Can the Visual System Measure Expansion Rates without using Optic Flow?

Abstract

As an observer moves towards a surface, the visual image of the surface expands over time. Traditional approaches to measuring this expansion rely on computing the divergence of the local flow field. A complementary approach which would work well in contexts with poor flow information would be to measure shifts in the Fourier amplitude spectra of textures over time (analogous to size changes). To test whether the visual system uses the information provided by such a measure, we have created a set of novel stimuli which contain, on average, no local flow. We filtered successive frames of spatiotemporally white noise with band-pass spatial filters whose peak frequency varied in inverse proportion to time. The resulting stimulus is temporally uncorrelated, but contains changes in the spatial Fourier amplitude spectrum consistent with a constant expansion rate. We tested whether subjects could consistently match the expansion rate of such stimuli, which have no local flow information, to the expansion in random-dot kinematograms designed to have no change in their Fourier amplitude spectrum over time. A temporal 2AFC task was employed to find points of subjectively equal expansion between the two types of stimuli. Several rates of expansion were tested and the start and end frequencies in the textured non-flow stimuli were randomised between trials. We found that the matched expansion rates of the non-flow texture stimuli were monotonic and nearly linear with the true expansion rates of the dot (flow) stimuli. Settings were also consistent across different start and end frequencies. The results suggest that the visual system can use the change in the amplitude spectra of textures over time to make judgments about expansion.