Subjective contours independent of subjective brightness

Transcription

1 Pereption &Psyhophysis 1981,29 (5), 5-54 Sbjetive ontors independent of sbjetive brightness OLIN WARE Sarborogh ollege, University oftoronto, OntarioMI IA4, anada Two theories of sbjetive ontors are distingished aording to the interrelationship of sbjetive ontors and sbjetive brightness effets. In one view, sbjetive ontors are illsory brightness gradients generated from groped loal brightness effets. In another view, sbjetive ontors are the edges of sbjetive forms reated on the basis of gestalt fators; sbjetive brightness is a seondary onseqene of form pereption. Two experiments whih se rating sales to separate jdgments of sbjetive ontor and sbjetive brightness are presented. The first shows that sbjets may jdge ontor to be strong when there is no sbjetive brightness gradient. In the seond, gestalt groping fators are shown to be more important than fators whih shold inflene brightness aording to loal effets theories. Both experiments spport the view that sbjetive brightness ors throgh interations at thelevel of form pereption. Sbjetive ontors are illsory edges pereived in regions that are physially homogeneos. These edges are sometimes pereived as like the edges of illsory objets (Kanizsa, 1955) and sometimes they seem to be brightness edges (Kennedy, 1979). This paper onerns the interation between the impression of "edgeness" and the impression of brightness. On this basis, two kinds of theories an be distingished. I Aording to the first, whose main exponents are Bradley and Dmais (1975) and Kanizsa (1975, 1976), gestalt fators reslt in the reation of a sbjetive form whih sbseqently ontrasts with srronding regions to seem brighter. 2 This kind of theory an be alled a "gestalt-ontingent" theory. Aording to the seond kind of theory, sbjetive ontorsand sbjetive brightness are the reslt of loal brightening and darkening effets. Ths, the ends of lines are held to inde loal ontrast effets (Day & lory, 1978; Frisby & latworthy, 1975; Kennedy & Lee, 1976), and loal assimilation may or between pattern elements (Day & lory, 1978; Kennedy, 1979). The theories of Day and lory, Frisby and latworthy, and Kennedy and Lee differ in aspets of the speifi loal effets postlated. However, they are similar in that they all inlde loal brightening at the ends of lines, and they inlde the idea that groping (either pereptal or physial) of these loal effets reslts in a region that is brighter or darker than srronding regions. The brightness gradient at the bondary of sh a region is the sbjetive ontor. This kind of theory an onveniently be alled a "loal-effets" theory for its emphasis on loal speial brightness effets. The athor's present address: Division of Physis, National Researh onil, Ottawa, Ontario KIA OR6, anada. One lear differene between the two types of theory is that, aording to loal effets theories, sbjetive ontors are brightness gradients and so jdgments of ontor strength and sbjetive brightness shold be interrelated in the following two ways: When there is no brightness, there shold be no sbjetive ontor, and jdged brightness and jdged ontor shold be positively orrelated aross displays with a gradient vale lose to one. onversely, aording to gestalt-ontingent theories sbjetive ontor may be largely independent of sbjetive brightness, sine the gestalt fators whih inde sbjetive form may be present while the ontrast fators may be absent. For example, the sbjetive ontor might be the pereived depth edge at the bondary of a form, or like a rak where the sbjetive form abts adjaent forms (Kennedy, 1975). In either ase, no brightness gradient is neessary for there to be ontor pereption. A stdy by lory and Day (1979) provided spport for loal effets theories by showing that versions of patterns isolminant with, thogh a different olor from, their bakgronds gave reded ratings of ontor and brightness ompared with versions that differed in brightness from their bakgronds. The form onstraints were held onstant throgh this maniplation while ontrast fators were altered. However, Gregory (1977) has arged that isolminane may be a ondition whih also weakens form pereption. If this were the ase, gestalt-ontingent theories wold also predit reded ontor and brightness, sine the gestalt wold be weakened by isolminane. The prpose of this presently reported researh was to frther assess the interrelationship of sbjetive brightness and sbjetive ontor. opyright 1981 Psyhonomi Soiety, In /81/55-5$.75/

