JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Effect of Forced-Choice vs Magnitude-Estimation Measures on theWaveform of Metacontrast Functions*

NAOMI WEISSTEIN, THoMAs JURKENS, AND THERESE ONDERISIN

Departinent of Psychology, Loyola University, 6525 North Sheridan Road, Chicago, Illinois 60626

(Received 24 January 1970)

Metacontrast functions were obtained for three subjects using forced-choice and magnitude-estimationresponse measures, and for a fourth subject using forced choice alone. The functions from these two mea-sures were in every case U shaped and did not differ significantly from each other, even though the stimuliused for each measure differed slightly, and the forced-choice study was done three months after the magni-tude-estimation study. Thus, U-shaped metacontrast functions are not artifacts of subjective responsecriteria. The relative merits of the subjective and objective response indicators as measures of metacontrastare discussed.INDEX HEADING: Vision.

Waveforms obtained in metacontrast experiments canbe divided into two types.' In the first type, the strengthof masking is monotonic decreasing with respect toincreasing intervals between target and mask. (Whenthe target is presented first, these intervals are desig-nated +At; when the mask is presented first, theseintervals are designated -At.) If only positive andzero values of At are considered, or only negative andzero values of At, then this waveform can be said to bea monotonic decreasing function of At, and has beenreferred to as such in the literature.' Although, if weconsider the entire range of At's over which meta-contrast effects are obtained, this first type of wave-form is also U shaped; the term U shaped has beenreserved for those metacontrast waveforms where themasking maximum occurs at some Atb'o. In general,monotonic waveforms are obtained when the ratio ofthe energy of the mask to that of the target is greaterthan around 10, and U-shaped waveforms are obtainedwhen the ratio is less than 10 (with energy defined asthe luminanceXduration product).'

Recently, experiments in metacontrast have em-ployed what are called subjective response indicators,such as magnitude estimations and rating scales.'These are used because of their versatility: They allow,for any given experiment, observation of a full range ofmasking interactions which may, for instance, includeenhancement at one or the other end of the maskingsequence.4 However, there are objections to thesemeasures: Subjects may change their detection criteriaas stimulating conditions change. In particular, experi-ments that obtain U-shaped metacontrast functionsusing subjective response measures are questioned onthese grounds.5 According to this view, U shapes reflectchanges of subjects' detection criteria as the delaysbetween target and mask change; the waveform isassumed to be simply an artifact of experimental pro-cedure.

On the other hand, U shapes have been obtained withforced-choice (objective) measures6; however, since,for the most part, these experiments presented stimuliin varying locations in the visual field, it is possible

that this spatial variation may have resulted in a Ushape. Thus, the question still remains: Are U shapesof metacontrast results to be regarded as experimentalartifacts?

To investigate this question, and to compare subjec-tive and objective response indicators in meta-contrast,two experiments were done. First, a magnitude-estima-tion experiment was done in which the target alwaysappeared in the same location in the visual field. Becauseintersubject variance in masking is considerable, inorder to have a valid comparison between measures,subjects must be held constant. Thus, in a forced-choiceexperiment, run subsequently, the same subjects wereused. The forced-choice experiment used, as far aspossible, the same stimulating conditions as the magni-tude-estimation experiment.

SUBJECTS

Four paid subjects were used. One of these was naive;the three others had at least one month experience onother metacontrast studies before beginning the magni-tude-estimation study, and at least three months onother metacontrast studies before beginning the forced-choice study.

APPARATUS AND METHOD OF PROCEDURE

Magnitude Estimations

Stimuli were transilluminated Kodalith slides, pre-sented dichoptically (target to right eye, mask to left)in a 6-channel binocular tachistoscope, ScientificPrototype, model GB. To find the interocular distanceand interocular angle for proper fusion, the target andmask were turned on for indefinite exposure. The sub-ject then adjusted the viewing angle of the right threefields of the tachistoscope, and the distance from theright to the left three fields of the tachistoscope, untilthe display appeared stabilized. In addition, two 900prisms for each set of three fields could be adjusted sothat the fields for the two eyes were superimposed.When the display was fused, it was turned off, a fixa-tion point was turned on, and this was also fused. Each

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VOLUME 60, NUMBER 7 JULY 1970

