The perception of color with achromatic stimulation

Yision Res. Vol. 11. pp. 591-612.

Perpamon Press 1971. Printed in Great Britain.

THE PERCEPTION OF COLOR WITH ACHROMATIC STIMULATION’ LEON

FESTINGER,MARK R. ALLYNand CHARLESW. WHITE

Department of Psychology,The Graduate Faculty, New School for Social Research, 66 West 12th Street, New York, N.Y. 10011,U.S.A. (Received 9 October 1970;

in revisedform 10 December 1970)

INTRODUCTION

THEREis a curious fact about color vision that has been known for well over 100 years. Under some conditions, repetitive black and white stimulation gives rise to the sensation of color. This experience of color has been produced, most commonly, by rotating a disc with a black and white pattern. For ease of communication we will, following CAMPENHAUSEN (1968c), refer to these as flicker colors. Over the years many patterns that produce some color sensation have been described in the literature. One of these, invented by BENHAM (1894, 1895), has been the subject of considerable research. The major facts about this pattern (a modification is shown in Fig. 1) are simple to state. If it is rotated under moderate achromatic illumination at somewhere between 5-10 c/s, the lines on the disc appear to be colored. If rotated counterclockwise, the lines labelled A in Fig. 1 appear reddish, the lines labelled B appear greenish, and the lines labelled C appear bluish. The colors are relatively unsaturated and there are considerable individual differences in how well they are seen.

FIG. 1.

A modificationof Benham’sdisc.

1The research work on which this paper is based was supported by NIH Grant No. 16327and NSF Grant No. GB-8178of which the principal investigatoris Leon Festinger. 591

592

LEONFESTINC~ER. MARK R. ALLYN AND CHARLES W. WHITE

The specific temporal order of presentation is critical. If the direction of rotation of the disc is reversed then the lines labelled C appear red and the lines labelled A appear blue. The lines labelled B remain green. In short, if there is a repetitive temporal sequence beginning with, let us say, 75 msec of black, followed by 25 msec of thin black lines and ending with 50 msec of white, a sensation of red is produced. If the black period is followed by 25 msec of white, 25 msec of thin black liries and another 25 msec of white, green is produced. If the black period is followed by 50 msec of white and then 25 msec of thin black lines, blue is seen. It is important for the production of these flicker colors that the black lines be thin. They must be wide enough to be clearly visible but if the lines are too wide no color is seen. There are quite a few other variables that affect the production of these flicker colors, but they are of less concern to us here. Extensive and careful explorations of these phenomena are reported by ROELOFSand ZEEMAN(195%) and CAMPENHAUSEN (1968a, 1968b, 1968~; 1969). A summary of much of the literature can also be found in SHEPPARD(1968). Possible theoretical explanations Today there is still no valid expJanation of this phenomenon, although one of&red by P&RON(1923) has tended to become widely, if uncritically, accepted. P&ON (1945) himself summarizes his explanation. He states: “The explanation of these colorations is found in the ditferences between the timeconstants of the establishment of sensation for the fundamental systems @I&RON, 1923). Tk blue system is the slowest to appear and to rsach its transitory maximum, the slowest ako to disappear; the red system is the quickest of the three. wha a pencil of white light is suddenly made to act, there is an initial unbalance which favours the red excitation, and when the light ceases to act the last phase in the pmistcnce of the image favouff blue. This phenomenon is, however, too brief for it to be seen norm&. The rotating disk of Be&am’s TOD. where a white half&k SNZX& a black. effects brief sti&ulations by whte light. Blackcircular &ipes disposed on the white sector shield co&ponding retinal areas from direct luminous stimulation. But at this level a diffusion of the excitation from the surrounding areas must be produced, extending to the whok of the stripe if this be thin enough; this diffusion will act selectively pn the chromatic systems. In the initial phase, when the unbalance is in favour of the red component. the diffusion of the excitation will lead to the production of a reddish coloration of the dark rings resulting from the short annular strip. In the second phase, the green coloration diffuses, the unbalance favouring mostly the second component. In tbe last phase, it is the blue coloration which, due to its slow growth, ma&a its maximum while the otben have already noticeably declined. The coloration obtained, red, orange, yellow, green, green-blue or blue. depends upon the position of the stripes on the white sector of the disk and upon the speed of rotation, that is to say, on the exact value of the time delay.” (MRON, translated by RREENE and Aem, 1952, pp. 156-l 57.)

Pi&on also presents a figure reproduced here as Fig. 2, that illustrates his conjectures. The red sensation comes to a peak early and drops off rapidly after the onset of white light. Green is next fastest and blue is slowest. Presumably whatever sensation is prevalent at the time diffuses over the black lines that are present at that time. The extent to which this explanation has been accepted may be judged from the following. MARRIO’IT(1962), for example, states: “It is fairly well atablished that the late&m of action of tbe difkrcnt mechanismsof colour vision differ, the red sensitive mechanism having a aborter ktcacy than the blue. This is tbe basis of various curious colour effects which can be produced by the IIIt of black and white objects. The best known of these is Benham’s Top. . . . For a detailed theoretical explanation of the phenomenon, see Pieron.” (MARRIOTT.1962, p. 289.)

Similarly, LEGRAND(1957) says : “With equal vahm of L there may be dilkcnccs due to colour, the 14 up mom quickly and dying away sooner than the blue. For this reason a rotating disc with sectors which are half red and

The Perception

of Color with Achromatic

593

Stimulation

RED YELLOW 45’ GREEN

SL”E

BLACK

WHITE. SECTOR 180.

SLACK SECTOR 180.

FIG. 2. Schema of the time courses of the three chromatic systems. (After

P&ON,

1952, p. 157.)

half blue gives an apparent displacement of the red in advance of the blue. In a similar way it is possible to explain certain phenomena (the fluttering heart phenomenon of Helmholtz, the FechnerBenham Top) which are classics in the vision of moving objects.” (LEGRAND, 1957, p. 302.)

