Compensation Method to Measure the Contrast Produced by Contours*
GEORG VON BtxksytLaboratory of Sensory Sciences, 1993 East-West Road, University of Hawaii, Honolulu, Hawaii 96822
(Received 28 March 1972)
An attempt is made to show that the brightness changes produced by a border can be measured by acompensation method in which the stimulus magnitude is reduced on one side of the border to such a valuethat the brightness inequality disappears. Experiments with rotating disks, similar to the color-mixing disks,were used to produce compensation for a specific luminance distribution with a border. The compensationmethod can be used to compare the lateral inhibition produced by Mach-band patterns with that whichaccompanies border effects. Also, comparisons are made of the similarities and differences of lateral inhibitionproduced by the two methods. The influence of the border has the advantage that sharp projection of theimage on the retina is not necessary, as in Mach bands. The difference between the Mach bands and theborder effect is discussed and used to explain why the compensation for the border effect changes with thedistance of the observer.INDEX HEADING: Vision.
As Ratliff pointed out,' the interaction betweencontour and contrast can be twofold. The phenomenonwhereby the contrast of brightness or color of twoadjacent areas produces a contour is very well known.It was first described precisely by Mach2 in 1865, 1866,and 1868. In the last decade, Mach bands, as thephenomena have come to be called, have been investi-gated by many researchers including Jung,3' 4 Werner,5Bergstrom and Rubenson,' and Ratliff.7 The psycho-logical observations of Mach bands have recently beenconfirmed by electrophysiological techniques, such asthose developed by Hartline.8 It has also been possibleto show that Mach bands do not appear only in visionbut that a very similar type of inhibitory phenomenonand contour enhancement can be observed on the sur-face of the skin and on several other sense organs.,""
The second type of interaction, namely, the effect ofthe contour on the contrast, was first described in detailby O'Brien." It was mainly he who showed that theinfluence of a contour on the brightness of the surround-ing surfaces is much larger than expected. Furtherexperiments by Cornsweet'2 and Fry and Bartley"confirmed O'Brien's results.
Ratliff' was able to show that the influence of aborderline on the brightness of the neighboring surfaceswas known in oriental art, especially during the Sung
dynasty in China. It was intentionally used in paintingand pottery to produce small changes of brightness,which gave a special delicacy to the art of that period.In this technique, the over-all reflectance of the glazeof a pot remains constant; however, the apparent bright-ness changes as the result of engraved border lines thatoutline the figures and forms.
The two types of contour-contrast phenomena de-scribed above strongly suggest that there are two typesof lateral inhibition in the retina. One, the Mach-band-type inhibition, acts over only a small area to producea contour, and the second, contour inhibition, influencesthe brightness of the area adjacent to the contour andextends over a much larger area. These two types ofinhibition have many similarities, but at the same timethey have important differences. The present study isdesigned to compare these two different types of inhibi-tion by means of a method that allows for comparisonwithin the same observer.
Recently, I developed a method for measuring opticalillusions that uses the amount of compensation neces-sary to overcome the illusions as a measure of the magni-tude of the illusion. For instance, the illusion of thebending of two parallel lines on top of a star was mea-sured by bending two wires in the direction oppositeto the illusion, so that the two wires looked straight.
The magnitude of the required bending was easy toobserve and gave a measure of the magnitude of thistype of optical illusion.'4
We can carry out this type of compensation not onlyin spatial displacements but also in luminance illusions.To measure the Mach bands, we developed a com-pensating luminance distribution that could be adjustedso that the Mach band disappears. Thus, the amountof compensating luminance became a measure of themagnitude of the Mach bands.
The effect of the borderline on the brightness of theneighboring surfaces has one advantage compared withthe Mach bands, namely, that it is much less dependenton the sharpness of the image formation on the retina.In this way, experiments on ,lateral inhibition can bedone without the continual problems of focusing andcorrection of visual defects. This seems to be importantwhen we are testing a large group of observers whosevisual capacities show great variability.
Most of the observations reported below were madewith a luminance of 100 mL using three well-trainedobservers. In addition, naive student observers wereused to verify the main results of the trained observers.