WAVEFORM OF METACONTRAST FUNCTIONS

eye viewed the display through a 2-mm artificial pupil.The target consisted of a lighted disk 10 in diameter;the mask, a lighted ring with 1°1' inside diameter,and 1023' outside diameter. The stimuli were illu-minated at 17.3 mL as measured by an SEI Zoomarphotometer, and target and mask were presented for16 msec each, against a dark background, 1° from alighted 4' fixation cross, which was lighted continuously.The rise time of the lamps in each channel was about100 psec. Luminance and duration were monitored byan oscilloscope that showed the output of photocellsplaced in each channel of the tachistoscope. Twenty-seven At's were run for each condition for each subject:-100 to +100 in 10 msec increments, and 125, 150,175, 200, 250, and 300 msec. Ten replications were runfor each At for each subject. For each replication, the27 At's were presented in random order. There were32 such random orders. Subjects were shown, at thebeginning of each session, the unmasked target (thestandard), and asked to assign to it a number thatrepresented its completeness (absence of fragmenta-tion, clearness of contour, etc.). For each maskingpresentation, subjects judged the completeness of thetarget in terms of the standard. All subjects assignedto the standard the value 10. Only the three practicedsubjects participated in this experiment, which wasdone three months prior to the forced-choice experi-ment.

Forced Choice

The stimuli were transilluminated slides, constructedto resemble as closely as possible the stimuli used in theparametric series described above. The stimuli werechosen so that the discrimination required of the sub-ject would reflect the nature of metacontrast-a dis-tortion, fragmentation, and dimming of the target.The target stimuli were the letter C with a 15' gap in oneof the four clock positions: 12:00, 3:00, 6:00, and 9:00.The C's had 10 outer diameter and 45' inner diameter.The mask was a ring with four 15' gaps at the clockpositions 12:00, 3:00, 6:00, and 9:00; it had 101' innerdiameter and 1°23' outer diameter. Stimuli were il-luminated at 17.3 mL and target and mask presentedfor 16 msec against a dark background, 1° from alighted 4' fixation point which was lighted continuously.Again, 2-mm artificial pupils were used. Therefore,the stimulating conditions were the same as in the mag-nitude-estimation experiment; the difference wassimply that the target had a gap at its edge and a holein the middle, and the mask had four gaps in it. Thegaps were the width of the stroke of the C.

Subjects were tested with 11 At's, corresponding tothose at which maximum masking for each subjecthad been found. For three subjects, these were from0 to 100 in 10-msec increments; for subject TJ, thesewere from -50 to + 50 in 10-msec increments. For eachposition of the gap, for each At, 8 replications wererun, yielding 32 trials for each of 11 At's. Each ses-

Gr

/I

6 -5 /

2

-4050 000150 200 250 300 -00 -50 0 50 00 150200250 300

FIG. 1. Metacontrast functions obtained with magnitude esti-mations (geometric means, solid line) and forced-choice responses(number correct out of 10, dashed line) for each subject for eachAt. Subjects, beginning with the upper left-hand corner, andreading clockwise, are BW, GY, JP, and TJ.

sion consisted of 44 stimulus presentations; for eachsession then, each position X At was presented once persession. The 44 different-position X At trials were pre-sented in random order; there were S orders.

RESULTS

A 3-way analysis of variance was run. responseindicator X the three subjects who participated in bothexperiments X2At. The geometric means for the 11At's used for forced-choice trials were used as entriesfor the magnitude-estimation condition; the numberseen out of a possible 10 (corrected by the standardguessing-correction formula) were used as entries forthe forced-choice condition. Thus, the raw forced-choice scores (Xi) were transformed by the guessingcorrection formula (Xi-8)/24 and then multiplied by10 to yield the expression Yi=10(Xz-8)/24, and theYi's used as entries for the forced-choice condition.

The only significant effect obtained was for At atp <0.01. Neither the response indicator, nor the re-sponse indicator X At interaction was significant.Thus, there is no significant difference of metacontrastwaveform as measured by magnitude estimations,and metacontrast waveform as measured by forced-choice responses. The analysis of variance model thatmust be used with repeated measures, however, leadsto a very conservative test because the error terminvolved is the subject X At interaction and/or thesubject X response indicator interaction, and subjectvariance is quite large.7 Failure to obtain significancehides, in this case, some obvious differences. Thesecan be seen in Fig. 1. Each point on each solid line is ageometric mean of 10 magnitude estimations at eachAt for a particular subject; each point on the dottedline, a Yt for each subject (see above). For the magni-tude estimations, the entire range of 27 At's are plotted.One subject participated in only the forced-choice

979July 1970

WEISSTEIN, JURKENS, AND ONDERISIN

experiment; thus no comparison function is availablefor her.

