Reduced Binocular Summation of Second–Order Contrast in Amblyopia

Abstract

Abstract: : Purpose:In normal observers, detection of first–order contrast (luminance modulations) improves under binocular viewing due to neural summation. We investigated whether amblyopes show binocular summation of second–order contrast (contrast modulations). We expected to find reduced binocular summation (less facilitation or suppression) due to impaired binocular vision in amblyopia and the binocular nature of second–order processing. Methods: We psychophysically measured monocular and binocular contrast detection thresholds in seven amblyopic (four strabismic and three anisometropic) and five normal (control) adults. Stimuli were contrast–modulated sinusoids consisting of an 8–c/deg carrier (first–order contrast) and a 1–c/deg envelope (second–order contrast). The task was to detect the envelope, which filled the central 50% of the carrier. We tested four carrier contrasts (10, 20, 40 and 80%), viewing was in free–space and the strabismic amblyopes wore prisms to obtain bifoveal stimulation. Results: Binocular summation was independent of carrier contrast in all observers. In controls, binocular thresholds were 10–28% lower (average 22% ± 2%) than the best monocular threshold. In amblyopes, binocular thresholds ranged from 8% lower to 8% higher (average 0% ± 3%) than the non–amblyopic eye threshold. These effects were independent of amblyopia type and severity (acuity loss), and the difference in monocular thresholds. Conclusions: Our results suggest suppression and/or reduction of binocular interactions in amblyopia. Reduced binocular summation could also reflect normal dichoptic masking produced by unequal, relative to detection threshold, first–order (carrier) contrast signals from each eye, although this is not supported by data from controls.

Keywords: amblyopia • pattern vision • visual cortex