Color naming of small foveal fields

Color naming of small foveal fields

Vision Res. Vol. 10, pp. 501-Y COLOR 1. Pergamon Press NAMING 1970. Printed in Great Britain. OF SMALL FOVEAL FIELDS CARL R. INGLING,JR...

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Vision Res.

Vol.

10, pp. 501-Y

COLOR

1. Pergamon

Press

NAMING

1970.

Printed

in Great

Britain.

OF SMALL

FOVEAL

FIELDS

CARL R. INGLING,JR.,~ HORST M. 0. SCHEIBNER~ and ROBERT M. BOYNTON Center for VisualScience,Universityof Rochester, Rochester, N.Y., U.S.A. (Received 29 May 1969; in revised form 8 December 1969)

INTRODUCTION WILLMER,in 1944, rediscovered K~)NIG’Sfinding (1894) that the central fovea was dichromatic for carefully-tiated sources subtending small visual angles, on the order of a third of a degree or less. The specific form of this dichromacy resembled tritanopia, or “blue-blindness.” WILLMERand WRIGHT (1945) subsequently quantified the effect for fovea1 fixation, and THOMSONand WRIGHT (1947) extended the measurements to eccentric fovea1 fields, showing that the effect was not an exclusive property of the fovea1 center. The characteristics of the dichromacy found under these small-field conditions are: (a) the fovea is indeed found to be dichromatic, since only two primaries are required to match the spectrum; (b) from the location of the two neutral points, the shapes of the hue-di~rimation curves, and the general appearance of the fields, the dichromacy resembles tritanopia; and (c) careful fixation is required to maintain dichromatic matches: if the eyes are moved following a match, the two fields will than differ in appearance. The data on small-field tritanopia have been explained in various ways. Of interest here are the differing inte~retations advanced by WRIGHT(1964) and BRINDLEY(1960, p. 237). It is Wright’s hypothesis that the tritanopic deficiency is caused by a virtual absence of blue receptors, particularly in the case of fovea1 fixation, where the effect is most easily obtained. Brindley emphasizes that an exact match of the type reported by Wright and his colleagues requires steady fixation. He indicates that color-matching with sequential presentation of the stimuli, rather than steady viewing of conventional bipartite fields, would fail to show the dichromacy. On this view, the tritanopia is apparently to be att~but~ to a type of adaptation in the blue system in which discrimination is much impaired with prolonged fixation. The present experiment was designed to test the adaptation hypothesis. A color-naming technique was used in preference to small field color matching because it has the following advantages: (a> it appears to be a sensitive method, as for example SCHEIBNER and BOYNTON (1968) have shown in assessing the extent of residual color discri~nation in supposed dichromats; (b) it provides a ready control over adaptation effects, since short test flashes to which the subject does not have time to adapt can be given; (c) the flash can be superposed upon background fields of various colors, allowing the general state of adaptation to be easily manipulated; and finally, (d> the effects of retinal eccentricity are more easily ‘The major substanceof this paper was given as a talk at the Renshaw Vision Conference, Ohio State University, 17-18 May, 1967. ‘Present address: The Institute for Research in Vision, The Ohio State University Research Center, 1314 Kinnear Road, Columbus, Ohio 43212, U.S.A. %esent address: W. G. Kerckhoff-Institut der Max-Pianck-Gesellschaft, D-6350 Bad Nauheim, Parkstrasse 1, Germany. 501

502

CARL R.

