Stereoacuity in the periphery is limited by internal noise

It is well-established that depth discrimination is finer in the fovea than the periphery. Here, we study the decline in depth discrimination thresholds with distance from the fovea using an equivalent noise analysis to separate the contributions of internal noise and sampling efficiency. Observers discriminated the mean depth of patches of "dead leaves" composed of ellipses varying in size, orientation, and luminance at varying levels of disparity noise between 0.05 and 13.56 arcmin and visual field locations between 0° and 9° eccentricity. At low levels of disparity noise, depth discrimination thresholds were lower in the fovea than in the periphery. At higher noise levels (above 3.39 arcmin), thresholds converged, and there was little difference between foveal and peripheral depth discrimination. The parameters estimated from the equivalent noise model indicate that an increase in internal noise is the limiting factor in peripheral depth discrimination with no decline in sampling efficiency. Sampling efficiency was uniformly low across the visual field. The results indicate that a loss of precision of local disparity estimates early in visual processing limits fine depth discrimination in the periphery.