1 ELSEVIER Ophthal. Physil. Opt. Vl. 19, N. 4, pp , The Cllege f Optmetrists. Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain /99 $ PII: SO (98) Bth clured verlays and clured lenses can imprve reading fluency, but their ptimal chrmaticities differ Anita Lightstne,a Tamsin Lightstneb and Arnld Wilkinsc %stitute f Optmetry, Lndn, UK, 4 Fairlight Curt, Tnbridge, Kent, UK and Visual Perceptin Unit, Department f Psychlgy, University f Essex, Wivenhe, Clchester, Essex, UK Summary Sme individuals read mre fluently when the text is clured: i.e., when clured sheets f plastic (verlays) are placed upn the page, r when clured lenses are wrn. Overlays prvide a surface clur whereas lenses mimic a change in the clur f a light surce. The neural mechanisms that underlie clur cnstancy ensure that the chrmaticity f verlays and lenses is prcessed differently by the visual system. We investigated (1) the relatinship between the ptimal clurs f verlays and lenses, and (2) hw reading rate is affected by a particular clur in verlays and lenses. In 100 patients we nted (1) the verlay(s) chsen frm amng the 29 cmbinatins f the 10 IO0 Intuitive which sample chrmaticity systematically and (2) the chrmaticity c-rdinates f the lenses subsequently chsen using the Intuitive a device prviding a light surce that can be adjusted in hue, saturatin and luminance independently. The relatinship between the chrmaticities f the verlays and the lenses shwed cnsiderable variatin. In a secnd study, patients attending the Specific Learning Difficulties clinic at the Institute f Optmetry, Lndn, were given verlays t use fr tw mnths. Seventeen wh derived benefit were examined using the Intuitive Clrimeter. Patients were asked t read alud randmly rdered cmmn wrds (Wilkins Rate f Reading Test): (1) with n clur, (2) with the chsen verlay, (3) with lenses matching the chsen verlay and (4) with lenses matching the Clrimeter setting. The aids increased reading rate significantly nly in cnditins (2) and (4). There was n significant imprvement when lenses matching the verlay clur were used, and under this cnditin the reading rate was significantly prer than in cnditins (2) and (4). The clur f a lens will imprve reading nly if it is selected under cnditins that mimic a change in the clur f a light surce: clured verlays give n clinically reliable guide t ptimal lens clur The Cllege f Optmetrists. Published by Elsevier Science Ltd. All rights reserved There has been cnsiderable cntrversy cncerning the use f clured verlays and clured lenses in the treatment f reading difficulty (see Evans and Drasd, 1991, fr review). Many early studies did nt appear in peer-reviewed jurnals and have been difficult t interpret due t design prblems such as selectin bias, sample size, hetergeneity f subjects, subjectivity f results, financial interests f investigatrs, and failure Crrespndence and reprint requests t Dr A. Wilkins Received: 22 December 1997 Revised frm: 23 Nvember 1998 t cnsider such factrs as placeb effects, cntrls and the phthalmic status f subjects (Menacker et cd., 1993). The study by Menacker frm which the pre- vius qutatin is taken was well cntrlled, but cm- pared the effects f nly a few clured lenses. Wilkins et az. (1994) presented evidence suggesting that the effects f clur are idisyncratic, and that, fr pti- mal effect, the clur had t be selected individually, and with precisin, each individual apparently requir- ing a different clur. This finding was supprted by a duble-masked placeb-cntrlled crss-ver trial which examined children wh had fund benefit frm 279
