PATTERN PERCEPTION AT HIGH VELOCITIES

Background: When objects are stationary, human pattern Vision is exqui sitely acute. A number of studies show, however, that Vernier acuity f or 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 analys is to low spatial frequencies. If correct, this implies that Vernier a cuity may not be subject to a strict velocity limit, and that with app ropriate low spatial frequency stimuli, Vernier acuity might be preser ved al high velocities. To test this notion, we measured Vernier acuit y and contrast discrimination using low spatial frequency periodic gra tings drifting over a wide range of velocities. Results: Vernier acuit y and contrast discrimination for low spatial frequency periodic grati ngs are both possible at velocities as high as 1000 deg sec(-1) When b oth are specified in the same units (as Weber fractions), Vernier acui ties are closely predicted by the observers' contrast discrimination t hresholds, Our results suggest that Vernier acuity is subject to a spa tiotemporal limit, rather than to a strict velocity limit. At temporal frequencies less than about 10 Hertz, Vernier acuity is independent o f velocity, but is strongly dependent on stimulus contrast. At high te mporal frequencies Vernier acuity is markedly degraded, and shows litt le dependence on contrast. Conclusions: Two mechanisms, which may have their neuronal counterparts early in the visual pathway, appear to li mit the perception of moving targets at low and high temporal frequenc ies. 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.