Simultaneous Brightness Contrast For Flashes of...

Simultaneous brightness contrast forflashes of light of different durations

Mathew Alpern

Measurements have been made of the magnitude of simultaneous brightness contrast on twoyoung adult male observers by a binocular brightness matching method. Five differentluminances of the inducing pattern were studied and the duration of the exposure was variedbetween 5 and 150 msec, along an arithmetic scale. For low inducing flash luminances, thelonger exposures show the greater contrast effect. However, as the luminance of the inducingpattern was progressively increased, the duration of exposure showing the largest contrasteffect systematically decreased. The curves resemble the Broca-Sulzer curves both in the•manner, at any given luminance, that the ordinate varies with duration and. in the way anygiven curve changes as luminance is varied. The data explain a previous contradiction be-tween results from psychophysical and. electrophi/siological experiments and therefore greatlystrengthen physiological theories of contrast.

s,Simultaneous brightness contrast is thefamiliar phenomenon in which an objectappears dimmer when seen against abright surround than against a dark one.This is illustrated in Fig. 1, which showstwo identical gray surfaces on differentbackgrounds.

Johannes Mueller10 believed that thiseffect was due to the reciprocal action ofadjacent retinal areas and Herings sup-ported this view, in disagreement withHelmholtz7 who had attributed the phe-nomenon to a more complex psychologicalprocess which he referred to as an "illusionof judgment."

There are a number of excellent reasonsfor supporting Mueller in this dispute, not

From the Departments of Ophthalmology andPhysiology, University of Michigan, Ann Arbor,Mich.

This investigation was supported in part by aResearch Grant NSF G( 10045) from theNational Science Foundation.

the least of which can be demonstratedwith the experimental arrangement illus-trated in Fig. 2. In this figure, a is a brightfield (the comparison standard) which ismaintained at a constant level (about 10FL. in these experiments) throughout; bis a similar field (the test patch) which isseen as far below the fixation point Z' asa is above it. The luminance of h (Bb) isvaried (the dependent variable) in order tomake a brightness match with a. c and c'are two similar rectangles (the contrast in-ducing patches) which are seen by theright eye on either side of b and theirluminance can be varied over a wide range(the independent variable).

If both b and c-c' are seen by the righteye and a by the left, then as soon as theinducing patches become very bright theymake the test patch appear much dimmerand its luminance must consequently begreatly increased in order to re-establishthe brightness match (open circles, Fig.3). On the other hand, if the inducing

47

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48 Alpern In ccsfigativc OphthalmologyFebruary 1963

Fig. 1. The phenomenon of simultaneous bright-ness contrast. The two gray squares have theidentical luminance but the one seen against theblack surround appears much brighter.

.05°

— pa

c b

2i

c'

JUU 1Fig. 2. The stimulus pattern used in these ex-periments, a is seen always by the left eye andc-c' always by the right eye. b is seen by theright eye except in the experiment illustrated inFig. 3 by the solid circles. Z' is the fixation pointviewed binocularly.

patches are exposed to one (i.e., the right)eye and the test patch is exposed to theother eye (solid circles, Fig. 3), no sucheffect occurs at all! On the contrary, whenthe inducing patch becomes very bright,the test patch actually increases slightly inbrightness. Thus, simultaneous brightnesscontrast requires the test and inducingpatches to be exposed to the same eye andthis is strong evidence for a retinal interac-tion effect along the lines proposed byMueller.

It is not too surprising that this is thecase, since modern electrophysiologicaltechniques have made it possible to un-cover exactly this sort of physiologicalinteraction of adjacent retinal areas bystudying the electrical discharge of singleretinal ganglion cells. Both Kuffler,0 in thecat, and Barlow,2 in the frog, found thatthe response of such cells evoked by illum-ination of the retina with light could beinhibited by illumination of surroundingretina. Nevertheless, there is one majordifficulty with the view that the effect illus-trated in Fig. 1 is due to retinal interaction.Barlow, Fitzhugh, and Kuffler3 found inthe cat retina at threshold that the effectwas much more pronounced for long flashes(380 msec.) than it was for short ones(7 msec). On the other hand, Chinetti0

found exactly the opposite result by use ofpsychophysical binocular brightness match-ing technique above threshold. Because ofthis obvious discrepancy, measurementsof the amount of simultaneous brightnesscontrast have been carried out for a widevariety of different durations (5 to 150msec.) and inducing patch luminances(over 3 log,0 units).

