Abstract
Stimulus variability is the 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. We show, using new analytical methods, the specific impact that two distinct sources of stimulus variability—luminance-pattern variation and local-depth variation—have on discrimination performance. First, we obtained a stimulus set, sampled from a natural stereo-image database with co-registered laser-based range-data at each pixel; this data set contains both natural luminance and natural depth variability. Next, we generated a second data set from the first, by flattening the local-depth profile of each stimulus; this data set contains near-identical luminance variability, but no depth variability. Stimuli were presented using a 2IFC paradigm. Each interval of each trial contained a unique stereo-image patch. The task was to report which patch appeared to be farther away. Each of three human observers collected 20,000 trials across two double-pass stereo-depth-discrimination experiments—one with each stimulus set. The stimuli in each trial were matched across passes, within and between experiments. The resulting stereo-depth discrimination thresholds indicate the absolute limits of discrimination performance at five disparity pedestals and three disparity-contrast levels. Analysis of each double-pass experiment indicates the relative importance of stimulus-variability and internal-noise in determining performance limits. And a novel quasi-quadruple-pass analysis, comparing the data from the two double-pass experiments, allows us to partition how luminance-pattern variability and local-depth variability limit performance in natural scenes. Our results show that luminance-pattern variation and local-depth variation have distinct and, surprisingly, almost completely dissociable effects on performance, and provide a rich picture of the factors contributing to human disparity discrimination performance in natural scenes.