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Abstract:
Size cues are known to affect the perceived distance between objects. If the physical distance between two objects remains constant, the larger the retinal image of the objects is, the closer to one another they are perceived to be. We tested whether size cues affect the perceived visual speed of self-motion. Ten subjects sitting in front of a panoramic screen (230 × 125° of field of view) were presented with constant-speed translations of the visual scene, which simulated a forward translation of the body. The physical size of the objects present in the scene was systematically varied, and the subjects had to estimate the speed of the translations. We used a 2IFC task, ie, two stimuli were presented successively and the subject had to indicate which one was faster. The experiment had four conditions, with two main factors: object size (big/small) and object familiarity (absolute size known/unknown). Eye and horizon heights were constant throughout all conditions. We observed a main effect of object size, with speed being underestimated for larger objects. Interestingly, this effect was significantly stronger for objects whose absolute size is known. These results suggest a top - down influence of size cues on the perceived visual speed of self-motion.
