EFFECT OF THE ISI ON THE VISIBLE PERSISTENCE OF A STIMULUS IN APPARENT MOTION

The persistence of briefly flashed stimuli undergoing a horizontal apparent motion is assessed as a function of the temporal interval (inter-stimulus interval or ISI) between successive locations. The main result is that the duration of persistence is increased when the ISI is reduced (within the range 1-15 msec). An increase of persistence also occurs when the spatial separation (Delta x) between successive presentations of the moving stimulus becomes larger, a well established result which is replicated here. In both cases, the elevation of persistence suggests that inhibitory processes, which are assumed to underlie the persistence-suppression, have become less efficient. According to the data, it seems that the spatio-temporal parameters of motion, and not the speed as such, are responsible for the strength of inhibition. Namely, optimal inhibition, and thus suppression, would need a minimum amount of time to take place, and would improve with proximity (i.e. with smaller Delta x). Finally, a persistence-suppression decrease is observed when the angular size of the flashed stimuli is reduced (i.e. when higher spatial frequencies become more predominant). A model of transient-on-sustained inhibition accounts well for these results.