experience. The common causes are refractive imbalance (anisometropia),
3 misalignment of the visual axis (strabismus),
4 asymmetric meridian power (astigmatism),
5 high refractive error,
6 and/or form deprivation resulting from congenital cataract
7 or ptosis.
8 The key clinical characteristic is reduced visual acuity in the amblyopic eye without any manifest ocular disease. The amblyope’s line visual acuity is often worse than the isolated single-letter acuity—a difference known as crowding.
9 The loss of acuity can be attributed in part to the loss of contrast sensitivity in medium and high spatial frequency (SF) mechanisms.
10 11 The degree of binocular imbalance strongly influences the depth of amblyopia.
3 12 In addition, amblyopes show deficits in a range of visual tasks. These include hyperacuity,
13 14 15 16 shape perception,
17 contour integration,
18 spatial interaction of surrounding visual objects,
19 20 phase sensitivity,
21 visual counting,
22 pattern vision,
23 stereopsis,
24 and motion-processing
25 deficits. Several psychophysical theories have been proposed to explain the abnormal visual perception in the amblyopic brain: (1) an increase in the size of cortical receptive fields,
13 with the peak of the SF tuning shifted to lower spatial frequencies; (2) a decrease in the contrast sensitivity of small cortical filters
10 11 ; (3) a decrease in the density of cortical neurons (i.e., undersampling)
26 ; and (4) an increase in spatial uncertainty or distortion, with the neural representation of the visual image being somewhat distorted at the cortical level.
26 27 28 29 Recent studies have shown that the loss of binocular vision may be critical in the development of amblyopia.
30 31