Contrast sensitivity of the amblyopic eye in central and peripheral retina with variation of stimulus field size-implications for its neural basis

Functional amblyopia is often assumed to be restricted to the central foveal region. The purpose of the present study was to establish the extent of the amblyopic deficit at 0°, 10°, and 20° retinal eccentricity using contrast sensitivity measures for sine wave gratings subtending various field sizes. Contrast sensitivity functions were shown to vary with stimulus field size, retinal location and degree of amblyopia. Foveally, the peak contrast sensitivity of the amblyopic eyes increased markedly with increasing field size, to reach the same magnitude as that of the contralateral non-amblyopic eyes for large fields. In the retinal periphery a significant difference in peak contrast sensitivity was found between the amblyopic and non-amblyopic eyes. High spatial frequency cut-offs were reduced both centrally and peripherally with all field sizes in the amblyopic eyes. The effect of increasing field size on the cut-off acuity was found to be smaller than the effect on peak contrast sensitivity. The most significant conclusions drawn from the results are that: 1) The amblyopic deficit in terms of both peak contrast sensitivity and acuity is not restricted to the central foveal region, and 2) Amblyopic eyes benefit to a greater extent from increased stimulus field size than non-amblyopic eyes in terms of peak contrast sensitivity. An hypothesis is proposed which suggests that the results obtained are due to fewer and/or less sensitive cortical neurons being driven by the amblyopic eye in humans with naturally occurring amblyopia.