A NEW PROCEDURE DESIGNED'1'0ASSESS CONTRAST AND COLOR VISION IN YOUNG INFANTS.
BY
@Michele Edith Mercer, B.Sc.(Honours)
A thesis submitted to the School of Graduate Studies in partial fulfillment of the
requirements for the de-rreeof Master of Science
Department of Psychology Memorial universityof Ne\lfounciland
December, 1989
St. John's Newfoundland
1+1
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distlibuer au veodre descopies de sathi\se dequeIQuemat1ier9et&OUSquelqueforme que oesoitpour mattre des exemplalresde cettetheseatadisposition despernonnes Interessees.
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AnQndUrinqpra ctic alproblem in studyinghuman visua l devel o pme ntis to obt a i n enough data to evaluate thevision of ind i v idua l infants. Withthisproblemin mind, wehave produ c e d ane.... test of ba s iccolorvi s i onusingMun s ell Hues .
.anda methodpa t t erne d afterthe TellerAcu ityCards (TAC).
Inourpr oc e d ure, we first evaluatean in fa nt' s sen s i t ivit y to luminance cont ra s t. The baby is shown lar ge grayca r ds (21.5x56 Cm) thathave a7.5 x12 emgray
"s tandard" patchof the same luminanceonthe left or the ri gh t side , and a gray "test "patchot differe ntluminan c e on the otherside. Like the TACprocedure,a"blindlf obse rveratte mpt sto correctly '[udqe the location of the testpatch. The procedure cont i nues untilthesmall e s t dete ctableluminanc e inc rem e nt and decrement is de t e rmined. Ne xt, wetestthe infant wi t h chromatic test pat c hes . To elimi na te brightnes s cues , the relative lumi nanceof the chromaticpabch and the gray bac kgroun dis varied systema t ica l l y (Tell er& Bornste in,1987) in equilumi na nt st e ps over a wide range (about1 logunit ) cente redaround anad u l t bri g ht ne ssmatch. The st e p size (thus,thenumber of cardsneededfo r each chromat ictest ) isdet e rmi ne dby theSUb j ect 'sse ns i t i vity to contras t in the firstph ase.
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We used the Color/ContrastCards to test 70 2- and 3 month-aIdswith four broad-bandchromaticstimuli , namely a red (dominantwavelength" 660 nm), a (580 nm) yellow,a (520 nm) gree n , and a (475 nm) blue. In approximately20 minutes, 8J%: ofa-e c ne h-ca de and 87.5%ofa-acnm-crde completedthe co nt r a s t phaseand atle a s t one ofthe four chromaticstimuli , and of these, 37\ of 2-month-oldsand34%
of a-acntn-eme completed allfour chromatic stimuli . Both groupsof infantswere significantlybetterat dete cting luminancedecrements than increments. 3-month -olds discriminated allfour chromatic stimuli fromgray. In contrast , z- mo nth- oIds discriminatedthe red and blue from graybutta i l e d to discriminatethe yellowand greenfrom gray at relative luminancesclose to the adul tbrightness match. Reasonsfor 2-month-oldsI "failures" are discussed in detail.
In general, theprocedurewassuccessful. Over a relatively sho rt period,we coul d te s t an infant 's sensiti v i t y to luminancecont rast, her/hischromatic- ac hr oma t i c discriminations ,and the relativeluminancesat whichth e infant "fails"to makethesediscriminati ons. In future, the Color/ContrastCards should prove to be clinical lyuseful for sc r e e n i ng youngerinfantsand handicappedchildren, as well as experimentally useful in providinginformationabout thedevelopment of colorvision and itsunderl y ing mechanisms.
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Acknowledgements
I would fi r stliketo expressmysince re appreciation to bothmy current and pr e v i ou s supervisors,Dr. Mary Courge and Dr. Russell Adams, respectively. Their contributions towards mygrowthand learni ngin the past few years will re ma i n immeasurable. I would also like to thankthe others on mythesis committee: Dr. carolynHarleyand Dr. Mike Sherrick who were very helpful in guiding the progress of this research. Also, specialth a nks to Lynn Brown and Lisa Grantfo r their assistance with the data collection. This researchwas poasLbj e because of space and staff provided by St.Clare's Mercy Hospita l and The GraceGeneral Hospital.
Alsoappreciatedare the parents who volunteeredtheir babies. This researc hwas supported by Natural Sciencesand Enginee ringResearch Council of Cana da Postgradua te Scholarships, Memorial universityof NewfoundlandGraduate Fellowships and teaching assistantships.
Finally , I wou ld li keto expressmywarmest thanks to myfamilyand friends for theirsupport and encouragement.
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nl!.l.eof Con te nts
INTRODUCTION:
Methodological Is s ue s••. . .• . . .•.•.•. •. .•.••••• .••
The Brig ht nessProblem••••.•• •.• .• .•• .. .• ...••.••
Re s ults FromStudiesUs i n g Systematicand
unsystemat icvariation ofLumi n a nce... ... 10 Fu rthe r Limitations inExisti ng Me a s u r e s of Color
Vision and a Prop osed Me t h od ol og i c al Remedy.. . 11
METHOD : 15
SUbj ects . • . .••..•.•. • .•....• . .••.. .. •.. ... . . . 15 Stimuli. .. . ... ... . .. ... ... . 15 Procedure. •..••.•••.•• • •• •••• • •••• •.• • •.• • •.. • •. 18
RESULTS: 21
Discrimina tionofLuulina nc e Con t r a s t. .... ... 21 Discrimina tionof ChromaticPatchesFroln
Achromatic Ba c k g r ou nds... ... . ... . . .. 22 Dev elop me ntal Changes.. .. . . ... . ... .. 24
DISCUSSION: 26
Evaluationofthe Color/Con t rastCard
Proced u re. ... ... . . . .... .. .... . . ... .. 26 Di s criminati on of Luminance Contrast... . .. 28 Di scri mi nati on of Ch r omaticPatchesFrom
Ac h r oma t i c Backgrounds.. .. .. . . .. ... .. .. . 29 (v)
Table ofCo n t e n tr;(cont.inJ.wU
PAGE TABLE.... . . ... . .... . .. .. . ... . .. . .. .. ... . .. .. . 40 FIGURES. .. .. .... ... . ... . . ... .. . .. .. .. .. ... . .... .. . 41 REFERENCES . ... ... . .... ... . .. ... ... . . .... . 4 6 F OOTNOTES. . .... ... .. .. . .. .... . .. ... . .... ... .. . . 54 APPENDI XA . . ... . ... .. . .... ... .. ... . ... . .... . ... 56
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ListoCTa b l e s
Munsellvalue s and calculated lumi na nc e s for bothac hr omat i c and ch romatic Mun s el l
papers... ... ... ... . .. . ... 40
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List of Figures
Cumulativefrequency distributionof 2- and 3- month-aIdsI detection of lumi na nc e
dil·erences... ... . ... ... . .. . .. ... 41 psrcentage of infants who fail to discriminate thech r oma t i c patches from a gray background.. . 42 Distribution of rslative lumi na nc esat which 2- month-aIdsfail to discriminatechromatic
patchesfrom a gray background..••• ••• • •. ..•. .. 43 Percentage of 2- and 3-month-oldswho
discriminatethe chronetIc patches from a gray background... ... ... ... .... ... . 44 Yellow/b lueand red/greenopponent response functions acrossthevi s i ble spectrum. . ... 45
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Introduction
With the advent of a number of new research methods for studying human infants (for a review, see Maurer, 1975), there has been renewed interest in the ontogeny of sensory processes, particularly in the early development of vision.
Asa reeine , significant advanceshave been made in understanding how infants perceive important aspectsof their visual environment, such as form,pattern, and contrast. A notable exception, however, has been the study of early color visionin which only the most fundamental questions have been addressed.
