Abstract
Visual cues to motion-in-depth include binocular signals, such as changing disparity (CD) and interocular velocity differences (IOVD), and monocular changing image size (CS) cues. Objects moving in depth at image locations above and below an observer’s line of sight also contain further monocular motion cues, arising due to both perspective projection and the statistics of natural scenes. Here, we assess whether such cues affect the perception of motion-in-depth. Participants were presented with pairs of fronto-parallel surfaces, moving towards the observer at a constant speed in a two-interval-forced-choice (2IFC) procedure. Stimuli were presented above or below the line of sight and were compared to a motion-in-depth stimulus presented at the centre of the screen, with participants asked to choose the faster interval. Varying the speed of the central stimulus allowed the recovery of psychometric functions describing the proportion of ‘centre faster’ responses as a function of speed. Points of subjective equality (PSEs) for these functions showed a bias to perceive stimuli presented in upper and lower visual fields as faster than stimuli presented at the centre of the screen, although this bias was most pronounced for lower visual field presentations. To assess the contribution of different potential sources for these biases, including visual field location and vertical motion due to perspective projection, in a series of control experiments we manipulated perspective motion signals and measured visual field biases at different reference speeds. Our results support roles for multiple stimulus factors and provide a basis for height-in-field cues for motion-in-depth.