2 2 O P8 Op 1 h10 N 4 ph 9 y9 t t 9 s:. h 9 i a c ( ev a 1lW A rt e l 9i t eer w. 9cul h aydl, e t 4 rk e deua r h t) leip i h n a f s e c ne y a i Cf d y m d s d i e p a e es ln c- t cni thnm v w f w s a l i t p s w u e t h e en spn h a ca r r d ats v t tl e tie a at c b n w t ut d il hi C ht 1t i t n T I as 9 f h su sche t e 7 f t trr e e rc 6ehsuee U c Cb a hs a S y ( rma m j awv h b en uml he a cau e c sal r fdn t e t v 0 N d. b it 0 e a sfh p 6 tmn ufe mt ) wad d eb sr ui. tenn fi r- mnrc hedt t l w ei e i t e s mv w n n rh r t ia dh s ei ( e uw dl n iece as 1 ( diw e t tn s c d ) 2 yti n hi l s w a k u e t s N i sa r h s e l ( i n e w e el f a l m n3 a i t es c l s a g ) l t hins che t vy R ( H e i1h s ap t 9pa i w dr a 6( s wn eli n 4 6 s a ci venvtd fm) s te etngeh t a r a b e t es t clf h fs ac f i e s fe i la e es n l b t cs r u a n d e t s T Ri a Re i h fa d f se n e ft i t a g e n. d r g T ( e ae 1W m t l st 9 si e w. t h 9 lp a i, e 6 ke s t ) ie u h n w i ch r n a a ati ec d l en c as i I f xh d tn v u t us i d di r ac wr pn i a ds r dm M n erm i m e rd a n m t es l t ln h c s t l Cp y w c a p is mc e te n ep f h l v da a y e r m t e t i ad hta se bs tie t e dex up efa wi e t a t a rf s t hn, S t t i y L h g pt r s Te i nn ce ha cs c ae a ed ude n s Cr, e i a ttd. I e l i f t heo n i nf a r p de e g r ist e r i i fu br c vf As fl er1 e ae t ue Dit t9 p1ne g wec, wr9 e w t r r f ihu h r e T rs i n e a sh a c ai f a r d ae nh n w l s e m gar at pf fu r p r s a e t l p pc a h iiel r( d e 1ts epa pi wi 2we d arp sut hn t 8 er gtlr sl wg i ra v b a f a m y t a f f ra t ht c in rsh ae t Tu ped e ee wn s hr nas ul 1ht r a) Ie t se e r i h s Te t ss i it hl e r h g m ie O s e ( h p a v1t a ww. er b e9 dt se i wx i r9 f w t lt i ah h ue cs pe ae xe u pn t n nt lss r w d v cweu a e e y eudp t d i h e i f rm e f g n we pa l br i da r u e at d m ves e c hr aiu v a ean a c p w v t r l hwe s h ev n e ir u e ddue ewt l w b ai trr sc ehla i t- rdcel ie s b u ev e Ts iq i s he nul a n pad e rceu na e en er ln an f ved fdeutt l y d u t ce ap t h p cn hde p le Ad n p e i ttn p ls a a ftf ihcud r ( r t -h mealrs L f f i e r t na n r ( e n ad s W Em r 1 l i e i vp t9r. v a l a aaf h9e, i c k ntu 1 J e a 91 e t l 9 a. 49 e n, a ; 7 wx eg r n ) a s i e dc. smr T c a h a f t n c ve l na l h er f d e e a lre u a f n n Tcl c rpn s yw uhua t w i f t e n a r l f th br yp r i d e n e e rs e nhk si t at e yf cipf sdl m- d r l e f b t v e s y Wh i ar y ie hu s n e vst fs es u na meti n e a6 wt rei l fd lt el lnm t e a a h ty w r ld ae i et h e i a s s ms y i wgpn a epwt echtd s nie i rate s v tc t p h bv rs e cue n a u tr p e m vr d l l r e if e( a el nada 1 sj uy m d ca i s nw f cu ly ht f r e h e wlff s uvi n r r e afme nre e s e l a w t e e ra a h n ya e r tar t a e s ed e 8 wys n l e s aaf i ep, t a s pn w f eihs vbt t d c l s T i n ml h g e t u e u e hwu h ar uc sw t etr a Isc rh eci hra t nse aea dt u c c n e w d s nl t bhn e ua a e tis r rts bul it al frs lret t neacri ai s a w d e ts ch e et hl i ( sf n h e lt L pp e mn e ( a i r 1 ms umwn t t 9 e 1 L a 9M a1 nz 5c 1 n9 I d ie 9C 9 pd 7 t s kwa a 85p a 1 t i e, nl 3Or t ; i, rb ne eah e t t nv mh h eii ea i e t udn ct n h rev h e an a lt l p c r i t tf c n ha w ml c be an e y ut s v re s d m i a f r f R k e e e r s d u I t f t n sh i w f i w t e rt e rn u T sh c e v d hc t e h - e i te l r s a fe h l b at c