INGLIXG,~R., HcasrIM.O.SCHEIBNER AXD ROBERTM. 3onron

measured, since in color naming there is no restriction on how small the fields can be made, whereas bipartite fields become meaningless as they approach a size where optical and/or neural integration become significant. When working with very small fields the above advantages of color-naming make the methodological problems of controlling adaptation and fixation easier than for colormatching. However, the interpretation of the results is not quite so straightforward. A color-naming experiment yields a set of frequencies of named hues as a function of wavelength (see Methods beIow). These frequency data can tell us if discrimination is present, even if we attach no significance to the particular names used. So long as two channels exist that have overlapping spectral sensitivity curves, and if these curves have different slopes, then discrimination-defined as a reliable difference in the ratio of frequencies of named colors as a function of wavelength-is possible. Conversely, different distributions of response frequencies across a spectrum of equally-bright stimuli confirm that the eye has more than one channel, just as does a color-matching experiment in which more than one primary is required. Taken by themselves, plots of color-naming frequencies do not in general tell us if more than two channels are operating. In particular, the use of n color names in a non-random fashion does not imply that there are n channels. Nevertheless, while there is no general way to make inferences about the number of channels in a visual system from plots of colornaming frequencies, in special cases such data can be used in rigorous arguments, given a few reasonable assumptions. In this study, the assumptions are these: (a) to the extent that the named frequencies at different wavelengths are simiiar, the lights at these differing wavelengths should appear similar if juxtaposed in a calorimeter field; (b) if the named frequencies are dissimilar, the lights should not match; and (c) if two response categories show a ratio that is constant with wavelength, then the categories are not being used discriminatively and may be collapsed into one category. Given these assumptions, the detailed features of the frequency distributions of color names can be used to determine whether or not certain features of small-field tritanopia are present under the color-naming conditions. For example: the studies by Willmer and Wright and Thomson and Wright demonstrate one, and indicate a second, neutral point in the small-field dichromatic spectrum ; also, the dichromatic chromaticity coordinates predict confusions should occur between certain pairs of wavelengths if these stimuli are equally bright. Both of these features may be confirmed or denied by examining plots of colornaming frequencies. Whatever color names are used at the neutral point, the same color names in the same ratios should occur at both neutral points. The same argument applies to pairs of wavelengths which match, or are confused; the same ratios of color names are again expected. In addition, it is not unreasonable to expect that some useful information can be derived from paying attention to which coIor names are used under particular conditions. The subjects, after all, have normal color vision and two of them have participated in other experiments where it has been shown that their color-naming responses are very similar, and that such names can be reliably attached to their chromatic sensations under favorable viewing conditions. METHODS Stimuli. The fixation target was a thin vertical luminous line, 3 min wide in an otherwise dark field. The

line had a 6-min gap in it; the subject fixated the center of the gap, and at a time when he considered his fixation to be precise, he released a button which presented the stimulus flash in the center. The stimuli,

Color Naming of Small Fovea1 Fields

503

but not the fixation target, were presented in Maxwellian view. The flashed field was circular and subtended 3 min of visual angle. Flash duration was 75 msec. The flashes were monochromatic (B & L 500 mm monochromator) and were chosen at 20-nm intervals throughout the spectrum, from 400 to 700 MI, although for some conditions wavelengths at the spectral extremes were not in&tded. For cent& &cation conditions, the stimuius appeared centered in the break in the line; eccentric conditions were given by moving the Cxation ,line relative ta the stimuhts so that the stimulus appeared to one side of the break in the line. There were two intensity levels and five fixation conditions: @5 and 15 log units above absolute threshold presented at eccentricities of 0, 5, 10, 30 and 60 min. Thresholds were determined for each subject individually by the method of adjustment at the 0 mm condition, which were then presented at the other eccentricities. For the 0-S intensity condition, some upward intensity adjustment of the long-wave stimuli was required for some subjects with increasing eccentricity in order to keep them &early above threshofdP The stimuti were randomly presented within an eccentricity condition. There were 16 presentations of each wavefength for each condition. To obtain su&icient test-flash intensity for the adaptation conditions, a slightly different Maxwellian view apparatus was used in which the monochromatic test and adapting fields were obtained with Schott Veril S-200 continuous running interference filters having IO-15 run halfwidth across the spectrum. The light source for this apparatus was a Xenon arc. An achromatizing lens was used for some conditions (Brmroru, and WYSZECKI,1357) so that the stimulus size remained reasonably constant in appearance for the various wavelengths, as it was not feasible to adjust the Maxwelfian lens for optimum focus for each (random) wavelength. Also a set of conditions was run in which blocking filters were used at short wavetengths to reduce whatever stray light may have been present, a precaution which seemed neces%uy because of the desaturation indicated by the responses given at these wavelengths. Use of the blocking filters had no notiable effect on the data. Scoring. The scoring of the color names was based upon a modification of that of BOYNTON,SCHAFER and NEUN (1964). Either zero, one, or two color names were reported, and the subject had at his disposal two optionai white judgment categories “‘white 2” and “white 1.” Each report received a total score of 3, which was divided among the response categories as described beIow. The permitted color names were “red,” “yellow,” “ green,” and “blue.” The first of two color responses always counted for twice the second. On the occasions in which no colar response was given, the white response then counted for 3. The instructions to the subject concerning how to treat the perceived chromatic content were: if negligible white (little or no desaturation), give no white response; if the amount of white perceived is less than one-half the total, give a “white 1” response; if the amount of white in the stimulus is judged to be greater than the chromatic content, give a “white 2” response (see Table I). TABLE1. EXAMPLHOF HOW MLOR-NAMINGRESPONSES WE% SCORED