2 SUBJETNE ONTOURS 51 EXPERIMENT 1 The idea behind the first experiment was to hold onstant a gestalt fator whih might be expeted to inde sbjetive ontors and to vary a fator expeted to alter the sbjetive brightness relationship between two regions of the display (aording to gestalt-ontingent and loal-effets theories). A set of displays was proded, with eah display divided into three regions, a entral horizontal strip flanked above and below by wider strips. By varying the relative densities of parallel vertial lines within the strips, sbjetive brightness between the lines old be maniplated. That is, aording to loal effets theories, when there are more lines in flanking regions, more line end brightnesses will be onentrated in the entral region and more darkening effets will or in the flanking regions. Hene the entral region will seem brighter than flanking regions. onversely, when there is a greater density in the entral region, the inded brightnesses will be reversed and the flanking regions willseem brighter. At some point in a series of displays, a point shold be reahed at whih brightening and darkening effets are balaned aross the three regions: At this point-aording to loal effets theories-the impression of a sbjetive ontor shold disappear. While relative line densities are varied, by holding onstant the nmber of lines terminating at the bondaries of the entral strip, a gestalt fator whih might be expeted to be important in inflening ontor strength old be held onstant. Illstrations of three displays made aording to these onstraints are given in Figre 1. (al 1 X X T en/ :5.a lj::: - I o Ol.2- l:: ~ l 1: VI ~ 1 19 ~.J:> 'x.8 E.~ 5! n '" '" -x (; " fator inflening relative sbjetive brightness of adjaent regions Figre 1. Tbree of a seqene of stimls patterns wberein tbe nmber of lines terminating along tbe entral bondary is beld onstant wbile line density in entral and nanking regions varies. Sbjetive ontors an be seen separating tbe regions. The preditions of the two types of theory are graphed in Figre 2. Aordingto loal effets theories, the impression of ontor shold be diretly related to the impression of a brightness differene between entral and flanking regions. As the impression of a brighter entral region is reded, ontor strength shold likewise be diminished ntil the entral and flanking regions seem the same and there is no sbjetive ontor. As the sbjetive brightness of the entral region is reded still frther, sbjetive ontor shold inrease with the inreasing brightness differene. This interation is diagramed in Figre 2(a). Aording to gestalt-ontingent theories, the sbjetive brightness shold vary in the same way as the relative line densities hange; when flanking regions ontain greater line density, the entral region shold seem brighter, and vie versa. However, the impression of a sbjetive bondary shold remain strong throghot the seqene of displays, sine the nmber of line terminations remains onstant. This relation is graphed in Figre 2(b). (b) X 1 x X X 1. ' enj::: I e g. Q iii b 1:.g t:> E > B VI is '" -t " fator Inflening relative sbjetive brightness of adjaent regions Figre 1. Two bypotbetial grapbs sbowing predited interrelationsbip of sbje:tive ontor and sbjetive brigbtness aording to two tbeories of sbjetive ontors. (a) Tbe predition of loal effets tbeories in wbib sbjetive ontors are brigbtness gradients. (b) Tbe predition of gestaltontingent tbeories in wbib ontor is independent of brigbtness.

