Modelling human motion perception. II. Beyond Fourier motion stimuli.

In the first part of this review a basic mechanism of motion perception was illustrated. The elementary motion detector (EMD) of the correlation type can account for the detection of "Fourier" motion stimuli in which the spatial intensity distribution on the retina is shifted over time. In recent years, novel classes of stimuli such as "drift-balanced" or "theta" motion (in which the picture elements carrying luminance contrast do not move, or move in the opposite direction to the traveling object defined by such element motion) were introduced into psychophysics. Such stimuli may play an important role in the understanding of "higher" visual processing which goes beyond the pure detection of motion. Thus, in the second part of the review, the question will be addressed as to what further processing steps, or more sophisticated mechanisms than the EMD, have to be assumed in order to understand more complex aspects of human motion perception.