COMPENSATION METHODS TO MEASURE THECONTRAST PRODUCED BY A CONTOUR
A contour effect that can be easily compensated isshown in Fig. 1. It is the same contour as the old Sung-dynasty or Korean potters used on their vases toproduce small variations of brightness without havingto change the paint or the glaze. This type of contourproduces a brightness distribution shown in the lowerdrawing, which increases the brightness of section (B).It can be easily compensated by decreasing the lumi-nance of section (B) to such a value that the brightnessesof section (A) and section (B) are the same. The
(A)
a). )CaC
E
en
cnC,0)C
.-D
I contour(B)
FIG. 1. The contour effect. If an equally illuminated surface ishiaccted by a darkj hand or a contour whose edges differ in sharp-ness of slope, as shown in the upper drawings of the luminance,then there is an increase of brightness of the area bordered by thesharp slope of the contour [side B]. The contour effect can beeliminated (compensated) by reducing the luminance in area (B)so that the brightnesses in (A) and (B) are apparently equal.
FIG. 2. A color-mixing-wheel pattern that (A) produces a con-tour effect, and (B) contains short spokes that can be adjustedto compensate the contour effect.
percentage decrease of luminance can then be taken asa measure of the magnitude of the contour effect.
To carry out the compensation measurements, wecan use an ordinary Mach-type rotating disk to producethe patterns. In Fig. 2, the heavy-shaded area with thesharp-pointed edges on each spoke was painted dullblack on a cardboard disk. When this disk was rotatedat fusion speeds, the points produced a borderline andan adjacent brightness decrease that extended to theouter end of the disk pattern. Around the same axiswas fixed the smaller compensation disk made of spokescut from black paper. These spokes could be adjustedrelative to the larger spokes so that they were eithersuperimposed on the larger spokes and, thus, notvisible, or rotated to various degrees out of alignmentsuch that they decreased the luminance in area (B) tothe degree necessary to give the appearance of equalbrightness on both sides of the contour. The degrees ofrotation were taken as a measure of the magnitude ofthe contour effect, as shown in Fig. 3. The two disks canbe mounted on a differential color mixer of the typethat permits a change of the relative positions of thetwo disks during rotation. In the latter case, the com-pensation measurement is extremely simple and canbe done very precisely.
The diagram in Fig. 3(A) shows a disk with thistype of compensation adjustment made from a viewingdistance of 50 cm and with a disk diameter of 27 cm(310 visual angle). It can be seen on this disk that theeffect is quite large, because the compensating spokeshad to be rotated about 80 relative to the disk. Sincethe white space between the spokes is 30°, this meansa luminance change of 37%.
INFLUENCE OF THE DISTANCE BETWEEN THEOBSERVER AND THE ROTATING DISKS
It was quite unexpected that the distance betweenthe observer and the rotating disk would play an im-
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November1972 MEASUREMENT OF CONTRAST PRODUCED BY CONTOURS 1249
portant role. However, Fig. 3(B) shows the results forthe same observer at a viewing distance of 200 cmwhere the disk subtends 80 of visual angle. In this case,the amount of compensation decreases from 80 to 2.4°as the viewing distance is increased from 50 to 200 cm.The amount of compensation for two trained observersat three viewing distances is shown in Fig. 4. A com-parison of these results with those in Fig. 3 indicatesthe large individual differences that we found, evenfor trained observers. Reduction of this and the largewithin-observer variability must await a better under-standing of the variables that are operating. However,it is clear that, under these conditions, the amount ofcompensation decreases as the viewing distance in-creases. This result is surprising when compared to thecase of Mach bands, for which viewing distance has theopposite effect; i.e., with increasing distance the widthof the Mach bands becomes much larger and the widthvaries proportionally with the distance. One possibleexplanation of this seeming discrepancy is that thesharpness of the Mach band producing the contourdecreases with the distance and therefore its effective-ness in producing the brightness difference also de-creases. However, a complete explanation of theseeffects will not be possible until the many variables thatinfluence contour brightness and Mach bands havebeen systematically studied.
LATERAL EXTENSION OF THECONTOUR EFFECT
The extent of the Mach-type lateral inhibition is ingeneral only a few mm, if the disk is seen from a shortdistance. But the lateral inhibition produced by anedge extends at least 12 cm from the edge, and perhapsfarther. We tested the question 'of how far the effectextends by producing a rotating disk with a diameter of54 cm. When viewed from a 50-cm distance, the bright-ness increase produced by the contour extended adistance of 15 cm' (170 visual angle) from the contourtoward the center of the disk. This width of extensionfrom the contour is one of the most surprising featuresof the inhibitory processes originating in contours.
