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Dorita H. F. Chang, Nikolaus F. Troje; Searching for a “super foot” with evolutionary-guided adaptive psychophysics. Journal of Vision 2010;10(7):784. doi: 10.1167/10.7.784.
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The walking direction of a biological entity is conveyed by both global structure-from-motion information and local motion signals. Global and local cues also carry distinct inversion effects. In particular, the local motion-based inversion effect is carried by the feet of the walker. Here, we searched for a “super foot”, defined as the motion of a single dot that conveys maximal directional information and carries a large inversion effect, by using a psychophysical procedure driven by a multi-objective evolutionary algorithm (MOEA). We report on two rounds of searches involving the evolution of 25–27 generations each (1000 trials/generation) conducted via a web-based interface. The search involved an eight-dimensional space spanned by amplitudes and phases of a 2nd-order fourier representation of the dot's motion in the image plane. On each trial, observers were presented with multiple copies of a “foot” chosen from a population of feet stimuli for the current generation and were required to indicate whether the perceived stimulus was right- or left- facing. The stimuli were shown at upright and inverted orientations. Upon completion of a generation, each stimulus was evaluated for its “fitness” based upon its ability to convey direction and carry an inversion effect from observer accuracy rates. The fittest stimuli were then selected to form a subsequent generation for testing via methods of crossover and mutation. We show that the MOEA was effective at driving increases in accuracy rates for the upright stimuli and increases in the inversion effect, quantified as the difference between upright and inverted stimuli, across generations. We show further that the two rounds of searches, beginning at different points in space, converge towards the same region. We characterize the “super foot” in relation to current theories about the importance of gravity-constrained dynamics for biological motion perception.
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