Two mechanisms for localization? Evidence for separation-dependent and separation-independent processing of position information.

The Weber function for separation--i.e. delta s as a function of separation s--is typically measured using a pair of targets presented roughly symmetrically relative to the fovea. With this paradigm, as the separation increases, the eccentricity of the individual targets increases also. To disentangle the effects of separation and eccentricity on the Weber function for separation, we systematically examined each of these variables and also examined the effects of target size and exposure duration. Separation discrimination thresholds were measured for average separations from 3 to 6 deg across a wide range of eccentricities, and for eccentricities of 2.5-10 deg for a range of separations. The dependence of threshold on target size was measured by varying the length of the stimuli from 1 to 120 min arc; the dependence on exposure duration was measured using durations of 100 and 500 msec at 10 deg eccentricity for comparison with data collected previously at smaller eccentricities. We found that for separations less than the eccentricity of the targets, thresholds depend primarily on separation; for larger separations, thresholds depend solely on eccentricity. In general, unless the targets are very small or quite brief, the spatial and temporal characteristics of the targets are not major contributors to the slope of the Weber function. Two mechanisms are proposed to account for thresholds in the two regions, one separation-dependent and one separation-independent.