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Zixin Yong, Po-Jang Hsieh; Speed-Size Illusion Explained by Empirical Ranking Theory. Journal of Vision 2016;16(12):1130. doi: 10.1167/16.12.1130.
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© ARVO (1962-2015); The Authors (2016-present)
Large objects are perceived to move slower than smaller ones with the same physical speed. This puzzling perceptual illusion, called the speed-size illusion, is observed in daily life and deemed to be one of the causes of railway crossing collisions. Despite its prevalence in our sensory experiences, no well-grounded explanation has been given. Here we show that the speed-size illusion can be explained by objects' speed occurrence frequency, which is intrinsically embedded in the natural environment statistics. We first conducted experiments to quantify this illusion by determining the subjective equal speeds of moving spheres with various sizes. Then we obtained objects' speed distribution statistics by simulating a virtual environment, where moving objects' size and speed projected onto an image plane. The simulation served as a proxy of retinal image formation process in the natural environment, whose object motions are ambiguous and computationally challenging to track. The simulation showed that for any given speed, larger object possess a lower percentile rank in its speed accumulation probability curve, which accounts for its lower perceived speed. Furthermore, the results of our psychophysical experiments can be predicted by the simulation to a good extent. It thus implies that the speed-size illusion is a result of past experiences accumulated in the environment, and perceptual qualities are empirically evolved according to the frequency of occurrence of the corresponding stimuli. Our study corroborates the Empirical Ranking Theory1 of visual perception. 1. Purves, Dale, et al. "How biological vision succeeds in the physical world." Proceedings of the National Academy of Sciences 111.13 (2014): 4750-4755.
Meeting abstract presented at VSS 2016
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