However, thispaucityof knowledge is not due to a lack of interest. Researchers are interested in examining infants'color vision for a variety of reasons. For example, a physiologicalpsych('~.ogiststudies early color vision because infonation about immature neural mechanisms provides insight into the functioning of adult neural mechanisms (e.g., photoreceptors, opponent channels). By studying the mechanisms in their simplest form, researchers are able to trace their development and understand these mechanisms in their mature state (e.g., Gordon and Abramov, 1977). A psychologist interestedin perceptionmay examine infants' color vision to better understand adults' perceptual processes (e.g. , Hurvich, 1981). Clinicians and others in the medical profession (e.g., Pease and Allen,
1988) tire interested in color vision to assist them in detecti ng early visualabnormalities(e.g ., cone deflciences)•
Me t h odol oq i cal Is~.
Despite numerous practical problems, researchers have managed to utilize three types of responses to assess non- verbalinfa nts' colorvision: refleKive, electrophysio- logical, and behavioral responses. One of the first reflexive measures used to test infants' color vision was the "eye-on-the-neck"reflex (the spontaneous jerkingof the head in response to euduen illumination). Peiper (1927), and Trinckerand Trlncker (1955) used the"e ye- o n- t h e - ne c k reflex"to measure infants' spectral sensitivity. Both studies reportedtha t, at photopiclev e l s , adult and infant spectral sensitivityfunctions are virtually identical.
More recently, researchers have tested infants' sensitivity to chromatic stimuli using ocular reflexes,such as the pupillary response (Young, Clavadetscher,, Teller, 198 7 ) , and optokineticnyst a gmu s (OKN is a reflexive oscillatory movement of the eyes elicited by moving la.rge stripes through the visualfield). For example, Anstis, Cavanagh, Maurer, & Lewis (1986) used a complex OKN task and found infants' and adults' spectral sensitivity to be virtually identical.
Other researchers (e.g., Barnet ,Lodge, &Armington, 1965; Dobson , 1976;Moskowitz-Cook , 1979) chosetodi r e ctly measurethe nervoussystem's re s pons eto ch romatic..timu li by usi nge1ectrophysiological tech niquessuch as the electroreti nogram (ERG) or the visual l:: evoked potential (VEP). For example, Dobson(1976) us e d VEPsto measurethe spectral sensitivityof both2-month-~l dsand adu ltsan d found tha t infantswererelatively moresensitive than adults in theshort -wave length re gio n of thespectru..
Behavioral measures have had the longest historyin the study of early colorvision. Ba ldwi n (l893) assessed hi s nlne-month-olddaughter'scolorpreferences by presentingher withcoloredpapers andobs e rv i ng her gr a s p ing behavior. He foundth a t she reachedmost for blues and reds,fol lowedby greensand browns. Gratping\<'<:' 5 la t e r us ed , in combination....it h forced-choiceand rein forcement procedu res (e.g . , Marsden, 1903; Valentine, 1914 , respectively) to measure infants ' abil ityto make chromatic discrimina tions. Although a convenient measure, grasping is neea reliableone becauseit is a percept ual -motor re s p ons e whic h li kely relieson more complexneural coordinatio n than isne ede dt~processchromatic information . Therefore, it is difficu l t to determi newhether an infant'sfailure to grasp for a partiC Ularch roma tlcstimulus is dueto her/his inabil ity to grasp or to he r /hi s inability to process chromatic in f orma tio n.
Presently , the most Successfu l and mo st popular beh a v i oral measuresemployinfantsI visual orienting be haviors, rather tha ncomplexmotorsk i lls. These include habit ua tion-dishabituation (e;g., Borns tein ,1975; Adams , Maurer, &Cavis, 1986), preferentiallooking (e.g., Fagan, 197 4 ) andforced-cho icepreferential looking (e.g., Peeples
&Teller, 1975; Packer, Hartmann&Teller,1984). In th e
habituation- dishabitua tionparadigm, an infant is presented repeated lywithst i muli th"tarethe same orsi mil a r, (e.g., a whiteli gh t of different lumina nces ) untiltheinf a nt habituatesor "be co mesbored"withthe stimulus. Once the infant decreaseshis lookingtime toa pre-specified criterion, a novel st i mulus (e.g., a redlight) is displayed and"lookingtimelt is compared to that forthe familiar sti mulu s. Ifth e infant increases hisloo k i ng time, or dishabituatesto the novel stimUlUS,this is taken as evidencethatthe infantcandiscriminatethe novel from the familiar stimuli.
Inatyp i c a l preferential loo k i ng (PL) procedure (Fa ntz, 1958 ) , aninfantis presented, over ase r i es of tria l s , with pairs of stimuli. Anadult ob s e rv e r, who is unc ...are of the posi tionofthe stimuli, judges the direction of the infantIs first fixation, orthe amount ofti me th at the infant spendsfi xa t ing eachsti mulu s. Currently, the most popul ar-te c hn i qu e is a versionof PL calledfo r c e d- choice pr e f e r en t i allooking (or FPL) (Teller,1979 ) . The
sUbj e c t is pre s entedwith a pair of stimuli,one of wh ich match e sthe backg r o und anda seco nd(test )stimuluswh ich diff ers insome way (e.g., isa different color orpa t t ern ). The loc ation of thi s test stimul us is variedfrom tri a l to tri al and anobserverisunaware of its l position. The observeris forced to choose on which side the test stimulus isloc a t edbas e d onthe sUbject'shead, eye, and body movements. If the observerIs choices are correct for a major.ity ofthetrial s (e.g., at least 75%of them), it 1s assumedthat the infan tcan di s c rimi na t e the testst i mu l u s from the background.
None t he less , re gardle s s of these procedural advances, a major methodo logica l problem must be consideredin the studyof any organismlscolor vision. ThisproblemIs to Ins ur-e that the SUbject discriminatesamong chromatic stimu lion the basis of differences in hue and not on the basis of brightness differences.
The BrightnessProb lem
Colorisde fin e dby three att ributes - hue,brightness, and saturation. Anadultwithnormal colorvision is capa b leof us i ng all three of these attributes to discrim i nateamong differently colored ob j e c t s. Howe v e r, even without theability toperc e i vehue or saturation, a colorblindadul tcanstill use brightnesscues to disc r imi na teamongmost chromatic sti mul i. A good example
of this is to consider a colorblind person viewing a colored photograph. He/she is easily able to discriminate objects of different IIhuell as long asthey differ in brightness.
Similarly, an infant with immature colorvis i o n may be able to discriminate among chromaticst i muli using brightness cues only.
In a first attempt toadd r e s s the brightnessproblem, researchers stUdying infantsI color vision have used chromatic stimuli that were matched in brightness by an adult. This is based on the assumption that if an adult could not discriminate chromatic stimuli on the basis of brightness, neither could a baby. However.-.t u d i e s of photopic spectral sensitivity (Dobson, 1976;Moskowitz-cook, 1979; Peeples& Teller, 1977) imply that infants and adults differ qualitatively in their sensitivity to the luminance of chromatic stimuli. For example, MoskowitZ-Cook (1979) usedVEPs to obtain spectral sensitivity curves for both infants and adults. She found that although the functions of older infants' (15-22 weeks) were similar to those of adults, the functions of younger infants (3-14 weeks) were slightly elevated(byabout 0.5log units) in the short- wavelength region. These results imply that infants and adults do not respond to the brightnessof chromatic stimuli in the same way; therefore, adult brightness matches are inappropriate for studying infants' chromatic vision.
