Functional architecture for binocular summation of luminance- and contrast-modulated gratings

Combining signals from the two eyes is fundamental to stereo vision. We studied binocular summation for horizontal luminance gratings (L), luminance gratings in noise (LM) and second-order gratings(contrast-modulated noise, CM). Noise centre frequency was 6 c/deg, rms contrast 0.2 or 0.1. We measured 2AFC detection thresholds for gratings (0.75 c/deg, 0.2 s duration) shown to one eye, to botheyes in-phase, or both eyes in anti-phase. Mean binocular thresholds were 5–6 dB better than monocular thresholds for L, LM and CM: close to linear summation across the eyes. We found full binocular summation for CM even when the 2D noise carriers were uncorrelated or anti-correlated between the eyes, implying binocular combination of envelope, not carrier, responses. Similarly, binocular summation for CM occurred with uncorrelated, oblique 1D noise carriers at both parallel and orthogonal orientations,suggesting that CM summation is insensitive to carrier similarity or composition. In all cases anti-phase signals showed no summation and very little cancellation, consistent with half-wave rectification before binocular summation. These results suggest an extended filter-rectify-filter model with two filter-rectify stages preceding binocular summation (FRFRB), similar to the architecture proposed for cat area 18 (Tanaka and Ohzawa, 2006 Journal of Neuroscience 26 4370–4382).