Apart from the vagueness concerning the diffusion of excitation to the thin black stripes, the critical aspect of the Pi&on explanation lies in the hypothesis about the differential rise and fall times of the different color sensations. If we return to Fig. 2 we notice that the difference between the red and blue peaks of sensation is almost equal to the duration of the white sector on the disc. Since under moderate illumination the disc should rotate at somewhere between 5 and 10 c/s for the colors best to be seen, the duration of the white sector is between 100 and 50 msec. We must then ask whether a time lag between the red and blue sensation maxima of perhaps as much as 75 msec is at all plausible. F&RON (1932) himself attempted to measure these differential latencies by psychophysical methods. In one experiment he measured the stimulus presentation time that yields a maximal sensation. That is, he employed the B&a-Sulzer phenomenon in an attempt to assess these different latencies. When the luminosities of his three colors were roughly equated, the difference in latency between red (640 nm) and green (530 nm) came to only 9 msec and the difference between red and blue (475 nm) came to only 24 msec. In a second experiment he measured the reaction time to the three different colors, that is, the time from the presentation of the light to the time that the subject sees the color. With this method he finds the difference in latency between red and green to be 11 msec and between red and blue to be 28 msec. Although the differences he found are in the direction required for his explanation of the Benham disc colors, they certainly do not approach the magnitude required. More than a decade later, LIANG and PIBRON(1947) attempted again to measure these differential latencies. They used the Pulfrich effect to measure the differential latencies of different colors that were equated for luminosity. The data obtained in this manner are even more disappointing. Averaging the results for three observers, the difference in latency between red and blue light is somewhere between 3 and 7 msec. Furthermore, the latency for green shows little or no difference from that for blue on these measurements. Indeed, on

594

LEONFESTINGER, MARKR.

ALLYNANDCHARLES W.

WHITE

some calculations the latency for green comes out to be longer than for blue. It seems difficult to explain the Benham disc colors on the basis of these differential latencies. Others who have investigated this question more recently also fail to find evidence to support the idea of large latency differences. GUTH (1964) concludes that “there is no general effect of wavelength on visual perceptual latency”. GREEN(1969) concludes “The available measurements show no signs of the blue curve being shifted toward the low frequencies as would be expected if the processes mediating blue vision were for some reason relatively slow” (p. 599). Thus, it turns out there are reasons for being skeptical of the Piiron explanation of the “flicker colors”. Others, of course, have also been dissatisfied and some have proposed other explanations. Several writers, in one form or another, have suggested that the color signal is coded for transmission in the neural pathways in a manner resembling a Morse Code. The earliest such suggestion was made by TROLAND(1921), who proposed : The individual nerve pulses could conceivably be spaced in different characteristic ways to rcpresent the various colors, luminosity still depending upon the total number of pulses arriving per unit of time. A stream of rapid impulses could be broken up by gaps of dB”t sorts, these gaps only being detected by the central color receiving apparatus . . . .” (TROLAND, 1921,p. 42.)

ROELOFSand ZEEMAN (1958) make a general statement, saying that this type of signal code might hold the promise of explaining the flicker colors. FRY (1945) presents specific hypotheses about a “temporal modulation theory”, saying: “In the mod&tion tbcory propored by tk writer each variation in chroma has its own chancteristic modulation pattern. . . . The rumed shape3have a artain amount of cxp&mmtal basis in the Be&am-Top phenomena but arc still to be rega&d as tentative.” (FRY, EMS, p. 114.) GEHRCKE (1958) also makes a specific theoretical proposal. He states that specific temporal patterns of change in the amplitude of electrical activity in the visual cortex produce the sensations of color. He calls these “current curves” and states (p. 97) : “The current curves are all to be conceived as the final current in the occiput and not as the initial current in retina and optic nerve”. Gehrcke presents no data concerning empirical observation in this paper but states several times that these current curves correspond to experimental facts, referring to a paper he published 10 years earlier in 1948. In that earlier paper he reports observations with rotating discs, some of which are similar in form to the Benham disc. The rciahhip, however, between the observations reported in the 1948 article and the theoretical model presented in 1958 is unclear. The most recent attempt to provide a theoretical explanation of the flicker colors is provided by CAMPENHAUSEN (1965a, 1%Sb, 1%5c, 1969) in a series of articles. He realized that in a pattern such as the Benham disc the temporal sequence of black followed by thin black lines and white is not an adequate description of what impinges on the retina. There is, in addition, a background that changes from black to white. In a series of ingenious experiments he demonstrated that if the background flicker is eliminated, the color disappears. He concludes from this evidence that lateral inhibitory processes are necessary for producing the flicker colors. In other experiments he produces good evidence to support the assertion that the inhibitory interactions necessary for the flicker colors take place on the retina. He shows that if a stimulus pattern, that when presented to one eye alone produces the sensation of color, is presented partly to one eye and partly to the other in a fused binocular image, the flicker

The Perception

of Color with Achromatic

Stimulation

595

is not present. He further shows that if two identical Benham discs are viewed independently by each eye, binocularly fused, the flicker colors are seen even if the two discs are not in phase with one another. In summarizing his theoretical position, Campenhausen states: color

“The rotating top is a source of periodic stimulus programs which excite neighboring retinal areas in a different way. The color sensations are induced by lateral interaction in the visual system consisting probably of phase sensitive inhibition of modulated flows of excitation by modulated flows of inhibition.” (CAMPENHAUSEN,1969, p. 677.)

This theoretical position does not seem to be in explicit contradiction to ideas concerning the possibility of temporally modulated neural codes. The lateral inhibitory interactions might help to produce these codes. The idea of a temporal Morse Code seems very attractive because, if true, it enables a very simple explanation of the flicker colors. The temporal succession of black and white stimulation may create a sequence of neural events that mimics the true temporal code. It seemed worthwhile to collect systematic data that might throw light on the plausibility of this explanation. The first question to ask is exactly how might the pattern of black, white and thin black lines on the Benham disc produce a facsimile of a temporal neural code. One possibility is that the thin black lines, because of blurring and because of area1 summation on the retina, actually produce an intermediate intensity of stimulation, somewhere between that of black and white. If this is, indeed, what happens then the Benham disc may be viewed as providing a temporal sequence of intensity changes, the specific sequence depending on where the thin black lines are placed. The differently timed intensity changes would produce different temporal patterns of firing frequency in the neurons. If this is correct, then one should be able to substitute a thick grey band for the set of thin black lines and still produce the color experience. Furthermore, unlike what occurs with black bands, one should be able to make a grey band quite wide without destroying the “flicker color”. We tried this and it turns out to be the case. The Benham disc colors can be produced with wide grey bands. The next question concerns the necessity of using a rotating disc or other means to create movement across the retina in order to produce the flicker colors. If we are creating a facsimile of a temporal code, the only thing necessary should be the proper time sequence of changes of intensity. In other words, it should be possible to change the intensity of a stationary spot of light and produce the appropriate colors by having the appropriate sequence of changes. We proceeded to collect data to answer this question. APPARATUS AND GENERAL PROCEDURE All the data to be presented in the following pages used the same apparatus. A schematic diagram of this apparatus is presented in Fig. 3a. The observer monocularly viewed a tripartite field through an artificial pupil of 2.5 mm in Maxwellian view. Figure 3b shows the general nature of this field. The test bars and background field were formed by a positive-negative pair of high contrast photographic transparencies (TI and Tz in Fig. 3a). Accurate alignment of the two patterns was accomplished by mounting one transpatency on a microscope stage. The entire circular field subtended a visual angle of 6” and was surrounded by a dark field. The matching field was reflected from a front-surface mirror (M) inserted partially across the combined test-background beam. The mirror was movable along a diagonal across the combined beam and was set so that the matching field extended one-third across the circular field. Test bars were used rather than a test circle and background annulus simply because observers felt the matching procedure was easier with the bars. The source for the matching field was the front opal glass surface of a BURNHAM(1952) calorimeter with a filter slide (FS) made up of Coming glasses 2404 (red), 4010 (green) and 5330 (blue). The observer varied

LEON FESTXNGER, MARK R. ALLYN AND CHARLESW. WHITE

596

1

PAL1II Fl

a

L2

Fz

MATCHING FlEl.0

TI

-6.