Teller and Bornstein (1987)describe two alternative methodsus e d to minimize brightnesscues. These techniques are termed the "unsystematic"andI;s y s t ema tic " variationof luminance . The unsystematicvariationof lUllli lla n c e is uses a wide range oflu mi nan c e s centeredaroundan adul t brightnessmatch. For example, if one were testing the discriminationof red from white, the luminanceof the white would be varied over a broad range, centered around the lumi na nc e at whichanadul t would perceive the red and white as equal. va r yi ng the luminanceprovidesthe baby withmany differentexamples of whiteand therefore, reducesthe likelihoodthat the infan t will discriminatethe red from the whitesolelyon the basis of any particularbrightne s s difference. Therefore,if an infant can discriminate the two stimuli on the basis of wavelength,one assumesthat he will respond differentially to the red and thewhite(s), de s pi te differences in brightness or not. On the other hand, if the infant failsto respond differentially, we concludethatthe infant cannot discriminatethe red from the whiteon theba s i s of wavelengthalone. Bornstein (19751and Adams, Maurer, andDa v i s (1986) used the unsystematicvariationof luminancein combination witha habituation-dishab ituationparadigmto test infants' color vision. Adams et al. presented neWbornswitha seriesot:
Whi tesquaresWhich, from trial to trial,varied in lumi n a n c e. Once the infant ha d habituated, or became
"bored", the infant was shown either a 630 nm red or a 480 ron blue of mid-range luminance and another white square of a novel luminance. The infants recovered (i.e.,looked longer) to the red but not to the blue. Because there was not a significant difference in the looking time between the white square and the blue square, Adams et a1. reasoned that newborns could not make the discrimination on the basis of wavelength information. However, the authors did note that this procedure also has limitations. Negative results may be due to the fact that infants have to both recognize and remember the stimuli presented on previous trials for successful discrimination. Therefore, habituation not only requires the ability to discriminate the stimuli, but also requires memory which may result in an underestimate of the infant's color vision. Moreover, an habituation paradigm assumes that the organism possesses the neural mechanisms necessary for habituation, an assumption that has been questioned with regard to young infants (Banks and salapatek, 1983).
The systematic variation of luminance procedure developed by Teller and her colleagues (Peeples and Teller, 1975) is a more refined method than the un::iystematic variation of luminance. The systematic version consists of two phases: Inche first (luminance contrast) phase, the infant views a large achromatic background that contains an achromatic test patCh, the luminance of which varies from
trial to trial. The index of an infant's sensitivity to achromatic contrast is the smallest luminance difference between the patch and the background that he/she appears to discriminate. In the second (chromatic) phase, the achromatic test patch is replaced by a chromatic test patch (e.g., a 650 nm red patch) ...hich also varies in luminance.
The luminances of the chromatic patch are centered around and includethe adult brightness match, and range broadly enough to insure that an infant's brightness match would be included. The spacing between luminanceswithin the range (i.e ., the step size) is set by the smallest difference in luminance that the infant can detect in the first phase.
Therefore, itis assumed that the infant will be presented with at least one chromatic patch that appears toma t c h the background in brightness. Thus, if an infant's performance falls to chance for even one luminance of the chromatic patch, this implies that tho infant is incapable of detecting the ...avelength information in that particular chromatic stimulus. On the other hand, if the infant discriminates the chromatic patch at all relative luminances (including, presumably, at least one pair that does not differ in brightness)I this implies that the infant can discriminate the chromatic stimUlUS from white on the basis of wavelength information.
Results From StudiesUs ingsystemati c and Un sy ste mati c Varia tio nor lA.l!dn ance
Inve stigators ha ve usedbothsystema t icand unsyst ematic variatio nof luminance tosuccess f u lly ev a l uate the earlydevelopmentof human col o r visio n. For examp le, Ha mer,Al ex and er , andTelle r (198.2) andPacker, Ha rtman n , and Teller (198 4) ha ve shown thatunlike adul t protan o p e s and deute r anopes , 2- and3-mo nth-ol dsare able toma ke Raylei g h dis cr ilninati on s (I. e.,discri minat i onsbetween pairsof wa velen gthsgreaterth a n 545nm). Also, in a related study, Var ne r, :ook, Schneck, McDon ald,and Teller (1985) fou ndthat, unlikeadu l t trita nope s, mos t 2-month- aI ds cou l ddisc ri mi na te a tritonpair ,specifically , B 416 nmblue fr om a 547 nmgre en . In te stsof chroma t ic- ac hr omaticdiscrimi na tions , Teller, Peeples, and Sekel
(1978) us ed FPLandfound that 2-mo nt h-o l ds were ableto discriminatemanywavelengths fromwh i te. Howeve r, 2-month- ol d s al so showedseveral limitation s inth eir col o rvi sion:
theyfaile dto discrt.inat e frollWhite,538 nmgree n, 561nm yellow- g ree n , andmid- purp l e.
Theco l o rvisionof infantsless than2 mont hs is even more limited. For exampl e,most I- mo nth- o lds faU to ma ke Rayleighandtr ita ndiscriminations (andpre sumably,li ke older infants ,wouldfail the whit e/yellow-gre enand the white/mid-purpl ediscrim inations) (Hamer et al., 1982 : Var ne r et a1., 19 8 5). Moreov er, Pa c ke ret a1. (19 8 4 1 found
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that even those zev 1-month-olds who were able to make successful Rayleigh discriminations required larqe (8 deg.) Bt!lnuli. Newborns show additional limitations; while they successfully discriminate from white, 630, 640 and 650 nm red, 575and5851lIlyellow, and 540 and 550 nm green, they fail to discriminate from white, 572 nm yallow-green, and 470, 475and 480 nm blue (Adams and ccuz-aqe , submitted;
Adams, Maurer,&:Cashin, 1985;Adams et a L,, 1986) until approximately 1 month of age (Maurer' Adams, 1987 ) . In addition, Adams (1989) demonstrated that newbornsare capable of making a Rayleigh discrimination, but this ability is limitedtostimuli that are very larqe (at least 16 deg.) and of wide spectralseparation (e.g., 545 nm green vs 650 nm red). collectively, these studies indicate that although newborns possess at least some rudimentary color vision, it improves significantlyover the first three months.
FurtherLimitationsin Existing Measures of Color Vision an d a Proposed MQthodologicalRemedy
Although these procedures appear to be successful in minimizing brightness cues, there are still limitations and problems. For example, the habituation-dishabituation method relies on group estimates of luminance sensitivity and may underestimate an individual infant'ssensitivity.
Therefore, such an infant may make an apparent chromatic-
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achromaticdiscriminationon the basisof brightness cue s , rather thanon wav elengthinformation. Although the preferential-lookingproc edure uses est imates ofind i vidua l infants' lumi nanc e se n sitivi ty to test chron:at ic-achromatic discrimi n a tio ns , it.isveryti me-c on sum ingand require s mu l t i plesessio ns;thus , manyin f a nt s becomefussyor sleepy, or failto return for further sessions. Bo t h of theseproblems , along with theunavailabilityof si mpl e, sta nd a r d izedequi pment , limitthe inte rp r e t iveandcli ni ca l value ofthe s ete c hn i qu es. Inot h e r words, an assessment of an indiv i d ua l in fa nt ' scolor vis ioncannot be dete rm i ned within on esho r t test ing session. Th is is imp o rta nt i fa test istoha ve pr ed i c t iveva l ue andwi de-spreaduse.
Si mil arprobl ems are facedbyre s e arch ers attemptingto studyotherimpo rtant visua l functions. In a recent and promisingat tempt toproduceamet hod toove r come the s e problems . Tellerand her co l l eag u es (McDona l d, Dobson , sebris, Baitch, Var ne r .&Teller . 1985) de sig n e d a set of
"Acu i tyCards"whi challow aqui c k, yet accura te,assessment ofvisua l ac u i tyin infants even a fe w ho urs old. The procedu re ,whic h is amodific a t ion of FPL, consis tsof presentingan infa nt witha se r i esof large gra y cards that, on either the ri ghtor left si de ofacentralpeephol e. contain a se t of bla ck and whit e st ri pes (gratings) that typically va ryinspa t ia l frequency from 0.2to 40 cyc les/deg r ee. Thesp aceave r ag e luminance of the gratings
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isequal to the luminance of the cards' backgrounds. An observer, ....ho is blind to the locationof the grating, watchep the baby through the peephole and jUdges the location of the stripes by observing the infant's head and eye movements. To test her/his assumptions, the observer can quickly rotate each card to position the grating on either side. It isas s umed thati fthe baby is capable of detecting the stripes of a particular spatial frequency, he/she will consistently orient to....ards them. If he/she does not see them, the entire card will appear gray due to the "fusion" of the stripes into the background. In this case,the infant will either continue to stare at the center of the card or look randomly from side to side.