3 52 WARE Method Sbjets. The sbjets were 14 individals between the ages of 18 and 3. Eah had 2/2 vision or better (orreted in some ases), as measred-by a Shnellen eye hart. Abot half the sbjets were stdents; the other half were from a variety of opations. Stimn and Apparats. The stimli onsisted of seven lo-m sqares of illstrationboard, eah dividedinto three regions, a 2-m entral strip flanked by two 4-m strips. Eah region ontained a nmber of vertial rled lines.4 ern wide. The ratios of line densities in inner and flanking regions were: 1:7, 2:6, 3:5, 4:4, 5:3, 6:2, 7:1. At the viewing distane of 114 em, the nmbers given in the above ratios orrespond to lines per degree of visal angle. Two standard, hosehold 6-W tngsten light blbs were arranged so as to ast light on the displays from a distane of 42 em at an angle of 45 deg from either side. The illmination so obtained was fond to be approximately 6 f as measred by a Mabeth illminometer. Proedre. The sbjets were first given a training program, whih onsisted of an illstrated test desribing some well-known sbjetive ontor effets. For example, Kanizsa's (1955) triangle pattern was reproded, and the sbjets were instrted that an illsory bright overlaying triangle was a ommon sbjetive impression obtained from this display. The sbjets were also instrted in the se of two rating sales. A 6-point sale ( throgh 5) was sed for rating sbjetive ontor, with representing no ontor and 5 representing strong ontor. Sbjetive ontor was defined as "the impression that a line or edge exists in the region indiated. This may appear as like a thin wire, a rak, the edge of a shape, or the edge of a bright region. The seond rating sale had 11 points, (-5 throgh +5, with -5 representing a darker region, representing no sbjetive brightness differene, and +5 representing a brighter region. Sbjetive brightness was defined as the impression that the spaes between the inding lines seemed "brighter, lighter, or darker" in the entral strip than did line interspaes in srronding regions. The displays were presented to eah sbjet twie, one in a seqene starting with the widest spaing in the entral region and one in the reverse seqene. Half the sbjets reeived one seqene first, and the other half reeived the other seqene first. 3 Sbjets were asked to rate the sbjetive bondary of the entral strip and the sbjetive brightness of the spaes between the lines in the entral strip. Reslts and Disssion The reslts are graphed in Figre 3. Their overall pattern is mh loser to that shown in Figre 2b than the pattern shown in Figre 2a. That is. the line representing ontor ratings is approximately straight and parallel to the X axis, showing that sbjets pereived strong ontor throghot the seqene of displays. Nevertheless. there is a slight dip in the enter of this line. On investigation, this dip was fond to be statistially signifiant. A Pearson orrelation of.89 (signifiant and the.1 level) was fond to exist between the mean normalized brightness ratings and sbjetive ontor ratings. The relationship between the two was allated to be = 1.17B by the method of least sqares, where represents the mean ontor rating and B represents the mean brightness rating. Notie that the gradient is slight ompared with the gradient lose to one whih might linespaing in flanking regions (Iines/deg.l Iii iii I 5 5 h T T 4 ~_1 T Vl-i 4 T 3 1~ ' '.s ' e.s VI a a E l... ::J : -1 : - 2 lt '.9 t""'i 2 ~ E -4-5 I, I I I I line spaing in inner region (lines/degl Figre 3. Graph showing mean brightness ratings and mean ontor ratings plotted against the line spaing In Inner and flanking regions of the display seqene. Vertial bars represent two standard errors on either side of the mean. be expeted were sbjetive ontors pereived brightness gradients. Notie, also, the large onstant whih indiates that ontor is strong in the absene of brightness. The priniple onlsion that may be drawn is that sbjetive ontors are not neessarily brightness gradients. However, the orrelation between sbjetive brightness and sbjetive ontor does sggest that sbjetive brightness might. to some extent, inflene pereived ontor. This might be de to some small inflene of loal brightness effets. Alternatively. gestalt- ontingent theories of sbjetive ontors an aont for this orrelation merely by allowing that a sbjetive form that differs in brightness from srronding regions will be more distint than one that does not. EXPERIMENT 2 The seond experiment was derived from a stdy by Kennedy (1978). He showed that the sbjetive disk was weakened as the radial inding lines forming a sbjetive disk (as in Figre 4a) beame more tangential to the disk's irmferene (as in Figre 4b). He interpreted this in terms of loal effets orring at the ends of lines; as the inding lines beame more tangential, these small bright regions wold be groped less and less within the region of the disk, the disk brightness wold be reded, and the disk bondary wold be orrespondingly reded. An alternative explanation an be derived from gestalt-ontingent theories. Perhaps altering the angle of the inding lines away from radial positions

