Iconic memory

Iconic memory is a type of short term visual memory (a sensory memory), named by George Sperling in 1960. Experiments performed by Sperling and colleagues provided evidence for a rapidly decaying sensory trace, lasting only approximately 250 ms after the offset of a display. [Averbach & Sperling, 1961; Sperling, 1960]

Background

Although recollection of four-to-five alphanumeric characters can commonly be reported after a single brief exposure (e.g., James McKeen Cattell, 1886), observers have frequently reported the phenomenal impression of many more elements immediately after display presentation (e.g., Gill & Dallenbach, 1926).

These two observations can be reconciled if it is assumed that visual memory [http://moodle.ed.uiuc.edu/wiked/index.php/Memory%2C_visual] consists of two parts: one, a rich, but rapidly decaying sensory trace of the entire stimuli display; and two, a short-term memory store of lower capacity, able to retain its contents over several seconds.

Provided the sensory trace decayed with sufficient speed, at the point of recall observers would only have the contents of the second, longer-lived, but lower capacity memory store available for report.

perling experiments

In a series of experiments, Sperling (1960) investigated and confirmed this bipartite model of memory.

In Sperling's (1960) experiments, observers were presented for a short period of time with displays composed of between 3 and 16 alphanumeric characters. Memory performance was compared under two conditions, referred to as whole report and partial report.

In the whole report condition observers were asked to recall as many elements fromthe display as possible. Observers were typically able to recollect four to fivecharacters, irrespective of how many other characters were present within thedisplay. This finding is consistent with previous studies of a similar kind (e.g.,Cattell, 1886), and suggests that whole report is limited by a memory system with acapacity of four-to-five items.

In Sperling's (1960) partial report condition observers were required to identify asubset of the characters within the visual display. At various intervals after theremoval of the visual display a tone was sounded. The frequency of the tone (e.g.,high, medium, or low) indicated to observers which particular set of characterswithin the display they were to report (e.g., the top, middle or bottom row). In orderto ensure that limits evident in the whole report task were not a factor forperformance in the partial report condition, all rows were composed of only three orfour characters (i.e., less than the limit associated with whole report).Performance in the partial report condition can be regarded as a random sample ofan observer's memory for the entire display. This type of sampling revealed that forshort intervals following exposure observers have a much better memory thansuggested by the whole report procedure. For instance, results using partial reportsuggest that observers retain about 12 characters from a 16-character array inmemory immediately after presentation of the array (Averbach & Sperling, 1961;Sperling, 1960).

By varying the time between the offset of the display, and the sounding of theauditory cue, Sperling (1960) was able to probe for changes in accuracy of reportover time. Using this technique, the initial memory for a stimulus display wasfound to decay rapidly after display offset; until approximately 1000 ms afterstimulus offset the overall memory span estimated by the partial reportapproximated that of whole report. In a series of experiments using partial report,Averbach showed that replacing the auditory cue (as used by Sperling, 1960) with avisual cue produced an equivalent pattern of results (for a summary of thisresearch, see Averbach & Sperling, 1961). Overall, experiments using partial reportprovided evidence for a rapidly decaying sensory trace, lasting for approximately1000 ms after the offset of a display (Averbach & Sperling, 1961; Sperling, 1960).Sperling (1960) named this type of store iconic memory.

Iconic memory was discovered to be quite fragile: eye movements occurring between stimulus offset and the onset of the cue were found to be highly disruptive, suggesting that iconic memory might be organized in a retinotopic manner. [Sperling, 1960] Further evidence of its fragility was demonstrated by the finding that iconic memory decayed much more rapidly if the stimulus display was immediately followed by a bright background rather than a dark background. [Averbach & Sperling, 1961]

Iconic memory in context

Interestingly, the majority of memory models up until the late 1960s considerediconic memory to be the only true visual trace (for representative ideas, seeSperling, 1963; 1967). The memory associated with whole report was not regardedas visual memory – perhaps not surprising given the use of alphanumericcharacters as stimuli – but as a form of categorical knowledge, encoded perhaps asa form of auditory trace (Sperling, 1967). For instance, Sperling (1967) proposed that information available for whole report was maintained viasubvocal rehearsal in auditory memory. Of course logically the information in iconic memory might alsohave been stored as a non-visual trace, but it appears for theoretical reasons this possibility was neverconsidered.

Iconic memory was viewed as a short-term sensory buffer, allowing time for sensory information to be recoded in a more permanent, categorical manner (Sperling, 1963, 1967). It was the pioneering work of Phillips (1971) that changed this view, and introduced the concept of a purely visual, short-term memory store.

Notes

References

Averbach, E., & Sperling, G. (1961). Short term storage of information in vision. InC. Cherry (Ed.), Information Theory (pp. 196-211). London: Butterworth.

Cattell, J. M. (1886). The inertia of the eye and brain. Brain, 8, 295-312.

Gill, N. F., & Dallenbach, K. M. (1926). A preliminary study of the range of attention.American Journal of Psychology, 37, 247-256.

Sperling, G. (1960). The information available in brief visual presentations.Psychological Monographs: General and Applied, 74(11), 1-30.

Sperling, G. (1963). A model for visual memory tasks. Human Factors, 5, 19-31.

Sperling, G. (1967). Successive approximations to a model for short term memory.Acta Psychologica, 27, 285-292.