Iso- and cross-orientation surround modulation of contrast detection and discrimination

Abstract

Background: Neurophysiological and psychophysical evidence reveals strong modulation of contextual stimuli on visual performance. Here we studied how contrast detection and discrimination in foveal vision are affected by surround stimuli at iso- or cross-orientations. Methods: The target was a small D6 grating (8 cpd, vertical) centered on a slightly larger sinusoid grating pedestal (8 cpd, vertical, contrast = 0–40%) with an abutting sinusoid grating surround (8 cpd, contrast = 0–80%, horizontal or vertical). Thresholds were measured with a temporal 2AFC staircase procedure. Results: (1) Iso-oriented surrounds have little effect on detection except at high contrast where they become suppressive. For discrimination, iso-oriented surrounds facilitate when the surround/center contrast ratio is <= 1 and suppress when the s/c ratio is >1. (2) Surprisingly, cross-oriented surrounds can facilitate detection and discrimination by a factor of up to 2, even in the dipper region of the tvc function. This effect is not due to reduced uncertainty, since the slope of the psychometric function is unchanged by the surround, and facilitation is not impaired by jittering the surround contrast. Cross-orientation facilitation is largely unaffected by the s/c ratio. Under each surround contrast condition, facilitation dips when the pedestal contrast is around 5–20%, and becomes stronger at lower and higher pedestal contrasts. The data also indicate that cross-oriented surrounds may facilitate detection and near-threshold discrimination (pedestal contrast <10%) via direct exCitation, and facilitate suprathreshold discrimination (pedestal contrast >=10%) via indirect disinhibition of masking. Conclusion: Our results suggest that surround effects participate in complex gain control processes in spatial filters, which are not easily explained by the standard spatial filter models, but are supported by recent neurophysiological findings.