MethodThe apparatus (Fig. 4) has already been de-

scribed in some detail elsewhere.1 The test patchh is formed by light which passes through thefield stop E after emerging from aperture N. Thisaperture is imaged in the plane of the pupil ofright eye by Maxwellian view. The comparisonstandard a is formed by light passing throughaperture M and then emerging through a similarfield stop at E. The image of M is focused in the


Simultaneous brightness contrast 49

Table I. Mean settings of brightness match(B,, = 6.57 FL. N = 30)

Log,, Be.o,

Observer

Duration(msec.)

51015202535455565758595105115125150

Steady

4.9

MA.

93.5243.0450.0468.0488.0621.0404.0338.0322.0263.0217.0224.0195.0183.0174.0188.0327.0

D.C.

1,2001,7101,7801,6101,2701,2101,050975881823811854745760713736457

4.4

MA.

27.039.387.2136.0140.0160.0250.0204.0160.0143.0139.0143.0151.0161.0156.0193.0137.0

D.G.

536674878968

1,1601,1901,000941805804733653640681690659240

MA.

14.226.764.886.597.8125.0128.0145.0143.0126.0118.070.276.854.253.967.560.5

3.9

D.C.

12413918827134737143543339035633634130527629321666.6

MA.

12.915.415.922.823.928.130.628.132.029.631.633.428.829.527.724.719.8

3.4

D.G.

18.618.826.733.842.177.5101.0114.0179.0182.0189.0176.0172.0155.0145.0118.023.3

MA.

10.211.812.614.116.317.520.028.828.828.131.727.430.025.123.921.714.5

2.9

D.C.

8.8113.2015.2014.6014.2014.6015.0017.0020.0022.4024.2025.1023.3023.1021.3022.8018.80

K

center of the entrance pupil of the left eye. Theinducing pattern c-d is obtained by light passingthrough a field stop at U after emerging from theaperture V. This stop is also focused in the planeof the entrance pupil of the right eye. The imageof each of these stops in the pupillary plane issomewhat smaller than the smallest possible naturalpupil size, and thus artifacts from variation inlight reaching the retina with changes in pupilsize are obviated. Rotating and fixed polaroidsK and L, and neutral filters at M, N, and Vpermitted variation of the luminance of the testfield continuously and that of all three fields instep fashion.

The roof prism and penta prisms were placedone above the other and so arranged that theposition of the image of N remained fixed whilethat for the image of M could be varied byrotating the platform on which the two prismswere mounted. In this way the interocular separa-tion of the two eyes was readily adjusted. Forthe special experiment in which the data illus-trated in Fig. 3 (solid circles) were obtained, itwas necessary to put both the test and the com-parison patch into the left eye. To do this, theplatform containing the roof and penta prism wasremoved and replaced by a special mountingcontaining two first surface mirrors (one abovethe other) which helped to image both M and Nin the center of the entrance pupil of the left eye.The amount of light reflected by the upper mirrorwas slightly greater than that reflected by theroof prism and this accounts for the slight verticaldisplacement of the two curves in Fig. 3 at the

= 100-

GJ 30-

l- 10-

0 1 2 3 4 5LOG INDUCING FIELD LUMINANCE FT-L

Fig. 3. Simultaneous brightness contrast com-pared when the test was seen by the right eye(open circles) and when the test patch was seenby the left eye (solid circles). In each case theinducing patch was seen by the right eye. Theordinate is the luminance of b (Bb) necessary toestablish a match to the standard which wasalways viewed by the left eye. The value of theluminance of the standard (B,,) for the open circleswas 12.6 FL., for the solid circles it was 13.9 FL.


50 Alpern Investigative OphthalmologyFebruary 1963

Fig. 4. Line drawing of the apparatus as viewed from above.

very low luminance levels. The sectored discs Iand II exposed the patterns for any desired dura-tion once every four seconds. In the experimentsshown in Table I, all the exposures were simul-taneous, i.e., the onset and cessation of test, in-ducing patches and of the comparison standardwere always the same.

