Unequal retinal and extra-retinal motion signals produce different perceived slants of moving surfaces.
Eye movements introduce retinal motion to the image and so affect motion cues to depth. For instance, the slant of a plane moving at right-angles to the observer is specified by translation and a component of relative motion such as shear. To a close approximation, the translation disappears from the image when the eye tracks the surface accurately with a pursuit eye movement. However, both translation and relative-motion components are needed to estimate slant accurately and unambiguously. During pursuit, therefore, an extra-retinal estimate of translation must be used by the observer to estimate surface slant. Extra-retinal and retinal estimates of translation speed are known to differ: a classic Aubert-Fleischl phenomenon was found for our stimuli. The decrease in perceived speed during pursuit predicts a corresponding increase in perceived slant when the eye tracks the surface. This was confirmed by comparing perceived slant in pursuit and eye-stationary conditions using slant-matching and slant-estimation techniques. Moreover, the increase in perceived slant could be quantified solely on the basis of the perceived-speed data. We found no evidence that relative-motion estimates change between the two eye-movement conditions. A final experiment showed that perceived slant decreases when a fixed retinal shear is viewed with increasing pursuit speed, as predicted by the model. The implication of the results for recovering metric depth estimates from motion-based cues is discussed.
Freeman
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School of Psychology, Cardiff University, PO Box 901, CF10 3YG, Cardiff, UK. freemant@cardiff.ac.uk
Fowler
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D003867:Depth Perception / Q000502:physiology*; D006801:Humans; D009039:Motion Perception / Q000502:physiology*; D011594:Psychometrics; D011698:Pursuit, Smooth / Q000502:physiology; D015349:Vision, Monocular / Q000502:physiology