Iconic memory and visible persistence

Abstract

There are three senses in which a visual stimulus may be said to persist psychologically for some time after its physical offset. First, neural activity in the visual system evoked by the stimulus may continue after stimulus offset ("neural persistence"). Second, the stimulus may continue to be visible for some time after its offset ("visible persistence"). Finally, information about visual properties of the stimulus may continue to be available to an observer for some time after stimulus offset ("informational persistence"). These three forms of visual persistence are widely assumed to reflect a single underlying process: a decaying visual trace that (1) consists of afteractivity in the visual system, (2) is visible, and (3) is the source of visual information in experiments on decaying visual memory. It is argued here that this assumption is incorrect. Studies of visible persistence are reviewed; seven different techniques that have been used for investigating visible persistence are identified, and it is pointed out that numerous studies using a variety of techniques have demonstrated two fundamental properties of visible persistence: the inverse duration effect (the longer a stimulus lasts, the shorter is its persistence after stimulus offset) and the inverse intensity effect (the more intense the stimulus, the briefer its persistence). Only when stimuli are so intense as to produce afterimages do these two effects fail to occur. Work on neural persistences is briefly reviewed; such persistences exist at the photoreceptor level and at various stages in the visual pathways. It is proposed that visible persistence depends upon both of these types of neural persistence; furthermore, there must be an additional neural locus, since a purely stereoscopic (and hence cortical) form of visible persistence exists. It is argued that informational persistence is defined by the use of the partial report methods introduced by Averbach and Coriell (1961) and Sperling (1960), and the term "iconic memory" is used to describe this form of persistence. Several studies of the effects of stimulus duration and stimulus intensity upon the duration of iconic memory have been carried out. Their results demonstrate that the duration of iconic memory is not inversely related to stimulus duration or stimulus intensity. It follows that informational persistence or iconic memory cannot be identified with visible persistence, since they have fundamentally different properties. One implication of this claim is that one cannot investigate iconic memory by tasks that require the subject to make phenomenological judgments about the duration of a visual display. In other words, the so-called "direct methods" for studying iconic memory do not provide information about iconic memory. Another implication is that iconic memory is not intimately tied to processes going on in the visual system (as visible persistence is); provided a stimulus is adequately legible, its physical parameters have little influence upon its iconic memory. The paper concl...

Frequency judgments: The problem of defining a perceptual event.

“…With only seven stimuli in each grid, and no masking stimulus, even the shortest stimulus duration (50 ms) was sufficient to produce a brief impression of a matrix .of letters. Such exposures typically produce correct immediate reports of a substantial number of letters when they are tested individually by location cuing (Sperling, 1960;Coltheart, 1980). Subjects in all groups were shown a sample grid and were told that they would be viewing a series of 4 × 4 grids containing symbols in different typefaces.…”

The visible persistence of stimuli in stroboscopic motion

“…100 msec, it should appear to temporally overlap stimuli that follow in less than 100msec. Previous estimates of the duration of visible persistence based on this method range between 100 and 300msec (Coltheart, 1980). When the distance and time between successive stimuli approaches zero, as in the case of real motion, the duration of visible persistence can be estimated by the length of an object's blur streak.…”

Perceptual consciousness overflows cognitive access

“…I mentioned that earlier work (summarized in [2,10]) showed that iconic memory is not based in early vision, but, in light of these findings, one sees that the so-called 'iconic memory' that was the subject of those Sperling experiments was an amalgam of two different kinds of memory: a rod-based 'pure iconic' memory lasting at most a few hundred msec and a much longer-lived fragile VSTM that is based higher up in the visual system and lasts up to 4-5 seconds. Indeed, Sligte et al [9] found neuroimaging evidence of long lasting spatiotopic representations that correspond to fragile VSTM in visual area V4 but not in the lower areas V1, V2 or V3.…”

“…By relying on this form of memory, we isolate consciousness from the immediate feed of the world and the retina. The neural locus of the high memory capacity demonstrated in the Sperling phenomenon is in brain areas that are candidates for the neural basis of conscious perception rather than in the retina or early vision [2,[8][9][10], and so the Sperling phenomenon may reveal the capacity of conscious phenomenology.…”