Abstract

Two experiments are described that are designed to investigate what determines the maximum spatial displacement detectable (dmax) for spatially broadband patterns exposed in a two-frame motion sequence. In experiment 1, dmax was found to be 1.63 times greater for a two-dimensional (2-D) broadband random pattern with a 1/f Fourier amplitude spectrum (equal contrast in each octave) than for a 2-D binary-valued random-dot pattern with a flat spectrum (higher contrast in higher-frequency octaves). In experiment 2, dmax was shown to vary in inverse proportion to the lowest stimulus frequency for random patterns with a one-octave bandwidth and normalized contrast. Furthermore, when these five one-octave patterns were summed together, dmax for this new five-octave pattern was found to be only 1.46 times lower than dmax for the lowest-frequency one-octave pattern presented alone. A model is described in which direction discrimination is based on the nearest-neighbor matching of zero crossings in the output of a single-spatial-filter bandpass in both spatial frequency and orientation. Data from the model show that the difference between dmax for the five-octave and the lowest one-octave patterns can be accounted for by the same filter passing some of the additional higher frequencies in the former pattern. Furthermore, it is argued that all the data can be accounted for by assuming that dmax is determined by the coarsest spatial filter activated by each stimulus. Modeling the results of both experiments suggests that the bandwidth of this filter is ~2.6 octaves and reaches peak sensitivity at ~0.47 c/deg. The model is shown to be capable of accounting for a wide range of other two-frame dmax data.

© 1996 Optical Society of America

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