The effects of color on brightness

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2 rticles The effects of color on rightness R. Beu Lotto nd Dle Purves Deprtment of Neuroiology, Box 3209, Duke University Medicl Center, Durhm, North Crolin 27710, USA Correspondence should e ddressed to D.P. (purves@neuro.duke.edu) Oservtion of humn sujects shows tht the spectrl returns of equiluminnt colored surrounds govern the pprent rightness of chromtic test trgets. The influence of color on rightness provides further evidence tht perceptions of luminnce re generted ccording to the empiricl frequency of the possile sources of visul stimuli, nd suggests novel wy of understnding color contrst nd constncy. Although it is nturl to imgine tht senstions of rightness re direct representtions of photometric intensity (luminnce), the mount of light returned to the eye from n oject nd the experience of rightness it engenders re relted only indirectly in wy tht is not yet understood 1. The perplexing nture of this linkge is nowhere more evident thn in the dependence of rightness on the surfces surrounding trget oject, phenomenon clled simultneous rightness contrst. Thus, gry trget on reltively drk ckground looks righter thn the sme trget on lighter ckground. As result of this contextul influence, most psychophysicl explntions of rightness ssume tht the visul system computes this senstion using luminnce rtios cross the contrst oundries in scene 1 3. In keeping with this interprettion, incresing the numer of luminnce oundries surrounding visul trget enhnces simultneous rightness contrst even when the overll luminnce of the surround is kept constnt 1,4,5. This consensus notwithstnding, n lterntive possiility is tht senstions of rightness re not determined y luminnce s such, ut y informtion out how the mount of light reching the eye from the ojects in scene (to which we susequently refer s the stimulus ) is most likely to hve een generted. The mount nd qulity of the light returned to the eye re determined y the illumintion of the ojects in scene, the reflectnces nd trnsmittnces of those ojects, nd the trnsmittnce of the spce tht intervenes etween the ojects nd the oserver. As n oserver cnnot compute the reltive contriutions of these fctors to given stimulus y direct nlysis of luminnces (ecuse informtion out these contriutions is not present in the stimulus), nd s successful response to the stimulus depends on proper evlution of these reltive contriutions, we rgue tht the visul system must resolve this dilemm y hving the stimulus elicit n ssocition (the percept) tht ccords with wht the source of the stimulus hs, on verge, turned out to e Here we tested the merits of this empiricl conception of visul processing y mnipulting the colors surrounding n chromtic trget. This pproch llowed us to exmine the effects of chnging the prole source of stimulus without ltering its luminnce. If computtions of luminnce rtios cross contrst oundries re the sis of rightness, then mnipulting equiluminnt colors should hve little or no effect. If, however, perceptions of rightness re ssocitions determined y the reltive frequency of the possile sources, then mnipulting colors should lter rightness ccording to the empiricl (sttisticl) informtion out illumintion, reflectnce nd trnsmittnce implicit in the spectrl return. Results Effects of colors consistent with light nd shdow Sujects were first shown the four scenes in Fig. 1, which consisted of two differently colored surrounds, ech with n identicl gry trget t the center. The right surround in ech scene is ten times more luminnt thn the left surround; s result, the centrl trget on the right looks drker thn the identicl trget on the left. To mesure this perceptul difference (which cnnot e fithfully reproduced in the ccompnying figures ecuse of the limittions of color printing), sujects djusted the luminnce of the centrl gry trget on the right until it mtched the trget on the left. Sujects next viewed the scene in Fig. 1, in which the two surrounds comprise 24 differently colored squres insted of single uniformly colored surfce. Since the colors in ech rry were the sme s those used in Fig. 1, the luminnce profile of the multicolored scene in Fig. 1 ws identicl to the uniformly colored scenes in Fig. 1. As efore, sujects djusted the trget emedded in the more luminnt (right) surround until the two trgets mtched in rightness. In this cse, however, the verge djustment required to mtch the trgets ws 53% greter thn the djustment required in the scenes with uniformly colored surrounds. Thus, incresing numer of equiluminnt colors in the two surrounds incresed rightness contrst. The effect on rightness elicited y incresing the numer of colors ws not simply consequence of the lrger numer of distinct surfces in the stimulus. Thus, when sujects were presented with the sme scenes segmented into rrys of 24 distinct tiles (Figs. 2 nd ), the verge difference in the djustments required to mtch the rightness of the trgets were not significntly chnged (56%, s compred to 53% in Fig. 1). Nor were these results consequence of greter numer of chromtic oundries in the stimulus, ecuse spectrlly identicl scenes cn induce different perceptions of trget rightness (Fig. 3). Thus, when sujects were presented with two multicolored scenes tht were spectrlly identicl ut rotted y 180, the verge djustment required to mtch the luminnce of the chromtic test tiles in the differently oriented scenes ws 46% greter for the presenttion in Fig. 3 compred to tht for Fig. 3. If the different perceptions of trget rightness in Figs. 1 nd 2 were 1010 nture neuroscience volume 2 no 11 novemer 1999

