Orientation tuning of binocular summation: a comparison of colour to achromatic contrast

Abstract

A key function of the primary visual cortex is to combine the input from the two eyes into a unified binocular percept. At low, near threshold, contrasts a process of summation occurs if the visual inputs from the two eyes are similar. Here we measure the orientation tuning of binocular summation for chromatic and equivalent achromatic contrast. We derive estimates of orientation tuning by measuring binocular summation as a function of the orientation difference between two sinusoidal gratings presented dichoptically to different eyes. We then use a model to estimate the orientation bandwidth of the neural detectors underlying the binocular combination. We find that orientation bandwidths are similar for chromatic and achromatic stimuli at both low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies, with an overall average of 29 ± 3 degs (HWHH, s.e.m). This effect occurs despite the overall greater binocular summation found for the low spatial frequency chromatic stimuli. These results suggest that similar, oriented processes underlie both chromatic and achromatic binocular contrast combination. The non-oriented detection process found in colour vision at low spatial frequencies under monocular viewing is not evident at the binocular combination stage.

A Normative Data Set for the Clinical Assessment of Achromatic and Chromatic Contrast Sensitivity Using a qCSF Approach

“…At the highest spatial frequency used for the two color conditions (2 c/d), the ratio drops to 1.5 for the RG and 1.6 for the BY condition, and to 1.5 for the achromatic conditions at 10 c/d. A similar trend of spatial frequency-dependent binocular summation was observed at subthreshold level 31 for the RG condition, with summation ratios of 1.78 (5 dB) and 1.58 (4 dB) at low and high spatial frequency (0.375 and 1.5 c/d). respectively.…”

“…However, this trend was not observed for the Ach condition, in which the ratio was 1.41 (3 dB) for both spatial frequencies. Furthermore, several other studies also have explored this issue at detection level for Ach, 32 and both Ach and RG conditions 31,33,34 with a low spatial frequency (0.5 c/ d).…”

The Whole is Other Than the Sum: Perceived Contrast Summation Within Color and Luminance Plaids

“…In sum, relative to component gratings, color plaids have lower perceived contrast summation than achromatic plaids and this chromatic summation difference is greater at the medium-spatial frequency and smaller at the low-spatial frequency. These results may be reflective of two separate processes: greater cross-orientation suppression in color vision (Kim et al, 2013; Medina & Mullen, 2009) and increased cross-orientation summation for low compared with medium-spatial frequency color stimuli (Gheiratmand et al, 2013, 2016; Gheiratmand & Mullen, 2014). …”

“…Moreover, this effect is greater for mid-spatial frequency stimuli; for this spatial frequency, chromatic plaids have the lowest perceived contrast summation while achromatic plaids have the highest. Recent research on cross-orientation effects in color vision has identified two different and specific mechanisms: subthreshold summation, which acts to increase contrast sensitivity to cross-oriented stimuli (Gheiratmand et al, 2013, 2016; Gheiratmand & Mullen, 2014), and cross-orientation suppression, which decreases contrast sensitivity when one cross-oriented component is at a suprathreshold contrast (Kim et al, 2013; Medina & Mullen, 2009). We next discuss how the presence of these mechanisms in suprathreshold plaid contrast perception is supported by our data.…”

“…They observed increased subthreshold summation between orthogonal gratings specifically for low-spatial frequency red–green color vision, indicative of detection mechanisms without orientation tuning (isotropic). Increased summation was also observed for binocular summation between dichoptic low-spatial frequency color gratings, but, unlike monocular vision, this summation mechanism is most likely orientation tuned (Gheiratmand, Cherniawsky, & Mullen, 2016). We, therefore, ask whether correspondingly greater cross-orientation summation, which elevates contrast sensitivity and can be characteristic of an isotropic mechanism, is also evident at suprathreshold contrasts for low-spatial frequency color stimuli.…”