Spatial Interactions in Interocular and Monocular "Blur Suppression".

ABSTRACT

SIGNIFICANCE: The suppression of blurred images in one eye by clear images in the other eye is thought to contribute to the success of monovision correction. We show that interocular suppression occurs also for low-contrast targets that are not blurred and, to a lesser extent, when clear and low-contrast targets are presented to the same eye. PURPOSE: A blurred target presented to one eye may be suppressed when a clear target is presented to the other eye. We sought to determine how this interocular suppression varies according to the separation between the blurred and clear targets and the magnitude of imposed blur. In addition, we examined whether a similar suppression occurs when the clear and blurred targets are imaged in the same eye. METHODS: Subjects (N = 4) viewed a clear 20/40 Sloan letter surrounded by four 2 × 10 min-arc flanking bars. In different blocks of trials, the gap between the letter and flanking bars varied from 0.5 to 4 bar widths. In addition, the flanking bars were either clear or spatially filtered to simulate 0.5 to 2 D of blur. The contrast required to detect the flanking bars was determined when the letter and flanking bars were presented either dichoptically or monoptically and compared with the thresholds for the bar targets presented alone. RESULTS: In both dichoptic and monoptic viewing conditions, detection thresholds for the blurred flanking bars are highest for the smallest spatial gap and decrease systematically as the gap increases. Thresholds are uniformly higher during dichoptic than monocular viewing, but the proportional change with the bar-to-letter separation is similar in both conditions. Surprisingly, the magnitude of imposed blur has very little influence on the magnitude of threshold elevation in either the dichoptic or monoptic viewing conditions. CONCLUSIONS: Because threshold elevation is nearly the same in the presence of 0 to 2 D of blur, we prefer to designate the phenomenon we studied as "contrast suppression." The similar spatial characteristics of suppression during dichoptic and monoptic viewing are consistent with contributions from a common neural mechanism.