J

Aw

-M

OG

4” ”

L3

--

,

FS

HG

P

Ro. 3. Schematic diqpam of the apparatus. (a) The optical system GM = glow moduktor tube; L - achnx~tic lens; F = filters; T = photoenphic tnumpamsy; ES = beam splitter cube; W = neutral de&~ wedge; M=tront~mirror;P~~ifjdP1p~~;S=32M)’Kt~t~~~;HG= beat &orb&g gkss; FS = filter slide; OG = opal glass: IB = optical integrating bar. (b) Tripartite visual 6eld.

X FIO. 4. Range of chromaticities possible with calorimeter.

The Perception

of Color with Achromatic

Stimulation

591

the chromaticity of the calorimeter field by manipulating the microscope stage on which the filter slide was mounted, thus changing the relative contribution of each filter to the calorimeter mixture. The range of chromaticities attainable by various combinations of the filters is indicated in Fig. 4 by the triangle drawn on the 1931 C.I.E. x,y chromaticity diagram. The observer controlled the luminance of the matching field with a balanced pair of Inconel neutral density wedges (w). Colorimeter matches were converted to chromaticity coordinates and luminance values in trolands from calibration curves of the filters and the light source used in the calorimeter (S). The light sources for the test bars and background field were Sylvania RI 131C glow modulator tubes. One tube (GMl) illuminated the test bars; another tube (GMZ) illuminated the background. The glow modulator tubes were driven in a square wave (rise and fall times less than 10 rsec) at 1000 Hz at approximately 25 mA peak current. Although the relative energy spectrum for these tubes has numerous bands in both the red and blue ends of the spectrum, the fields illuminated by the tubes had a generally white appearance. Different glow modulator tubes have perceptible differences in chromaticity and luminance. We attempted to visually match the test and background fields using appropriate Kodak color compensating filters and Wratten neutral density filters (Fl and F2). For example, the balancing filter combination used with two particular tubes included ND 0.80 and ND 060 in the test beam with ND 1.00, ND 040, and CCO25C in the background beam. These changed somewhat when tubes had to be replaced. The time duration and intensity of each glow modulator tube flash was controlled by driving circuits. Timing circuitry was modified from a Digital Equipment Corporation Logic Lab to provide a series of consecutive periods which were variable in duration. The retinal illuminance produced by each glow modulator tube during each period was varied by changing the duration of the on period of the 1000 Hz duty cycle from 0 to 95 per cent of the duty cycle. Chromatic changes in the glow modulator tube were perceptible if the on period of the duty cycle was less than 20 per cent. To avoid such real color changes we never used on periods of 20 per cent or less except for completely off. The output of the timing circuits was continuously monitored during the experimental sessions with a dual-beam oscilloscope. The general task of the observer was to adjust the luminance and chromaticity of the matching field until it matched the appearance of the test bars in hue, saturation, and brightness. Most experimental sessions lasted about 45 min, during which 20-30 matches were made. The test signals were always presented in different random orders for each session. The observer was unaware of which specific test signal was presented. The three observers were each tested on the American Optical Company H-R-R pseudoisochromatic plates and on the Farnsworth-Munsell lOO-Hue Test. All three were diagnosed to have normal color vision on the H-R-R plates and were rated either superior or high average in hue discrimination on the lOO-Hue Test. Flicker colors from a stationary light source Our first attempt was to see whether or not, by imitating the Renham disc pattern of temporal intensity changes (thin black lines interpreted as intermediate intensity values), we could produce the “flicker colors” using a stationary source of light. Thus, for example, if the light were off for 75 msec, on at an intermediate level for 25 msec and on fully for 50 msec, this sequence should produce the sensation of ted. Other appropriate sequences should, similarly, produce other color sensations. Using the apparatus we have described we tried, systematically, to produce such flicker colors in the test bars. The results are quite clear and unequivocal. If the background field is dark or if it is illuminated at some steady level, little or no flicker color is seen with any of the signals we tried. We tried again to produce the flicker colors in the test bars of our apparatus. This time the background field was made to flicker on and off in a square wave, the off phase of the background in synchrony with the off phase of the test bars. When this is done, the flicker colors are seen and, in general, the various signals in the test bars produce the appropriate color sensations. This finding completely agrees with CAMPENHAUSEN’S (1968a, 1968b) results. We can, then, come to the conclusion that it is not necessary to have a moving pattern in order to produce the sensation of color from achromatic light. It can be done with a stationary source. CAMPENHAUSEN’S (1969) suggestion concerning the importance of lateral inhibitory processes in the production of the flicker colors seems plausible and we shall return to this question later on when we present data specifically collected to explore this matter. First, however, let us examine the exact effects produced by the various different temporal signals. Exact measurement of the Jlicker colors For these measurements each test bar subtended a visual angle of 17 min of arc; the spaces between the bars, a visual angle of 65 min. The test signals in the test bars consisted of four consecutive periods. During the first period, which was 75 msec in duration, the glow modulator tube illuminating the test bars was turned off. During the next three periods, each 25 msec in duration, the duty cycle was varied to produce the changes in luminance

598

LEON FESTINGER.MARK R. ALLYN AND CHARLES W. WHITE

J-l A-l RED

GREEN

BLUE J-4

CYAN

J-l

MAGENTA -l--b

-2 YELLW

1

B l

:

zrl

100 YSEC.

FIO. 5. Physical signals used to produce Bicker colors.