The procedure begins with large stripes (low spatial frequency) and progresses with increasingly smaller stripes.
The point at which the observer cannot jUdge the location of the stripes is an estimate of the infant's visual acuity.
This acuity card procedure can usually be completed within approximately 5 minutes as compared to the traditional FPL procedure which usually requires upwards of 1 hour, often across mUltiple sessions. In addition, the TAC method yields the same estimates as the traditional FPL method (McDonald et al., 1985).
Because the Teller Acuity Cards have proven successful in efficiently assessing infants' visual acuity, a variation of this procedure may be useful in measuring other visual
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functions. The present study attempts to develop 4 new test of infants' color vision by employing Teller's version of the FPL procedure, and stimuli constructed with Munsell Hues - a widely known, standardized color system. Also, to best control brightness cuea , the procedure incorporates a two- phase systematic variation of luminance (Teller et al .•
1978). In the first phase,the infant's sensitivityto luminancecontrast is measured using a set of achromatic contrast cards. In the second phase, the cards are altered in order to asses s the infant's ability to discriminate chromatic stimuli, representing various spectral regions, from achromatic backgrounds of greaterI lesser. and equal luminance. The numberot backgrounds needed to evaluate each infant's ability is determined by his/her sensitivity to luminance contrast in phase 1.
Thus, the general purpose of the present study is to design an efficient technique to assessyoung infants' contrast andcol o r sensitivity. More specifically, the goals are (1) to design a procedure that allows the rapid and simple,yet accurate assessment of infant color vision , (2) to collect sufficient data from individualin f a nt s to determine whether the procedure has diagnostic value, (J)to determine the earliest age at which the color/contrastCards can be used, (4) to compare our resures withthose obtained with more lengthy procedures (e.g ., FPL - Teller et a L;, 1978). and (5) to make statementsabout the developmental
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state of early color vision mechanisMs (e.g., photoreceptors, opponent channels).
Metbod
~.
The sUbjects were 19 female and 16 male 2-month-old infants (M age'" 9.20 weeks: s.d.- 0.61 weeks) and 24 fellale and 11 male 3-month-old infants (H age= 13.09 weeks;
s.d. ..0.10weeks). All infants were at least 38 weeks gestation and at least 2500 grams at birth. An additional 18 infants (11 two-month-olds and 7 three-month-olds) were tested but not included in the sample: 12 (13.6%) because of incomplete data, and 6 (6.8\) because of a procedural error.
An infant was designated as incomplete when he/she failed to complete at least the contrast phase and one chromatic condition. Infant fussiness accounted for all incompletions.
illJnJill·
The cards' backgrounds were constructed by mounting 56 em long x 21.5 em wide pieces of gray Munsell matte paper onto 1 em thick stiff board. On each card, two smaller 12.5 em long x 1.5 em wide Munsell patches were mounted on thinner (1/4em) board and attached to the backgrounds with Velcro. The nearest edge of each smaller patch was located
15
1.5emto the leftand the right of a1em central peephole.
To prevent damage, both thebackgroundsand the patcheswere laminated.
The Munsell colornotationsystemid e nti fie scolor in terms of threeattribute s- hue(W'av e leng t h) , valu e (brightness/l uminance), andch roma (s aturation). Thesca l e defini ngeach of the s e attributes consists of numbe rs whi c h represe nt, to adults, steps (incre mentsor de creme nts) of equa lspa c i ng. These sc a le s allowprecise ident ifica tion anddescripti onofcol o runde r standardi llumination and vi e wi ng cond i tion s.
The hue nota tion cons i stsof bothhueini tials (e . g., B for blue ) for the tenmajorhue families , and huenume r al s (e.g •• lOBfor dee p blue )formore precise specificatio nof spectral location. Thevaluenota tionin dicatesthe brig ht nessof a co lo r inrelation to a neutral gray sca le , whichexten ds from abs o l ute black (symbolizedby N0/)to absolute whi t e (symbolizedbyN10/). The chr oma notation ind icatestheamo u nt ofsat ur a tion of agi v enhue ra ngi ng from /1 (very desaturated)to/16 (extremely saturat e d).
In the cont r ast phas e of this experiment, weused achr omatic (gray) patche swhic hare designated inthe Munsell system only by va l ueand notby ch rcma or hue. The graypatch eshad values(l umi na nce s ) rangi ngfrom N 2/
(1.09 logcd/ m2) toN9.5/ (1.86 log cd/m 2) (s ee Table 1 for
16
other vatueeand luminances],and were mounted on a mid-gray (N 5/) backqr-cund (1.36 log cd/m2).
InsertTable1 about here
In the chromatic phase, the achromatic backgrounds ha.d values (luminances) ranging from N 2/ (1.09 log units) to N 9.5/ (1.86 l.u.) andthe chromaticpatches had valuesof 4/ (1.331.U.),5/ (1.371.U ) , 6/ (1.491.U.), and 8/ (1.67 1.u.), for red, blue, green, and yello..., respectively'. All of the chromaticpatches were equallysa t u r a t e d ( /12). These chromatic st i mul i were chosen because they had spectral characteristicssimilar to those used in the only other study of 2-month -oldsI chromatic-achromatic discriminationsus i ng a FPL method(Teller et; aI., 1978 ) .
The luminances of the stimuliwere measured.in li.tY.
with a Minolta Chroma Meter Cr.-lOa and a Macbeth
Illuminometer. stimuli wereilluminatedunder diffuse white light with eIE chromaticityXand Y.. coordinates of 0.31 and 0.31, respectively. This correlateswith a color temperature of approximately 7000 deg. K. (eIEIll umi na nt C) the illuminantfor whichthe s eMuns ell hueswere standardized. Under the s e conditions, thedo mi n a nt wavelengthsofth e chromaticpatchesweredetermined to be 660 nm for the red, 520 nm for the green, 475 nm for the bl ue , and 580 nm for the yel low. The chromatic st imu l ihad
17
chromaticity X andl::coordinates of 0.50, 0.29~orred, 0.20, 0.20 for blue, 0.26, 0.47 for green,and 0.47, 0.46 for yellow. Excitationpurity values were calculated from the chromaticity coordinates to be 0.43for red, 0.57 for blue, 0.30 for green,and 0.81 for yellow.
Procedure.
We used a version of forced-choice preferential looking (FPL) most similar to that used with the Teller Acuity Cards, a modified staircase procedure to determine threshold, and a systematic variation of luminance to control for brightness cues. In our version, the infant fiits on the mother's lap and an observer holds the cards approximately 45 em from the infantts eyes. At 45 em, the test patches subtend a rectangular field of 9.5 x 16 deg.
Because a second experimenter selects and changes the cards, the observer is always"b lind1l to the characteristics of the stimuli (e.g., location, luminance, and/or hue, of the test patch)•
In general, our version of FPL proceeds as follows.
The experimenter places two patches on each card - one that matches the background luminance, and a second test patch that differs from the achromatic background, either in luminance (the contrast phase) or in chroma and/orluminance (the chromaticphase). The experimenter then carefully passes the card to the observer so that the observer cannot
18
seethe stimulitobepre s e nt e d. Theobs e rv er holds the car dinfront of the inf a nt, andviews her /hi m thro ug ha cent ra l peepholein the car d. The observer rotate s th e card several ti me sto alternate the pos i t i on of the test pa t c h and then,either jUdge s the positionofthete s t patch (based on thehe ad and eye movements of the infant), or conc ludesthat the infant cannotdiscriminate the patchfrom the background .