Scoring of responses Response(s) “Red” “Red-yellow” “Red-yellow-white “Red-yellow-white “Yellow-red-white “Yellow-white 2” “Yellow-white 1” “White 2”

Red

1” 2” 1”

: 4/3

Yellow 0 2,:

213 0

l/3 413 :

x

0

Green

x 0 0 0 0 0

Blue

White

0 :

0 0 1

a0 0 0 0

: 2 1 3

RES WLTS

Four subjects served in the experiment; three extensively and one part-time. From the extensive data collected, we have selected representative samples for presentation here in graphical form. Figure I shows the data for three subjects for the central foveal condition. Figure 2 shows data for selected peripheral and high-intensity conditions, Figure 3 shows the results for fovea1 color-naming, when the test stimuli are superposed upon a green (540 nm) 4For data on the variation of threshold with eccentricity (see STILES,1949; BRINDLEY,1954).

504

CARL R. INGLING,JR., HORST ,&I. 0. SCHEIBNER A.VD ROBERT M. BOYSTO?;

background. Figure 4 shows results for stimuli upon a blue (440 nm) background. Both backgrounds were approximately 1000 trolands; the test stimuli were 0.5 log unit above threshold on the respective backgrounds.

Wavelength,

nm

FIG. I(a).

HS 0’

400

600

500 Wavelength. nm FOG. I(b).

7oo

Color Naming of Small Fovea1 Fields

500 Wavelength,

nm

FIG. l(c).

FIG. 1. Color-naming responses for three subjects, for central fovea1 presentation, at an intensity O-5 log unit above absolute threshold. Each response is worth 3 points (see text); the maximum possible point value for any function is 48.

40-

L__&LX_Li

I

500

Wavelength,

nm

FIG. 2a(l).

SOS

Wavelength, frc;. 2a(2>.

FIG. 2b.

nm

Color Naming of Small: Fovea1 Fields

Wavelength,

507

nm

FIG. 2c.

FIG.2. (a) Data for subject C.I. 30’ eccentric fixation, 1.5 log units above threshold, and for 60 eccentric fixation, 1.5 log units above threshold. (b) Data for subject H.S., 60’ eccentric fixation, 1.5 log units above threshold. (c) Data for subject R.M.B., 60’ eccentric fixation, 0.5 log unit above threshold.

P 40

!

/

Wavelength, FIG.

J

/’ ‘t,

HS 0’ Green Background

3. Data for subject H.S., for foveally-presented ground.

nm

stimuli presented against a green back-

CARL R. IXGLING, JR., HOIST M. 0. SCHEIBNER AND ROBERTM. BOYXTON

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Color Naming of Small Fovea1 Fields

509

DISCUSSION

Centralfovea not dichromatic. The central fovea, under the conditions described (careful