u = 4x/d; u = 6y/d; where d = 2x + 12~ + 3. ~FFig.6achpointistbeavcrepofsixmatc~.The~usaofthe~~disgrmdtown inthelamr~portbnofthe~~~t~ssctlonthPtbexpPadodintbctodhridurlobgvcr charts. To cvahmte stnt&kdly tb 4tBmwma aatongthcmatchcsofthctcmpomlrip*katwowayamly& ofvariaDa(~Xtsmporal~b)wM~ssparatelyfortheuaadfortboovPlusl.Tbc~ ciWtofobservcmandthc~XMcractionwen~~tkwitbinalbcmxvar&ncc. Themaincff~ofripnabwPstestedaOainstthesignalsXobsemrsintersction.S~tbemrfodkaof the temporal signals was wt for both the u values (F (6.12) = S-02) and for the o values (P (6.12) = 30.45). all the pomibic pats of the sewn temporal signal means were compared by Ctcst8 using the within cells estimate of valiana. The results of t&se individual I-test comparisons arc also shown in Fig. 6. Pain that arc not si@@cautly ~~t(pz~lO)oaboththeupodthcodhrauioasareconrrstod atkutoncdimanio n with p between O-10 and O-05 am

by doubJe Iha.

l’airs~f;~

linked by single lines. Unamncct significantly different with p < O-05for at least one dimension.

The Perception

of Color with Achromatic

0.40 -

Stimulation

OBSERVER

0.35 -

599

MA

G . R

v 0.30 -

N

<

‘NM

c

.B

I

.B

0.25 t-

,

0.20

0.25

, 0.30

, 0.35

, 0.40

0

0.20

U FIG. 6. u, v Coordinates

0.40

0 60

U of color matches to the various temporal signals.

For each observer the green, yellow, and blue signals produced relatively strong ticker colors. All three signals were consistently displaced from the neutral signal in the expected directions. The “red” signal was always matched reddish with respect to the neutral matches, although only for LF did it differ from neutral at the 0.05 level of significance. The magenta matches exhibited more interobserver variability than the other signals. Always reddish and diierent from the neutral matches, the magenta signal was matched like the red signal by LF and CW, and like the yellow signal by MA. The cyan signal matches did not differ from the neutral matches in chromaticity, and only for LF is the difference from neutral in the proper direction. The interaction of signals with observers was reflected primarily in the gnzen and magenta matches of MA, which were more saturated, that is, displaced farther from the neutral matches, than the corresponding matches of LF and CW.

THE

EFFECT

OF VARYING

TEST

BAR

WIDTH

Various writers have pointed out that the colors elicited by the Benham disc are strongly affected by the width of the stripes (see COHEN and GORDON,1949). For example, ROELOFS

and ZEEMAN(1958, pp. 347-348) remark that a stripe with a width of 1 mm may evoke “a fine red color” while one with a 20 mm width “has fine red boundary lines” and “shows a transition from reddish brown to a somewhat violet grey toward the middle”. Our interpretation of this is that the thin lines on the Benham top serve to produce intermediate intensities of stimulation on the retina, something that thick bars would not do. Evidence from WESTHEIMER and CAMPBELL(1962) and KRAUSKOPF(1962) shows, for example, that the retinal illumination of a point 2 min of visual angle from a rectangular illuminance increase is about 20 per cent of the maximum. CAMPENHAUSEN (1968b)

600

LEON Frsni%ER, MARK R. ALLYN AND CHARLES W. WHITE

reports that if the lines on the Benham top subtend such a visual angle, strong chromatic sensation is evoked. Much wider lines would produce areas of absence of stimulation and thus would not produce effective flicker colors. If our interpretation is correct then the effect of test bar width on perceived flicker color might not exist with our procedure for producing the colors. Since the intensity of the test bar is directly varied temporally, the flicker color should be uniform and the same regardless of the width of the test bar: In order to examine this question data were collected on two additional test bar widths in a manner identical to that used previously with a test bar width of 17 min of arc. The two new widths were 34 min and 66 min of arc. The separations between test bars were, respectively, 132 and 129 min of arc. Each observer made six matches with each signal and each test bar width. Three way analyses of variance (line width X observers X signals) were performed. A highly significant main effect due to signals was again obtained (F = 4858 for u and 26.79 for v, df = 6,12). However, there was no significant main effect due to line width (I: = 0.61 for u and 2.92 for a, df = 2,4). There is, however, some evidence for a possible interaction between line width and signal. While this interaction is not significant for the u coordinate (F = 1.28, df = 12,24), it is significant for the v coordinate (F = 240, df = 12,24). However, post hoc comparisons between each line width within each given signal failed to reveal any significant differences. Inspection of the data shows a tendency for increase in line width to lead to a slightly higher v value for the “blue” signal and a slightly lower v value for the “green” signal. This seems the likely source of the significant interaction. Thus, there seems to be little evidence for any consistent, systematic change of perceived flicker color with changing line width within the range we studied. It is also important to note that at no time did any observer report that the color was confined to a region adjacent to the contour of the test bar. Regardless of the width of the test bars the spatial distribution of the color was seen as uniform. FLICKER

COLORS WITH SINGLE FLASHES

There is another question that must be dealt with. The fact that the flicker colors can be produced with a stationary light source certainly indicates that large movement of the stimulus across the retina is not necessary. Someone might propose, however, that some movement across the retina was necessary. This might be produced by eye movements that occur. We have not demonstrated that such eye movements are irrelevant. Ideally, one would like to test this by producing a stabilized image on the retina to see if the flicker colors were still perceived. CAMPENHAUSEN (19688) mentions that Trinoker observed good colors on a rotating Benham top after paralyzing the eye muselea with curare. We felt we could explore this question in a slightly different way. Instead of the repetitive sequence of signal period and dark period one could present single flashes of the signal sequence to the observer. The entire visual field would be dark and then one 75-msec signal sequence would be presented and the observer would be asked to match the perceived color. After the match was made (from memory of necessity) another single signal sequence would be presented so that the observer could further adjust the match. In this manner matches were obtained with which the observer felt quite comfortable. It is quite unlikely that eye movements during a 75msec period would be very sign& cant. Certainly slow drifting of the eye would have little significance. Saccadic flicks would

601

The Perception of Color with Achromatic Stimulation

SlWGLE - (cy&

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MATCHES

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U FIG. 7. A comparison of color matches to single cycle and repeated cycle temporal signals.

presumably only rarely coincide with the particular 75 msec period in which the signal sequence occurred. Small eye tremor, however, does of course occur. At any rate, we can maintain that with the use of single signal sequences we have materially reduced the occurrence of eye movements. Figure 7 presents the data for these single flash matches averaged over the three observers, each of whom made three matches to each signal. Also shown in Fig. 7, for comparison purposes, are the matches to the repeated signal sequences averaged for the three observers. It is apparent that the flicker color is still perceived in a single signal sequence. Differences that occur are not systematic and the perceived color is relatively unaltered. It seems unlikely that eye movements have much to do with the production of the flicker colors. THE APPARENT