Measurements were made at five different in-ducing flash luminances and sixteen different dura-tions. In a given experimental session only asingle inducing patch luminance level was studied.Six matches were made at each exposure dura-tion, beginning first with the shortest flash andthen the next shortest, and so on. At the verybeginning and at the very end of the session, six-matches were made of the value of h requiredto match a (5 msec, exposures) when the induc-ing flashes were not exposed at all. Five repeti-tions of each experiment were carried out on eachof the luminance levels of the inducing patternand for each of the observers. A third observercompleted only two experimental sessions undereach condition. Nevertheless, his results con-firmed in every way those obtained with theother observers.

In order to be certain that the matches estab-lished between the test patch and the standarddid not themselves vary with duration even inthe absence of the contrast inducing field, acontrol experiment was carried out in which theduration of a and b were varied in absence ofthe inducing pattern. There were no differencesin these measurements for different durationswithin the precision of the measurements (±10per cent).

Results

The mean results of the 30 brightnessmatches for each of the two observersunder each condition are summarized inTable I. While differences between the twoobservers exist, particularly in the magni-tude of the effect at the higher inducingpatch luminances, similar trends exist inboth sets of data, and so the mean resultsare plotted in Fig. 5.

When the luminance of the inducingpatterns is very low, the longer flasheshave the greater ability to reduce thebrightness of neighboring flashes. How-ever, as the luminance of the inducingpatterns increases, the flash duration whichis associated with the largest ability to re-duce the brightness of surrounding patternsbecomes shorter and shorter.

A second feature of the results in Fig. 5is the close parallel between flash bright-ness and flash contrast induction thatthey suggest. Broca and Sulzer5 showedthat the intensity of a light required tomatch constant high intensity flashes ofdifferent durations was at a maximum forintermediate duration flashes. While thiseffect has been familiar for a long time,


Simultaneous brightness contrast- 51

no one really understands it.'1 Perhaps forthis reason, attempts to attribute the phe-nomenon to some measurement artifactappear from time to time. (For the mostrecent of these, see Raab, Fehrer, andHershenspn.") In Fig. 6 are plotted twodifferent measurements of the phenomenonmade on different observers and withdifferent stimulus conditions. The resultsnonetheless are quite similar. In the ex-periments from which the lower set ofresults were obtained, binocular brightnessmatches were established between twostimulus patterns (similar to a and b inFig. 2). a was held at a constant intensitybut its duration was varied, while b washeld at a constant duration (750 msec.)and its luminance was varied to make thebinocular brightness match. The separationof the patterns sufficed to obviate the possi-bility of any interaction between the twoflashes and in this way the possible artifact

LOG INDUCING FLASH LUMINANCE

1000

8 0 0

6 0 0

'400

2 0 0

K

0 50 100 150FLASH DURATION ( MSEC)

Fig. 5. Mean data of two observers for themagnitude of B6 necessary for a match with awhen the duration of the entire pattern (illus-trated in Fig. 2) was varied. The center of b wasseparated from the center of c (and c') by% degree. Each point is the mean of 60 measure-ments.

100 200

FLASH DURATION -msec.

Fig. C. Two examples of measurements of theBroca-Sulzer curves. A, The test patch was ex

given on the curve and its durationvaried as the independent variable, The de-pendent variable was the energy (luminance xduration) of b needed for the match. The dura-tion of b at any given luminance level of c-c'was held fixed but was varied between luminancelevels in the following way: for the top curve30 msec, for the next curve 15 msec, the onebelow that 10 msec, and the bottom curve 5msec. All flashes synchronized at off. Tungstenlight had an approximate color temperature 3.0(10)3 degrees absolute. B, The test patch wassimilar to a in Fig. 2, the matching flash similarto b. The test flash luminance was held fixed atthe value given on each curve and its durationvaried as the independent variable. The match-ing flash duration was held fixed (0.75 sec.) andits luminance varied to establish the match (de-pendent variable). The flashes were so synchro-nized that the test patch always appeared in themiddle of the exposure of the matching flash.The light source was the same as that from whichthe data in A were obtained except that aWratten No. 74 filter was placed in both beams.


52 Alpern Inveslij-nth : OphthalmologyFebruary 1963

suggested as an explanation for the effectby Raab and associates11 has been avoided.The similarity of these curves to these inFig. 5, both in the way they vary with flashduration at any given luminance level andthe way any given curve changes asluminance level is changed, is very re-markable indeed.