The phases of the functions obtained for subjects TJand JP, with each response indicator, are quite similaralthough not identical; for BW, there is a considerablephase difference between functions obtained with thedifferent response measures. There is no consistency ofthe differences of magnitude between the two measures:For TJ, the forced-choice measures indicate less mask-ing; for JP, the measures indicate about the sameamount of masking; for BW, the forced-choice measuresindicate more masking. The difference of what ismeasured by the two types of response indicators maypreclude any meaningful comparisons about theirmagnitudes. However, the relative magnitudes of thetwo types of measures may be compared at leastordinally. By such a comparison, it is clear that theordinal relations vary from subject to subject. Simi-larly, there is no consistency of the differences of phasebetween the two response measures. For JP, theforced-choice maximum occurs at a Al 20 msec less thanthe magnitude-estimation maximum; for TJ, themaxima occur at about the same At's, although themagnitude-estimation function clearly starts its descenttowards maximum masking at earlier Al's. Finally,for BW, the maximum occurs fully 60 msec later forthe forced-choice measure than for the magnitude-estimation measure.

A difference of phase was not entirely unexpectedbecause a small change of stimulus pattern can producea shift to the left or right of a metacontrast function,and introducing a gap and a hole in the target stimulusmay represent a substantial change. Moreover, theforced-choice study was separated from the magnitude-estimation study by a period of three months. Becausethese differences are not consistent from subject tosubject, and because for two of the subjects, the wave-forms are fairly similar, there seems to be no indicationthat one or the other of these response measures iserroneous. The reasons for the phase shifts are notclear; metacontrast maxima tend to shift phase fromstudy to study and subject to subject. The stimulusparameters that lead to these shifts need further study.

In any case, the critical test is whether forced-choiceindicators yield U-shaped waveforms, i.e., maximafor At> 0 because the objection to magnitude-estimationmeasures that yielded U-shaped metacontrast functionswas not on the basis of phase shifts but simply whethera maximum could be obtained at any At&0. Here, thedata are quite clear. In every case-for every subject,for each indicator-the maximum masking occurs atA/#-0. Treatment means for magnitude-estimnationsand forced-choice responses were compared separately.The mean difference between the three and the four

subjects' scores, respectively, at At= 0, and at thatAl at which the maximum masking occurred was testedagainst the standard error of the residuals as computedfrom the analysis of variance,7 i.e., [(Sr/1n)(Eaij)}.For the magnitude estimations, masking is greater atAI#0O at p <0.005; for the forced-choice responses,masking is greater at AI#-0 at p<0.025. Thus, Ushapes are established in every case.

DISCUSSION

It is clear from this experiment that U-shaped func-tions-functions whose masking maxima occur atA15- 0-exist for metacontrast, no matter what re-sponse indicator is used. What is surprising and un-expected is that there were no statistically significantphase shifts or amplitude differences in either directionfor the forced-choice functions as compared to themagnitude-estimation functions, even though differentstimuli were used, and the experiments were separatedby a period of three months. As discussed, failure tofind significant differences is a joint function of theconservative statistical test used and of the fact that,whereas there were differences between waveforms forthe two measures for some subjects, there was no con-sistency to these differences. Magnitude estimationsare preferred for metacontrast experiments becausethey provide a more versatile and informative measure.This experiment does not indicate that waveformsobtained with these measures are artifacts of thechange of response criteria. Although it might betheoretically simpler if there were no such functions,they exist. A theory to explain them has been offered.'This theory has led to a number of fruitful predictions.Because such functions appear to be associated withcentral, contour processes, they have the potentialfor greatly increasing our understanding of higher-ordervisual processing. Magnitude-estimation measures,which allow the fullest range of effects to be observed,seem to be highly useful and versatile tools for furtherinvestigation of this area.

REFERENCES* This work was supported in part by a grant to the first author

from the National Eye Institute (EY 00143).' N. Weisstein, Psychol. Rev. 75, 494 (1968).2 C. W. Eriksen, J. F. Collins, and T. S. Greenspon, J. Exptl.

Psychol. 75, 500 (1967).3 See discussion and review in D. Kahneman, Psychol. Bull.

70, 404 (1968).E. Donchin and D. B. Lindsley, Vision Res. 5, 59 (1965).

aC. WV. Eriksen and D. H. Marshall, Psychon. Sci. 15, (4),195 (1969).

0 N. Weisstein and R. N. Haber, Psychon. Sci. 2, 75 (1965);M. S. Mayzoer, Al. H. Blatt, W. H. Buchsbaum, R, T. Friedel,P. E. Goodwin, D. Kanon, A. Keleman, and W. D. Nilsson,Psychon. Sci. 3, 79 (1966).

7 A. L. Edwards, Experimental Design in Psychological Reserch:(EcHlt, Rinehart and Winston, New York, 12968).

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