Extrafoveal viewing reveals the nature of second-order human vision.

For the majority of visual tasks, performance in extrafoveal vision can be equated with that at the fovea simply by a change in spatial scale of the stimuli (magnification). We sought to exploit this association to examine the nature of second-order vision. More specifically, we investigate the relationship between the scale of second-order vision and the scale of its first-order input. We find that sensitivity to second-order stimuli can be equated across visual space, but only for stimuli that are magnified in every respect (identical scaling of both first- and second-order characteristics). In other words, sensitivity to stimuli which poses a fixed ratio between the scale of first-order input and second-order spatial scale can be equated across the visual field using a single magnification factor. Moreover, stimuli which poses quite different ratios of first- and second-order scale can be equated across eccentricity using the same magnification factor. This argues for a strict relationship between second-order vision and the scale of its first-order input and reflects a parallel arrangement of dedicated second-order mechanisms having a common eccentricity dependence.