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 blank inter-stimulus interval, and a coherent offset in spatial position is added to dots in one frame relative to dots in the other frame. Direction discrimination performance is limited temporally to inter-stimulus intervals below about 100 msec (Tmax). Experiments are described in which temporal smoothing was applied to the onset and offset of each frame in the kinematogram. Tmax was found to increase in proportion with the time constant of the temporal smoothing function. An explanation based on contrast-dependent responses in simple motion detectors cannot accommodate the results. Instead, the increase in Tmax with temporal smoothing, and analogous increase in spatial limit (Dmax) with spatial blurring, can be related to the spatiotemporal frequency content of the stimulus. Random-dot kinematograms can be viewed as continuously drifting patterns that have been discretely sampled at regular spatiotemporal intervals. Sampling introduces artefacts (alias signals), which become more intrusive as sampling rate declines (i.e. inter-stimulus interval or spatial displacement increases) and consequently limit 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 confirmed that the sampling account can predict the proportional increase in Tmax and Dmax limits as filter time or space constant increases.