Determinants of the spatial properties of cone-rod interaction

Photopic increment thresholds can be elevated by 0.2-1.9 log units, depending on the diameter of a concentric scotopic background. This cone-rod interaction displays spatial properties that resemble the spatial sensitization (Westheimer effect) observed in the isolated scotopic and photopic systems. This raises the possibility that the spatial properties of the interaction are determined by the same mechanisms or pathways that determine the spatial properties of either scotopic or photopic vision. When annulus backgrounds are used, the spatial properties of interaction match those of the scotopic system but not the photopic system. When disk backgrounds are used, the spatial properties of interaction match those of neither photopic nor scotopic systems. Thus, under some conditions, the scotopic visual system alone is sufficient to determine the spatial properties of cone-rod interaction. Under other conditions, additional complications arise. The results are discussed in terms of the center-surround model that has previously been applied to cone-rod interaction.