Local translation detection: Evidence for velocity tuned pooling of spatio-temporal frequencies

P R Schrater, D C Knill and E P Simoncelli

Published in Investigative Opthalmology and Visual Science Supplement (ARVO), vol.38 pp. S-936, May 1997.

This paper has been superseded by:
Mechanisms of visual motion detection
P R Schrater, D C Knill and E P Simoncelli.
Nature Neuroscience, vol.3(1), pp. 64--68, Jan 2000.

Purpose: Local image translations have a simple characterization in the spatio-temporal frequency domain: the power spectral density of a translating pattern lies on a plane passing through the origin. The orientation of the plane specifies the velocity of the translation. To efficiently measure local velocities the visual system should selectively pool the outputs of the early spatio-temporal filters whose peak frequencies lie in a given plane. This kind of selective pooling creates velocity tuned mechanisms. We performed psychophysical experiments to test for such preferential pooling mechanisms. Methods: Using a 2-AFC discrimination paradigm, we measured signal power thresholds for detecting filtered noise signals embedded in temporally white noise. The signals were samples of spatio-temporal gaussian white noise filtered by one of two possible configurations of 11 band pass filters: planar or 'scrambled'. In the planar configuration, the 11 filters were arranged to form an annular ring centered on a specific plane in frequency space. In the scrambled configuration, the planar configuration of filters was modified by inverting the sign of the temporal frequency of 5 of the 11 filters. The two sets of signals thus constructed had equivalent temporal and spatial frequency fingerprints, considered independently, but differed in whether or not the signal power was concentrated around a single plane in frequency space. Experiments were run using two different noise patterns - noise which was both temporally and spatially white and noise which was temporally white but was spatially filtered to have the same spatial frequency fingerprint as the signal. The latter noise was used to eliminate the possibility of using a purely spatial cue for detection. Results: Detection thresholds were about 40% lower for the planar configurations of spatio-temporal signal power than for the scrambled configurations. This significant difference was enhanced to about 65% in the noise condition which eliminated the spatial structure cue. Conclusions: The results are consistent with the hypothesis that local translation detection is mediated by mechanisms which selectively pool planar configurations of power in the spatio-temporal frequency domain.

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