Abstract
Purpose: Veridical perception of the environment requires the integration of multiple depth cues. Previous work has shown that relative motion (RM) and binocular disparity (BD) combine in shape perception, and that BD and size change (SC) interact in time to contact estimation. How do these multiple cues combine in signalling motion through space? We used the radial speed illusion (in which an expansion pattern appears faster than a rotation pattern of the same local dot speed) as a measure of the contribution of BD and SC to the perception of motion-in-depth.
Design and Methods: Points of subjective equality (PSE) were obtained from 3 naive observers between an expanding random dot pattern, signalling approaching motion-in-depth, and a rotating pattern. Each dot was a narrow-band Difference of Gaussian. The disparity of the RDK's was manipulated in three experiments: in Experiment 1 BD was fixed at one of 7 levels during each trial, signalling a fixed depth plane; in Experiment 2 BD was varied throughout the trial, signalling receding motion-in-depth. In Experiment 3 disparity and shrinking dot size signalled receding motion-in-depth.
Results: Experiment 1: For all observers defining the expanding pattern at a fixed depth plane made no significant difference to the magnitude of the speed illusion.
Experiment 2: For all observers conflicting dynamic disparity signalling receding motion made no impact on the speed illusion.
Experiment 3: Both receding dynamic disparity and diminishing size cues combined did not reduce the speed illusion.
Conclusions: The relative motion information contained in an expanding radial motion pattern is such a strong cue to motion-in-depth that incongruent size change and disparity cues, whether fixed or dynamic, have no impact on the magnitude of the radial speed illusion. In terms of cue combination theory this represents a strong example of cue veto that should be reflected in cue combination models.
Acknowledgement: R. Watson funded by ESRC award R00429934458