BRIGHTNESS

OF THE FLICKER

COLORS

Changes in the appearance of the test bar as a function of the specific temporal modulation were not restricted to hue and saturation. Apparent brightness also changed. Such brightness differences have been generally ignored by the color-naming procedures of most flicker color experiments. There exists scant information about the magnitude of these differences. There is one report (VERREST and SEKI, 1964) of a series of Munsell matches made by one observer for a Benham top with four sets of bands, each of which was formed by a black 45” arc. As the rate of rotation was varied from l-67 c/s to 13.67 c/s in 1.67 c/s steps, the luminous reflectance of the matching Munsell chips increased for each set of bands. The rates of increase were not equal, however, and above 3.33 c/s the blue bands were consistently matched with lower retlectance chips. WSKJN II/~---H

LEONFESI-INOER, MARK R. Aum

602

AND CHARLFSW. WHITE

Since, in our experiments, the observers could vary the hue, saturation and the brightness of the calorimeter to match the test bar, we had quantitative data on the perceived brightness of the test bar for each signal. The data reveal large differences in apparent brightness for signals that had the identical time averaged intensities. The results, in terms of the retinal illuminance of the calorimeter light needed to match the test bar, are presented in Table 1 for the various signals and various line widths that were used. TABLE 1. MATCHINGRETINAL ILLIJMINANC~S FORTHEVARIOUS SIGNALS(Log trolands) Flicker color signals

Width of test bar Neutral

Red

Blue

Yellow

Cyan

Magenta

Green

17 min 34 mio 66min

2-94 2.88 2.91

2.93 2.86 2.95

2.25 2-25 2.37

2.68 2.66 2.72

2.11 2.12 2.16

242 2.40 2.47

2.62 2.56 2.62

Means:

2.91

2.91

2.29

2.69

2.13

2.43

2.60

2.78

2.76

2.76

2.60

2.60

2.60

2.57

Time-iImaOsd retina1 iuuminance

The last row of the table shows the time-averaged retinal ilhuninance of each of the temporal signals. It is only of interest, of course, to compare the matching retinal illuminance within those signals that had, essentially, the same time-averaged ilhnninance. The neutral, red and blue signals are physically equal in terms of their time-averaged retinal illuminance but, as may be seen, the blue signal is matched about O-6 log units lower than the other two signals. The two diE&rences are each sign&ant (p < DOl). Similarly, the other four signals comprise a group that are physically similar in timeaveraged retinal illuminance. Again one of these, the cyan signal is seen as significantly darker (p < @Ol) than each of the other three. These other three do not diEer sign&antly among themselves. It should be noted that the absolute levels of the match@ retinal illuminance and the time-averaged retinal illuminance are not expected to be equal since the test and matching 6&s had quite different surrounds. This result was surprising to us. The apparent brightness of the tlicker colors seems to depend on when, during the signal period, the intermediate intensity occurs. Both the blue and the cyan signals have the intermediate intensity periods occurring toward the end of the signal period and they appear dark. The magenta signal, that also has a late intermediate intensity period also appears less bright than the yellow or green signal although not sign%caxrtIy so. In or&r to understand the phenomenon better it was decided to determine how these differences in apparent brightness were affected by changes in the duration of the temporal signal. To collect these data we used the 17 min of arc test bars and arranged the timing circuitry so that the temporal signals could be varied along the time axis without changing the relative durations within each signal. The dark period was always kept equal in duration to the total signal period, Five durations of the signal plus dark periods ore used: 18,36, 72, 144 and 288 maec. The corresponding frequencies are, approximately, 56.28, 14,7 and 3.5 c/s. The 144 maec duration corresponds closely to the time period used in the preceding experiments. The procedure is equivalent to varying the rotation speed of a Benham disc.

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603

Stimulation

Each of the seven flicker color signals was presented once at each of the five durations within any one experimental session. The order of presentation was random. The same three observers each made three matches to each signal at each duration. Before presenting the data on brightness matching it is worth noting that the chromaticity matches confirm the often stated finding that the flicker colors are produced best between 5 and 10 c/s. The most saturated colors were produced by the 7 c/s signal.

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FIG. 8. Matching retinal illuminance as a function of signal frequency.

The brightness matches for each signal at each frequency are shown in Fig. 8. The plotted values are the means for all three observers. It may be noted in the figure that, at 28 and 56 c/s, those signals that have equal time-averaged retinal luminance are matched very similarly. The differences that we have been discussing emerge only at lower frequencies. At 7 c/s, the differences are about the same as the brightness differences obtained previously. The blue, cyan and, to a lesser extent, the magenta signals appear darker. At 14 c/s the differences are less marked. The blue signal is no longer darker than the red or neutral. The cyan and magenta signals, however, are still somewhat darker than the yellow and green. Large differences in matching brightness also exist at 3.5 c/s but these are harder to interpret. At 7 c/s and above there was little perceived flicker in the test bar and the observers had no difficulty matching the brightness. At 3-5 c/s, however, the flicker was pronounced, alternate light and dark periods being quite distinct. It was very difficult to match the “average brightness” and the matches are quite unreliable. The ordering of the perceived brightness does, however, tend to parallel the ordering at 7 c/s.

