Weichselgartner & Sperling (1985) ‘Continuous Measurement of Visible Persistence’, notes

by Corry Shores
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[The following is summary and quotation except for my bracketed commentary. Please consult the original text, as not all of this article was clear to me.]

Erich Weichselgartner; George Sperling

‘Continuous Measurement of Visible Persistence’

Brief Summary:

The authors conduct experiments meant to determine the duration of visual persistence, and they introduce the notion of ‘perceived brightness’, which might be similar to the concept of ‘presentedness’ in the phenomenological study of the specious present.

Notes:

The authors will examine the decay of persisting visual images.

A brief visual stimulus presented to a subject is not perceived to end abruptly but to fade out gradually.
[p.711]

There have been two primary ways that the duration of persisting images have been tested: 1) inferring from the accuracy of subjects’ reports, and 2) depending on subjects’ subjective reports of their experiences.

The time difference between the physical termination of the stimulus and its perceptual termination has been investigated in a variety of paradigms. These paradigms fall into two main classes: those that infer visual storage from the accuracy of subject's reports and those that depend on subjective reports (e.g., Brindley's [1960] Class A and Class B procedures).
[711]

The first type (accuracy procedures) are generally either partial report tasks or picture completion tasks. In partial report (like Sperling 1960), the subject views a flash of a matrix of letters and must report on a randomly selected portion. In picture completion, subjects must compare successive images.

In the partial report paradigm (Sperling, 1960), the observer views a brief flash of a matrix of letters. Afterwards, a tonal cue that can be precisely located in time is used to request report of a randomly selected row of the stimulus. The decline of response accuracy with cue delay indicates the duration of short-term visual storage (iconic memory— Neisser, 1967). Picture completion paradigms require the observer to integrate information from two successive flashes in order to identify a target letter (e.g., Eriksen & Collins, 1967) or to detect a missing dot in a regular dot matrix (Hogben & DiLollo, 1974).
[711]

In subjective procedures, the subject makes their own judgment to how long the stimulus remains in their sensory memory. They can do this by adjusting clicking sound [see section 2.3 of Sperling 1967] [711B]

For the most part, both these objective and subjective methods indicate a similar duration for sensory memory. However, there are some important discrepancies. [Please see the last full paragraph of p.711, as it is not clear to me how to summarize it. It suggests that there is a function to describe the decay curve, but “no one has come up yet with a measure for those decay functions.” (712Aa)]

WS then distinguish two classes of studies on visible persistence. (a) Studies that are “concerned with the the time difference between stimulus termination and perceptual termination”, and (b) “studies concerned with the total phenomenal duration of the stimulus that require reliable judgments of both onset and termination, the estimated stimulus duration being the time difference between judged onset and termination”. [712Aa.b] However, “There are no statements concerning the actual form of the rise and decay curves or of the complete representation of the stimulus as a function of time in the subject's visual system.” [712Ab]

Thus what is needed is “a method to measure the entire moment-to-moment time course of visible persistence.” [712A] But, the term ‘visible persistence’ already has various meanings. So WS will use another term to be more specific: temporal brightness response (TBR), which “describes how the perceived brightness of a brief visual stimulus changes as a function of time.” [712A] [note, this could be similar to the notion of presentedness in the phenomenology of the specious present.] “The purpose of this article is to prove the feasibility of measuring the TBR and to measure TBRs to brief flashes for 3 observers.” [712A]

Elaborated Synchrony Judgment Paradigm

The following will be an elaboration of Sperling’s (1967) method. It will be an “intermodal synchrony judgment paradigm.” [712A] In it, the subject views two adjacent stimuli: (a) a reference stimulus that is “presented at the beginning of the trial and remains on with constant luminance during the trial,” and (b) “a test stimulus of varying luminance, which is presented and terminated sometime during the middle of the trial. Luminances are are adjusted so that from the observer's point of view the test stimulus initially appears dimmer than the steady-state reference stimulus but increases in intensity until eventually it becomes as bright or brighter (Figure 1).” [712A]

