Pattern perception at high velocities

BACKGROUND When objects are stationary, human pattern vision is exquisitely acute. A number of studies show, however, that Vernier acuity for lines is greatly impaired when the target velocity exceeds about 5 deg sec-1. The degradation of line Vernier acuity under image motion appears to be a consequence of a shift in the spatial scale of analysis to low spatial frequencies. If correct, this implies that Vernier acuity may not be subject to a strict velocity limit, and that with appropriate low spatial frequency stimuli, Vernier acuity might be preserved at high velocities. To test this notion, we measured Vernier acuity and contrast discrimination using low spatial frequency periodic gratings drifting over a wide range of velocities. RESULTS Vernier acuity and contrast discrimination for low spatial frequency periodic gratings are both possible at velocities as high as 1000 deg sec-1. When both are specified in the same units (as Weber fractions), Vernier acuities are closely predicted by the observers' contrast discrimination thresholds. Our results suggest that Vernier acuity is subject to a spatiotemporal limit, rather than to a strict velocity limit. At temporal frequencies less than about 10 Hertz, Vernier acuity is independent of velocity, but is strongly dependent on stimulus contrast. At high temporal frequencies Vernier acuity is markedly degraded, and shows little dependence on contrast. CONCLUSIONS Two mechanisms, which may have their neuronal counterparts early in the visual pathway, appear to limit the perception of moving targets at low and high temporal frequencies. Taken together with other recent work the present results suggest that the process of spatio-temporal interpolation in pattern analysis can operate at very high velocities.