fixation of a 75 msec 3’ subtense flash) is clearly not dichromatic. Figure 1 shows the presence of a blue response category which generally is used in a discriminative manner (i.e. does not parallel another response category); this eliminates the possibility of obtaining equal frequencies in all response categories, which is required if there are to be confusion pairs. The data of subject H.S. do show that, for a limited range of wavelengths (between 440 and 480) there probably are confusion pairs, but this small range in which such pairs are present does not match the range predicted from the dichromatic chromaticity coordinates. Despite the result that the tritanopic deficiency found by color-matching is not present under these conditions, the central fovea does show some features that accompany smallfield dichromacy. The dichromatic tendency seems to be superposed upon the trichromacy. There are two nearly neutral regions strongly indicated, approximately where they appear in the Willmer and Wright data. (Neutral points cannot be precisely located in the present study because of the 20 nm stimulus intervals.) Yellow and green are clearly separated by a white neutral region, and no yellow-green or green-yellow flash was ever reported (although Fig. 1 shows that some flashes of a given wavelength might be called yellow on some trials and green on others). On occasion, if accommodation was poor and the field was blurred so that it appeared larger than 3 min, flashes in the normally yellow-green region of the spectrum would still not appear yellow-green; instead, the stimulus would break up into separate patches of either yellow or green. The nearly neutral regions occur at the expense of the chromatic responses; to the observer, there is very little chromatic content indeed in the stimuli at 400 or 560 nm. Although many of the features of the present data are paralleled in the data of Willmer and Wright, it appears very likely that the use of a sequential color-matching procedure as suggested by Brindley would yield trichromatic matches, although not normal ones. Eccentricity and intensity. The effects of the variables of fovea1 eccentricity and intensity are that, in general, an increase in either tends to favor a return to normal trichromacy. The nearly neutral regions become less conspicuous, and yellow and violet tend to be restored; see Fig. 2. “Red” responses in the shortwave end of the spectrum are almost totally absent for fovea1 flashes. Their absence is somewhat unexpected, and it is not clear why reducing the field size should have such an effect. The red response is given at short wavelengths for larger field sizes, higher intensities and eccentric positions. Indiuiduaf differences. Of the three subjects for whom almost all conditions were examined, the data for C.I. and H.S. were similar, and differed from the data on subject R.M.B. (see, for example, Fig. 1). For all conditions, R.M.B. gave fewer green responses. RICHARDS (1967) has recently found two types of color normal; it would be of interest to know whether the three observers in the present experiment are the same type. The adaptation hypothesis. To test the adaptation hypothesis under the present conditions, the test flashes were superposed upon adapting fields of various wavelengths. For a green adapting field, all chromatic response categories are still used discriminatively; see Fig. 3. When a blue adapting field is used (Fig. 4), although some flashes appear blue, the blue and green curves appear to carry the same information. KRAIJSKOPF and SREBRO (1965), in a study using point sources (1.3 min subtense, 0.7 msec flash duration, wavelength range 464-688 nm) close to threshold, report that blue and green responses reflect the spectral sensitivity of a single mechanism. However, in contradiction to the present study, their result VU,ON 10/6--e

510

CARL R. IXGL~XO,JR., Hoas~ M. 0. SCHEIBNERAXD ROBERTIV. Bovxro~

was obtained with no adapting field. Thus, in a study in which the stimuli were similar to those used in our fovea1 conditions, their analysis indicates a true dichromacy for the central fovea. No expfanation is offered for this difference, although it may relate to the fact that their stimuli were closer to threshold than ours. Clearly, the idea that fovea1 tritanopia is caused by the absence of a receptor group is not tenable. However, our fovea1 data indicate that the idea that all of the features of fovea1 and small-field tritanopia are due to adaptation effects arising from the steady fixation required to make color matches is also not tenable. Apparently blue receptors are scarce, but not entirely Iacking, in the centraf fovea. Their scarcity accounts for the pronounced desaturation at 400 and X0-80nm, and must be somewhat similar to the desaturation shown by near-dichromats at the appropriate neutral points. Why steady fixation produces dichromacy is not so clear, but it appears to be related to the Troxler effect, normally observed in the peripheral retina. Since acuity is known to be poor for the blue mechanism (BRINDLEY, 1960, p. 234fl it may be possible to achieve such effects even foveally with careful fixation. If the basis for the dichromacy is a fovea1 Troxler effect, then this implies that the cone systems of the retina must, to a considerable degree, independency show the type of adaptation which characterizes Troxler fading. Specifically, the activity of the redgreen system, which small eye movements appear capable of sustaining, does not prevent the blue-yeliow system from turning off. ~~~uf~~~io ~~~~ld-~r~c~~ [email protected] To some extent the results of the present study can be subsumed under the Bezold-Brticke hue shift effect. Increasing fiefd size and the eccentricity, and/or decreasing adaptation, has an effect similar to increasing intensity. All four variables affect the contribution of the yellow-blue system. For small, dim, steady, foveallyhated fields, the eye is dichromatic; the contribution to vision of the yellow-blue system is virtualiy eliminated. For large, bright, fIashed fields the poor acuity and low sensitivity of the blue mechanism is not as apparent, and the biue-yellow system increases its influence, spreading the range and increasing the number of blue and yellow responses to spectral stimuli. Acknowle&ements--This research was done during 1965-66 and was supported by Grant NB-00624, from the National Institute of Neurological Diseases and Blindness. CARL &ormo received support from ao NSF Cooperative Feil5~~~, and Hoas~ SC=received support from a NATO fellowship, REFERENCES BEDFORD.R. E. and WYSZECK~,G. (1957). Axial chromatic

aberration of the human eye. J. ugt. Svc. Am. 47,

564465.

