Differences in the neural basis of human amblyopias: the effect of mean luminance

The effect of reducing background luminance on contrast thresholds for different forms of amblyopia was investigated. As expected from previous acuity studies strabismic and organic amblyopia exhibit dramatically different behaviour; for a particular spatial frequency the degree of strabismic amblyopia reduces with reducing luminance whereas organic amblyopia does not. Unexpectedly, there is a similar difference in the behaviour of strabismic and anisometropic amblyopia. This difference suggests that these two forms of “functional” amblyopia do not have identical neural bases. An hypothesis concerning the extent of the abnormality within the visual field is advanced to explain these luminance dependent results.