* tP(A) ¶.<P (B)
^Xvbolt50cm 200cm
FIG. 3. Actual setting of the color-mixing wheels compensatedfor a constant brightness inside and outside of the contour andviewed at distances of 50 and 200 cm. At 200 cm, the contour effectbecomes quite small.
A
08 -
En0)
C)
-aC
C04 -G
aon
C£a.E0C-
4.
0
compensation
1 2 3 4
distance in metersFIG. 4. The decrease of contour effect is measured by degrees of
compensation that results from the decrease of distance betweenobserver and the rotating disks.
USE OF A ROTATING PRISM TO PRODUCECONVENIENT MEANS TO INVESTIGATETHE COMPENSATION OF THE BRIGHT-
NESS CHANGES PRODUCEDBY A CONTOUR
A simple method was devloped that allows manytypes of contours and luminance distributions to beproduced. A window was cut in a piece of black paper,as illustrated in Fig. 5, and a triangular shape, cut fromanother piece of paper, was placed so as to partition thewindow into left and right sections (A) and (B). Arectangular-shaped black paper was placed on the loweredge of section (B). If we look at this window througha rotating prism, as indicated in Fig. 6, the rotating
a.
FIG. 5. A square window viewed through a rotating prism isused to produce measurable contour effects. The adjustabletriangle produces the contour whereas the lower edge of area (B)is adjustable upward to produce compensation. The ratio b/a atequal brightness gives the compensation ratio.
:ft-
I
GEORG VON BtKESY
out with scissors from the black paper. To facilitatethis, the whole opening was placed on a light box andprojected with a lens on a very finely ground glass plate.This projected image on the glass plate was viewed froma distance of 25 cm through the rotating prism. A high-quality photographic lens was used to project thepatterns on the ground glass plate. The height of theimage of the window projected on the stimulus screenwas 6 mm or 1.40 of visual angle. Compensation wasdone by moving a stop in the (B) area up and down inthe window.
CONTOUR WITH THE LARGESTCONTRAST FORMATION
To investigate the degree to which the change ofbrightness depends on the shape of the contour, weplaced seven different types of contours, as illustratedin Fig. 7, into a window. The tip of each contour wasalways placed in the middle of the window. The rightsides were always vertical but the left sides had differenttypes of slopes. If a window contained only a contour,then the area (B) was always brighter than the area(A). To make the compensation, the horizontal stopwas moved from below into the area (B), as shown inFig. 5 and as described above, until areas (A) and (B)had equal brightness.
The values in Fig. 7 indicate to what degree the heightof the window in the area (B) had to be reduced to
positioningstage ba = 0.6
FIG. 6. Schematic diagram of the rotating-prism apparatus. Theapparent position of the window is rapidly moved in the verticaldirection by the use of a rotating prism. In this way, the heightof the window in Fig. 5 determines the luminance of the stimulusseen by the eye from a distance of 25 cm, on a finely ground glassplate.
prism will produce an apparent displacement of thewindow into the vertical direction four times duringevery rotation, thereby transforming a spatial patterninto a luminance pattern. During rotation, the observersaw a lighted rectangle that is divided by a thin border-line such that the right side is brighter than the left.Because the luminance of any vertical strip of thewindow is given by the height of the window, com-pensation could be accomplished by moving the loweredge of area (B) up and down until brightness equalitywas attained. A micrometer-screw assembly providedprecise movements and measurement of the amount ofdisplacement of the adjustable edge.
The rotating prism has to be carefully made. Wemade it from Lucite; it was milled on the axis of themotor. By microscopic observation we found little or no
visible displacemeiit of tild patllern ill tlhe horizontaldirection. During the experiments, we found that it isextremely convenient if the window is as large aspossible, so that the different edges could be simply cut
0.65 0.62
0.65
0.80
0.73
0.78
FIG. 7. Different shapes of contours introduced in the windowof the rotating prism and their associated compensation ratios.The largest contour effect is obtained with a simple triangle.