[As we can see, the luminosity of the test image will become at least as bright as the test image, maybe even brighter.] The instant when the luminosity seems to match between test and reference stimuli is called the match time. This method wants to know when the match time occurs. “In a time interval around match time, the subject is presented with a click.” [712Ba]

If the subject perceives the click before they perceive the two stimuli matching, they indicate with a left-hand response key. But if the click occurs after they seem to match, then with a right-hand response key. [712B]

Or, the subject might be asked to press the right-hand key if at the time of the click the test was brighter than the reference stimulus. Otherwise [if it seemed the same or dimmer] then they press the left-hand key.  [If the test is dimming and they press the right-hand key, then this would suggest the brightness persisted beyond its actual luminance.]

Repeated judgments result in a psychometric function, the probability of a right response as a function of time. The point of subjective equality (pse) is the 50% point of the psychometric function; it corresponds to the time at which the brightness of the reference matches that of the test. A psychometric function obtained with a particular luminance of the reference stimulus determines only one match time. The entire TBR function is obtained by obtaining match times for a full range of luminances of the reference stimulus, and by determining match times near the onset and also near the termination of the test stimulus.
[712B]

Verification Procedure

[The actual stimulus change has a luminance function, its change over time.] They will for example have a light bulb increase linearly to its maximum at 300 ms, then dim to off in another 300 ms. While the stimulus is rising, WS expect the match times to increase with increasing luminance.

Our primary interest is in the TBR function of a very brief stimulus. However, it is important to first demonstrate that a subject can make reasonable match time judgments with a slowly varying stimulus for which the TBR can be assumed to approximately track the temporal luminance function of the physical stimulus. An example of such a physical stimulus is the gradual fading out of a light bulb turned off with a dimmer. Therefore, we first determine TBRs for a control condition in which test field luminance increases linearly during a 600-ms period, stays at its maximum for 300 ms, and then turns off linearly during another 600-ms period. The data from this control condition with real "physical persistence" can be used to evaluate the method. For example, when the rising part of the test stimulus is under investigation (onset trials), we expect the match times to increase with | increasing luminance of the reference stimulus, and when the decaying part of the test stimulus is under investigation (termination trials), we expect the match times to increase with decreasing luminance of the reference stimulus. After Experiment 1 (with ramped onsets and terminations) we proceed to Experiment 2, which determines TBRs for brief stimuli. [712-713]

General Method

Overview

There are two experiments, the verification and the main experiment. Both use the same apparatus and a similar method.

Apparatus and Stimuli

Spatial arrangement

Spatial Arrangement:

As depicted in figure 2, the stimuli were two square-wave gratings.

Stimulus intensity:

Reference took on 5 different luminances [see table 1].

Procedure

Individual trials:

(1) Reference stimulus first turned on. (It remains on for 3,000 ms)

(2) Secondly, the test stimulus is turned on. (It remains on for 1,500 ms in Experiment 1, and 31 ms in experiment 2.)

(3) Click sounds. “On the first trial, the onset time of the click was randomly chosen within an interval of ± 200 ms around starting points (initial values), which were determined in preliminary experiments (see below).”

(4) Afterward, subject must decide whether click occurred before or after the instant of perceived brightness match between two visual stimuli. Either click before or click after choices available. [714B]

Blocks of trials:

Ran a number of trials varying brightness according to step procedure (below).

Staircase procedure:

WS used the stair-case procedure described by Lewitt (1971) [Please see the original article to be sure of its content. As far as I can summarize, it seems the problem that WS are contending with is that the subject cannot just press a button when it seems the stimuli match. For, this involves a motor response whose accuracy might be unreliable and variable. To narrow down to greater accuracy, WS suggest this step method. In it, the subject reports only after the stimuli have finished. The subject reports if the clicked seemed to come before or after the apparent match. If before, then the click is moved forward a step. If after, then moved back. It then seems by means of calculating this data, it can then be better approximated when it is that the subject perceives the match. The following diagram is from Levitt (1971) and it depicts this step ‘staircase’ procedure:

]

Experiment 1

Experiment 1 consisted of two phases. In Phase 1, stimulus parameters for Phase 2 were empirically determined. In Phase 2, the TBR for a ramped test stimulus was measured using the parameters from Phase 1.
[715A]

Method

Procedure

Phase 1:

The first phase determines appropriate luminances for the reference stimuli and also time for occurrence's of the click. They used the step method.

