Effects of blur and eccentricity on differential spatial displacement discrimination

Differential spatial displacement discrimination thresholds were determined for stimuli consisting of blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. The tasks were similar to the two-point discrimination acuity task and the three-dot alignment hyperacuity task. Thresholds were determined as a function of eccentricity along the horizontal meridian of the visual field (from 45 degrees nasal to 65 degrees temporal). The spatial spread or blur parameter of the blobs was adopted as a scale parameter. The results show that the performance of the visual system in differential spatial displacement discrimination tasks becomes progressively more homogeneous for a progressive increase in the blur parameter of the stimuli. Scaling (i) the three-blob alignment results with estimates of the cortical magnification factor and (ii) the two-blob separation discrimination results with their corresponding neural blur parameter shows an impressive isotropy and blur scale-invariance for the mechanisms mediating differential spatial displacement discrimination across the visual field. These results are interpreted in terms of a scaled sampling lattice model of the visual system, in combination with an automatic scale-selection mechanism.