Motion perception: a 'quasi-linear' mechanism tuned for temporal frequency, and a 'non-linear' mechanism tuned for velocity per se

The aim of these experiments is to investigate the sensitivity of the human visual system to two types of image motion: non-Fourier and comparable Fourier motion. Contrast thresholds were measured for detection and direction discrimination as a function of spatial frequency and velocity in the near-periphery. The stimuli were cosine wave carriers modulated by Gaussian envelopes. For non-Fourier motion the micro-patterns were displaced such that the sinewave carrier remained stationary while the envelope drifted. For Fourier motion the whole stimulus was drifted smoothly. In the near periphery (0.5-1.5 degrees), contrast thresholds for detection and direction discrimination are the same for stimuli drifting with 'Fourier motion' and show that the underlying mechanism is comprised of units tuned for temporal frequency. For 'non-Fourier motion' stimuli this is not the case. Firstly, the detection and direction discrimination thresholds are not the same. Detection remains constant across a wide range of velocities, whereas direction discrimination shows band-pass tuning with respect to velocity. Secondly, the mechanism that underlies the perception of such motion is comprised of units tuned for velocity per se and not temporal frequency.