e t th t f h ie p pl vwi e F t li T een d i nf i p. u h r ewa g hsm atr s b m e c y au tl h nl he r f p d t c ei tt ivs st c c f hih pae be e l s u e t ei s Cn h ln T i s e g et ah n tel ar cu c ne g hs v fti e d et, r eu C i a s s d i lt pi e m h g tl a v s p ra rtil r iwh l lit cur e ci etim ugeam a p t w ca f el ii sg a wxi t tn ule v at g h h T ca a y hc a h h vu ra b t t e 4 er i r t h a s h u tn la c e b e f v hd iit a e 1a e gnu n p5 r pb hdr ( 9 e i as taet m l. r e p w ea a i c n is l t h d n se u hf a - e tx + m1 sr en 1 ns ct i tdu ai g ti ie f O s w na eu s i lrn vr p w t y e Aef c ebe h s f t s n a a ce r ec l h c vn tl e iete e e v w t ic i sh tsa c t te hi l a h f h pe b lpt i U e da l r f phsn C i rt ei ureu l s h a ip e a s gh tu t f d ia hy r ha f f mtt s cn c ave ih it h e l teci b lfe ch r e c l T a e w h ul s n w a e ns s s ths t due ti. i h T erf sdhm v li ac h resi ei
3 Clured verlays and lenses can imprve reading fluency: A. Lightstne et al n3 0.4 Figure 1. Average chrmaticitv f lenses assciated with each f the 10 verlays. Lines extend frm -1 t +I EG.:. either side G the mean. u between each lens and its assciated verlay was estimated by calculating the CIE clur difference, delta E&,. T simplify the analysis, an equal energy illuminant with luminance 100 cd mp2 was assumed (althugh similar results were btained with ther luminances). The average value f delta,?$, verall was In each f 1000 Mnte Carl trials the lenses were randmly reassigned t verlays and the average delta _$& re-calculated. The mean f the averages btained in the Mnte Carl trials was 61.0 with a standard deviatin f 1.1, The value f delta l&, btained when the lenses were apprpriately assigned t verlays was therefre significantly lwer, indicating that n average the similarity between the clur f an verlay and the clur f a lens was greater than might have been expected by chance. The mst impressive feature f Figure 1, hwever, is nt the similarity between the clur f an verlay and that f the lens, but the variability between the tw. The difference in chrmaticity between verlays and lenses was 55/61 = 90 h f the difference btained with randm allcatin. The average hue angle (/zuv) f the verlays (Chrnicle and Nimm-Smith, 1992) was 56.3 degrees whereas that f the lenses was 9.3. The difference was significant (z = 1.83, p = 0.03) indicating that there was a weak tendency fr lenses t have mre reddish shades. The average suv f the verlays was 0.57 (sd. 0.06) and that f the lenses 0.94 (s.d. 0.47). The difference was very highly significant by a l-test (JJ < 0.000) indicating that n average verlays were less saturated than lenses. The verlays and lenses differed nt nly in hue and saturatin, but als, f curse, in their phtpic density (the amunt f light they absrbed weighted fr the spectral sensitivity f the eye). The lenses were selected using the Intuitive Clrimeter which allws fr the selectin f a hue and saturatin at cnstant luminance, but the clur was then matched by lenses that varied in density with hue, sme pigments such as blue being darker than thers such as yellw. The average phtpic reflectance f the verlays was 67Oh (s.d. 15OA) and the average transmissin f the lenses was 40 h (s.d. 20 h). The Spearman rank crrelatin between the density f the lenses and the density f the assciated verlay was nly Given that sme pigments are darker than thers, the weak crrelatin in density is again cnsistent with the weak assciatin in clur between lenses and verlays.
