TEMPORAL FILTERING ENHANCES DIRECTION DISCRIMINATION IN RANDOM-DOT PATTERNS

In conventional presentations of random-dot kinematograms, two frames of random dots are presented in temporal sequence, separated by a blan k inter-stimulus interval, and a coherent offset in spatial position i s added to dots in one frame relative to dots in the other frame. Dire ction discrimination performance is limited temporally to inter-stimul us intervals below about 100 msec (T-max). Experiments are described i n which temporal smoothing was applied to the onset and offset of each frame in the kinematogram. T-max was found to increase in proportion with the time constant of the temporal smoothing function. An explanat ion based on contrast-dependent responses in simple motion detectors c annot accommodate the results. Instead, the increase in T-max with tem poral smoothing, and analogous increase in spatial limit (D-max) with spatial blurring, can be related to the spatiotemporal frequency conte nt of the stimulus, Random-dot kinematograms can be viewed as continuo usly drifting patterns that have been discretely sampled at regular sp atiotemporal intervals. Sampling introduces artefacts (alias signals), which become more intrusive as sampling rate declines (i.e. inter-sti mulus interval or spatial displacement increases) and consequently lim it discrimination performance. Temporal smoothing or spatial blurring extends performance because it removes alias signals generated by high spatiotemporal frequencies in the pattern. Computational modelling to estimate the Fourier energy available in random-dot kinematograms con firmed that the sampling account can predict the proportional increase in T-max and D-max limits as filter time or space constant increases.