Dur i ng the contrast phase of the experiment, the infaLt 'sluminancesensitivityismeasured (see AppendixA for a simulation of the enti reprocedure ). First, th e observer shows the infant a card withthe larg e st lumi na nc e difference (greate stcontrast) becveenthe achromatictes t patch and theachromaticbackground ~either thela r ge s t increment (t owa r d s White) or larg e s t de cre me nt (toward s bla c k ). Ifthe observer correct ly jUdges the side where the test patchis loc a t ed, the contrastis decreasedfor the next card. To increase observer uncertainty, the amountof contras t on thissec o nd car dvariesacrossbabies. If the observeragain cor r e c t l y jUdges"i;hat the infant is able to detect this luminancedifference, stilla smaller difference is tested. However, if the observer jud gesthat the infant cannotmake the discrimina tio n, a larger differenceis te s t e d (if one exi sts ) . The smal lest luminance difference tha t the infan t candi s c r i minate is a measure of his/her contras t sens itivity . Th i s stai rcase pr oca du r e is then
19
repeated until we have determined the infant's sensitivity to both luminance increments and decrements.
In the chromatic phase, we use the information from the infant's performance in the contrast phase to test his/hl:o!r discrimination of chromatic patches from gray backgrounds. For each infant, the smallest luminance difference detected during the contrast phase determines the step size (I.e., the size of the luminance increment or decrement between adjacent cards), and thus, the number of backgrounds that are required to test each wavelength during the chromatic phase. In other words, the better the infants' luminance sensitivity, the greater the number of cards that need to be used during the chromatic phase (see the simulation in Appendix A). The selection of the appropriate step size assures th.'lt the infant wjll be presented with at least one card in which he/she cannot discriminate the chromatic test patch from the gray background on the basis of brightness.
Therefore, i fthe infant can discriminate the chromatic patch from the background fori l lcombinations, including at least one in which the patch and background match in brightness, he/she is probably capable of making that discrimination on the basis of wavelength information.
conversely, i fthe infant fails to discriminate the chromatic patch from theb~ckgroundfor one or more combinations, then we assume that he/she is unable to make the discrimination on the basis of wavelength information.
20
The order of presentation for the differentbackground luminancesused during the testing of each chromatic stimuluswas counterbalanced acrossbab i e s , as is the order ofthe four chromatic stimuli. The procedure continued untilte st i ng with allfour chromatic stimuli....as completed, or untilthe infant became uncooperative.
Re su l ts
The procedure vas successful. 83% of 2-month-oldsand 87.5%of a-ncneh- ordecompleted the contrast phase and at le a s t one of the chromatic sti muliin an average time of 22.15minutes (range
=
14.11- 33.45) ar.d 19.18minutes (rangeIII12.12 -28.17),respectively . Of these infants, 37% of 2-month-oldsand 34% of 3-month-oldscompletedall four chromatic stimuli inanaveragetime of 20.92minutes (range..13.50- 31.11) and20.85minu tes (range"14 . 5 4- 26.39), respectively .Discriminationof I.uminance Contrast.
Figure1displays the cumulative distribu.tion functions forth e smallestluminanceincrements and decrements detected by 2- and 3-month-oldinfants. A cumulative distribution was chosen becauseitsshapeis most
21
Insert Figu r e1 about he r e
comparableto those derived froll other ps yc hophys i c a l measures (e.g . , FPL, Teller etal. , 1979). As Figure 1 shows, both 2- and 3-month- oldsappearnore sensitiveto lumi na nc e decrementsthan toincrements. This is indicated by a more steeplyris ingslopeand by a smallerseen for decrements than for increments(meansindicatedbyarrows on Figure1). On the average , 2-mont h -o ldsdetecteda decrement of 0.20lou. (8%:Mi ch e l s o n contrastz) anda incrementof 0.26lou. (n contrast). For a-ee ne n- o rds,the mean lumi na nc e decrement and incrementdetected was0.19 l.u. (7.5\contrast) and0.30 l.u . (10\ contrast), re s pe c t ive ly. Wilcoxon testsfor matchedsamplesconfin thatboth 2-lIlonth-olds(1 "3.41 ;R<0.05) and3~month-olds
tz...3. 45 ; Il<0.05) are significantlybetter at detecting luminancedecrementsthan Ine reeenea .
pisc ri mina t i o n of Ch romatic Pa t ch e s
F rom
Achromatic Backgrou nds.For an infant to show evidenceofdi s crimi nating a chroma t i cpatchfromgray backgro u ndsonthe ba s isof wave leng t h inf orm a t i on,the obse rve r had to cor rec t l y gue s s the location of the chroma t i c patch for allrelative lumina nc e s (i.e., al l thepa tch/backgroundcombina tions ).
22
If the observerwasnotable to make a decision astothe locationof achr oma tic patch for one ormo r eofthe relativeluminances , it was con cludedthat the infa nt could not dis crim inatethat parti cular chr omat i cst imulus from gray , at leastnot on the bas i s of wavelength info rma t i o n. Figure2shows the percentageof infantswho, for each chr oma t i csti mu lus,appe ar to show this pattern of
Insert Figure 2 abouthere
"failure". Thisoccured when 12of22 (55 %) 2-month-old s were testedwith yellow, 8 of 23(35%)weretestedwith green, 5 of 25 (20%)we re tested with red, and5%(on ly 1of 21SUbj e c ts ) werete stedwith blue. In con t r as t,virtua lly noneof the3-mont h - ol ds showthi spattern of "fa i l ure"with any of the chromat ic st i mu li. However, in order tosta te that 2-month-olds , • • •qr oup , candisc r imi na teaparticular chromaticstimulus from gray ,significantly morethan 50%of the infants ha d toshow evidenceof makingthat
discriminat ion. Chi-squa r e analysesreveale d that a-acntm- olds sho wedev id e nceof dis c riminatingthe660 nmre d [,,2 (l,D"'25) ...9.00 ,R<.01 ] and the 475 nmblue(X2{1,n=2l )= 17.19, R<.001Jpatches from gray butnot the 520nmgrean [X2{l,n= 2J ) =2. 13, R>.05) or the580 nm yellow [X2 (1,n"22) '"0.18, 2>.05] patche sfrom gray. In contrast, chi-squareanalyses rev ealedthat 3-month-olds showed
23
evide nc e of discriminating all four chromaticstimulifrom gray. There werenosex difference s inperforma nceon any of these e sur es ,
With this procedure,it isalsopossibleto determi ne therelativelu. ina nce (s ) at whichan inf ant"fails·to discri .inate a chromaticpatch from an ac h r oma tic backgrou nd, and comparethesewiththetypicaladult brightnes smatch. Figure3 shows the distributions of
InsertFigure 3 about here
relati v elumi nanc esat which 2-mont h-oldsappear to"fail "
to discriminatethe red,yel low , and gre en patchesfr om the achromaticba ckqround s. For eachof the red,yellow ,and green patches,the se failu resappear to cluster aroundthe typical adult bright ne ss matc h. The ditferen ces in lWlli na nc e betwe e nthe adultmatch and the infa nts'
"fa ilu r e s · wereall within+/-0.18lou. for the red (M..
-0.06i.u.},
-r»
0.20 lou. for the yellow(M· +0.05lou.) and within+/-0. 22 Lu, for the green US- -0 .13i.u.),peyel opmgnta l Change s.
Figure1 illus t rates th a t 2- and3~month -()ldsare very similarintheirab il i t y to detect lumina nc e diffe rences.
Mor e over, Mann-Whi t ney U testsfor independ entsamples confinedstat i s t i ca lly tha t there are nosi g ni f i c a nt
24
differencesbetween 2- and 3-month-olds ' ability to detect either luminanceincrements Ul- 0.996 ;R> .05) or decrements Ul=0.636:R>.05).