4 282 Ophthal. Physil. Opt : N 4 Discussin Althugh statistically significant, the relatinship between the clur f an verlay and that f the assciated lenses was weak, and the variability between verlay and lens clur was large. Althugh the assessment f ptimal clur is knwn t be f mderate reliability fr bth verlays (Jeanes et ul., 1997) and Clrimetry (Wilkins, 1997), the questin arises as t whether the weakness f the relatinship is due simply t the errrs assciated with subjective measurement. Jeanes et d. (1997) shwed that in children wh persisted in the use f an verlay, the verlay increased their rate f reading bth befre and after experience f its use. The rate was assessed using a simple Rate f Reading Test in which the children were required t read alud randmly rdered cmmn wrds (Wilkins et al., 1996). The Rate f Reading Test was used in Study II t assess the respnse t clured verlays and lenses. The test-retest reliability f the test is high when test and retest are mnths apart (Wilkins et al., 1996). When test and retest are in immediate successin, the reliability is even higher, as is evident in a mre recent study. In this recent study (in preparatin) 283 children aged 7-8 read with an verlay, then withut, again withut and then again with the verlay. The average rate f reading n the tw trials withut the verlay was 81 wrds per minute, and the difference in reading rate frm the first t the secnd assessment withut verlay (i.e., n tw immediately successive trials, taking accunt f sign) averaged 0.20 (standard deviatin 14.6 wpm, 18%). The Spearman rank crrelatin between the children s perfrmance n these tw trials was The crrelatin fr the tw trials with the verlay was 0.93, indicating that the effects f practice did nt influence the crrelatin. The Rate f Reading Test was used t cmpare reading fluency using lenses that matched the Clrimeter setting with the fluency using lenses that matched the verlay clur. Study II Methds The participants were all patients at the Institute f Optmetry wh were being investigated accrding t the clinical prtcl described briefly abve, and in mre detail by Lightstne and Evans (1995). They tk part immediately fllwing assessment with the Intuitive Clrimeter and befre the patient had had the pprtunity t see the trial lenses. (The Clrimeter assessment invlved judging the appearance f randm letters arranged t resemble text, and the text f the Rate f Reading text was nt viewed in the Clrimeter.) Ethical cmmittee apprval fr the study was btained. All patients were given the pprtunity t pt ut, but nne did s. The ptmetric characteristics f the patients are shwn in Table 1. The criteria fr nrmal bincular visin were (1) near pint f cnvergence f 10 cm r better; (2) assciated heterphria f up t 1 prism diptre hrizntally and stable; (If the assciated heterphria was 1 prism diptre, this was nly regarded as acceptable if the dissciated heterphria was lw, the recvery n cver test was rapid and the fusinal reserves were gd); (3) fusinal reserves met bth Sheard s and Percival s criteria. Fr mst subjects, the amplitude f accmmdatin matched the age-related nrms and where this was nt the case, the accmmdative lag and/r the accmmdative facility was in the nrmal range. All measurements were subjective, except fr ne subject where readings were judged by bservatin. A cmbinatin f trial lenses matching the clrimeter setting was selected by the examiner (AL) using established techniques (Wilkins, 1993): the match was made by cmparing the appearance f tw apertures, ne revealing a surface in the Clrimeter illuminated with light f the chsen clur, and the ther revealing a surface illuminated with flurescent light, crrelated clur temperature 3500 K. The secnd aperture was cvered by the apprpriate cmbinatin f clured trial lenses. The matching was undertaken by AL, a female with nrmal clur visin and gd discriminatin (2 errrs n the Farnswrth Munsell 100 hue test). The lighting available in the examinatin cubicle was flurescent, crrelated clur temperature 