Summation of texture segregation across orientation and spatial frequency:electrophysiological and psychophysical findings

Objects are usually segregated from ground by several visual dimensions. We studied texture segregation in checkerboards defined by gradients in spati al frequency, orientation or both frequency and orientation, using Gabor-fi ltered noise patterns. Saliency was measured electrophysiologically using t he visual evoked potential (VEP) associated with texture segregation ('tsVE P') (an associated component in the visual evoked potential), and psychophy sically by a 2AFC task. Spatial frequency and orientation stimuli evoked pe rcepts of texture segregation and tsVEPs in all 11 subjects. The tsVEPs to combined stimuli were larger than those to each dimension alone, but smalle r (74%) than the algebraic sum of tsVEPs to both individual dimensions. Psy chophysical detection rates differed significantly between all conditions ( P < 0.001), with highest rates for the combined stimuli. The findings sugge st that segregation based on a combination of 'orientation' and 'spatial fr equency' is more salient than that based on either of these alone. The sign ificant deviation from full additivity in the tsVEPs suggests that simultan eous contrasts in spatial frequency and orientation have a common processin g stage. (C) 2000 Elsevier Science Ltd. All rights reserved.