Spatial localization: interpolating first- and second-order visual structure.

Human observers are able to localize the relative position of objects defined by Gaussian variations in either luminance (1st order) or contrast (2nd order). However, positional sensitivity is significantly poorer for 2nd- than for 1st-order stimuli. These judgments require the visual system to construct a representation of pertinent variation from a number of individual retinal samples-a process known as interpolation. We compared 1st- and 2nd-order interpolation mechanisms to examine whether differences in this process underlie differences in positional sensitivity. Observers were required to judge the relative position of two vertically separated 1st- or 2nd-order Gaussian distributions. The distributions were discretely sampled, and both sample number and separation were systematically varied. Results showed that with fixed sample separation (1.9 or 7.7 arcmin), optimum localization was obtained with a minimum of 4-6 samples, for both 1st- and 2nd-order stimuli. When sample number is maintained above this critical value, marked changes in sample separation (0 to 9 arcmin) had relatively little impact on thresholds for both 1st-order and 2nd-order stimuli. These results suggest that both 1st-order and 2nd-order interpolation mechanisms are limited by sample number rather than separation, and require a similar number of samples to mediate positional judgments.