To detenn inewhethe r thereare si g n i fica n t ch a ng es between 2- and a-nc ne n s - c r e e ' ab il i t y to discriminateeach of thech r omatic sti mulifromgray,we performed ch i -squa re tests(see Fig u re 4). Theresults of thes eanalys e s indic a te thatbetween2- and 3-months, there were
Ins ert Fi g ur e 4ab out here
si gni ficantimpro ve ment s in thein fants ' abil ity to discrimin atethegre en (X2{l,H-43) - 8.54;R<.005]and the yellow [X2{I,li=o4 3 )...15.89:R<.001] fr om 9ray. Because2- month-oldshadalready shown evidenceof di scriminatingred and blue, asex pe c t e d , there we r e no additional improvemen ts between 2and 3months foreit he r the red[X2{l, H=47) ..
3.84 ; R>.05 ]or the blue [x2Cl,li=45 ) = 1.15;R>.05] .
25
m eeu ••ioll.
The Color/Cont rastCards provedto be an efficient procedureforasse s s i ng the contra st andcol o r vis i o nof 2- and 3-month-oldinfants . In a relatively sho r t sessi onwe obtained information about an in f a nt'sse ns i t iv i ty to luminancecontrast,his/herabilitytodisc r i mi na t e chromatic fromac h ro maticsti mul i, and the rela t ive luminances at whi chthein fa nt "fails" to mak e achromatic- ac h r oma tic di scrimination . We found that2- and a-e cnen- aIds aresens i t i v e to luminanc edifferences, especially to luminance decrements. Most 2-mo nt h- o l ds ar e ableto discriminateboth red and blue from gray at all lumina nces, but many fail to discriminateyellowand green from gray at relative luminancesclose to the adult brightnessma tch. In contrast, 3-month-olds di scrimin ateall fourchromatic stimuli from gray.
Evaluation of the Color/Contrast Card Procedure . The main purpose of this expe rimentwas to designa rap id and acc urateprocedureto assess an infant'scolor vi sion. Although the procedureis not asefficient as that used vit h theTellerAcui tyCa r ds, it still requires much less time and provide s mo re informationthan al t erna tive color visionproceduressuch as Teller'sve rs ion of FPL
26
(e.g. ,Var neret a1., 19 85) andha b i t u a t i o n - d ish a b i t u a t i o n (e .g. ,Adams,19 8 9 ). Our Color/Contrast.Cards allow one to measure in a singlesession, no t on l yan in fant's sens i ti v itytolu mi nan c eco n t rast , bu t alsohis/herabil ity todiscriminateseveral chromatic stimuli from gray.
Although36t of thein fa n ts completedtesting with .11four chromat ic st im u l i ina singlesession (and85t withat least one st imulus), therear e a numberof ways toimp r o v e the procedure. First, if we enhance t.he salienc e ofthe st i mul i, theyshouldca p t u r a and holdtheinfant'sattention more easily, an d thus hastenth e procedure. This couldbe acco mp l ish e d by increasing stimu lUSsize ana satur!'ltion, an d / o r by usingpatternedst i mu li such asgrat ingsor checkerboardscomposedof chromatic elements. Ase c o n d improve mentto shortentheprocedurewo u l d be torestrict the rangeof background lumi n a nc e s used inthe chromatic phase. This is just i f i ed by the fact that none of the infant s"f ail e d " the chromat ic-achroma ticdiscriminationsat luminancedifferences gr e a t e r th a n plusor minus 0.22log units. Thus,the entire range of backgroundlu min a n c es couldbe reduced from. 0.8lou. (+or- 0. 4 1.u.) to 0.5 lou . (+ or - 0.25l.u.). A thirdimpro v e me n t wo ul d be to include
"c a t c h " t rialsbothwithin the contrastand thech r o matic phases. Inthiscase,the observer would present the infan t with a cardco n t a ini n gtwo achroma tic patchesthathave the sa me luminanceas theba c kgrou n d. The observer would also
27
beunaware at which pointthese "catch " tri als wouldoccur.
Thismod i fic ati on shou l d impr ove the ob jectivi tyofthe procedur e in two ways. Firs t, it will better insure th at the observer is certain of theloc at i onofate st patch beforemaking a decision. seco nd ly, th e incl usio n of
"blankll trialswil l providethe obs erv e r withong o i ng exampl e s ofthe infant's beha v i o r whenpresentedwithan
IIindiscriminable "pair of stimu li.
piscr imina tio n ofLumi nanc e Cont rast .
The find ingthat infa ntsarebet te r at detecting decrements tha n inc rementsin lumina nceis cons istentwith previous studieso( in f ants ' sensitivitytocont ra st (Pe e ple s andTeller , 197 5 ;'re t re e , Peep les , andseket, 1978 ). For exa mp le, Telleret a1. pre s ente d 2-month-olds with an ac hroma t i c bar embeddedina grayscreenand found that infantswerebe tter at detect ingsmaller differlilDcesin lumina nc e when thebar wa sdi mmertha nwhen it wasbrig ht e r thanthe scree n. Inadditiun, the 2-mon t h-oldsinTeller et aL,'s st Udyshowed ne arl y identica l mean pez-formance fo r luminan c e inc reme nts(10%) and de cre ments (8%) as did the 2- mont h-aIds in the present study (9%for increments, 8% for decremen ts) . ThUS, althoughinfa nts can discr i mi na t e relativelysma l l lumi na nce differe nCe S , the y aremuch less sensitivetha n adu l ts who, under idea l condit ions , ar e able to detect luminan c e dif ferencessmalle r than 0.3% (Ca mpbe ll
28
, Robson , 1968 ). However , like infants , adults ar e better atde tecting luminan c ede cre me ntstha nincr e me nts (R. D.
Hamer ,pers onal communicat ion , May 4, 198 :J). It is not known why humansdi splaythispatternofasymmetry.
However , the fac t that bot hinf ants andadu l tsshow this asymmet r y, impl ies that themecha nismsmed iat ingcontras t detection(e.g .,B/W oppo ne nt ch annels , late r al inh i bition) may be similar ininfan tsandad u l ts, butthatthe in f a nts ' mecha n ismsare much we a ker. Inother words, themechan isms med iatinginfa nts' dete ction ofcontras t may be
qua ntitati vel y, ra t h erthan qualita t i vely , differe nt from thoseof adults'. Fi na lly, thepre se nt findingthat 2- and a-encneh-olds do not differ intheir sensitivity toluminanc e contras t complementspr ev ious findings . These re sults show that, forst i mul i of very low sp a tial frequ e ncy (like thos e use d in thepre s ent study) , cont r astsensi tivi ty doe s no t improve betw e en 2 and3 mo nths (see Atki ns on , Bradd ick, and Hoar , 1977; Banksand Salap atek, 1978).
~tio nofCh roma t i c Patches FromAchrp1Il5lli.!L
~.
Al though 3-month-old s showedevidenceof disc ri minating all four chro maticst i muli fr omthe ach romatic ba c kg r ounds , 2-mon t h -ol dsshowed adi ffer ent patt e r nof ee surus. Fi rs t, wefound that a-acne n - otds , like 3-mont h - o l ds, appear to dis crimi nate 660 nm red and475 nm blue fromgray. Th i s
29
resultcomplementsearlier reportstha t a-aceen -erdscan discriminatea 633 nmre d and a48 6 nm bluefrom gray (e.g., Peeples, Telle r , " Sekel, 1978 ) . Studiesof even younger infants (Adams et a1., 1986 , experiment 2;Ma ur e r &Adams, 198 7a , experiment 2; Ad ams , 198 9 ) have found that newborns are able to discriminate(;30,640, and 650 nm red from gray and, after 1 month, candiscriminate475 nm blue from gray.
