How distinct sources of nuisance variability in natural images and scenes limit human stereopsis

Abstract

Stimulus variability, a form of nuisance variability, is a primary source of perceptual uncertainty in everyday natural tasks. How do different properties of natural scenes contribute to this uncertainty? Using binocular disparity as a model system, we report a systematic investigation of how various forms of natural stimulus variability impact performance in a stereo-depth discrimination task. In two closely related double-pass psychophysical experiments, that utilized a massive stimulus set sampled from a stereo-image database of real-world scenes, each of three human observers responded twice to ten thousand unique trials containing twenty thousand unique stimuli. New analytical methods reveal, from this data, the specific and nearly dissociable effects of two distinct sources of natural stimulus variability, luminance-contrast variation and local-depth variation, on discrimination performance, as well as the relative importance of stimulus-driven-variability and internal-noise in determining performance limits. Further, between-observer analyses show that both stimulus-driven sources of uncertainty make stimulus-by-stimulus responses more predictable (not less), are responsible for a large proportion of total variance, and have strikingly similar effects on different people. The consistency across observers raises the intriguing prospect that image-computable models can make reasonably accurate performance predictions in natural viewing. Overall, the findings provide a rich picture of stimulus factors that contribute to human perceptual performance in natural scenes. The approach should have broad application to other animal models and other sensory-perceptual tasks with natural or naturalistic stimuli.