Unilateral amblyopia is a visual disorder that arises after selective disruption 
of visual input to one eye during critical periods of development. In the 
clinic, amblyopia is understood as poor visual acuity in an eye that was 
deprived of pattern vision early in life. By its nature, however, amblyopia has 
an adverse effect on the development of a binocular visual system and the 
interactions between signals from two eyes. Visual functions aside from visual 
acuity are impacted, and many studies have indicated compromised sensitivity in 
the fellow eye even though it demonstrates normal visual acuity. While these 
fellow eye deficits have been noted, no overarching theory has been proposed to 
describe why and under what conditions the fellow eye is impacted by amblyopia. 
Here, we consider four explanations that may account for decreased fellow eye 
sensitivity: the fellow eye is adversely impacted by treatment for amblyopia; 
the maturation of the fellow eye is delayed by amblyopia; fellow eye sensitivity 
is impacted for visual functions that rely on binocular cortex; and fellow eye 
deficits reflect an adaptive mechanism that works to equalize the sensitivity of 
the two eyes. To evaluate these ideas, we describe five visual functions that 
are commonly reported to be deficient in the amblyopic eye (hyperacuity, 
contrast sensitivity, spatial integration, global motion, and motion-defined 
form), and unify the current evidence for fellow eye deficits. Further research 
targeted at exploring fellow eye deficits in amblyopia will provide us with a 
broader understanding of normal visual development and how amblyopia impacts the 
developing visual system.