Because a-acntn-eree are ableto discriminate some chromatic stimulifromachromatic stimulionth e basis of wavelengt h, they must possess at least dichromaticcolor vision (I.e., possess two functioning receptortype s with different spectral sensitivities) . Assuming these receptors are similarto the adul t re c e pt o r s [i. e,, rods, or one of the three cone types- short-wavelength-sensitive (SWS), mid- wavelength-sensi tive (MWS), or long-wa v ele ngth-s e ns i t ive
(LWS) cones],it is likely that 2-month-oldspossess either a rod/c o ne receptor combinationor a cone/cone combination. The lattercombination is more likely because both human psychophysical evidence and infrahuman physiOlogical evidence (see Hurvich , 1981) ha ve yet to show that rods and conescombine the irinputat post-receptoral levels (I.e., withinanoppone nt cha nnel) .
The findingtha t many 2-month-olds fa il to discriminate a 580 nm yellowand a 520 nmgreen patC hfrom an achromatic back'.Jround is alsoconsistentwith Teller et at.'s (1978) re s ults tha t 2-mon th-oldsmay haveaneutral zone (aba nd of
30
wavelengthsindiscriminable from white) in the greenand yellow spectral regions. In addition, Tel leret; aj, .rs SUb j ec t s , like our's , failedto discriminategreen and yellowfroman achromatic background at relativeluminances nea r the adultbrightnessmatch(a ll differences<0. 25 lou.). Yo ung e r infantsalsoappear to reveal a neutralzo ne intheye l low-g r e e n region. Adams(sUbmi t ted ) usedthe ha bit ua t i o n procedureto testne wb or ns ' abilityto discriminate16deq, 565and 572 nm yellow-greensquares from Whi teand found thatnewborns did notshow ev i d e nc e of makingeitherdiscrimination.
There are threepos sibletyp e sofex p lana tio nto accountfor the failureof 2-month-oldin fi! nts to discriminatebroadband520 nmgreen and580 nm ye ll o wfrom gray:thosewhich are basedon 1) receptoral immat ur i t i es, 2) post-rec eptoral /neu ral immaturitie s ,orJ) mo tiva t i on a l fa ctor s. Themostobvious receptoral explanationis that young infants lack one of thethree canetypes,and thus havedichromatic col or vision. Most clas sical ad u l t dichromatspossessat leastone neutral zone in the sp e c t r um. For protanope s (thosepresu med to be lackingthe LWScone s), this zone isa small bandof wavelengths cen t e r e d at about 496 nm; for deuterano pes(t hoselacking the MWS cones), at about 496 nm; and for tritanop es (those la Cki ng the ssecones),at about 575nm(Hurvich,1981 ) . In the pre sent st ud y, 2-mo nt h- o l ds fail to discriminate a
31
chromaticstimulus (580 nm yellow) thatisveryclos e to the predicted tritanopic neutralpoint and a se c o nd (520 nm green) at a spectral locationnot predicted by any of the cl ass ical dichromacies. Thus, tromthe"mis sing cone"
perspectiv e,thebes t explanationfor the s ece eu r.c sand tho seof Teller eta1. (1978) is that a-ncnen-otds ma.:lI·be tritanope-like dichromats,possibly witha broa derneutral zone. Moreover,in a previous attemptto isolate SWS cones in Inrenee, Pulos, Teller,and Buck (1 980) used chromatic adaptati on, a techn ique commo n l y usedtoisol a t eSWSco nes inadults. This procedurere vealed tha t 3-mon th-olds , bu t not 2-month-olds, appear to pos sessfunction al SWS co n es . Howe v e r , in a morerecent 'it u d y ,Va r ne ret aI., (1985) found that2- mon t h-o l d s are capableof making tritan
discriminations,a result sug g e s t i n g that2-mo n t h - olds pos ses s fu nct i ona l sws cones.3 In addition, Volbrechtand Werner (1986) report electrophys i o log icalevide n c e suggesting the presence of SWS cones inl- month- ol d in fa nts. However,dueto great methodological differencesbetween the studiesofVa r n eret al.,Pulos et al. ,Volbrechtand Werner, and the present study, Whether ornot 2-month-olds possess tu n c t i o na l SWS mechanismsand whether thisis an explanation asto why 2-month-olds fail to dis criminate broadbandgreenand yellowfrom white isstill unc e rta in.
Another possible explan a t i on based on receptoral fa ctorsis that 2-month-oldspossess all three type ,,; of-sc n e
but that at least one of these receptors is sparse or poorly developed. The rationale for this explanation stems from anatomical and behavioral studies of other subject populations. For example, the density of peripheral cones in adults decreases with increasing eccentricity (Curcio.
Sloan, Packer, Hendrickson, and Kalina, 1987). As a result, adults using their peripheral vision require large targets to successfully discriminate chromatic from achromatic stimUli, especially when those targets are located at greater eccentricities (Gordon and Abramov, 1977).
Similarly, cats have a relativelylow retinal cone density (Steinberg, Reid, and Lacy, 1973) and also require large stimuli to make chromatic discriminations (Loop, Bruce, and petuchowski, 1979). Recent evidence shows that the density of human retinal cones is not adultlike until at least 6 months after birth (Abramov, Gordon,Hendrickson, Hainline, Dobson and LaBossiere, 1982; Yuodelis and Hendrickson, 1986). Therefore, one would expect that 2-month-olds, like newborns (Adams, Maurer, and cashin, sUbmitted), may also require large stimuli to make successful chromatic- achromatic discriminations, at least for chrenae tc stimuli in some spectral regions (e.g ., the yellow and green regions). Future studies designed to examine 2-month-olds1
discriminations of achromatic :from chromatic stimuli of varying size (e.g., with yellow and green patches larger than those used in the present study) may help to evaluate
33
the explanation that 2-month-olds' colorvision limitations areba s e d onsparee or immature cones.
Additional support for the weak cone explanation is provided by examining re sul t s from studies of adult saturationdiscrimi natio n (for a review, see Heia and Graham, 1966). For adults , the yellow-greenregion appears the le ast saturated (i.e ., most like white) (Boynton, 1979). If 2-month-oldshave weak or poorly developed cones, the entirespectrum wil l appear desaturated (Adams and Courage, sUbmitted), especiallythe yellow-greenregion. Therefore, the yel lowand greenpatches used in the present study may have appeared very desaturatedto the infants,making the patches less discriminablefrom the gray background. To furthe r evaluatethisexplanation, future studies are needed which examine2- mont h-old s' abilityto discriminate chromatic patchesofva ry i ng saturation from an achromatic background.
An alternative explanation to account for infa nts ' apparentdiscrimination failures isone based on a post - receptoral (or neural) li mit a t i on . It isnow well establishedthat, following initial processing by the photoreceptors,chromatic and lumi na nc e information is processed withinthe opponent cha nnel s (Hurvich, 1981). Hurvich argues that adUl tspossess three opponen t channels:
a lumi n an c e (or B/W) channel composed of the weighted sum of LWS and MWS outputs ,a R/G channelcomposed of the weighted
34
difference of LWS and MWS cone outputs, and a DI Ychannel composedof the wei ghteddifferencebetweenSWSco ne s and thesum ofth e LWS andMWSconeoutputs. Infantsmay have functional cones buttheirapparentlyweak colorvision ma y be causedby ei t he r adysfu nt i ona l or absent oppone nt channel, most li ke l y theB/Y channel . Howeve r ,even th i s explanationis unlike l y : theneutral zo nespredic ted for an ad u l t with a dysfunct ional B/Ychannel wou l d fall around 475 nm and 575 nm(pork o r ny , Smith, verriest,, Pinc ke rs , 1979). This prediction is made be c au se, at thesewa v elengths, the RIGcha nne lsh ows little, or no, activity (seeFi gure 5)•
InsertFigure 5 abouthere
In the presentst udy . 2-mo nt h-o l ds displa y onl yone of the s e neutral zones (575 nra], Thus, an explanation bas ed ona dysfunctional B/ Ychannel isinsufficientto fu lly ac count for their failuretodiscrimin ate 520nmgree nand 580nm ye l l owfr o m gray.