4 SUBJETIVE ONTOURS 53 (a) / / --j -: -: / -- Figre 4. A version of Kennedy's (1978) demonstration witb wbib be sbowed tbat perpendiiar inding lines (a) give a stronger sbjetive disk tban do obliqe inding lines (b). (a) Figre 5. Two figres wbib inde sbjetive triangles. Sb jets report a stronger sbjetive effet in (b), in wbib inding lines are obliqe to tbe triangle. redes the gestalt for a disk, and, therefore, also the brightness effets. Reently, Parks (198) sed a maniplation involving a sbjetive triangle to disentangle form pereption.and brightness. With this pattern, he was able to prode a figre in whih the less perpendilar the inding lines, the greater the impression ofstratifiation. in depth between the sbjetive triangle and srronding regions. He onlded that this was evidene against the idea that line-end brightness ases sbjetive ontors. However, stratifiation and ontor strength are not neessarily related (Kennedy, 1975), and so the present experiment was designed to se a similar maniplation, involving an inded sbjetive triangle, and have sbjets rate both brightness and ontor. In Figre 5a, the inding lines are perpendilar to a trianglar region; any line-end brightness shold be maximally effetive within this triangle. In Figre 5b, the inding lines an be seen as omplete line triangles overlaid by an opaqe white triangle, a mh stronger gestalt. The inding lines in this (b) seond pattern impinge at 6 deg, whih shold rede brightness aording to loal effets theories. Ths, loal effets theories predit stronger brightness and ontor effets in Figre 5a and gestalt ontingent theories predit stronger brightness and ontor effets for Figre 5b. Method The sbjets and the training proedre were the same as those for the previos experiment. The stimli onsisted of patterns as reproded in Figres 4 and S. They were drawn on 8Yz x II in. piees of illstration board and had the following dimensions. The entral disks in the patterns reproded in Figre 4 were 4 m in diameter, and the sides of the sbjetive triangles in Figre S measred S.S em. Reslts The reslts are set ot in Table 1. Figre 4a was given a signifiantly higher rating for both ontor and brightness than Figre 4b, onfirming Kennedy's observations. Figre 5b reeived signifiantly stronger ratings for both ontor and brightness than Figre 5a, spporting the view that gestalt fators, and not loal brightness effets, an better aont for the stronger effet in Figre 4a. GENERAL DISUSSION Experiment 1 established that sbjetive ontors are not neessarily brightness gradients. Experiment 2 showed that gestalt fators are more important than the angle of lines impinging in the determination of sbjetive brightness and ontor effets. Both of these findings are inompatible with loal effets theories of sbjetive ontors, and both are in aord with the gestalt-ontingent view that sbjetive ontors are the edges ofsbjetive forms. The problem remains as to the exat mehanism whereby a sbjetive form omes to seem brighter or darker than its srrondings. Kanizsa (1976) sggested that the mehanism is ontrast. Ths, the sbjetive strip formed in Figre 1 will ontrast with the flanking strips to seem brighter when the flanks ontain a greater density of line elements, and ontrast with srronding regions to seem relatively darker when the flanks ontain fewer elements. One problem with this explanation is that thin lines ontribte little to a region's overall density, and it is hard to see why they shold exert mh inflene in the form of ontrast on adjaent regions. Indeed, it was the effiay of thin lines in inding sbjetive ontor and bright- Table 1 Mean Brightness and ontor Ratings for the Patterns Given in Figres 4 and 5 Pattern Figre ontor Brightness Sbjetive Disk With Perpendilar Inding Lines 4a 3.1t 2.t Sbjetive Disk With Obliqe Inding Lines 4b Sbjetive Triangle With Perpendilar Inding Lines Sa Sbjetive Triangle With Obliqe Inding Lines 5b 2.8t 1.9* Note-Signifiane levels indiate signifiantly greater mean ratings for a given figre ompared with its omparison figre (two-tailed test). "p <.5 ip <.1