Discussion

The results in Fig, 5 make it possible tobring the psychophysical experiments oncontrast into agreement with the electro-physiological data obtained on the catretina. In the latter case Barlow and co-workers3 found, at threshold, that a verylong flash was a much more powerfulcontrast inducer than a very short one.Chinetti11 found the opposite result psycho-physically but he used flashes which werevery much above threshold. Fig. 5 showshow a short exposure has a greater abilityto induce contrast than a longer one ata high intensity and a smaller ability toinduce contrast than a long flash at, ornear, threshold.

Thus, the present experiment by bring-ing the psychophysical and electrophysio-logical data into accord greatly strengthenstheories of contrast based on retinal inter-action effects such as the one Mueller pro-posed many years ago.

It is not possible, however, to be nearlyas certain when the question is raised asto whether or not the similarity of thecurves in Figs. 5 and 6 is anything butfortuitous. The results illustrated in thesefigures are from different observers andquite different stimulus conditions so thatfrom the available data it is not yet pos-sible to say that those stimulus flasheswhich have the largest ordinate on theBroca and Sulzer plot also have thelargest ability to induce a contrast effectupon neighboring flashes. Apparatus limita-tions, among other technical difficulties,have so far prevented an exhaustive anal-ysis of this important empirical question,but a preliminary attempt to do so is illus-trated in Fig. 7. For the Broca-Sulzer

Fig. 7. The relationship between the ability of aflash to induce a contrast effect and its ordinatein the Broca and Sulzer experiment. Broca-Sulzer data from N. B., B« = 146 FL. Contrastdata from M. A., B,, = 79 FL. Each point repre-sents a flash of a different duration. B\ is thevalue of Bt, obtained in each experiment understeady state conditions.

curve, rectangle a was exposed to the lefteye for a variety of durations but held ata fixed intensity (146 FL.), while rectangleb (exposed to the right eye) was held at afixed duration (750 msec.) while itsluminance was varied so that a brightnessmatch was obtained. The contrast experi-ment was exactly like those from whichthe data in Fig. 5 were obtained. The in-ducing flash luminance was 79 FL.; thetest flash luminance was 6.57 FL. Takinginto account the difference in the area ofthe inducing pattern in the contrast ex-periment as compared to that of the testflash in the Broca-Sulzer experiment, thestimulus conditions were approximatelycomparable, although different observerswere used in the two cases. The resultsfrom each experiment revealed that theintermediate flashes had the greatest effect,the maxima in the two experiments beingapproximately the same (85 msec, in thecontrast experiment, 95 msec, in theother). In each case, what is plotted inthe figure is the ratio of B\, needed toestablish a brightness match with a foreach flash duration and that required to


Volume 2Number 1

Simultaneous brightness contrast 53

1

do so when a and b (and in the contrastexperiment, c and c') had reached a steadystate (B'b). While the data do seem toshow some sort of linear relationship be-tween the two very different kinds ofmeasurements, the extent to which themeasured points fall on the empirical lineis not nearly as satisfactory as one wouldwish, particularly at the upper right-handpart of the graph. The relationship be-tween ordinate on the Broca and Sulzercurve and ability to induce contrast isclearly not 1:1, since the slope of the linein the figure is only 0.26. This means thatunit increase Bh/B\ in the contrast experi-ment is associated with an increase of 3.84in Bb/B\ in the Broca and Sulzer settings.

The relation between the ordinate of theBroca and Sulzer curves and the ability toinduce contrast in neighboring flashesillustrated in Fig. 7, as well as the rela-tions illustrated in Fig. 3, makes it verytempting to devise a neurophysiologicalmodel relating the brightness of a flashand its ability as a contrast inducer in amore specific way. However, with thedearth of data presently available it doesnot seem worth while to attempt any de-tailed theoretical analysis of possible rela-tionships between these two differentresults of retinal excitation. Furthermeasurements need to be made with iden-tical flashes in each of the two experimentson the same observers before more specifictheoretical treatment can prove profitable.

REFERENCES1. Alpern, M., and David, H.: The additivity

of contrast in the human eye, J. Gen. Physiol.43: 109-126, 1959.