3 rticles Fig. 1. Effect of incresing color informtion consistent with prticulr condition of illumintion on perceived rightness. () Four scenes consisting of equiluminnt gry trgets emedded in uniformly colored surrounds tht differ in luminnce (the surround on the right is in ech cse 10 times more luminnt thn the surround on the left). () A scene in which ech of the four equiluminnt colors in () re roken up in ptches such tht ech color is represented six times in the two surrounds. A greter verge djustment ws required to mke the two trgets look the sme in the scenes with multiple equiluminnt colors in the surrounds thn in the scenes with uniformly colored surrounds (p < 0.001). (As responses of sujects to the vrious uniformly colored rrys in () were not sttisticlly different, dt were comined.) Figures only pproximte scenes shown to sujects ecuse some of the colors used re outside the printer s gmut; furthermore, effects re weker thn on the computer screen ecuse the comined presenttion llows the informtion in one scene to ffect tht in the others. determined y the numer of spectrl oundries, then spectrlly identicl ut differently oriented scenes like those in Fig. 3 should induce the sme perceptions of trget rightness. The results documented in Figs. 1 3 do not, therefore, support the ide tht senstions of reltive rightness rise from computtions of luminnce rtios cross contrst oundries. Nor did the scenes tht generted enhnced differences in trget rightness vry in perspective, surfce curvture or contour junctions, which re other chrcteristics on which such computtions might e sed It is pprent, however, tht the rightness difference of the two chromtic trgets ws lwys enhnced when the informtion provided y the equiluminnt colors in ech rry mde it more likely tht the left side of the scene ws in shdow nd the right side in light. Effects of color inconsistent with light nd shdow If the differentil rightness of the two trgets in these figures is incresed y constelltion of equiluminnt surrounding colors consistent with two differently illuminted rrys, then constelltion of equiluminnt colors inconsistent with this possiility should mke the two trgets pper more similr in rightness. To produce stimuli hving the sme luminnces nd hues ut inconsistent with the possiility tht two sides of these scenes re differently illuminted, we incresed the sturtion of the red nd lue squres in the drker rry in Fig. 2 while mintining their luminnce. The rtionle for this pproch ws tht stimulus perceived to e more sturted is likely to hve risen from well illuminted surfce, wheres n equiluminnt stimulus perceived to e less sturted is more likely to hve risen from less well illuminted surfce (ecuse, if the mount of light coming from two surfces is the sme, the surfce reflecting nrrower rnge of wvelengths, nd thus perceived s more sturted, will typiclly hve een under stronger illumintion). We therefore presented sujects with scene similr to tht in Fig. 2, ut in which the sturtion of the red nd lue tiles in the less luminnt (left) rry ws greter thn the corresponding rry in Fig. 2 (Fig. 4 nd ). Under these conditions, the djustment required to mtch the trget squres in Fig. 4 ws much less thn the djustment needed in Fig. 4. Although it is difficult to explin this result in terms of luminnces or luminnce rtios, from n empiricl perspective, the greter sturtion of red nd lue tiles in the left rry in Fig. 4 mkes it less likely tht this prt of the scene lies in shdow thn the left rry in Fig. 4 (which hs exctly the sme luminnce profile). Reltionship to rightness constncy These oservtions on rightness contrst er eqully on the phenomenon of rightness constncy, in which different luminnces induced similr perceptions (insted of the other wy round). Two trget squres tht differ fourfold in luminnce (Fig. 5) hve een plced on the surfces of multicolored cue (Fig. 5; the trgets re colored here to help distinguish them from the surrounds). Despite the fourfold difference in luminnce, the two trgets pper similrly right. This striking percept suggests tht constncy rises in this instnce ecuse the concordnt informtion provided y the 24 distinct, equiluminnt tiles on surfces of the cue in Fig. 5 enhnces the proility tht the surfces hve similr reflectnces, ut re differently illuminted. In support of this hypothesis (nd consistent with the oservtions in Figs. 1 4), constncy egins to fil when the colored tiles surrounding the trgets re replced with gry surrounds tht hve the sme luminnces s the corresponding sides nture neuroscience volume 2 no 11 novemer

