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Stuart Anstis, Alan Ho; Apparent speed of a rotating disk varies with texture density . Journal of Vision 2014;14(10):1333. doi: https://doi.org/10.1167/14.10.1333.
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© ARVO (1962-2015); The Authors (2016-present)
A circle of white dots on a black surround was made to rotate on different trials at speeds ranging from 5.6째/s to 360째/s (= 0.016 to 1 rev/min). We found, by a matching method, that the apparent speed of rotation increased as the number of dots increased. Result: when compared across all speeds with a circle of eight dots, four dots appeared to rotate 16% more slowly, and sixteen dots appeared to rotate 33% faster. Same results were found for second-order random-texture dots that rotated on a background of the same texture. (However, if 16 dots were alternately 8 black and 8 white dots on a grey surround, their apparent speed was the same as for 8 dots.) Both spatial and temporal factors played a part in this illusion. Speed = Distance/Time, or equivalently, for a drifting grating, Speed = Temporal frequency/Spatial frequency. So dots spaced far apart round a circle had a long spatial period (equivalent to low spatial frequency for a grating), and perceived speed was judged partly as the time for an object to move through its own diameter, so four far-apart dots seemed to move more slowly than 16 close-together dots (JF Brown 1927, 1931). Also, sixteen dots moving past a fixed point had 4 times the temporal frequency of four such dots, which contributed to their appearing to move faster.
Meeting abstract presented at VSS 2014
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