Interocular ND filter suppression: eccentricity and luminance polarity effects

Depth and extent of interocular suppression was measured in binocularly normal observers who unilaterally adapted to neutral density filters (0, 1.5, 2 and 3ND). Suppression was measured by dichoptically matching sectors of a ring presented to the adapted eye, to a fixed contrast contiguous ring presented to the non-adapted eye. Other rings of alternating polarity were viewed binocularly. Rings were defined by luminance (L), luminance with added dynamic binary luminance noise (LM) and contrast modulating the same noise (CM). Interocular suppression depth increased with increasing ND density nearing significance (p=0.058) for 1.5ND. For L and LM stimuli, suppression depth across eccentricity (±12 deg visual field) differed for luminance increment (white) versus luminance decrement (black) stimuli, potentially confounding eccentricity results. Suppression for increment-only (white) luminance stimuli was steeper centrally and extended across the visual field, but was deeper for L, than LM stimuli. Suppression for decrement-only (black) luminance stimuli revealed only central suppression. Suppression was deeper with CM than LM stimuli suggesting that CM stimuli are extracted in areas receiving predominantly binocular input, which may be more sensitive to binocular disruption. Increment (white) luminance stimuli demonstrate deeper interocular suppression in the periphery than decrement (black) stimuli, so are more sensitive to changes in peripheral suppression. Asymmetry of suppression in the periphery for opposite polarity luminance stimuli may be due to interocular receptive field size mismatch as a result of dark adaptation separately affecting ON and OFF pathways. Clinically, measurement of suppression with CM stimuli may provide best information about post-combination binocularity.