Why do certain spatial after-effects increase with eccentricity?

Abstract

Aim. The shape-frequency and shape-amplitude after-effects (SFAE and SAAE) describe the shifts in perceived shape-frequency/shape-amplitude of sinusoidal-shaped contours following adaptation to contours with slightly different shape-frequencies/shape-amplitudes. When measured using pairs of adaptors/tests positioned above and below fixation, both after-effects increase with eccentricity. Why? We have considered the following explanations: (i) scaling (magnification) of contour-shape receptive fields with eccentricity; (ii) reduced spatial interactions between the contour pairs when presented peripherally; (iii) less rapid decline of adaptation at test onset in the periphery; (iv) greater positional uncertainty in the periphery. Methods. We measured SFAEs and SAAEs as a function of eccentricity using a staircase procedure. At each eccentricity, we varied stimulus scale, the spatial separation between the contour pairs, and the time interval between adaptor offset and test onset. We also compared shape-frequency/shape-amplitude discrimination thresholds between center and periphery. Results. We found: (i) similar-size after-effects for all scales at each eccentricity; (ii) only a small increase (~10%) in the after-effects with increased spatial separation between the pair of contours when eccentricity was held constant; (iii) similar temporal rates of decline of adaptation for center and periphery, and (iv) comparable center-to-periphery ratios for shape discrimination thresholds and shape after-effects. Taken together, the results are inconsistent with all the above explanations except positional uncertainty. Conclusion. The increase in the SFAE and SAAE with eccentricity is best explained by increased positional uncertainty in the periphery.