3500 K, with a cntributin frm lcal halgen filament lamps, giving a cmpsite illuminatin with UCS chrmaticity crdinates f u = 0.27, v = 0.52 (measured with a Minlta Clur Analyser). Under this illuminatin the lenses prvided a chrmaticity similar t that selected in the Clrimeter (CIE delta Ez+,= 11.6 (s.d. 3.54)). The luminance f the white paper n which the Rate f Reading passages were presented was abut 70 cd -2 m. A secnd cmbinatin f trial lenses was prepared matching the CIE UCS 1976 chrmaticity f the verlay that the participant had been using. The match was such as t allw fr the increase in saturatin btained when the verlay was in cntact with the page f text, and s the lenses had a saturatin apprximately twice that f the verlay. The chrmaticity match was btained by direct measurement using the Mnlite tele-spectrradimeter and the match f the clur appearance cnfirmed by eye. The clur difference was small: CIE Delta fa,,= (s.d. = 5.86) and was due mainly t differences in L* (i.e., transmissin/reflectance). Nevertheless, the luminance f the text btained with the verlay and with the
5 Clured verlays and lenses can imprve reading fluency: A. Lightstne et al. 283 lenses was similar: the Spearman rank crrelatin between the phtpic reflectance f the verlays and the phtpic transmissin f the lenses that matched them in chrmaticity was The participants read the entire passage frm the Rate f Reading test. The passage was presented n white A5 paper (landscape rientatin) and the paper lay n the grey surface (reflectance abut 50%) f a small desktp (0.36 m wide and 0.30 m deep). The time taken t read the passage was recrded, and errrs and missins nted. The average number f wrds crrectly read per minute was calculated (Wilkins et d., 1996). The test was administered under the fllwing cnditins: (1) with n clur, (2) with the chsen verlay, (3) with lenses matching the chsen verlay and (4) with lenses matching the Clrimeter setting. The cnditins were presented in randm rder and participants were nt aware f the purpse f the study r the cmparisns being made. Althugh the examiner was aware f the experimental cnditins, ther studies using this test have shwn reading rate t be little affected by explicit mtivatin frm the examiner (in preparatin). The clur difference between the lenses matching the Clrimeter setting and thse matching the verlay was delta ELI,= 60.8 (s.d. = 27.7). The frmer had a mean phtpic transmissin f 23.8Oh and the latter 54.9%, with a mdest crrelatin between the tw transmissins (Spearman rank crrelatin, I = 0.58). Results The number f wrds per minute read under each f the fur cnditins is shwn in Tuble 2. An analysis f variance with repeated measures revealed a significant main effect f grups (F(3,54) = 5.4, p = 0.003) which accunted fr 23Oh f the within-subject variance. Pst-hc pairwise cmparisns using the Peritz prcedure with an experiment-wise alpha rate f 0.05 (Tthaker, 1991) revealed significant cmparisns between the nil cnditin and the chsen verlay and als the nil cnditin and the lens matching the Clrimeter setting. Nne f the ther cmparisns was significant. The planned cmparisn between the nil cnditin and the verlay match failed t reach significance (p = 0.1, l-tail t-test), indicating that lenses matching the verlay did nt significantly imprve reading speed. The planned cmparisn between the lens matching the verlay and the lens matching the Clrimeter setting was significant (p = 0.03, l-tail t- test) indicating that the lens matching the Clrimeter setting resulted in significantly faster reading than the lens matching the verlay. There was n cnsistent crrelatin between reading speed and lens transmissin. Fr lenses matching the
