Iconic memory

Although 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 consists of two parts: one, a rich, but rapidly decaying sensory trace of the entire stimulus 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. In an elegant 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-to-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 from the display as possible. Observers were typically able to recollect four-to-five characters, irrespective of how many other characters were present within the display. 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 a capacity of four-to-five items.

In Sperling's (1960) partial report condition observers were required to identify a subset of the characters within the visual display. At various intervals after the removal 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 characters within the display they were to report (e.g., the top, middle or bottom row). In order to ensure that limits evident in the whole report task were not a factor for performance in the partial report condition, all rows were composed of only three or four characters (i.e., less than the limit associated with whole report). Performance in the partial report condition can be regarded as a random sample of an observer's memory for the entire display. This type of sampling revealed that for short intervals following exposure observers have a much better memory than suggested by the whole report procedure. For instance, results using partial report suggest that observers retain about 12 characters from a 16-character array in memory 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 the auditory cue, Sperling (1960) was able to probe for changes in accuracy of report over time. Using this technique, the initial memory for a stimulus display was found to decay rapidly after display offset; until approximately 1000 ms after stimulus offset the overall memory span estimated by the partial report approximated 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 a visual cue produced an equivalent pattern of results (for a summary of this research, see Averbach & Sperling, 1961). Overall, experiments using partial report provided evidence for a rapidly decaying sensory trace, lasting for approximately 1000 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).

Interestingly, the majority of memory models up until the late 1960s considered iconic memory to be the only true visual trace (for representative ideas, see Sperling, 1963; 1967). The memory associated with whole report was not regarded as visual memory – perhaps not surprising given the use of alphanumeric characters as stimuli – but as a form of categorical knowledge, encoded perhaps as a form of auditory trace (Sperling, 1967). For instance, Sperling (1967) proposed that information available for whole report was maintained via subvocal rehearsal in auditory memory. Of course logically the information in iconic memory might also have been stored as a non-visual trace, but it appears for theoretical reasons this possibility was never considered. 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.