604

LEON FESTINGER,MARK R. ALLYN AND CHARLESW. WHITE

The possible role of lateral inhibition

It seems possible to account for the observed differences in apparent brightness of the ticker colors in terms of temporal variation in the magnitude of lateral inhibitory effects. It has been shown, at least in the Limulus (HARTLINE, 1969; LANGE, 1965), that the temporal course of inhibition is rather similar to the temporal course of excitation. After some delay there is a transient peak that is an overshoot of the magnitude of inhibitory effect, following which the inhibition settles down to some steady level. It will be recalled that, in all the data we have reported, the signal in the test bar is accompanied by a synchronously flashing background. The onset of stimulation in the background may be expected to initiate an inhibitory effect on the excitation produced by the signal in the test bar. Furthermore, we can expect the magnitude of inhibition to follow its known temporal course. The perceived brightness of the test bar would then be affected by the exact time at which the transient inhibitory overshoot occurred. If this inhibitory overshoot coincided with large excitation, the test bar would appear darker than if the inhibitory overshoot coincided with low excitation from the test bar. If we assume that the inhibitory impulses are delayed a few msec in relation to the excitatory impulses, we would then expect the inhibitory overshoot to occur relatively early in the period of excitation set off by the signal in the test bar. Hence the blue and cyan signals, in which the intensity of the light is high at the beginning of the signal, would suffer more from the impact of this early occurring inhibitory overshoot than would the other signals. There are two ways, both relatively indirect, in which we could check this explanation. If the brightness differences are, indeed, due to the transient inhibitory overshoot, then such brightness differences should not be present if the background is always on at a steady level rather than flashing on and off. With the background always on at a steady level the inhibitory impact will have settled down to its steady state. This was checked systematically for two observers who matched the seven flicker color signals with the background held steady at the same intensity as the on period of the background in the previous experiments. For these matches the signal period was 75 msec and the off period in the test bar was also 75 msec. The results were quite clear. With a steady background the brightness of signals with the same time-averaged retinal luminance were all matched very closely together. The previously observed differences no longer existed. Moreover, we may remind the reader that under these circumstances the perception of color also disappears. Another way of indirectly testing the explanation of the apparent brightness differences is to introduce a spatial separation between the test bar and the flickering background. With a spatial separation the total impact of lateral inhibition from the background should be very much reduced, or perhaps, even eliminated. We would expect that the introduction of this spatial separation would, hence, reduce or eliminate the apparent brightness differences in the test bar for the different temporal signals. To examine this, we used test bars of 17 min of arc width and a separation on each side of 83 min of arc between the test bar and the background. The separation stripes were dark all the time. Once again the various temporal signals were flashed in the test bar and the background flashed full on-full off in synchrony with the signal. The results were again clear, although somewhat surprising in view of the small separation between the test bar and the background. The apparent brightness differences were entirely absent. It is important to note that, in addition to eliminating the apparent brightness differences, this procedure also eliminated the flicker color. No color was perceived for any of the signals. They were all matched very close to neutral.

The Perception of Color with Achromatic Stimulation

605

The effect of asynchrony between test bar and background

It is highly suggestive that the two circumstances we explored that destroy the apparent brightness differences among the different signals also destroy the appearance of the flicker colors. It certainly seems, as CAMPENHAUSEN (1969) has maintained, that lateral inhibitory

effects are needed for the production of the flicker colors. But in what way, exactly, do these lateral inhibitory effects operate? We decided to investigate whether the exact timing of the lateral inhibitory overshoot is an important factor. All of the data we have presented thus far were collected with the background flashing on and off in synchrony with the signal in the test bar. Introducing asynchronies between the two should, clearly, affect the exact timing of the inhibitory overshoot and should affect the perceived colors. These data were collected for only observer LF, using the 17 min of arc test bar. The timing apparatus was modified to permit changes in the onset asynchrony between test bar and background. Each of the seven temporal signals was matched at each of nine onset asynchronies for a total of 63 signal-asynchrony combinations. These 63 combinations were presented, one each in random order, within one experimental session. Over four such sessions, four matches were obtained with each combination. The duration of the signal and the background on period was 75 msec, as was the duration of the off period. The asynchronies used, in msec, were: -35, -25, -15, -5,O, f5, +15, +25, +35. A minus sign indicates that the background flash onset occurred before, and a plus sign indicates that it occurred afrer, the onset of the test bar signal. At large asynchronies the chromaticity and brightness matches became quite unreliable. In a large number of instances the observer could not make a match. At positive asynchronies of 15, 25 and 35 the predominant perception was of short light flashes in a very black field. At negative asynchronies of 25 and 35 the perception of brief dark periods in a very bright test bar were obtained. Something can be said about the asynchronies of + 15 and -25, however, since the observer did succeed in making several matches at these asynchronies. At +15, when matches were made, they were very dark and the chromaticity moved in the direction of blue. At -25 the matches were very bright and the chromaticity moved in the direction of yellow. For the asynchronies from - 15 to + 5 msec all the signals were matched by the observer without great difficulty and, hence, data can be presented. For the red, yellow, green and neutral signals there is little or no change in brightness at these different asynchronies. For the cyan, magenta and blue signals, however, the brightness increases steadily as the asynchrony changes from + 5 to - 15. These data are shown in Fig. 9 in which the matching luminance, in log Trolands, is plotted against the asynchrony. It does seem that, as the transient inhibitory overshoot is advanced in time so that its effect begins to take place before the excitatory rise, those signals that showed unusual apparent darkening appear brighter and brighter. The data again indirectly support the idea of the importance of the transient inhibitory overshoot. The data on the chromaticity matches are shown in Figs. IOa and lob. They are drawn separately only for purposes of clarity. It can be seen in Fig. 10a that for the neutral, cyan, red, yellow and green signals variation of asynchrony within this range makes only a little difference in the color that is seen. The negative asynchrony does reduce the perceived difference between the yellow and the green signal and the positive asynchrony makes the neutral signal move toward blue. There are no differences for the other signals. Two of the signals show marked changes with different asynchronies and these are shown separately in Fig. lob. A negative asynchrony of 15 completely destroys the perceived blue

LEON FESTWGER, MARK

R. ALLYN AND CHARLESW. WHITE

26M B

color and the blue signal is matched very close to neutral. The magenta signal is matched with magenta only for the zero asynchrony. For the -5 and - 15 asynchronies it is matched like the red signal. It is interesting that the two signals that are strongly afFected by the diffmnt asynchronies are the two signals that have an intensity decrement occurring 50 mSec after the signal onset. The negative asynchrony means that the background flash

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starts before the signal flash and, accordingly, stops before the signal flash has ceased. It seems as though the release from inhibition by the early cessation of the background flash reduces the effect of the intensity decrement in the late signal period. Eliminating the late decrement in the magenta signal makes it appear like the red signal and cancelling the late decrement in the blue signal makes it appear like the neutral signal. Flicker colors with a constant background