Together, the isolated brightness and temporal matches of Phase 1 pinpoint the brightness and time of the perceived brightness peak of the test stimulus.
[715]

Phase 2:

[Recall that “temporal brightness response (TBR)” “describes how the perceived brightness of a brief visual stimulus changes as a function of time.” (712A)]

In Phase 2 of Experiment 1, the TBR was determined for a ramped test stimulus by means of the psy -|- chophysical method described above in the Procedure section.
[715|716]

Results

The results of Experiment 1 can be found in table 1. Figure 3 shows the temporal brightness response times for the 3 subjects. The match times are open squares. “The five data points on the left side of the TBR are the match times for the onset judgment, and the five data points on the right side of the TBR are the match times for the termination judgment.” [716A, see for more detail]

The TBRs are comparable to the stimulus slopes. “In all cases of the onset judgment, the match times were ranked in an ascending order with increasing luminance, and in all cases of the termination judgment, the match times were ranked in an descending order with decreasing luminance.” [716B]

Discussion

[See text pp.176-178]

Conclusion:

From the results of Experiment 1, we conclude that the elaborated synchrony judgment method yields reasonable temporal brightness response functions, and there are pronounced individual differences.
[718B]

Experiment 2

Experiment 2 applies the paradigm of Ex1 to a 31ms flash to determine its TBR.

Method

Procedure

Phase 1:

A reference stimulus was found whose intensity matched the peak of the test stimulus.

Phase 2:

“Phase 2 of Experiment 2 determined the TBR for the 31-ms pulse with the elaborated synchrony judgment paradigm.” [718A]

Results:

Ex2 indicates match times. Results are shown in figure 4, which indicates the TBR for each subject.

General Discussion

Duration of Visible Persistence

Here the authors discuss “three fallacies in thinking of persistence as a concept that is adequately described by a single number, its duration.” [719A]

[skipping to]

Conclusion:

The elaborated synchrony judgment paradigm utilizes a highly refined form of introspection to trace out an entire temporal brightness response function to a test stimulus. The paradigm applies to a wide variety of possible temporal waveforms. For very brief test flashes, both the onset and the termination phases of the TBR differ widely across observers, with overall durations of the TBR varying from about 200 to about 500 ms.
[724B]

Weichselgartner, Erich, and George Sperling. "Continuous Measurement of Visible Persistence." Journal of Experimental Psychology 2.6 (1985): 711-725.

http://weichselgartner.de/erich/full_text/EW+GS_Visible_Persistence_JEP_1985.pdf

http://www.ncbi.nlm.nih.gov.sci-hub.org/pubmed/2934504

Also a reference to:

Levitt, H. "Transformed Up‐Down Methods in Psychoacoustics." The Journal of the Acoustical Society of America 49 (1971): 467-477.

http://bdml.stanford.edu/twiki/pub/Haptics/DetectionThreshold/psychoacoustics.pdf

http://scitation.aip.org/content/asa/journal/jasa/49/2B/10.1121/1.1912375

Sperling (1960) ‘The Information Available in Brief Visual Presentation’, notes

by Corry Shores
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George Sperling

‘The Information Available in Brief Visual Presentations’

Brief Summary:

One way to know how long images stay in our mind after they disappear is to have people see a flash of objects then recall them after the image disappears. If they try to recall as many as possible regardless of position or type, then it is a whole report. One theory is that the image persists in the mind after the stimulus physically disappears, and when reporting, we select from the whole group, but that selection is arbitrary for the most part. One problem with the whole report method is that it alone does not tell us if the unselected parts persisted or not. If the whole image remains to our awareness, then this would be a persistence of the image, and if not, then either it does not persist or it does only partially. To find out which possibilities is true, Sperling employs a ‘partial report’ method. Here, the subject is to remember only a part of the set of flashed objects, but which part they might not know until shortly after the image has disappeared. If it can be shown that it does not matter from which part they are to report on, that suggests all of it remains in their mind’s eye, and they select from the whole. The partial report experiments here described show that not only does it not matter which part, and thus that the image probably persists, but also that subjects do better when they only need to recall part. One explanation for this again supports the image persistence theory: in whole reports, the subject must consume time deciding which figures to report, but in partial reports, that selection is made for them, giving them more time to make reports while the image persists in their minds.

Summary notes:

We often remember less than we original saw.

So we see a lot, but we can only remember part of it. [In the following, it seems our concern is not just with the quantity of content recalled in immediate sensory memory, but also with the duration of it. After the visual impression fades, it might become encoded as memory in a form which is not the sustained visual impression that remains in our vision. So we do not what to ask the subject to recall more than memory allows them too. They only need to report a portion of what they saw, and the question is, for how long does the impression stay in their minds so that they can still look at in it in their minds and withdraw information from it.?]

In order to circumvent the memory limitation in determining the information that becomes available following a brief exposure, it is obvious that the observer must not be required to give a report which exceeds his memory span. If the number of letters in the stimulus exceeds his memory span, then he cannot give a whole report of all the letters. Therefore, the observer must be required to give only a partial report of the stimulus contents. Partial reporting of available information is, of course, just what is required by ordinary schoolroom examinations and by other methods of sampling .available information.
[p.1]

After the stimulus terminates, the subject can be instructed to recall a certain part of the stimulus.

The experiments described in the following were conducted “to study quantitatively the information that becomes available to an observer following a brief exposure.” [2]

Letters were arranged in following sorts.

The stimulus array was flashed for 50 msec [see p.3]. Subjects were to report what they saw [depending on instruction for whole or partial reporting]. They were scored both for number correct and positions correct. [p.4]

Experiment 1: Immediate memory

Sperling writes:

When an S is required to give a complete (whole) report of all the letters on a briefly exposed stimulus, he will generally not re- | port all the letters correctly. The average number of letters which he does report correctly is usually called his immediate memory span or span of apprehension for that particular stimulus material under the stated observation conditions. An expression such as immediate-memory span ( Miller, 1956a) implies that the number of items reported by S remains invariant with changes in stimulating conditions. [p.4-5]

All 12 types of arrays [‘of stimulus materials’] were used.

Results:

the average number of correct letters contained in an S's whole report of the stimulus is approximately | equal to the smaller of (a) the number of letters in the stimulus or (b) a numerical constant-the span of immediate-memory which is different for each S. The use of the term immediate-memory span is therefore justified within the range of materials studied.
[5-6]

Experiment 1 showed that, regardless of material, Ss could not report more than an average of about 4.5 items per stimulus exposure.
[p6]

Experiment 2: Exposure duration

In the previous experiment, the exposure duration to the stimulus was short, 0.05s. In order to determine whether the 4.5 item limitation results from the shortness of the duration, we need to vary that duration.

Procedure:

Subjects were given tasks from prior experiment, except at varying durations of exposure to the stimulus: 0.015, 0.050, 0.150, 0.200, and 0.5oo second durations.