4 rticles of the cue in Fig. 5 (Fig. 5c). This effect is even more striking if the relevnt surfces re depicted so s to leve uncertin their rrngement in spce, which further decreses the proility tht the surfces re differently illuminted (Fig. 5d). In short, contrst nd constncy re not fundmentlly different perceptul phenomen, ut superficilly different mnifesttions of the sme empiricl process. DISCUSSION The perception of visul trgets clerly depends on their context. But how the surroundings of n oject influence its perception hs een much deted. As noted, explntions of simultneous rightness contrst re generlly predicted on the computtion of luminnce rtios cross contrst oundries, the enhncement of this effect y n incresed numer of different surfces eing ttriuted to the greter numer of contrst oundries in the scene 1 4. As we hve shown, however, the sme enhncement cn e elicited y surrounding the test trgets with surfces mde up of multiple equiluminnt colors. Wheres the color in these scenes incresed the numer of distinct surfces y virtue of spectrl differences, the luminnce of the stimulus nd numer of luminnce oundries remined the sme. How, then, cn this nd the other effects of color on rightness tht we descrie e rtionlized? The explntion implied y ll these oservtions is tht empiricl informtion provided y color chnges the reltive proility of the possile sources underlying the stimuli, thus chnging the perception of rightness. In Fig. 2, for exmple, only single spectrl qulity is ville to indicte the provennce of the light; with this limited informtion, the reltive contriutions of reflectivity, illumintion nd trnsmittnce to the light coming from the vrious components of the scene remin highly uncertin. When, however, numer of different spectrl returns re present in the stimulus s in Fig. 2 nd, moreover, re ll consistent with prticulr stimulus source (two differently luminnt regions rising from differences in their lighting), this miguity is reduced. As result, the perceived rightness of the identicl chromtic trgets emedded in the surrounds is chnged in ccordnce with the incresed proility tht the trgets re differently reflective ojects in light or shdow. By the sme token, since illumintion usully comes from ove, when pir of multicolored rrys is depicted such tht the less luminnt rry lies ove the more luminnt one (Fig. 3), the stimulus is consistent with shdowed surfce lying ove similrly colored surfce in direct light. When, on the other hnd, the sme stimulus is rotted 180 (Fig. 3), this likelihood is reduced; the scene in Fig. 3 is therefore more consistent with the upper nd lower rrys eing eqully illuminted. As result, the rightness difference of the two trgets in these rotted ut otherwise identicl scenes is not the sme. In Fig. 4, the difference in trget rightness cn e similrly explined in terms of the empiricl significnce of sturted nd unsturted colors. Incresing the sturtion of some of the squres in the drker rry increses the likelihood tht the surfce is more reflective nd/or more intensely illuminted (see ove). Thus this chnge diminishes the likelihood tht the rry on the left is in shdow, therey decresing the difference in pprent rightness of the two trgets. The empiricl significnce of differently sturted colors lso provides n explntion of the so-clled Helmholtz-Kohlrusch effect, which refers to the fct tht more sturted colors pper righter t equiluminnce thn less sturted colors 15,16. Fig. 2. Effect of multiple colors on the reltive rightness of equiluminnt trgets is not due to the numer of distinct surfces in the scene. () The sme scenes s Fig. 1, with the uniform surrounds now prtioned into 24 distinct surfces. () The sme scene s Fig. 1, similrly prtioned into distinct surfces. Despite this prtioning, the difference in the verge djustment etween the single nd multicolor rrys ws the sme s in Fig. 1. (As there ws no sttisticl difference in the responses of sujects to the different colors in, these dt hve gin een comined.) Thus, the greter difference in the trget rightnesses in Figs. 1 compred to Fig. 1 is not simply the result of n incresed numer of discrete surfces in the surround. For the resons given in Fig. 1, the effects re weker in the figure thn in the scenes presented on the computer screen nture neuroscience volume 2 no 11 novemer 1999

rticles Fig. 3. Effect of spectrlly identicl scenes on rightness perception.

However, when the sme stimulus is rotted, s in (), it ecomes less consistent with this possiility.

