Abstract
The Olzak-Thomas (1999) model of spatial vision proposes an initial stage of weighted linear filters followed by a stage of nonlinearities and then a stage of specialized higher-level summing circuits. Among the nonlinearities is a contrast gain control mechanism that pools the output of the first stage over different orientations and/or spatial frequencies. One of the summing circuits is specialized to signal orientation information. It is broadband with respect to spatial frequency but more narrowly tuned to orientation. We measured the orientation bandwidth of the contrast gain control pool and summing circuits associated with orientation judgments. To determine the bandwidth of the contrast gain control pool, orientation discrimination performance for vertical 3 (or 15) cpd luminance gratings was measured for the grating alone and in the presence of a 15 (or 3) cpd mask, as a function of mask orientation. Two-cue stimuli were used to measure the bandwidth of higher-order orientation summing circuit. Two-cue stimuli were initially composed of two near-vertical gratings of widely different spatial frequency, each presenting a cue to discrimination. They could vary together in one condition (i.e., both left vs. both right), or could vary in opposition. Superiority in performance in the varying-together condition (configural effect) indicates the operation of a summing circuit. The presence of a configural effect was measured as a function of the base orientation of the second component (the first was always varied around vertical). Our results show that for nearly all observers, the masking effect slowly decreases until the orientation of mask reaches 80 degrees, where it disappears. The configural effect disappears abruptly when the orientation of the second cue reaches 4 degrees off vertical. These results indicate that the gain control pool is widely tuned with respect to orientation, while the higher level summing circuit is quite narrowly tuned.
This work was supported by NIH Grant EY13953 to LAO.