Anothe r post-re ceptoralexplanat ionisone bas edon immaturereceptivefields. Hamer et al. (19 8 2)sug g e s t tha t in fa nt color vis ionis ana taecu eto adult peripheralcol o r vision. Adultsmay require increasinglylarg etargets wit h greater eccentricitiesto succes sfullydis cr i min ate chromati c from achr o ma t i c stimul i, not only becaus e of a receptorallimitation due a paucityofcones, butalso
35
becauseof a ne ur al convergence problem due t) large receptive fields. Similarly, young in f a nt s may fail to discriminate certain chromatic stimul i (e.g.,greenand yellow) from achromatic stimulibecause of large receptive fields at higher levelsof the infants' visu al syst em.
Large sum..·"ion areaswould cau se small st i mul i to be fused withthe i r sur rounds , resulting in a degradation of the wavelengthinformationreceivedfromthe receptors.
However, further stud i es are needed to determine whether the large stimulus requirementis due to limitati onsat either (or both)the receptoral and/o r neural level(s)•
A finalneurally ba s e d explan ationis that2-month- olds' colorvis i on re s embl e sthatofanadu l t who has an acquired color deficiency. For example,one pos sibilitymay be a Type III B-'{ deficiency (Pokorny et a1.,1979). These patientspossess a broad neutralzone that includes the yellow-green re gion. This deficiency ismost prevalent amongst elderly people, usually because of deqene-..dtionof an important visual st r uc t ur e, suc h asthe optic nerve, and/orthevi s ua l cortex. withinfants,however, the color deficiencymay be due to an immatur e structure (see Banks and Salapatek , 1983) rather than a degeneratedor damaged st r uc t u r e. For example,anatomical st udies have shown that the development of the human visual cortex isnot complete untilabout 6 months of age (ccnet, 1939-1963). Recent evidenceshows that , at leastinhi g he r primates, seve r a l
36
layers of the primary visual cortex (e.g•• la ye r s 4C-beta, 2, and 3) areessential in the processing of chromatic inf o rma t i on (Livingston and Hubel, 1988 ) .
Finally, a third type of explanationis oneof general
"mot i va tio n a l" factors. Young infantsmay be able to discriminate yellow and green from gray, but these chromatic stimulimay be no more interestingor morepreferred tha n the achromatic background. Forexample, Bornstein (1975) testedboth 4-month-old infants 'and adults' color preferences and found that wavelengths in the green to green-yellow region were relativelynon-preferredfor both infantsand adults(cf. Adams, 1987b). If thisistrue for 2-month-old inf a n t s , theirmotivationto look at the green or yellow patch may be no greatertha n thatfor the gray background.
unf or t u natel y , the present stUdy does not allow us to determine definitivelywhic!:l explantion(s)is(are) the most accurateinexplaining why 2-month-oldinfants succeed in discriminating660 nmre d and 475mil.blue from gray but fail to discriminate 580 nm yellow and 520 nm green fromgray.
However,on balance,the evidence doesnot point to the absence of a specificcolor visionmechanism. This is because 2-month -oldsshow atLe ast;a rUdimentaryabilityto perform successful luminance, Rayleigh, and tritan diacriminationsand do not show neutral points in all the spectral lo c atio n s predicted for subjectswho lack an
37
opponentcha nnel, This implies that 2-month-oldspossess three functional co n e types and threeneuralpathways capableof pre s erving rece pt o r a l information3(see Varne r et aL, 1985). Rather, it ismorelikely that the s emechanis ms arepresent butar everywe akand/ or immature.
Theoretically, anadultwit h weakcolor vision me chanism s WOUl d , like young infants, beab le tode t ect wa veleng t h inf o rma t i o n only with relativelylarge stimu l us sizes (Packe r et aL, 1984), be ab le to ma ke only verybro a d chr oma t ic discriminati on s (creveae eccner, Brown, Ankrum, and Tell e r , 1988),and show aselective loss of chroaatIo- ach r o mat i c discrimi n a tio ninthe mid -spectr a l region (present sttl'-'Y).
Insumma ry , the Color/Contra stCardsappe arto hold much pro mi se as a proc edure fo r te sting an indi v idual inf ant' s ab i lity to discriminate amo ngstimuli differing in cont r a s t and/or chroma ticcharacteristi c s. The card shav e al readypr ov e n to be an impro vement overot he r experimental p:.:ocedur e s becauseof their quic kness , portabil ity, and potentialfor individualas ses sme nt. Furthe r mod ificatio ns of the ca r ds to assesschromatic-achroma ticdisc rimina tion s . aswell as sa t ura tionandch roma tic dis c riminations (e.g., Ray l e igh and tritan dis crimin ations) ehcu kd imp ro ve boththe cl i n i c a l and experimentalus e .ulnessof theColor/Contrast cere-s, For examp le, the proc eduremay prov etobe a va l u a ble method for screeni n g infant s and non-ve r bal
3B
handicapped children for congenital color vision defects, cone deficits, or central nervous system problems.
Moreover, any additional modifications of the procedure sho u l d helpus in the continuingeffort to pinpoint the nature of the mechanisms underlying the early development of human colorvi s i on.
39
Table 1
Mun s e ll val ue s and calculatedlumina ncesfor bothachromatic and chro ma ti c Munsellpap e r s
Papertype
Achroma tic
'::h romati c
Muns el l value
4.5
5.5
9.5
R4 B5 G 6 Y B
40
Luminance (logcd/m2)
1.0 9 1.16 1.2 8 1. 33 1. 36 1.38 1.43 1. 61l 1. 68 1.85 1.86
1.33 1.37 1.49 1. 67
40;.:n:::um.:.:b::a:::r_O:::f~l::.n:.:fan=t:.s
+- _
20
o '---_-L_ _ '-_+_----'_ _---'-_---'
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Lumina nce difference (log cd/ m2)
- 2- mon th-olds-+-
3-monlh-olds Figure 1. Cumulative Frequency Dis t ribution of 2-and 3-month- o lds ' Detec t i onof Lumina n ce Di ffe rences .41
,., 01Inl ants whofall
60~':"":::':'=:'::-~"':-_---,
60
475 520 580 660
dominant wavelength (nm)
_ z-month-otds . 3- mon t h-old s Figure2. Percentage of infantswhofail todiscrimin ate the chromaticpatc hes froma qray background.42
$ of 2-mo-olds who fall
40;:...::.:....:::--=---..,
30 .... ..
20
10
o
L---I~--'---L..--'--l_-'__l'--'-0.4 -0.3 -0.2 -0 .1 0 0.1 0.2 0.3 Log relative lumin ance(cd/m2)
620nm green -l-660 nm red -1<-660nm yellow Figure J. Dist r i butionof relati ve lumi n a nces at wbich 2-month - o lds fa i l to. dis crimina t e chroma ticvatches fr om a gray background.
43
'l> of Infants whosucceed
120;:....:~~:.::....:.c:..:.::.-'-'---...,
100
~.
e o _ .
eo _ .
40 .
20 _ _ _ _ _ _ .
o
L---'- ..L.._ _--'- - ' - - - - J476 620 sao eeo
dominant wavelength (nm)
~ 8-monlh-olds
-+-
2'month'0Ids Figu re 4. Percentageof2~and3-mont h-ol d swhodiscrim- inat e thechromat ic patches f~omagray background.44
Relativeresponse
0.6;..:.:.:::..:..::..-:c:.::..:...---,
0,4
0.2
0.0
~---=:"<-I---__+---'-'''''+_i-0.2
-0.4 .
-0 .6
450 600 660 600 Wa velength (nm)
650 700
red/ green -f- blue/ yell"", Figure S. Yellow/ Bl ue and Red/Green response func ti ons across the visible spectrum .
45
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