5 rticles Fig. 3. Effect of spectrlly identicl scenes on rightness perception. Becuse illumintion is ssumed to come from ove, the spectrl returns in the scene in () re consistent with the lower rry eing in light nd the upper rry eing in shdow. However, when the sme stimulus is rotted, s in (), it ecomes less consistent with this possiility. As consequence, the identicl gry trgets t the center of the lighter nd drker rrys were perceived to differ more in rightness in () thn in (), s indicted y the djustments sujects mde to equlize their ppernce (p < 0.001). Finlly, Fig. 5 mkes the importnt point tht this empiricl explntion of rightness contrst works eqully well to explin rightness constncy, which is simply nother mnifesttion of the sme proilistic process tht genertes color percepts (see lso ref. 5). In neurl terms, perceptions of rightness (or color) on wholly empiricl sis would require first, tht the developing niml e endowed with neurl networks intrinsiclly ised during the course of evolution to elicit pproprite ssocitions in response to spectrl stimuli, nd second, tht the synptic weightings of such networks continue to e modifile y feedck from experience during postntl development (see ref. 17 for review). The instruction for the evolution nd developmentl modifiction of the relevnt visul circuitry presumly stems from the success or filure of visully guided ehvior in response to the perception of visul stimuli (y nturl selection in phylogeny, nd y the feedck of neurl ctivity on the formtion nd mintennce of synptic connections in ontogeny). CONCLUSIONS These results indicte tht the perceived rightness of ny light returned to the eye is mnifesttion of its most likely provennce, rther thn its photometric vlue reltive to the luminnces of other elements in the scene. Evidently, these nd presumly ll other visul percepts re determined y the gmut of informtion including color relevnt to the prole contriutions of illumintion, reflectnce nd trnsmittnce to the stimulus. Becuse different spectrl returns convey, in their own right, empiricl informtion out these reltive contriutions, n importnt corollry is tht the perception of color is itself mnifesttion of the empiricl significnce of spectrl return 18. Perhps color contrst nd constncy effects, such s those demonstrted y Lnd 3,19,20, cn lso e understood empiriclly in terms of the reltive proilities of the possile sources of such stimuli. METHODS Construction nd presenttion of computer grphics. All test grphics were creted with Power Mcintosh G3 computer, using Adoe Photoshop 5.0 softwre nd the stndrd Mcintosh color plette. The stimuli were displyed on clirted 48 cm (digonl) color monitor (Sony Multiscn 300sf; monitor resolution, 1024 ( 768; scn rte, 75 Hz, noninterlced). The computer interfce for ech experiment ws creted with Director 6.0 (Mcromedi, Sn Frncisco, Cliforni). Sujects with norml cuity nd trichromtic vision (the uthors nd 8 nive volunteers) oserved the screen from distnce of 60 cm in n otherwise drkened room fter dpttion to the mient light. The stimuli presented to sujects were specified y the RGB settings of the computer (nd were, therefore, device dependent). Fig. 4. Effect of inconsistent color informtion on the reltive rightness of equiluminnt trgets. The pir of rrys in () is the sme s tht in (; sme pir s in Fig. 2); however, the spectrl return of the red nd lue tiles in the less luminnt rry on the left hs een ltered so s to increse the sturtion of these tiles while mintining their luminnce. This chnge cused sujects to mke smller verge djustment to equlize the ppernce of the trgets in () thn in (); p < 0.001; see text. As in the previous figures, the effects were stronger on the computer screen. nture neuroscience volume 2 no 11 novemer