BOYNTON,R. M., SCMFER, W. and NEW, M. E. (1964). Hue-wavekngth

relation measured by color-naming method for three retinal locations. Science N. Y. 146,666668. BRDDLEY, G. S. (1960). ~~y~io~ogy of the Bet&u and Visuuf faraway. Edward Arnold, London. BRINDLEY,G, S. (1954). The summation areas of human colour-receptive mechanisms at increment threshold. J. Physiof. 124, 400-408. K~NIG, A. (1894). tier den menschiichen Sehpurpur und seine Bedeutung fiir das Sehen. S. B. Akad. Wiss. Berlin 577-598. KRAUSKOPF,3. and SREPRO,R. (1965). Spectral sensitivity of color mechanisms: derivation from fluctuations of color appearance near threshold. Scipnfe N. Y. 15#,1477-1479. RXHARDS, W. (1967). Differences among c515r normals: Classes I and II, J. opr. Sec. Am. 57,1017-1055. SCKEIB?~ZR, H. M. 0. and B~YNTON,R. M. f1968). Residual red-green d~sc~~ation in dichromats. J. opt. Sot. Am. 58, 1151-1158. STILES, W. S. (1949). Increment thresholds and mechanisms of colour vision. Documenta Ophthal. 3,138-163. THOMSON,L. C. and WRIGHT, W. D. (1947). The colour sensitivity of the retina within the central fovea of man. J. Physiul. 105, 316-331. WILLMER,E. N. (1944). Colour of small objects. Nature, Land. 153, 774-775.

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Color Naming of Small Fovea1 Fields

WUIMER, E. N. and WRIGHT, W. D. (1945). Colour sensitivity of the fovea centralis. 119-121. WRIGHT, W. D. (1964). A new look at 37 years of research. Vision Rex 4, 63-74.

Nuirrre, Lo&. 156,

Abstract-Using a color-naming procedure, it is found that the central fovea is not dichromatic, although it tends toward tritanopia. The trichromacy can be eliminated by presenting the test flashes against a bright blue background. More nearly normal responses are obtained by increasing the intensity and/or retinal eccentricity of the stimulus. Support is given for Brindley’s hypothesis that dichromatic color-matching for small fovea1 fields is due to a selective adaptation of the blue-mediating system which occurs during fixation. R&&-Par une technique de couleur nomm&, on trouve que la fovea centrale n’est pas dichromatique bien qu’elle tendre vers la tritanopie. On peut Climiner le trichromatisme en prCsentant les &lairs tests sur un fond bleu brillant. On obtient des r6ponses plus pr&s de la

normale en accroissant l’intensit6 et/au I’excentricid n%nienne du stimulus. On coniirme I’hypothbse de Brindley selon laquelle les dquations color&s dichromatiques pour de petits champs fovkaux proviennent d’une adaptation dlective, par suite de la fixation, du systeme qui engendre le bleu.

Zusammenfassung-Mit Hilfe eines Verfahrens der Farbbenennung findet man, dass die zentrale Fovea nicht dichromatisch ist, obgleich sie Anzeichen einer Tritanopie zeight. Die Trichromasie kann dadurch beseitigt werden, dass man die Priif-Lichtblitze gegen einen hellen blauen Hintergrund darbietet. Man erhalt Antworten, die den normalen nahekommen, durch Erhahen der Intensitlt oder such durch Vergri%sem der retinalen Exzentrizitlt des Reizes. Die Ergebnisse stiitzen Brindleys Hypothese, dass die Miiglichkeit dichromatischer Farbab gleiche mit kleinen fovealen Feldem von einer selektiven Adaptation herriihrt, die w&rend des Fixierens das Blausystem betrifft. IIpW B’ZlOsIb3OBaIGUiMeTOIIFN%iNia3bIBaHH% IIBeTOB), HaiiaeHO, ¶TO UeHTpaJ&~ SaCTb t$OBezI3IbliOt 06nacr~ He IIBIIIIeTCa AIiXpOMaTE’IeCKO~, XOTII II EMeT TeHAeBLIBfo K TpaWHonmi. T~RX~OM~~EJJ MOxceT 6bn-b 3~ posarra npa npeAaaBJlexmi TeCTOBbIX BcnbmreK aa apxo~ cIiAeM aoRe. Pea=, donee 6na3me K HO~M~JII,HHM,6r~mr norryneHsr PesroMe -

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