Utility of the computer as tachistoscopein a particular p~ychophysical setting*

JOSEPH J. MEZRICHUniversity of Michigan. Ann Arbor, Michigan 48104

A discussion is provided covering the use of the'computer as a tachistoscope.

There is very little question that the introduction ofthe computer into the psychology laboratory hasenriched the experimental repertoire of the psychologist.The impact of the laboratory computer has been quiteevident in the field of visual perception. The use ofdynamic visual noise by Uttal (e.g., 1969), the use ofrandom-dot cinematograms by Julesz (e.g., 1971), andthe study of stereotyped scanpaths for eye movementsby Noton and Stark (1971) are but a few of theapproaches to the analysis of visual processing thatwould be impossible without the use of the computer.

However, the vast majority of experiments beingperformed on the processing of information by thevisual system do not require exotic stimulus presentationor data collection schemes. It is reasonable to ask whatadvantage is gained by employing a computer in suchexperiments, where more traditional apparatus is clearlyadequate, Some insight concerning this question can begained by examining the impact of on-line use of thecomputer on the procedures of a specific, traditionaltype of experiment as the degree of involvement of thecomputer increases. In particular, we will consider thestudies of Reicher (1968). Wheeler (1970), and Mezrich(1973) on the word superiority effect.

In 1968, Reicher demonstrated that a four-letter wordis more easily recognized than a single letter when bothare briefly displayed for the same duration. Forexample, a S presented with a brief visual display of thestimulus WORD and then asked to decide whether theletter D or the letter K was present in the stimulus ismore likely to be correct than if the stimulus D precededthese response choices, It should be noted that either Dor K can be used to construct a four-letter word with theinitial letters WOR, so the possibility of response bias isabsent. The phenomenon does not depend on the criticalletter position of the word. Reicher's finding is generallyreferred to as the word superiority effect.

Some details about Reicher's experimental methodare necessary for our discussion, Each stimulus trial wascomposed of a sequence of three visual displays: aprestimulus blank field with a fixation point. followedby the stimulus. which was followed by the two

'The valuable advice and assistance of J, E, Keith Smith andFrank M, Goode are gratefully acknowledged. The workreported in this paper was supported by Public Health ServiceGrant GM-01231-08 to the Uruversit v of Michigan.

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response alternatives located adjacent to a visual noisemask superimposed on the stimulus location. The noisemask was identical for every trial and consisted of amatrix of superimposed Xs and Os. The fact that thisnoise mask is far from random turns out to beunimportant. Reicher typed his presentation materialson white cards and used a Scientific PrototypeModel GB three-channel tachistoscope as his displaydevice.

An important part of the experimental procedure isthe determination of the appropriate stimulus exposureduration for each S. For a two-alternative forced-choicesituation, a stimulus exposure duration that insuresresponse accuracy at some level between 50% and 100%correct is necessary. Reicher used what he called amodified up-down threshold procedure in a preliminarysessionwith each S to ascertain the appropriate exposureduration. One hundred and twenty stimuli werepresented in blocks of 10. Based on performance for agiven block. the exposure duration for the next blockwas adjusted up, down, or not at all. The exposureduration determined from the last block was used forthe experimental sessions,which occurred on subsequentdays. Reicher does not give full details about thescenario of this preliminary session, but it would appearto have taken at least 1 h.

It should be clear that there is little justification forassuming that such a technique yields a reliable estimateof the appropriate stimulus exposure duration for a S insubsequent sessions of the experiment. The problem ofthe instability of operating levels in psychophysicalexperiments is well known (e.g., Green & Swets, 1966).The effects of fatigue, motivation, and practice within asession all contribute to, or detract from, theperformance of the S, There is no guarantee that thestate of the S is constant within a session, much lessbetween sessions that occur on different days.Fortunately, the word superiority effect seems to occurat all accuracy levels in the 50% to 100% correct range,even though the magnitude of the effect does depend onthe value of the accuracy level. However. the problem ofoperating-level fluctuations makes these results lessconvincing than they could be.

To a great extent, Reicher was forced to separate thepreliminary session from the experimental sessionsbecause a conventional tachistoscope was used. Thesomewhat clumsy procedure of having the Econtinuously monitor a S's performance level and adjustthe tachistoscope exposure setting practically insures afairly long preliminary session, To precede a longexperimental session with such a preliminary sessionwould surely introduce a significant amount of fatigue

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and boredom, very likely making the estimate of theappropriate exposure duration as unreliable as if the twosessions were separated by days. Thus. the proceduralconstraints imposed by his equipment were quite severe.

