Two sources of error in pop-out localization

An odd-one-out stimulus, such as a vertical bar among horizontals, pops out from the background and is easily detected, but its location may be slightly ambiguous. Four observers were asked to pinpoint these stimuli on thousands of trials, in 5 x 5 and 9 x 9 arrays of Gabor patches. We found they made frequent errors toward neighbors of the target. Over a range of performance from 41% to 96% correct, the frequency of neighbor errors was well described by a linear function of the total error frequency, a function that might result from mixing together two spatial distributions--one broad, the other narrow. We suggest that these represent two sources of error in pop-out localization; one might correspond to a higher visual area with imprecise retinotopic mapping, and the other to a more fine-grained localization process in primary visual cortex.