Minimal conditions for the perception of rotary motion

Abstract.— The necessity of simultaneous length and direction changes for the perception of rotary motion in depth was studied using patterns of one to four segments of a horizontal line. The patterns changed in length only, according to one of four waveforms: A sinusoidal waveform, with both the left and right half of the pattern changing at the same rate (simulating a parallel projection of rotary motion); a sinusoidal waveform, with asymmetric left-right changes (simulating a polar projection of rotary motion); a linear waveform, the same for both the left and right halves (a simple expansion and contraction); and a linear waveform on which the asymmetries of a polar projection were superimposed. Most reports described rotary motion in depth. Both the linearity and the right-left symmetry of the waveforms affected the proportion of rotary motion responses, with the proportions greater for the sinusoidal waveforms and for the asymmetrical waveforms.