Another aspect of Reicher's procedure that we wouldlike to focus attention on is the postexperimentalquestioning of Ss. As is typical for experiments of thisnature, the 5 is called back for discussion at least a dayafter his participation in the experiment. This delay isused so that the E can Have the relevant performanceindicators in hand to help guide his questioning.Maximum yield from discussions with Ss about factorsthey suspect may have affected performance occurswhen the experiment is still very fresh in the S's mind,and when the E has available performance indicators toguide the discussion. Because traditional data-collectionprocedures were used, Reicher had to compromise onthe first of these conditions in order to exploit thesecond. Surely, a significant amount of detail isforgotten by the S when he has to wait at least a day toreport his experiences. It is probably obvious thaton-line use of a computer enables one to insure presenceof the conditions for maximum discussion yield. Furtherdiscussion on this point appears below.

Two years after Reicher found the word superiorityeffect. Wheeler (1970) performed a series of experimentsto explore a number of hypotheses about sources of theeffect. His conclusion was that the effect is quite real,but a simple explanation of its cause is elusive. Wheelerused a PDP-8 computer connected to an oscilloscope as atachistoscope. The machine was also used for datacollection and for some on-line computation.

A major difference between the visual displays usedby Reicher and by Wheeler was the noise maskemployed. It was pointed out above that the fact thatReicher did not use a random noise mask wasunimportant. This is known primarily because Wheelerused a random noise mask at the offset of each stimulusand still got the effect. His mask consisted of a randompattern of dots, different on every trial. Such a mask iseasily generated with a computer-controlled display, butis impractical with a traditional tachistoscope display.The use of a nonrandom display to mask patternedvisual stimulation introduces potential artifacts, makingReicher's results less convincing than they could be.Thus, the accessibility of random noise masks illustratesone of the advantages of the computer as tachistoscopein this experimental setting.

We pointed out, above, the disadvantages of Reicher'sprocedure for determining the stimulus exposureduration. Wheeler apparently used a similar algorithmfor determining this experimental parameter (precisedetails were not given in his paper); however, theadvantage of computer control gives the algorithm a bitmore reliability. For a typical session in Wheeler'sexperiment, which lasted about I h, the S responded to384 stimuli. The first half of these stimuli were used todetermine the appropriate stimulus exposure durationfor the second half, or the first half hour was spent as

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preliminary session for the second half hour. Because acomputer was used, Wheeler was able to accomplish witha S in an hour what it took Reicher two long sessionsover a period of :2 days to do. The simple act ofcomputerizing the data-collection procedure enables theS to run through the preliminary and experimentalsessions at a fairly rapid pace. thereby shortening thedurations of both sessions. As the durations of thesession, decrease. so does the problem of operatinglevelinstability. Thus, another advantage of the cornpuler inthis experiment becomes evident. It will be seen shortlythat one can make more efficient use ot the computerfor minimizing the instability problem.

The last comment we would like to make aboutWhce le rs method concerns his approach topostcxperimental questioning of Ss. Whereas Reicherpostponed S questioning until performance indicatorswere available, Wheeler engaged in immediatepostexperimental interview. Unfortunately, he did notprogram the computer to print out performance levelsprior to this interview, thus the conditions for maximumdiscussion yield were not met.

We performed yet another set of experiments on thernysterious word superiority effect (Mezrich, 1973). Weconfirmed the findings of Reicher and Wheeler. and alsofound that forcing Ss to vocalize all stimuli reverses theeffect. making letters more recognizable than words. ACRT controlled by a LINC-8 computer was used as atachistoscope. The computer was also used for datacollection and on-line computation.

We used a dynamic random noise rnask.! Acontinuously changing display of lines of random length,orientation, and location was painted over the stimuluslocation at stimulus offset, giving the visual effect ofsmall sticks flying about. The use of this type of randommask was partly due to convenience and partly forreasons of aesthetics. We were concerned aboutpreventing Ss from becoming bored during ourexperiments. Making an otherwise dull displayinteresting to watch was intended as a boredominhibitor. Clearly. a noise mask such as ours is impossibleto generate with a conventional tachistoscope.

In constructing the actual stimulus display, the Eusing the computer as a tachistoscope has a few morethings to worry about than does the E using a traditionaltachistoscope. In our displays. a letter was constructedfrom a grid of points rather than from continuousstrokes. We, therefore. had to be sure that thedistinctiveness and clarity of a briefly presented letterwas not affected by its being spatially discrete. it wasnot. The stimulus exposure duration for a CRT display isnecessarily quantized. since the duration of exposure isan integral multiple of the duration of a single paintingof the stimulus. In our experiments. we avoided anydifficulties due to quantization by using a verybrief-duration painting cycle. Wheeler's solu tion to thisproblem was, apparently, to use a storage-scope in thestore mode. Slow drifts in luminosity of the CRT displayand a long time constant CRT phosphor are two

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equipment properties that the E has to guard against inselecting the oscilloscope that his computer will control.Adequate CRTs are readily available, so this is not aserious problem. Any disadvantages caused by using aCRT display in the type of experiment we are discussingare insignificant.

In reviewing the approaches of Reicher and Wheelerto the problem of determining the stimulus exposureduration, we pointed out that, because of instability ofoperating levels, it is important to have a quickdetermination of the appropriate exposure duration,followed by a quick run through the experiment. In ourexperiments, a very quick exposure-durationdetermination was achieved by using a Markov designprocedure due to Smith (1961).

