||Attentional sampling of multiple wagon wheels.
Attending to periodic motion stimuli can induce illusory reversals of the direction of motion. This continuous wagon-wheel illusion (c-WWI) is maximal for stimuli with a temporal frequency of 10 Hz, and this has been taken to reflect discrete sampling of motion information by visual attention. An alternative view is that it is caused by adaptation. Here, we attempt to discriminate between these two interpretations by asking participants to attend to multiple stimuli: the discrete sampling account, but not the adaptation account, predicts a decrease of c-WWI temporal-frequency tuning with set size. We presented one to four rotating gratings that occasionally reversed direction while participants counted reversals. We considered reversal overestimations as manifestations of the c-WWI, and determined the temporal-frequency tuning of the illusion for each set size. Optimal temporal frequency decreased with increasing set size. This outcome favors the discrete sampling interpretation of the c-WWI, with a sampling rate for each individual stimulus dependent on the number of stimuli attended.