Two definitions of persistence in visual perception

It is well known that the perception of a visual stimulus does not terminate with the offset of that stimulus, but decays with time. Although a variety of specific methods have been developed to study this phenomenon, these methods have not always yielded consistent results. We believe that some of the discrepancies in the literature on visual persistence have arisen because different researchers have based their estimates of persistence on qualitatively different features of the visual residual. Efron (1970) asked subjects to temporally align the onset of a click with the offset of a brief flash. Persistence, defined as the interval between flash offset and onset of the indexing click, was found to decrease with increasing flash intensity. Bowen, Pola, and Matin (1974) asked subjects to adjust the temporal placement of a probe flash so that it was coincident with the apparent offset of a spatially separated test flash. Persistence was defined as the interval between test flash offset and onset of the indexing flash. Again, measured persistence decreased with increasing test stimulus intensity. The same result has been reported by Hansteen (1971) using a procedure resembling that of Bowen et al. In a somewhat different approach, Haber and Standing (1969) presented a repetitive cycle in which a stimulus was followed by a blank interval. Subjects were asked to adjust the duration of the blank interval between successive stimulus presentations so as to produce a discontinuity. Once again, persistence, defined by the duration of the blank interval between successive stimuli, declined with increases in the intensity of the test stimulus. Other researchers, however, using different procedures, have reported that stimulus intensity has the opposite effect on persistence. In one of a series of experiments, Sakitt (1976) asked observers to temporally align the onset of a click with the disappearance of the residual (icon) of a briefly exposed letter array. Her measure of persistence, the temporal inter-