Our findings thus far have completely confirmed the fact that, using signal sequences analogous to those on the Benham disc, a flickering background is necessary for the production of the flicker colors. The presumption is strong that lateral inhibitory interactions are crucially important. Furthermore, it seems that the exact timing of the transient inhibitory overshoot from the background flash with respect to the temporal signal is relevant. If this timing is changed by varying the synchrony between the signal flash and the background flash, the flicker color can be seriously affected. What are the implications of these facts for a temporal modulation theory of color coding? It is possible, of course, that a temporal modulation theory is false and that the flicker colors are produced entirely through some complicated inhibitory-excitatory interactions. On the other hand, one might still maintain that a temporal modulation theory has validity but that the signal sequences of the Benham disc do not closely resemble the true neural temporal color code. A flickering background might be necessary with such signals so that the inhibitory transient overshoots can function to alter the neural firing rates at specific times so as to bring the total temporal modulation pattern closer to that which is needed. To maintain this view, however, implies that a flickering background is not absolutely necessary for the production of the flicker colors but is only necessary with Benham disc type signals. One would have to demonstrate that one could produce adequate, controllable, flicker colors with the background constant. There is some encouragement for this point of view. FRY (1933a, 1933b) reports chromatic changes depending upon the duration and spacing of light pulses using rotating sectored discs through which light passes. Under these conditions there is no background flicker. The data are, however, scanty. The issue is clearly of central theoretical importance and, consequently, we proceeded to see whether or not, by conjecturing the net result of the modification of the signal by the inhibitory transient, we could reach a better approximation to the neural code. If we could, we then might be able to produce the flicker colors with a constant background. The first consideration we had to guess about concerned signal frequency. When we used Benham disc type signals we employed a frequency of slightly less than 7 c/s, each cycle representing 75 msec of signal and 75 msec dark. It seems reasonable to guess, however, that the dark period is important only in helping to produce the transient inhibitory overshoot when the background light comes on. If we dispense with the necessity of the inhibitory overshoot, we can dispense with the dark period. We thus decided to simply use signal durations of 75 msec or approximately 13 c/s to start with. The next problem was to guess what a better approximation might be to the neural color codes. We started considering the Benham type blue signal since it produced rather good, reliable, color experience with a synchronously flashing background. It will be recalled that this signal was full on for the first 50 msec and then at an intermediate level of luminance for the last 25 msec of the signal period. What would be the effect on this signal of the transient lateral inhibitory overshoot. It seems plausible that, as the inhibitory overshoot develops

608

LEONF&STINGER. MARK R. ALLYN AND CHARLESW. WOE

in time, the effective neural activity generated by our signal decreases from a maximum at the beginning of the signal period until the inhibition reaches its maximum. At that point the effective neural activity would start to increase. But somewhere in the vicinity of this last point the signal itself takes a sharp drop. Therefore, it might be reasonable to try, as a better approximation of the blue signal, a steadily decreasing signal luminance. With the apparatus we were using we could not produce smooth changes in the luminance of the signal but, using eight one-shots to control the timing of successive luminance levels, we tried to approximate smooth functions. We were also technically restricted in another way. The glow modulator tubes showed chromatic changes if they were operated with an on time of less than 20 per cent of the duty cycle. Consequently, as in the previous work reported here, we restricted ourselves to levels of entirely off and the range from 20 per cent to 95 per cent on time in the duty cycle. If the background was off entirely, the signal appearing in a totally dark field, all we saw were very bright flashes. If, however, the background was kept on at a cons&t level so as to su5ciently reduce the apparent brightness of the signal, we did obtain flicker color. A good blue was obtained if the luminance of the signal steadily decreased from maximum on to the minimum on value that we used over a 40 msec period followed by 35 msec off.

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The Perception

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of color matches to temporal signals with constant ‘background.

Thus encouraged, we attempted the same kind of reasoning for the other Benham-type signals. Since we had never produced a very good color with the red Benham-type signal we were less sure of our reasoning but imagined that a steadily increasing luminance of the signal might be a reasonable approximation, Such signals did, indeed, produce a reddish color. We had difficulty obtaining a green signal until it occurred to us that the Benhamtype green signal might imply that the neural code for green operated at twice the frequency as the code for blue. With a steep down ramp going from maximum on to zero in 10 msec followed by 30 msec off, we succeeded in producing a green flicker color. Reasoning that a mixture of such a green temporal code with the blue temporal code might have, as one of its effects, a slower down ramp but with the same frequency as the green signal, we tried a 20 msec down ramp followed by 20 msec off and, indeed, this did produce a cyan flicker color. After this exploration we settled on seven signals to use in collecting systematic data. The three observers used throughout these experiments, and one additional observer, each made six calorimeter matches to each signal. The observers were unaware of which signal they were matching. The background was always on at a retinal illuminance of 750 trolands. The maximum retinal illuminance of the signal was 5000 trolands. Instead of bars as used in the previous experiments, here the signal area was a disc 3.5 deg dia. The background was an annulus whose outer diameter was 6 deg. This change from bars to a disc was made because several persons, talking with us about the previous experiments, had raised questions as to whether a disc-annulus arrangement would work equally well. The seven signals we used are shown in Fig. 11 as smooth curves. The color labels attached to each signal represent our guesses, as explained above, about the flicker color each would produce. The data for the four observers, each point representing the mean of six matches, are shown in Fig. 12, plotted on U, u coordinates. Examination of this figure shows

610

LEON FESTINGER, MARK R. ALLYN AND CHARLESW. WHITE

clearly that we did produce consistent flicker colors using these signals with a constant background. The blue, green and red signals, in particular, produced reasonably good colors for all observers. With the exception of observer CW, the cyan signal also produced a cyan color. For only two observers, LF and JL, did the Magenta signal produce magenta. The so-called yellow signal was not consistently seen as different from neutral. A comparison of Fig. 12 with Fig. 6 shows that, except for yellow the colors we produced with the constant background were at least as good as the colors produced with the Benham-type signals and flickering background. DISCUSSION

AND

SUMMARY

We began the series of experiments reported in this paper in the hope that we would be able to say something, pro or con, about the idea that the flicker colors are produced by artiticially creating, somewhere in the visual system, a temporal pattern of neural firing that mimics a true temporal Morse Code for color. We have argued that the thin lines on the Benham disc probably function to produce a level of illumination on the retina that is intermediate between that produced by black or white. We have shown that, interpreted in this way, the flicker colors can be produced by temporal intensity changes of a stationary stimulus display. We have also shown that repetitive sequences are not necessary; single flashes of the signal sequence produce the color experience. In short, movement of the stimulus across the retina does not appear to be necessary to produce the flicker colors. This finding is quite consistent with ideas concerning the creation of a facsimile of a temporal code. If this kind of theory is correct the only necessary thing to produce flicker color would be a sequence of intensity changes at the retina that produced the proper modulation of firing rates. We have also confirmed the finding of CAMPENHAUSEN(1968b) that, using signal sequences analogous to those on the Benham disc, a flickering background is necessary for the production of the flicker colors. The presumption is strong that lateral inhibitory interactions is a crucial factor. We have presented further evidence that the exact timing of the transient inhibitory overshoot from the background Hash with respect to the temporal signal is important. If this timing is changed by varying the asynchrony between the signal flash and the background flash, the flicker color can be seriously affected. These lateral inhibitory interactions could, conceivably, be complicated enough so as to open up a whole series of possible theoretical explanations of the flicker color phenomenon. On the other hand, the findings concerning the importance of lateral inhibition do not at all rule out a temporal modulation theory. If such a theory is correct, however, it implies that Benham disc type signal sequences do not even closely resemble the true temporal code. In order for them to produce the flicker colors it is necessary to modify them by means of the effect of the transient inhibitory overshoot from the background. We proceeded to try to guess signals that might be closer approximations to the true neural temporal code so that the flicker colors could be produced with a constant background. We were successful in doing this. It seems clear, then, that the flickering background and the lateral inhibitory transient it produces, is not necessary for the production of the flicker colors. The data we have obtained support the temporal modulation theory of color coding. If we are correct, then we can assert that the signals we used in the last experiment with a constant background are a better approximation to the true neural code than are the Benham-type signals. We can not yet specify the exact nature of the temporal code, how-