Results:

The main result is that exposure duration, even over a wide range, is not an important parameter in · determining the number of letters an S can recall correctly. Both individually and as a group, Ss show no systematic changes in the number of letters correctly reported as the exposure duration was varied from 0.015 to 0.500 sec. The invariance of the number of letters reported as a function of exposure durations up to about 0.25 sec. for the kind of presentation used ( dark pre- and postexposure fields) has long been known ( Schumann, 1904) .
[p6]

Experiment 3: Partial Report

[The controls experiments above tell us the full total quantity of the amount of information available to immediate recollection. This experiment will determine if the subject has more information than she can indicate in the immediate memory report. What this would suggest is that in the full reports, the subjects could have recalled different parts of the arrays, because all of it is available, but it decays, and the subject needs to report as much as possible, choosing arbitrarily from the retained image. If the subjects are able to recall the same number of figures but at varying spatial locations, then this suggests there is such a retained image from which a partial report can be selected.]

Experiments 1 and 2 have demonstrated the span of immediate-memory as an invariant characteristic of each S. In Experiment 3 the principles of testing in a perceptual situation that were advanced in the introduction are applied in order to determine whether S has more information available than he can indicate in his limited immediate-memory report.

The S is presented with the stimulus as before, but he is required only to make a partial report. The length of this report is four letters or less, so as to lie within S's immediate-memory span. The instruction that indicates which row of the stimulus is to be reported is coded in the form of a tone. The instruction tone is given after the visual presentation. The S does not know until he hears the tone which row is called for. This is therefore a procedure which samples the information that S has available after the termination of the visual stimulus.
[p.6]

Procedure:

Arrays with only two lines were used. Right after they disappear, the subject heard either a high or low tone, indicating whether to report the upper or lower row. [It then seems cards with more rows were shown and cued with  more tones. see p.6Bd]

Results:

The subjects improved, at first averaging 4.5 and later 5.6 letters [compare to the 4.5 limit of the control whole report sessions.] The diagram below shows in fact that partial reporting yields higher recollection [the lower curve is average number of figures recalled in whole reporting, the middle in partial reporting. Perhaps it is for this reason. In both cases, the image decays rapidly. In whole reporting, it might take more time to decide where in the retained image to select the figures to report. But in partial reporting, those briefly time-consuming choices are made already for the subject, allowing for them to draw out more information in the short time that the image remains in their vision.]

In Fig. 3 the number of letters available as a function of the number of letters in the stimulus are graphed as the upper curves. For all stimuli and for all Ss, the available information calculated from the partial report is greater than that contained in the immediate-memory report. Moreover, from the divergence of the two curves it seems certain that, if still more complex stimuli were available, the amount of available information would continue to increase. [p.6]

Experiment 4: Decay of Available Information

Part 1: Development of Strategies of Observing

We will now explore the decay of available information by delaying the cue telling which parts to recall. They used these variations on the cue time (even placing it before and during the stimulus): “0.05 sec. before stimulus onset (-0.10 sec.), ±0.0-, +0.15-, +0.30-, +0.50-, + 1.0-sec. delays after stimulus off-go”. [p.8]

Each subject went through all of these variations in delay, going either in ascending or descending order.

Results:

Look first in fig.5 to the leftmost panel (5a). These are the results of one subject giving partial reports at varying delay times. (Arrows tell what order the sequence went in on a whole, thus here began with ascending). We see that the lines go down, indicating that there is a decay, and much of the visual information is lost after 0.25s. The second session (5b) began with descending, and the results were not so orderly. The third session (5c) had more trials and also had a part where the the cue came before the stimulus.

The variability of 5b may have resulted from the subject choosing a strategy where she first guesses which row will be requested, and is sometimes right and sometimes wrong. Another subject described changing strategies after 0.15s  from trying to remember all equally and guessing the row, and his diagram [see p.9]

Part 2: Final Level of Performance

The experimenters attempted to eliminate this factor of choosing a strategy of guessing the row. Three proposed ways are: increasing the number of figures, making selection less useful; have tones come slightly before the stimulus; have the experimenter say something that would change the subject’s approach to the experiment, for example “not testing memory but reading, don’t read the card until hear the tone”. [It is not clear to me if the following results come from an experiment implementing some or all or none of these, but it seems at least some. Given that the results show a variation of times, it is not clear that the second modification was used any differently than before.]

