Abstract
In textures composed of black and white dots, dot density and/or dot contrast can be modulated to produce one-dimensional, oriented gratings. Just as Mulligan and MacLeod (1988, Vis. Res. 28:503) found strong reciprocity between density and luminance for dots viewed against a darker background, there is strong reciprocity between density and contrast: detection thresholds for in-phase modulations of density and contrast are 30% - 55% lower than detection thresholds for density and contrast modulations that are 180° out of phase (Morgan, MacLeod, & Solomon, 2021, Vis. Res. 192:107948). This large effect of phase supports the existence of two psychophysical channels: one that is excited by both density modulations and contrast modulations (and is thus quiescent when those modulations are presented 180° out of phase) and one that is relatively insensitive to density or contrast modulations (and thus remains stimulated regardless of the phase angle between density and contrast). We attempted to obtain further evidence for the architecture of these mechanisms by dichoptic experiments in which density-modulated and contrast-modulated textures were present either in the same or opposite eyes. As is the case for orientation-defined textures (Kolb & Braun, 1995, Nature 377:336; Solomon, John, & Morgan, 2006, Vis. Res. 46:1488), out-of-phase modulations of density and/or contrast were much easier to detect when they were in opposite eyes. These dichoptic advantages imply that the mechanism sensitive to both density modulations and contrast modulations is present in a monocular channel. When modulations of density and contrast were presented to opposite eyes, we found a large effect of phase angle: detection thresholds were highest when these modulations were presented 180° out of phase. This result seems incompatible with a purely monocular mechanism that is exclusively sensitive to contrast modulations or density modulations. Consequently, we conclude that this latter mechanism receives input from both eyes.