Orientation tuning of binocular summation in color vision assessed with subthreshold summation

摘要

Previous work by our laboratory found that color vision lacks orientation tuning at low spatial frequencies for monocular presentation (Gheiratmand et al JOV, 2013; Gheiratmand & Mullen, Sci Rep, 2014). Our current research aims to investigate whether orientation tuning is gained when low spatial frequency monocular color signals are binocularly summated. We assess color and luminance vision at low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies using the psychophysical method of subthreshold summation. By using low, near-threshold contrast levels, we are theoretically able to bypass the processes of contrast normalization and access underlying neural detection mechanisms. Grating stimuli are presented at threshold levels both monocularly and dichoptically over a wide range of orientation differences. Orientation bandwidths of binocular mechanisms are computed using a probability summation model in which monocular signals are binocularly combined with a non-linear transducer and spatially combined using Minkowski summation. Preliminary results point to binocular orientation tuning in color and luminance at both low and mid spatial frequencies. Therefore, tuning is acquired at the binocular level for low spatial frequency color vision. Intriguingly, low spatial frequency color vision summation ratios are higher than other conditions over all orientation differences. We speculate that this result may indicate that under binocular conditions, low spatial frequency color vision has access to both tuned binocular and isotropic monocular signals.