Fig 2. Spectral properties of translating motion stimuli.

a Space-time plot of a one-dimensional noise pattern moving to the right at constant speed (v = 60° s^-1). b Magnitude spectrum of translating noise, with darkness indicating larger frequency contribution. For rigid motion toward the right, components cluster around a line connecting first and third quadrant (in green) via the origin, with slope indicating true velocity. Little magnitude is assigned to frequencies in the quadrants signaling leftward motion (in brown). c Left, space-time plot of a discretized square-wave grating (v = 60° s^-1, λ = 60°, step width = 2°). Right, quantification of energy indicating right- and leftward motion, respectively. Energy was simply averaged within quadrants color-coded above. Energy distribution is not affected by half-wave rectification (i.e., zeroing either components below or above mean luminance for ON or OFF respectively). d Corresponding illustration and quantification for discretized reverse-phi stimulus. Energy distribution reverses for full-wave stimulus but signals veridical direction when rectification is applied (ON or OFF).