Stohr, R. Eric. M.S., Department of Psychology, Wright State University, 2003. Effects of Size Change on Speed Judgments of Frontal-Parallel Motion. Given that size changes of objects provide an indication of motion in depth and that humans are capable of detecting small differences in speed, this study sought to explore the perceptual ramifications of changing the size of objects during a speed discrimination task. Three experiments were conducted for this study, two speed judgment experiments and a depth matching experiment, to determine if observers interpret the size change as moving in depth and incorporate perceived depth into judgments of speed. In additon, stimuli for all experiments were presented on three backgrounds (blank, textured dot field, and optic flow field) to determine any effect of contextual background information. The stimulus for the two speed judgment experiments consisted of a white circular disk that moved right to left across the display, while maintaining, increasing, or decreasing in size. The constant virtual speed experiment stimulus coupled a hyperbolic rate of size change with a deceleration along the frontal parallel plane to simulate constant motion in depth along three “virtual” trajectories. On the other hand, the constant orthogonal speed experiment stimulus maintained constancy in terms of both the rates of size change and the frontal parallel (i.e., plane orthogonal to the line of sight) speed, which perceptually indicated acceleration (contracting stimulus) or deceleration (expanding stimulus) into depth. Both speed experiments used a two-interval, standard versus comparison, forced-choice procedure in which the task of the observer was to indicate the interval that had the fastest speed. Within each of the speed experiments, each stimulus (e.g., constant size, expanding, or contracting) served as the standard to which each other stimulus was compared (nine comparison conditions). The depth experiment stimulus was similar to the constant virtual speed experiment except that nine virtual trajectories were simulated. The task of the subject was to match the stimulus with one of the nine trajectories suggested by a reference schematic. The averaged depth “ratings” were plotted by simulated trajectory to determine if the observer perceived depth (slope of 1) or no depth (slope of 0). The results indicated that a dissociation existed between the peceived depth suggested by the depth experiment and what was used (i.e., retinal speed while ignoring size change) to judge speed in the constant virtual speed experiment. However, this dissociation was not completly carried over to the constant orthogonal speed experiment. One observer appeared to exclusively use retinal speed, while other observers were greatly affected by the size change, particularly for contracting stimuli (perceptually equivalent with acceleration away from the observer). No effect was found for background. Three conclusions were drawn from this study. First, the observers were able to interpret the size change stimuli as moving in depth. Second, when making speed judgments, most observers can ignore the size change as long as it is not associated with an acceleration into depth (contracting stimulus in orthogonal speed experiment). Third, using a center-tracking strategy is the most efficient means to judge speed based on the retinal speed of the stimulus. In sum, what you perceive is not necessarily what you use to produce speed judgments.