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ever, since we can only conjecture about the effect on actual neural firing rates of the physical intensity changes of the signal. Do the dark periods in our signals, for example, produce comparable periods of lower firing rates or do they permit neural firing rates to go below spontaneous firing levels? Many such unclarities exist. It would be unreasonable to assume that the photic stimulation we employed accurately drives the neurons in a corresponding manner. We intend to pursue these questions further, hoping to reach even better descriptions of this hypothesized temporal code. REFERENCES BENHAM,C. E. (1894). The artificial spectrum top. Nuture, Land. 51,200. BENHAM,C. E. (1895). The artificial spectrum top. Nature, Lend. 2, 321. BURNHAM,R. W. (1952). A calorimeter for research in color perception. Am. J. Psycho/. 65, 603-608. C~ENHAUSEN, C. v. (1968a). Untersuchung des Fechner-Benhamschen-Farbphlnomens. P@gers Arch. ges. Physiol. 300, 98. CAMPENHAUSEN, C.V. (1968b). aber die Farben der Benhamschen Scheibe. Z. vergl. Physiol. 60, 351-374. CAMPENHAUSEN, C.V. (1968~). ‘Uber den Ursprungsort von musterinduzierten Flickerfarben im visuellen System des Menschen. Z. vergl. Physiol. 61, 355-360. CAMPENHAUSEN, C.V. (1969). The colors of Benbarn’s Top under metameric illuminations. Vision Res. 9, 677-682. COHEN, J. and GORDON, D. A. (1949). The Prevost-Fechner-Benham subjective colors. Psychol. Buli. 46 97-l 36. FRY, G. A. (1933a). Modulation of the optic nerve current as a basis for color vision. Am. J. Psychol. 45, 48W92. FRY, G. A. (1933b). Color phenomena from adjacent retinal areas for different temporal patterns of intermittent white light. Am. J. Psychof. 45, 714-721. FRY, G. A. (1945). A photo receptor mechanism for the modulation theory of color vision. J. opr. Sot. Am. 35, 114-135. GEHRCKE,E. (1948). Neue Versuche fiber Farbensehen. Ann. Physik 2,345-354. GEHRCKE,E. (1958). Uber die Stromkurven von Wechselstromen im Sehorgan, welche sich aus der physiologischen Optik ergeben. Optik 15,94-97. GREEN. D. G. (1969). Sinusoidal flicker characteristics of the color sensitive mechanisms of the eve. Vision Res. 9, 591-w.. GUTH, S. L. (1964). The effect of wavelength on visual perceptual latency. Vision Res. 4, 567-578. HARTLINE.H. K. (1969). Visual receotors and retinal interaction. Science. N. Y. 164. 270-278. KRAUSKO~F,J. (1962). iight distribution in human retinal images. J. opt..Soc. Am. 52, 10461050. LANGE,G. D. (1965). Dynamics of inhibitory interactions in the eye of Limulu.s: Experimental and theoretical studies. Ph.D. Thesis, Rockefeller University. LEGRAND, Y. (1957). Light, Colour und Vision (translated by R. W. G. Hmr, J. W. T. WALSHand F. R. W. HUNT), Chapman & Hall, London. LIANG, T. and PI~RON,H. (1947). Recherches sur la latence de la sensation lumineuse par la methode de l’effet cbronost&oscopique. Ann. Psychol. 43-44, l-53. MARRIO~, F. H. C. (1962). Color vision: Other phenomena. In The Eye (edited by H. DAVSON),Vol. 2, The Visual Process, Academic Press, New York. P&RON,H. (1923). Le m&anisme d’apparition des couleurs subjectives de Fechner Benham. Ann. Psycho/. 23, l-49. P&RON, H. (1932). La sensation chromatique. Ann. Psycho/. 32, l-29. WRON, H. (1945). Aux Sources de la Connaissunce: La Sensation de Vie, Gallimard, Paris. P&RON,H. (1952). The Sensations: Their Functions, Processes and Mechanisms (translated by M. H. PIRENNE and B. C. ABBOTT),Yale University Press, New Haven. ROELOFS,C. 0. and ZEEMAN,W. P. C. (1958). Benham’s Top and the color phenomena resulting from interaction with intermittent light stimuli. Acta Psychol. 13, 334-356. SHEPPARD,J. J. (1968). Human Color Perception .Elsevier,-New York. TROLAND.L. T. (1921). The enicrma of color viAon. Am. J. Phvsiol. Oaf. 2. 23-48. VERRIEST,~ G. and SEMI, R. (196). Les chromaticitts des coulkurs subjectibes suscit&s par la rotation du disque de Fechner-Benham. Rtv. d’Opt. 43, 53-63. WESTHEIMER, G. and CAMPBELL,F. (1962). Light distribution in the image formed by the living human eye. J. opt. Sot. Am. 42, 1040-1045.

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LEON FESTINGER,MAJU R. ALLYN AND CHARLE-SW.WHITE Ahstraet-A series of experiments demonstrated that the flicker colors can be produced by appropriate changes in the intensity of a stationary light source. If the intensity changes resemble those produced by a Benham Top, then lateral inhibitory effects from a flickering background must be present to produce the colors. Patterns of temporal intensity changes were discovered by means of which the flicker colors were produced with a constant background.

R&um&Une s&ie d’exptriences a d&.nonti que dea couleurs vacillantes peuvent Ctre produites par dea changements approprits dans l’intensid d’une source de lumitre stationnaire. Si ks chanpemcnt.5 d’intenaitt -blent B c&s produites par une Toupic de Benham, alors lea dfets inhibitcurs lat&aux d’un ani& plan vacillant doiwnt i?tn pr&ents pour produire ks coukurs. Des typea de ament de I’iotu3siti tempo&e ant Ctt d6couverts. a I’aide desqueJs les coukurs vacillantes ont tti produites avaz un arri& plan constant.