6 rticles c d Fig. 5. Reltion of these oservtions on rightness contrst to understnding rightness constncy. () The colored trgets on the upper nd lterl surfces of the cue re identicl to the corresponding tiles in the susequent pnels. () The trget tiles in () hve een emedded here in multicolored surfces, the vlues of which were precisely chosen to represent the mount of light tht would e reflected y two identicl surfces under the implied conditions of illumintion. As in previous figures, the tiles in ech surround re equiluminnt, the overll luminnce of the two surrounds differing fourfold. (c) The sme stimulus s in (), ut with the colored trgets now in uniform gry surrounds (luminnce profiles of scenes nd c re identicl). (d) The relevnt fces of the cue in (c) hve een oriented so s to leve their positions in spce uncertin. This demonstrtion shows tht the sme multicolored surrounds previously used to elicit contrst cn, given the pproprite empiricl significnce, e used eqully well to generte constncy. Testing. Sujects were sked to djust the rdince of the trget in the more luminnt (right) surround of the scenes presented until it mtched the perceived rightness of the trget in the less luminnt (left) surround (in these instructions no ttempt ws mde to hve the sujects distinguish etween lightness (surfce ppernce) nd rightness in the sense of source intensity; thus we hve used the term rightness in its generl mening through out this report). The luminnce of the colors in the computer-generted scenes ws mesured photometriclly with n opticl power meter (Model 371R, Grsey Optronics, Orlndo, Florid) under the relevnt test conditions. For Figs. 1 4, the luminnces of the light nd drk surrounds were 55 cd per m 2 nd 5 cd per m 2, respectively, nd for Fig. 5, 65 cd per m 2 nd 12 cd per m 2, respectively. The projected scenes mesured cm on the monitor s screen. Otherwise, these djustments were mde under the sme conditions nd with the sme methods s in our previous studies of rightness Sujects repeted ech experiment three times on three seprte occsions; verge responses for ll trils, plus or minus the stndrd error, re presented; sttements of significnce re sed on Student s t-test. ACKNOWLEDGEMENTS We thnk Dvid Coppol, Dvid Fitzptrick, Don Ktz, Tom Polger, Len White nd Mrk Willims for helpful criticism. This work ws supported y NIH grnt #NS RECEIVED 11 JUNE; ACCEPTED 21 SEPTEMBER Adelson, E. H. in The Cognitive Neurosciences 2nd edn. (MIT Press, Cmridge, Msschusetts, in press). 2. Wllch, H. The perception of neutrl colors. Sci. Am. 208, (1963). 3. Lnd, E. H. Recent dvnces in retinex theory. Vision Res. 26, 7 21 (1986). 4. Ktz, D. The World of Colour (Kegn Pul, Trench, Truner, London, 1935). 5. Brown, R. O. & McLeod, I. A. Color ppernce depends on the vrince of the surround colors. Curr. Biol. 7, (1997). 6. Willims, S. M., McCoy, A. N. & Purves, D. The influence of depicted illumintion on perceived rightness. Proc. Ntl. Acd. Sci. USA 95, (1998). 7. Willims, S. M., McCoy, A. N. & Purves, D. An empiricl explntion of rightness. Proc. Ntl. Acd. Sci. USA 95, (1998). 8. Lotto, R. B., Willims, S. M. & Purves, D. An empiricl sis for Mch nds. Proc. Ntl. Acd. Sci. USA 96, (1999). 9. Lotto, R. B., Willims, S. M. & Purves, D. Mch nds s empiriclly derived ssocitions. Proc. Ntl. Acd. Sci. USA 96, (1999). 10. Purves, D., Shimpi, A. & Lotto R. B. An empiricl explntion of the Cornsweet effect. J. Neurosci. 19, (1999). 11. Gilchrist, A. L. Perceived lightness depends on perceived sptil rrngement. Science 195, (1977). 12. Knill, D. C. & Kersten, D. Apprent surfce curvture ffects lightness perception. Nture 351, (1991). 13. Adelson, E. H. Perceptul orgniztion nd the judgment of rightness. Science 262, (1993). 14. Gilchrist, A. L. et l. A new theory of lightness perception. Psychol. Rev. (in press). 15. Kohlrusch, A. Der flimmerwert von lichtmischungen. Ber. D. Ges. Physiologie Exp. Phrmkologie 3, (1920) 16. Yguchi, H. & Iked, M. Helmholtz-Kohlrusch effect investigted y the rightness dditivity. J. Illumin. Inst. Jpn 64, (1980). 17. Purves, D. Neurl Activity nd the Growth of the Brin (Cmridge Univ. Press, Cmridge, 1994). 18. Purves, D., Polger, T. & Lotto, R. B. Color vision nd the four-color mp prolem. J. Cogn. Neurosci. (in press). 19. Lnd, E. H. Color vision nd the nturl imge. Proc. Ntl. Acd. Sci. USA 45, (1959). 20. Lnd, E. H. Color vision nd the nturl imge. Prt II. Proc. Ntl. Acd. Sci. USA 45, (1959) nture neuroscience volume 2 no 11 novemer 1999

Check your understanding 3

Check your understanding 3 1 Wht is the difference etween pssive trnsport nd ctive trnsport? Pssive trnsport is the movement of prticles not requiring energy. Movement of prticles in ctive trnsport uses energy. 2 A gs tp in the