Displacement sensitivity in amblyopia

Although defined as reduced visual acuity, spatial uncertainty and distortion have been proposed as the basic anomaly in functional amblyopia. The primary purpose of this study was to assess spatial abnormalities within the centralmost 1 degree of the amblyope's visual field (functionally critical for acuity) by determining displacement thresholds for a small target in the absence and presence of stationary reference contours. With reference contours absent, 3 of 4 amblyopes demonstrated 1) elevated displacement thresholds over a range of target movement durations and 2) substantial constant errors in discriminating rightward from leftward displacements. Conversely to normals, displacement thresholds for severe amblyopes apparently were determined by target velocity for movenent durations up to about 0.5 sec and by target displacement for longer durations. With reference contours, responses were less abnormal for the 2 amblyopes tested. Amblyopes' elevated thresholds and constant errors for displacement found in this study are interpreted as resulting from both their spatial abnormalities and poor eye movements.