2. Barlow, H. B.: Summation and inhibition inthe frog's retina, ]. Physiol. 119: 69-88, 1953.

3. Barlow, H. B., Fitzhugh, R., and Kuffler, S.W.: Change of organization in the receptivefields of the cat's retina during dark adapta-tion, J. Physiol. 137: 338-354, 1957.

4. Brindley, C. S.: The physiology of theretina and the visual pathways, London,1960, Edward Arnold & Co., p. 155.

5. Broca, A., and Sulzer, D.: La sensationlv.minen.se en fonction du temps, J. dePhysiol. 4: 632-640, 1902.

6. Chinetti, P. J.: The effects of reduced ex-posure duration on simultaneous brightnesscontrast, Ph.D. thesis, University of Wiscon-sin, 1957.

7. Helmholtz, H. L. F.: Handbook of physio-logical optics, 11 (Trans, from the 3rd Ger-man edition by J. P. C. Southall). OpticalSociety of America, 1924, pp. 264-296.

8. Hering, E.: Zur Lehre vom Lichtsinne,Sitzungsb. d. Wiener Akad. G8 ( a $ f e ) : 186-201, 229-244, 1873. " * "

9. Kuffler, S. W.: Discharge patterns and func-tional organization of mammalian retina, ].Neurophysiol. 16: 37-68, 1953.

10. Mueller, J.: Elements of physiology (Trans,from the German by W. Baly), London,1842, Taylor & Walton, vol. 2, p. 1187.

11. Raab, D., Fehrer, E., and Hershenson, M.:Visual reaction time and the Broca-Sulzerphenomenon, J. Exper. Psychol. 61: 193-199, 1981.

DiscussionDr. Hermann M. Burian, Iowa City, Iowa. Dr.

Alpern's paper is a difficult one to discuss, if bydiscussion is meant the raising of critical ques-tions. His work is always so careful and meticu-lous that one is hard put to find fault with hismethodology or his results. This also applies tothe paper which he has just presented to us,which as anyone familiar with Dr. Alpern's workrealizes, is only a tile in the mosaic of his workon contrast phenomena.

In this particular study he has given us first ofall an elegant demonstration that simultaneouscontrast is indeed due to the interaction ofadjacent retinal areas, as was taught by Johannesvon Miiller and Hering. Nevertheless, I am some-what uneasy about the fact that he makes binoc-ular brightness matches, even though he hasassured me yesterday when I discussed this withhim that this is a well-accepted technique.

Dr. Alpern has furthermore confirmed withrefined methods the observations of Broca andSulzer. This confirmation is of value since it canno longer be maintained that the effects noted byBroca and Sulzer might be measurement artifacts.

Lastly, a question about Dr. Alpern's premisethat there is an obvious contradiction betweenthe neurophysiological results of Kuffler and Bar-low and the psychophysical findings of Chinetti.Since the former worked at threshold, the latterwith flashes very much above threshold, therewas no reason to assume a priori that the resultsof these investigations should give comparableresults. In fact one might have rather expected adiscrepancy. Dr. Alpern has confirmed the differ-ence in response to light flashes at thresholdintensity and at supraliminal intensities and thus


54 Alpern Investigative OphthalmologyFebruary 1963

removed any question about a contradiction be-tween the p.sychophysical and neurophysiologicaldata.

I wish to thank Dr. Alpern for his courtesy insupplying me with his manuscript in advance ofthe meeting.

Dr. Alpern (closing). Binocular brightnessmatching was introduced in 1934 by W. D.Wright (Proc. Roy. Soc. s.B., 115: 49-87) andsince then it has proved to be an extremely power-ful tool in our efforts to comprehend a variety ofcontrast and adaptation phenomena. However,there are limitations to the kinds of questions one

can ask with these methods, and I have littledoubt that as our knowledge increases we will berequired to develop better techniques in order toanswer the most basic remaining questions.

While I agree with Dr. Burian that we do notknow enough about the physiology of brightnessperception to predict a priori that suprathresholdand threshold light flashes would behave similarlyin a lateral inhibition paradigm, this was clearlythe most parsimonius assumption. The fact, asthese experiments demonstrate, that this assump-tion was invalid makes the ultimate understand-ing of brightness perception more, not less,difficult.


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