After a brief instructional period, a S was run through96 stimulus trials at 60-msec stimulus exposure duration.This warm-up session, which lasted about 15 min, wasincluded to eliminate practice effects before theexposure duration for the experimental session wasdetermined. After a 2-min break, the S was run throughwhat we call the timing phase of the experiment. Thealgorithm for this timing phase was based on thefollowing considerations. Let Pi be the probability of acorrect detection for a given type of stimulus material atexposure duration t i . Only single-letter stimuli were usedfor the timing phase, so Pi represents the hit rate forletters. The probability of two consecutive hits atexposure duration t i is, therefore, Pt. If we find theexposure duration at which the probability of twoconsecutive hits equals the probability of not gettingtwo consecutive hits, then we have found t i where p~ =I - Pt , or Pi = .707. In our algorithm, we started at anarbitrary exposure duration, 60 msec, and inspectedeach consecutive pair of responses on-line. Let H denotea hit, and M denote a miss. The four possible outcomesfor a pair of responses are: (H,H), (H,M), (M,H), and(M,M). If (H,H) occurred, the exposure duration wasdecreased by 10 msec for the next pair of stimuli. If oneof the other three outcomes occurred, the exposureduration was increased by 10 msec. Actually, thealgorithm also inspected the first response of a pair,before the second trial was initiated, and automaticallyincreased the exposure duration by 10 msec if a missoccurred on that trial, since (H,H) was not possible forthat pair. We used lO-msec increments because ofdisplay constraints. This hunting procedure continueduntil the exposure duration hopped back and forthbetween two lO-msec separated exposure durations fourtimes in a row. The program had then found twoexposure durations, to and to + 10 msec, such thatabout midway between them the probability of twoconsecutive hits equals the probability of not gettingtwo consecutive hits, or equivalently, the probability ofa hit is .707. The exposure duration was set at to +10 msec for the following experimental session.

Two important advantages are offered by thisprocedure. First, it gives a very quick exposuredetermination. It usually took less than 5 min for the

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stopping criterion to be reached. Second, it gives a morerigorous basis, on statistical grounds, than a modifiedup-down threshold procedure for assuming that we havefound the exposure duration we want. This procedurereflects our philosophy about the optimal way ofinsuring desired operating levels. After practice effectsare eliminated by a warm-up session, a very quick probeis used to set the S at the operating level desired andthen the experiment proper is run. On-line use of thecomputer makes the quick probe easy to implement.

We have pointed out the conditions that we feel arenecessary for obtaining maximum yield from S's reports,i.e., immediate access to the S's insights and to therelevant performance indicators at the termination ofthe experimental session. In our experiments, the S'sresponse to the last stimulus of the experiment broughtin a subroutine that did the appropriate computationsand printed out the desired performance indicators. Thisprintout included a summary of accuracy levels andconfidence-level judgments. By the time the S got out ofhis chair, the E had a complete summary of the S'sperformance. Thus, the Sand E could immediatelydiscuss factors that may have affected performance, asindicated on the computer printout. In our pilot studies,this minimal-delay feedback was a critical input that ledto the realization that vocalization might reverse theword superiority effect. The availability ofminimal-delay feedback, for use in immediatepostexperimental discussion with pilot Ss, is animportant asset for structuring an experiment. This assetis not available to Es who do not use a computer.

In examining the utility of the computer astachistoscope in studies on the word superiority effect,we have pointed out a number of procedural constraints,imposed by the use of traditional apparatus in atraditional type of experiment, that are easily avoidedby computerization. We have focused our attention onthese studies of the word superiority effect in order tobe concrete. The points we have considered, however,are really quite general and are applicable to a number ofexperimental settings.

REFERENCESGreen, D. M., & Swets, J. A. Signal detection theory and

psychophysics. New York: Wiley, 1966.Julesz, B. Foundations of cyclopean perception. Chicago:

University of Chicago Press, 1971.Mezrich, J. J. The word superiority effect in brief visual displays.

Elimination by vocalization. Perception & Psychophysics,1973, 13, 45-48.

Noton, D., & Stark, L. Scanpaths in saccadic eye movementswhile viewing and recognizing patterns. Vision Research,1971,11,929-942.

Reicher, G. M. Perceptual recognition as a function ofmeaningfulness of stimulus material, Technical Report No.7,Human Performance Center, University of Michigan, 1968.

Smith. J. E. K. Stimulus programming in psychophysics.Psvchometrlka, 1961, 26, 27-33.

Uttal, W. R. Masking of alphabetic character recognition bydynamic visual noise. Perception & Psychophysics, 1969, 6,121-128.

Wheeler, D. D. Processes in word recognition. CognitivePsychology. 1970, 1.59-85.

NOTE1. The program for the dynamic visual noise mask is due to

Frank M. Good,',

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