Functional amblyopia is an optically uncorrectable decrease in visual acuity, i.e., spatial resolution, with no apparent pathological or morphological cause. There are several etiological origins for amblyopia in the human population; anisometropia and strabismus are the two most prevalent etiological factors. Anisometropic amblyopia is a decrease in visual acuity (spatial resolution) that is the result of a large difference in refractive error between the two eyes. Strabismic amblyopia is characterized by an eye misalignment early in life which is associated with a decrease in visual acuity.

The initial site of the defect in amblyopia is the primary visual cortex. The amblyopic eye is placed at a competitive disadvantage to the nonamblyopic eye which results in a reduction in the synapses or cells responding to the amblyopic eye. The conditions associated with amblyopia must be present prior to the end of the critical period of neural development for amblyopia to develop. The neurophysiological systems of anisometropic and strabismic monkeys have differences in the distributions of binocular cortical cells. Anisometropic amblyopic monkeys only exhibit binocular cells that are tuned to low spatial frequencies. If a neuron has a high spatial resolution, it is unlikely that the neuron will be binocular. However, strabismic amblyopic monkeys exhibit few binocular cells and, moreover, there does not appear to be a spatial frequency dependent distribution of these binocular cells. The strabismic subjects typically displayed few binocular cells, regardless of the spatial frequency tuning of the cell.

If the amblyopic eye is compared to the nonamblyopic eye, two important differences are noted. First, the amblyopic eye has a lower contrast sensitivity at high spatial frequencies (and sometimes at all spatial frequencies) than the nonamblyopic eye. This also results in a decrease in visual acuity for the amblyopic eye. Second, the amblyopic eye demonstrates a longer latency for neural transmission than the nonamblyopic eye.

There is very little published on adult amblyopia therapy. This is because clinicians initially believed that amblyopia could not be treated after the end of the critical period. The prevailing theory was that the synaptic contacts between cells could only be modified during the critical period. In children, the majority of the improvement in acuity (80%) takes place in the first 6 weeks of therapy. There is a direct relationship between the hours of patching and the improvement in acuity. Most of the improvement in acuity occurs after the first 100 hours of patching.

There is a linear dose-response function for amblyopia treatment. The logMAR acuity increases 0.1 log unit or 1 chart line per 120 hours of patching. The response does not differ between the types of amblyopia. It has also been demonstrated that there is only a minimal benefit to patching more than 2 hours/day.

In a Pediatric Eye Disease Investigator Group (PEDIG) study, 189 amblyopic children were treated with 2 hours of patching per day. The amblyopic eye acuities ranged from 20/40 - 20/80 before treatment. The average improvement in acuity after 4 months of treatment was 0.24 logMAR (2.4 lines improvement on a logMAR chart). Another study used amblyopes with poorer acuity (20/100 - 20/400 before therapy). After 4 months of patching, the average increase in acuity was 0.48 logMAR (or an increase of 4.8 lines on a logMAR chart). The average acuity went from 20/250 (i.e., 1.10 logMAR) before treatment to 20/63 (i.e., 0.50 logMAR) after treatment. The results were the same for anisometropic amblyopes and strabismic amblyopes. In another study with amblyopes with starting acuity worse than 20/100, the improvement was 3.7 ± 2.5 logMAR chart lines of acuity.

Recent studies have suggested that adult amblyopia can be treated with non-standard therapy. Patching combined with active, near, threshold-training tasks and continuous feedback is employed in these studies. These studies indicate that amblyopia can be treated after the critical period ends. It is suggested that these perceptual learning tasks improve performance in the amblyopic eye by decreasing neural noise. Standard amblyopia therapy typically employed in children has not been investigated in adult amblyopes. The purpose of this study is to use standard amblyopic therapy to determine if there is an enhancement in visual performance in the amblyopic eye of adults.