Detection of an orientation singularity in Gabor textures: effect of signal density and spatial-frequency.

This work presents evidence for a second stage of spatial filtering in early vision. This second stage operates on the output of the well known linear spatial filters and integrates their thresholded responses with a center-surround weighting function. The evidence for the existence of this second stage comes from experiments where observers have to detect a Gabor signal with known parameters (target) among a varied number of other Gabor signals having orthogonal orientation (distractors). Detection performance on this task depends on the number of distractors and the distance between them: when the number of distractors is small performance deteriorates with an increasing number of distractors; however, when the number of distractors becomes larger performance improves with an increasing number of distractors. This improvement depends on the spatial-frequency of the signals and their spatial separation. Best performance is achieved when the spatial separation between signals is larger than three times their center wavelength but smaller than nine times their wavelength, implying a second stage filtering with a center size of six wavelengths and a total size of 18 wavelengths. This second stage of filtering may underlie our ability to detect certain texture boundaries preatentively.