Temporal Integration in Visual Memory

Iconic memory has often been likened to a sensory store whose contents drain away rapidly as soon as the inducing stimulus is turned off. Instances of short-lived visible persistence have been explained in terms of the decaying contents of iconic store. A fundamental requirement of this storage model is that strength of persistence should be a decreasing function of time elapsed since the cessation—not since the onset—of the inducing stimulation. That is, strength of visible persistence may be directly related—but not inversely related—to the duration of the inducing stimulus. Two complementary paradigms were utilized in the present studies. In the first paradigm performance was facilitated by visible persistence in that the task required the bridging of a temporal gap between two successive displays. In the second paradigm (forward visual masking by pattern), performance was impaired by lingering visible persistence of the temporally leading mask. Both paradigms yielded evidence of an inverse relationship between duration of inducing stimulus and duration of visible persistence. More specifically, in a task requiring temporal integration of a pattern displayed briefly in two successive portions, performance was severely impaired if the duration of the leading part exceeded about 100 msec. This suggests an inverse relationship between duration of inducing stimulus and duration of sensory persistence and allows the inference that visual persistence may be identified more fittingly with ongoing neural processes than with the decaying contents of an iconic store. In keeping with this suggestion, two experiments disconfirmed the conjecture that lack of temporal integration following long inducing stimuli could be ascribed to emergence of unitary form separately in the two portions of the display or to the triggering of some sort of discontinuity detection mechanism within the visual system. In added support of a "processing" model, two further studies showed that the severity of forward masking by pattern declines sharply as the duration of the leading mask is increased. This pattern of results is equally unsupportive of a storage theory of iconic persistence as of perceptual moment theory in any of its versions. This is so because both theories regard interstimulus interval rather than stimulus-onset asynchrony as the crucial factor in temporal integration. Neither can the results be explained in terms of receptor adaptation or of metacontrast suppression. The theory of inhibitory channel interactions can encompass the more prominent aspects of the results but fails to account for foveal suppression and for some crucial temporal effects.