Motion capture depends on signal strength

When flickering dots are superimposed onto a drifting grating, the dots app ear to move coherently with the grating. In this study we examine: (i) how the perceived direction of a compound stimulus composed of superimposed gra ting and dots, moving in opposite directions with equal speeds, is influenc ed by the relative strength of the motion signals; (ii) how the perceived s peed of a compound stimulus composed of superimposed grating and dots, movi ng in the same direction but at different speeds, is influenced by the rela tive strength of the motion signals; and (iii) whether this stimulus is dis criminable from its metameric speed match. Dot signal strength was manipula ted by using different proportions of signal dots in noise and different do t lifetimes. Both the perceived direction and speed of these compound stimu li depended upon the relative motion-signal strengths of the grating and th e dots. Those compound stimuli that appeared coherent were not discriminabl e from the speed-matched metameric compound stimuli. When the signals were completely integrated into a coherent compound stimulus, the local motion s ignals were no longer perceptually available, though both contributed to th e global percept. These data strongly support a weighted-combination model where the relative weights depend on signal strength, instead of a winner-t akes-all model.