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