Fig. 2.

Motion energy and temporal subsampling. (a) A 1D horizontally drifting sinusoidal grating as shown in the space-time plot on the left is represented in the Fourier domain by a pair of spatio-temporal components, in diagonally opposite quadrants (Right). (b) When the same motion stimulus is subsampled in time, spurious motion components appear in the Fourier spectrum. We estimate the intensity of motion perception in the actual and opposite directions as the sum of motion energies over the corresponding quadrants of the Fourier spectrum (marked in the figure as “real” and “illusory” motion, respectively). (c) Assuming that this simple subsampling mechanism contributes half of the total motion percept, and using low-pass motion sensitivity envelopes derived from experimental data, the model predicts rates of illusory reversals for 1st-order and 2nd-order motion that closely resemble those observed experimentally (compare with Fig. 1 c and d).