Motion-detecting mechanisms: Dual input devices?

Displacement thresholds for luminance step edges were measured for a wide range of contrasts and mean luminances. Thresholds for extended edges (longer than about 0.5°) are determined not by contrast but rather by the amplitude (L_max-L_min) of the luminance change produced by the displacement. Arguing from the standpoint of the Marr-Ullman model of movement detection, we had expected that thresholds might be jointly determined by both contrast and amplitude. Using a range of edges of different lengths, we found that differential effects of luminance and contrast can be revealed: for short edges (less than about 0.5°) thresholds are influenced by both amplitude and contrast, while for more extensive edges only amplitude has an influence. The results are consistent with the properties of a mechanism that has two separate inputs, one from a spatial operator that is contrast-dependent and one from a temporal operator that is amplitude-dependent. The spatial operator is markedly sensitive to changes in edge extent, the temporal operator much less so. The output of the spatial operator saturates early as a function of contrast.