Interocular difference detection is facilitated as well as suppressed by surround masks

Interocular (between-eye) contrast differences (ICDs) elicit an impression of lustre, a cue to their detection. Recently Kingdom, Jennings & Georgeson (JOV, 18(5):9, 2018) provided evidence that ICD detection is an adaptable dimension of vision, in keeping with the idea that ICDs are detected by a dedicated binocular differencing channel, termed B-. Here we study the properties of the putative channel using surround masking rather than adaptation. Observers were required to detect ICDs in the form of interocular phase differences between horizontally-oriented 0.5 cpd test luminance gratings in a circular 2 deg diameter window. The test gratings were surrounded by 0.5 cpd horizontally-oriented luminance mask gratings that were interocularly either in-phase or anti-phase. ICD thresholds for a 10% contrast test increased with the contrast of the anti-phase surround, indicating surround suppression. With the in-phase surrounds ICD thresholds decreased gradually with contrast, indicating surround facilitation. The results are consistent with a B- channel that is subject to inhibition from surrounds containing interocular differences but which benefits from surrounds that are interocularly matched, suggestive of a mechanism that plays a role in perceptually segregating regions with and without interocular differences.