Eccentricity dependence of orientation anisotropy of surround suppression of contrast-detection threshold.

Both neurophysiological and psychophysical data provide evidence for orientation biases in nonfoveal vision-specifically, a tendency for a Cartesian horizontal and vertical bias close to fixation, changing to a radial bias with increasing retinal eccentricity. We explore whether the strength of surround suppression of contrast detection also depends on retinotopic location and relative surround configuration (horizontal, vertical, radial, tangential) in parafoveal vision. Three visual-field locations were tested (0°, 225°, and 270°, angle increasing anticlockwise from 0° horizontal axis) at viewing eccentricities of 6° and 15°. Contrast-detection threshold was estimated with and without a surrounding annulus. At 6° eccentricity, horizontally oriented parallel center-surround (C-S) configurations resulted in greater surround suppression compared to vertically oriented parallel center-surround configurations (p = 0.001). At 15° eccentricity, radially oriented parallel center-surround stimuli conferred greater suppression than tangentially oriented stimuli (p = 0.027). Parallel surrounds resulted in greater suppression than orthogonal surrounds at both eccentricities (p < 0.05). At 6° the horizontal center was more susceptible to suppression than a vertical center (p < 0.001) for both parallel and orthogonal surrounds, while at 15° a radial center was more susceptible to suppression (relative to a tangential center), but only if the surround was parallel (p = 0.005). Our data show that orientation anisotropy of surround suppression alters with eccentricity, reflecting a link between suppression strength and visual-field retinotopy.