Lecturer's Précis - Sperling (1960)

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First published online 13:44 BST 30th May 2002, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 14th January 2010

An earlier version of this material appeared in Smith (1996; Chapter 2). It is repeated here with additional detail and supported with hyperlinks.

 

Sperling's (1960) Partial Report Paradigm

See firstly the supporting commentary for this material.

This story begins in 1890 with an observation by William James concerning the ability of the visual system to maintain what he called "after-images". Thus:

"If we open our eyes instantaneously upon a scene, and then shroud them in complete darkness, it will be as if we saw the scene in ghostly light through the dark screen. We can read off details in it which were unnoticed whilst the eyes were open. (James, 1890:645; emphasis added.)

This ability was studied by George Sperling, at the time a post-doctoral researcher at Bell Telephone Laboratories, Murray Hill, New Jersey, in 1960. He showed that a more or less complete, almost photographic, but very rapidly decaying image, is maintained by the visual system for up to half a second after the disappearance of an external stimulus; in other words, that there was a new type of memory to be taken into account before short term memory (STM) was ever reached. This new type of memory came to be called either sensory memory, because it seemed to be a property of the pre-perceptual sensory pathways, or very short term memory (VSTM), because it decayed so quickly.

Sperling's chosen methodology was the partial report paradigm. This is an experimental design wherein recall of only part of a stimulus array is required. This makes it a powerful paradigm when investigating rapidly decaying memory, because it keeps the length of the act of recall - otherwise a major confounding variable in its own right - to a minimum. The paper thus starts from the simple presumption that more is seen than can actually be remembered. The fuller argument goes like this:

Sperling's Rationale: If a subject is shown an array of items for a short period of time, and then asked to report only part of that array, the amount of information actually present in memory at the moment of asking can be calculated by multiplying up the partial report score in the proportion whole:part. Thus if you only reproduce one third of the array you need to triple your score, and so on. Selection of the part array should be randomised, so as to prevent the subject guessing in advance where to attend. The giving of the recall instruction is known as "cueing".

Sperling describes seven separate experiments, four of which are summarised below. The stimuli in all cases were tachistoscopically presented letter cards. Letters and/or digits were arranged on these cards in single rows of three to seven symbols per row, or as matrices of two or three rows by three or four symbols per row, as now shown .....

Sperling's (1960) Partial Report Paradigm - Typical Stimulus Card: Here are two specimen stimulus cards, namely a four-over-four and a three-by-three letter array. 

PICsperling1960a.gif

 

Redrawn from Sperling (1960; Figure 2). This version Copyright © 2002, Derek J. Smith.

 

Base line performance in the experimental set-up was established by showing each card for 50 msec. (ie. 1/20 sec.), and scoring correct recall. Responses were recorded in writing on a blank response grid containing one blank response box for each stimulus character. A response character's identity and position both had to be correct to be scored as such. Subjects typically scored perfectly on the three- or four-letter cards, but averaged only 4.5 right answers on all the others. This represents a reliable upper limit on how much information can be taken in, or "apprehended", at a single glance, and is known as the span of apprehension. Performance remained the same even if the exposure time went down as low as 15 msec., and increased only marginally if it was increased to 500 msec. Exposure time per se was not an important variable, therefore.

The simplest partial report condition was then created by showing two-row stimulus cards followed immediately by a high or low tone. The high tone signified that the subject was to recall only the top row, and the low tone signified that the subject was to recall only the bottom row. Some practice was necessary, but performance on the six-item cards eventually stabilised at an average of 2.8 per three-item line, or an equivalent 5.6 items per card. This was significantly greater than the 4.5 items averaged in the original total report condition. Here is the basic sequence of events, expressed diagramatically .....

Sperling's (1960) Partial Report Paradigm - Basic Sequence of Events: Here is a schematic representation of what happens to a four-over-four letter array during a single partial report trial. One or other of the rows would be cued after stimulus offset (either immediately or after a controlled delay period). Subjects are then able to ignore the non-cued row altogether, leaving them only four letters to recall. Since this is below the 4.5 item span of apprehension (see above), a perfect response is expected, as shown. 

PICsperling1960b.gif

 

Copyright © 2002, Derek J. Smith.

 

A more complicated partial report condition was created with the three-row stimulus cards by introducing a third, mid-range, tone to cue recall of the middle row. Performance on the 12-item cards now rose to 3.03 per four-item line, or an equivalent 9.1 items per card, more than double the performance in the total report condition.

As noted in the rationale paragraph above, the interpretation of these data is that the subject becomes free when cued to switch all his/her attention to the single row indicated by the cueing tone, but that s/he could only do this if some sort of image of the entire card still existed in the visual system. (There was nowhere else the information could come from, remember, because the screen itself had already gone blank when the cue was given.)

With the basic paradigm in place, Sperling proceeded to manipulate the delay between stimulus offset time and cue onset time. This would show how long the sensory memory image was available to be exploited. A variable delay was introduced, with cue onset either preceding stimulus onset by 50 msec., coinciding exactly with it, or else following it by 150, 300, 500, or 1000 msec. The three-over-three and four-over-four card layouts were used, and the results are shown below.

 

Sperling's (1960) Partial Report Paradigm - Typical Performance Curve: Here is a plot of accuracy of recall (%) against cueing delay (seconds). The solid black bar marks the period of actual stimulus exposure. Note the rapid (ie. steep) decay of the memory trace in the first 150 msec. after stimulus offset, and the half second limit to the zone of VSTM advantage (highlighted in red).

PICsperling1960c.gif

Redrawn from a black and white original in Sperling (1960; Figure 7 (Subject NJ)). This version Copyright © 2002, Derek J. Smith.

 

Sperling's results have since been verified many times, and similar effects have been found in other sensory modalities, namely hearing (auditory) and touch (haptic). Visual sensory memory was subsequently termed iconic memory (Greek eikon = image), and its auditory analogue was termed echoic memory (see Darwin, Turvey, and Crowder, 1972). Iconic memory is also often known as the Sperling Store after its discoverer.

 

References

Darwin, C.J., Turvey, M.T., & Crowder, R.G. (1972). An auditory analogue of the Sperling partial report procedure: Evidence for brief auditory store. Cognitive Psychology, 3:255-267.

Smith, D.J. (1996). Memory, Amnesia, and Modern Cognitive Theory. Cardiff: UWIC. [ISBN: 1900666006]

Sperling, G. (1960). The information available in brief visual presentations. Psychology Monographs, 74(11), Whole No. 498.