Bischof, W. F. and Di Lollo, V. (1990). Perception of directional sampled motion in relation to displacement and spatial frequency: Evidence for a unitary motion system. Vision Research, 30, 1341-1362.
Perception of directional motion was studied by displaying two images (F1 and F2) in rapid succession. The two images were identical except for a horizontal displacement of F2 with respect to F1. Observers reported the direction of horizontal motion over a wide range of displacements. The stimuli in Experiment 1 were one-dimensional gratings with spatial frequency between 0.125 and 6 c/deg. Motion was seen at all displacements to almost 0.5 cycles (counterphase) and remained invariant across spatial frequencies. In Experiment 2 the stimuli were band-pass filtered random-dot patterns. The bandwidth of the filters was 1 octave, and centre frequencies ranged from 0.75 to 12 c/deg. In every case, the response functions exhibited quasi-periodic oscillations related to structural properties of the images. One-dimen sional analyses based on autocorrelation did not provide a satisfactory account of the data. By contrast, the data were fitted successfully by a two-dimensional analysis that integrated the responses of neighbouring motion detectors so as to yield a smooth motion flow field from which left-right directional motion could be derived. Practically and conceptually, the outcome supports a unitary motion system as distinct from separate systems subserving short-range and long-range motion.
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