4timulus
lens
lightbox
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November1972 MEASUREMENT OF CONTRAST PRODUCED BY CONTOURS 1251
produce brightness matches for different types ofborders. As can be seen, a simple triangle, as is shownat the top of Fig. 7, produces the maximum contoureffect. It is quite possible that this maximal effect isattributable to the fact that a triangle also produces awhite Mach band on the lower side of the frame wherethe triangle meets the frame. But I do not think it isa very strong effect because the contours on the rightside have the same white Mach bands.
DIFFERENT SENSITIVITY FOR LUMINANCECHANGES ON THE LEFT- AND RIGHT-
HAND SIDES OF THE CONTOUR
In trying to compensate the areas (A) and (B) forequal brightness, we were surprised to find that thedisplacement of a stop placed in the area (B) producesa much-more-visible change of brightness than if a stopin area (A) is similarly displaced. If we make the heightof the window in the areas (A) and (B) equal to 4 mm,by use of horizontal stops in both areas, then an upward1-mm displacement of the stop in the (B) area is, ingeneral, visible. But if we move the stop in the area (A)down to expose the full 6-mm opening, it is not possibleto detect any brightness change.
If we produce any small localized disturbance ofluminance on the lower edge of the window of either thearea (A) or (B) (e.g., the tip of a pencil inserted into thewindow area at the middle of the stop), these smallchanges, which do not extend along the whole length ofthe window, are immediately visible. The sensitivityin this case is exactly the same in area (A) and area (B).We have to conclude that the contour does not affectthe visibility of small variations of the height of thewindow, when they extend only a few mmn in the hori-zontal direction. The contour affects only the bright-ness produced by a very evenly illuminated window. Ihave the impression that a closer investigation of thisdifferential brightness change produced by the contourwill give some clues as to how the nervous system in theretina works, because this study clearly indicates thatsmall irregularities are much more important for visionthan uniform brightnesses.
Area (A) is very interesting from the point of view ofneural inhibition. Because its brightness sensitivity fora small change in a small area was not modified andonly the sensitivity to changes of uniform brightnesswas affected, it is interesting to determine whether ornot a small change of the stimulus distribution canproduce Mach bands. To investigate this, we used thewindow illustrated in Fig. 8. The lower part was exactlythe same as before but the upper edge consisted of black
(A) (R)
FIG. 8. Superposition of a Mach-band type of inhibition pattern(top line) on a contour pattern (triangle, bottom line).
paper that had a discontinuity to produce Mach bands.The discontinuity could be moved easily from area (A)to area (B). We found that there was practically nochange of the white and black Mach bands produced bythe upper edge when it was moved from area (A) toarea (B). Thus, Mach-band formation in the modifiedarea is of the same type as in the unmodified area.
This seems to indicate that inhibitory phenomenaproduced by Mach bands and those produced bycontours are different, even though the mathematicaltreatment may be quite similar. Therefore, it seems tome justified to distinguish two different types of inhibi-tion: the Mach type for short extensions and the Heringtype'" for inhibitions that extend farther away from thediscontinuities. This distinction was already proposedin an earlier paper, on different grounds.'
REFERENCES
* This investigation supported by PHS Research Grant No.06890 from the National Institute of Neurological Diseases andStroke.
t Deceased 13 June 1972. See Phys. Today 25, No. 9, 78(1972).
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52, 303 (1865); 54, 131 (1866); 57, 11 (1868).3 R. Jung, Studium Generale, Vol. 24 (Springer, Berlin, 1971),
p. 1536.4 R. Jung, in Zukunft der Neurologie, edited by H. G. Bammer
(Thieme, Stuttgart, 1967).6 H. Werner, Am. J. Psychol. 47, 40 (1935).6 S. S. Bergstrom and B. Rubenson, Vision Res. 10, 1057 (1970).7 F. Ratliff, Mach Bands: Quantitative Studies on Neural Net-
works in the Retina (Holden-Day, San Francisco, 1965).8 H. K. Hartline, Science 164, 270 (1969).9 G. v. B6k6sy, Physik. Z. 29, 793 (1928).'° G. v. Bekesy, J. Gen. Physiol. 50, 519 (1967)."V. O'Brien, J. Opt. Soc. Am. 48, 112 (1958).1" T. N. Cornsweet, Visual Perception (Academic, New York,
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