Results:

The data indicate that, for all Ss, the period of about one sec. is a critical one for the presentation of the instruction to report. If Ss receive the instruction 0.05 sec. before the exposure, then they give accurate reports : 9 1 % and 82% of the letters given in the report are correct for the 9- and 12-letter materials, respectively. These partial reports may be interpreted to indicate that the Ss have, on the average, 8.2 of 9 and 9.8 of 12 letters available. However, i f the instruction is delayed until one sec. after the exposure, then the accuracy of the report drops 32% (to 69% ) for the 9-letter stimuli, and 44% ( to 38%) for the 12-letter stimuli. This substantial decline in accuracy brings the number of | letters available very near to the number of letters that Ss give in immediate-memory ( whole) reports. [p.11-12]

[Experiment 5, skipped. It experiments with different pre- and post-exposure fields.]

Experiment 6: Letters and Numbers

[It could be that what is being remembered is a result of remembering locations. The following experiment reduces the importance of location.]

Procedure:

Subjects saw arrays mixed with both letters and numbers, and afterward were asked to recall only one or the other. [It seems this experiment also had trials asking for top and bottom rows too, regardless of symbol/number. see page 14Ad]

The results show that reporting either letters or numbers only is little better than immediate memory. [But it is better with locations. So asking for locations is a better method for finding how long images stay in our minds. Presumably there is not an unwanted advantage with positions, because in both cases the cue can come after the fact (and not be from guessing location ahead of time, see experiment 4). So presumably whether the subject is asked for types or for locations, in both cases, the image would have been equally available to their sensory memory.]

The failure in Experiment 6 to detect a substantial difference in accuracy between partial reports of only letters (or only numbers) and whole reports clearly illustrates that partial reports by position are more effective for studying the capacity of short-term information storage than partial reports by category.
[p.16]

[Experiment 7: Order of Report, skipped. From the results: “The results obtained in this experiment support the conclusions that both a position preference and the order of report ordinarily correlate with the accuracy of response, but that probably neither are necessary conditions for response accuracy.” p.19]

Discussion:

Sperling has two questions regarding why subjects remember more in partial reports than in whole reports.

(a) Why is the partial report more accurate than the whole report ? (b) Why does the partial report retain this added accuracy only for a fraction of a second after the exposure?
[20]

One way to answer [question b]  is with the subjective accounts of the subjects. They say that the image stays in their vision even when the tone sounds 150 msec after the image has physically disappeared.

The answers proposed are a systematic elaboration of an observation that is readily made by most viewers of the actual tachistoscopic presentation. They report that the stimulus field appears to be still readable at the time a tone is heard which follows the termination of the stimulus by 150 msec. In other words, the subjective image or sensation induced by the light flash outlasts the physical stimulus at least until the tone is heard. The stimulus information is thus "stored" for a fraction of a second as a persisting image of the objective stimulus. As the visual image fades, its legibility (information content) decreases, and consequently the accuracy of reports based upon it decreases. [p20AB]

Sperling then notes that sensation is not instantaneous. So the fact that the subject still sees the image after its extinction partly results from the fact that it takes time for the sensation to register in the subject’s awareness. But this is not necessarily related to the persistence of the image. It only refers to the delay. So what matters is not the delay after the extinction of the stimulus but rather the duration it remains before decaying.

There is other evidence, besides such phenomenological accounts, that suggests that information is available in the form of an image for a short time after extinction of the physical stimulus. In the first place, it is inconceivable that the observers should stop seeing the stimulus at exactly the moment the light is turned off. The rise and fall of sensation may be rapid, but they are not instantaneous. The question is not whether the observer continues to see the stimulus after the illumination is turned off, but for how long he continues to see the stimulus.
[p. 20]

Sperling then cites other research which would estimate the persistence of vision to be from 0.05 to 1.0 sec, and most probably around 1/6 of a sec.