5 54 WARE ness effets that led Frisby and latworthy (1975) to first introde the idea of speial line-end ontrast mehanisms. Therefore, it 1s worth onsidering an alternative to the ontrast explanation. Day and lory (1978) and Kennedy (1979) have already offered assimilation as a loal effets mehanism. However, there is also the possibility of assimilation based on the gestalt. Simply stated, when the visal field is divided into sbjetively defined regions, these regions may take on a tint of the olor and brightness of inding elements ontained within them. This theory has the advantage of aonting for the effiay of lines in inding sbjetive effets in that thin lines relatively widely spaed are known to be among the most effetive inders of assimilation (Helson, 1963; Helson & Rowles, 1959). A seond reason for giving assimilation a role in sbjetive effets is the disovery that olor assimilation, and not olor ontrast, ors in ertain versions of Kanizsa's triangle pattern when the inding elements are olored (Varin, 1971; Ware, 198). Finally, there is some anedotal evidene whih links sbjetive ontors to assimilation effets. Writing abot Bezold's (1876) assimilation demonstrations, Evans (1948) noted that "these effets are so strong to most observers that a hazy line is seen vertially throgh the figre at the ends of the blak line part of the figre." Apparently, sbjetive ontors are not inompatible with Bezold assimilation! REFERENES BEZOLD, W. VON. The theory of olor. Boston: Prang, BRADLEY, D. R., & DUMAIS, S. T. Ambigos ognitive ontors. Natre, 1975,257, DAY, R. H., & JORY, M. K. Sbjetive ontors, visal aity, and line ontrast. In J.. Armington, J. Kraskopf, & B. R. Wooten (Eds.), Visalpsyhophysis and physiology. New York: Aademi Press, EVANS, R. M. An introdtion to olor. New York: Wiley, FRISBY, J. P., & LATWORTHY, J. L. Illsory ontors: rios ases of simltaneos brightness ontrast? Pereption, 1975, 4, GINSBURG, A. P. Is the illsory triangle physial or imaginary? Natre, 1975,257, GREGORY, R. L. Vision with isolminant olor ontrast: I. A projetion tehniqe and observations. Pereption, , HELSON, H. Stdies of anomalos ontrast and assimilation. Jornal of the Optial Soiety of Ameria, 1963, 53, HELSON, H., & ROWLES, F. H. A qantitative stdy of the reversal of 1assiallightness ontrast. Amerian Jornal ofpsyhology, 1959,72, JORY, M. K., & DAY, R. H. The relationship between brightness ontrast and illsory ontors. Pereption, 1979,8,3-9. KANIZSA, G. Margini qasi-perettivi in ampi on stimolazione omogenea. Rivista di Psiologia, 1955,49,7-3. KANIZSA, G. Some new demonstrations of the role of strtral fators in brightness ontrast. In v. S. Ertel, L. Kemmler, & M. Stadler (Eds.), Gestalt Theorie in der modernen Psyhologie, KANIZSA, G. Sbjetive ontors. Sientifi Amerian, 1976, 234(4), KENNEDY, J. M. Depth at an edge, oplanarity, slant depth, hange in diretion and hange in brightness in the prodtion of sbjetive ontors. Italian Jornal of Psyhology, 1975, 2, KENNEDY, J. M. lllsory ontors and the ends of lines. Pereption, 1978, 7, KENNEDY, J. M. Sbjetive ontors, ontrast and assimilation.. In. F. Nodine & D. F. Fisher (Eds.), Pereption and pitorial representation. New York: Praeger, KENNEDY, J. M., & LEE, H. A figre-density hypothesis and illsory ontor brightness. Pereption, 1976,5, PARKS, T. E. The sbjetive brightness of illsory figres: Is stratifiation a fator? Pereption, 198,9, VARIN, D. Fenomeni di ontrasto e diffsione hromatia nell' organizzazione spaziale del ampi perettivo. Rivista di Psiologia, 1971,65, WARE,. olored illsory triangles de to assimilation. Pereption, 198,9, NOTES I. There is a third kind of theory whih stands otside this dihotomy. This is Ginsbrg's (1975) theory that at some level in the visal system sbjetive forms (and hene their ontor bondaries) are present bease of spatial Forier filtering of the display. In this theory, form and brightness are present in one operation. The present stdies do not address this third alternative. 2. This refers to a display with blak pattern elements on a white grond. In a display with white elements on a blak grond, the effet will be pereived as a darkening. 3. The two experiments reported in this paper were part of a larger stdy in whih sbjets were reqired to rate 6 different patterns for ontor and brightness. The display seqene reponed here was given a blok within other bloks of displays. There was no evidene that prior exposre to other patterns sed in the stdy inflened mean ratings. (Reeived for pbliation Febrary 28, 198; revision aepted Marh 6, 1981.)