Ideal observer for heading judgments.

Several aspects of the viewing situation affect the ability to determine heading from optical flow. These include the amount of depth variation and number of texture elements in the scene, the location and amount of the visual field stimulated, and the position of the focus of expansion within the stimulus. Without a quantification of the discrimination information provided by the stimuli presented to the observer, it is impossible to determine how much of an observed change in performance reflects the properties of neural mechanisms and strategies employed by the observer. To enable a better quantification, we developed an ideal observer for the discrimination of heading from random-dot flow fields. Internal noises of the ideal observer were set by the results of single-dot velocity discrimination experiments. We compared human and ideal observer performance in discriminating headings with different patterns of flow (e.g. radial vs laminar) presented on different parts of the retina. Efficiency--the ratio of ideal and human thresholds--was fairly constant for the various flow patterns and retinal eccentricities. This outcome indicates that most of the variation in human observers' ability to estimate heading from the flow patterns and retinal loci considered here is due to changes in the discrimination information provided by the stimulus after measurement by the visual system. In the discussion, we show how the ideal observer can be used to quantify the spatial distribution of heading discrimination information for any observer translation through any scene represented by dots.