These estimates of the persistence of the visual sensation vary from a minimum of 0.05 sec. (Wundt, 1899) to almost one sec. (McDougall, 1904). The most representative estimates are in the neighborhood of l /6 sec. (cf. Pieron, 1934), a figure that is in good agreement with the results.¹³ [ft 13: Measurements of the persistence of sensation have almost invariably used techniques which have at most questionable validity. Wundt's method depends upon masking, the effect of the persisting stimulus upon another stimulus. The masking power of a stimulus may be quite different from its visibility. McDougall's measurements, as well as those cited by Pieron, depend upon motion of a stimulus across the retina. Such measurements are undoubtedly influenced by the strong temporal and spatial interactions of the eye (Alpern, 1953). Schumann's ingenious application of the method of Baxt to the determination of persistence is probably the only experiment that utilizes pattern stimulation. The other methods have not been tried with pattern stimuli although there .is, a priori, no good reason why they have not been. The possibility that the persistence of pattern information is quite different from persistence of "brightness" has not been investigated.] [Citing these sources: Wundt, W. An introduction to psychology. London: Allen & Unwin, 1925 (reprinted.) Transl. from 2nd German ed, by R Pinter. Edinburgh, Ballentyre Press, 1912. ((see pdf, where much of the bib info is overwritten by hand.)). McDougall, W. The sensations excited by a single momentary stimulation of the eye. Brit. J. Psychol., 1904, 1, 78-113. Pieron, H. L'evanouissement de la sensation lumineuse: Persistance indifferenciable et persistance totale. Ann. psychol., 1934, 35, 1-49. Alpern, M. Metacontrast. J. Opt. Soc. Amer., 1953, 43, 648-657.]
[p.20]

Sperling again on the persistence of vision:

This then is the evidence-phenomenological reports, the effects of the postexposure fields, the known facts of the persistence of sensation, and the detailed characteristics of the responses-that is consistent with the hypothesis that.information is initially stored as a visual image and that the Ss can effectively utilize this information in their partial reports. In the present context, the term, visual image, is taken to mean that (a) the observer behaves as though the physical stimulus were still present when it is not (that is, after it has been removed ) and that (b) his behavior in the absence of the stimulus remains a function of the same variables of visual stimulation as it is in its presence, The units of a visual image so defined are always those of an equivalent "objective image," the physical stimulus. It is as logical or illogical to compute the information contained in a visual image (as was done in Experiments 3 and 4) as it is to compute the information in a visual stimulus.
[p.21]

But as indicated above, the subject behaves ‘as though’ it were present. Perhaps in fact it is not visibly present but only seems be in their behavior.

"Visual image" and "persistence of sensation" are terms suggested by the asynchrony between the time during which a stimulus is present and the time during which the observer behaves as though it were present. Although asynchrony is inevitable for short exposure durations, there is, of course, no need to use the term "visual image" in a description of this situation. One might, for example, refer simply to an "information storage" with the characteristics that were experimentally observed. This form of psychological isolationism does injustice to the vast amount of relevant researches.
[p.22]

Persistence of Vision and Afterimages

[It would seem the difference so far used with the terminology is this: persistence of vision is the very immediate lingering impression, and the afterimage remains shortly after, less than longer-term memory.]

Between the short persistence of vision and the remembrance of a long-passed event, there is an intermediate situation, the afterimage, which requires consideration. In discussing afterimages, it will be useful to distinguish some phases of vision that normally follow an intense or prolonged stimulus. First, there is the "initial" (or primary, or original) "image" (or sensation, or impression, or perception, or response). Any combination of a term from the first and from the second of these groups may be used. The initial image is followed by a latent period during which nothing is seen and which may in turn be followed by a complex sequence of afterimages.
[p.22]

George Sperling. The information available in brief visual presentations. Psychological monographs: General and applied, Vol. 74, No. 11. (1960), pp. 1-29.

http://www.citeulike.org/group/1696/article/995840

http://aris.ss.uci.edu/HIPLab/staff/sperling/PDFs/Sperling_PsychMonogr_1960.pdf