Abstract

The findings of Levinson and Sekuler¹ and of Watson et al.² suggest that flickering gratings, which can be represented as a sum of two moving gratings, are detected by two independent rightward and leftward sensitive motion mechanisms. However, the recent work of Stromeyer et al.³ shows a failure of this independence above absolute threshold and points to a motion opponent mechanism. To further investigate these issues, we have been exploring the space of stimuli consisting of sums of rightward and leftward moving gratings, of equal spatial frequencies and speeds, but with varying contrasts. We can represent these stimuli by plotting the rightward and leftward component contrasts on vertical and horizontal axes. In this space, points on the diagonal correspond to counterphase flickering gratings, while those along the axes correspond to single moving gratings. Other points in the space represent mixtures of flicker and motion. We have measured discrimination thresholds in the various possible directions in this space around points on the axes, on the diagonal, and in the regions between. These thresholds define contours somewhat analogous to ellipses of confusion in color discrimination. The data cannot be fit with existing motion models. We present a model in which two kinds of mechanism (opponent motion detectors and counterphase flicker detectors) operate in parallel. These detectors consist of energy extracting mechanisms like those described by Adelson and Bergen⁴ followed by static nonlinearities.

© 1985 Optical Society of America