The suppression effect of simulated anisometropia.

Although there is evidence that differential interocular image clarity degrades binocular performance (for example, reducing stereo sensitivity) little is known about how this sensitivity reduction occurs. As there is evidence that suppression scotomas are induced in the more defocused eye, the presence and approximate size of suppression scotomas induced with uni-ocular defocus was investigated in five binocularly normal subjects. The targets comprised a fusional lock and a suppression check for each eye and the task of each subject was to report the presence of the suppression check with varying amount of defocus. There was a range of simulated anisometropia over which suppression did not occur (approximately +/- 0.50 DS). Thereafter the probability of suppressing a small foveal target increased approximately linearly until the target was always suppressed. Greater amounts of simulated anisometropia produced larger suppression scotomas. However, less anisometropia was required to produce an equivalent sized suppression scotoma if greater amounts of fusional detail were provided. These results suggest that the decrement in binocular performance which accompanies simulated anisometropia may occur because of localized inhibition in the more defocused eye and is not solely due to loss of resolution in that eye.