6 284 Ophthal. Physil. Opt : N 4 T 2 Mean rate a f. reading (wrds b crrectly read l per factrs e cannt easily explain the imprtance f viewing minute) and standard deviatin (shwn in parentheses) fr cnditins. The individual differences in ptimal clpetfrmance by the 17 subjects in the fur cnditins f Experiment 2: namely with n clur, with the chsen ver- ur remain difficult t explain, particularly since these lay, with lenses matching the chsen verlay and with differences appear t depend n viewing cnditins, lenses matching the Clrimeter setting but nt in any easily interpretable way: the verlay clur des nt appear t be strngly related t the Nil Chsen Lens Lens lens clur. Perhaps sme f the individual differences verlay matching matching chsen Calrimeter in beneficial clur reflect differences in susceptibility verlay setting t perceptual distrtin acrss the visual field. Obviusly, were the size f an verlay t be increased Mean rate f reading prgressively there wuld cme a pint at which the (s.d.) (28.5) (27.8) (23.8) (29.3) entire field was clured and the effect f the verlay was equivalent t that f lenses. When relatively small Clrimeter setting Spearman rh was 0.29 and fr lenses matching the verlay rh was -0.14). The difference in reading speed in the tw cnditins was nt crrelated t the difference in transmissin (rh = 0.01). General Discussin The findings f Study II cnfirm thse f Wilkins et a (1996) and Jeanes l e a (1997). in t demnstrating Z an. imprvement in reading speed with the apprpriate clur, but indicate that this apprpriate clur depends critically upn the viewing cnditins under which it is selected. The increase in reading speed with the lens matching the calrimeter setting cannt easily be attributed t a placeb effect because the participants were unaware f the purpse f the study and, in particular, they were nt infrmed that ne lens matched their verlay and ne matched the Calrimeter setting. The weakness f the relatinship between the clur chsen fr use as an verlay and that chsen fr a lens cannt nw be attributed simply t the variability inherent in the techniques fr subjective measurement. This is because f the findings f the secnd study which shwed imprvements in reading fluency cntingent upn clur, but nly when the clur was selected apprpriately fr the viewing cnditins in which it was used. The average difference between the clur f lens matching the Clrimeter and the clur f the lens matching the verlay was small. Nevertheless it was nly the frmer lens that resulted in a significant increase in reading fluency. This finding cnfirms the findings f the duble-blind study (Wilkins e a 1994) in shwing t Z just hw precise. must, be the selectin f clur fr ptimal benefit. The present findings have imprtant implicatins fr theries that seek t explain the effects f clur n reading. The findings make it mre difficult t attribute the beneficial effects f clur t peripheral cular factrs such as variatins in accmmdatin: these verlays are used, the part f the visual field that is clured may r may nt include all thse parts that are susceptible t perceptual distrtin. The relatinship between the clur chsen fr use in verlays and in lenses may perhaps reflect individual differences with respect t which parts f the visual field are susceptible t distrtin, and the extent t which these are clured by the verlay. Such a pssibility will shrtly be explred. One imprtant practical cnclusin frm the present studies is that the clur f the verlay may give nly misleading guidance as t the clur ptimal fr use in lenses. Practitiners shuld be warned that it is nt acceptable practice t ffer lenses tinted t match the clur f a selected verlay. Lenses tinted in this way may ffer relatively little benefit. Acknwledgements The authrs thank the patients wh tk part in the study and they are als grateful fr the helpful cmments frm tw annymus referees. References Chrnicle, E. and Nimm-Smith, I. (1992) Applicatin f sme statistical methds fr cmparing hue-angle data. Clur R a A e 1n 3 p s 7d 7 p e Evans, B. J. W. and Drasd, N. (1991) Tinted lenses and related therapies fr learning disabilities: a review: O a Pp no hh 1 2 d p yt 1 0 J R M e J L. ae Sa. e, N r. P tn, w. t, D a W A. J n ( i P.. d 1u l rc 9s k f f c v r l B e Je r a r r a sf P 8 5 s 8 3 y, 1 c L E H ( a C.. v1 na t n i9 idn lh a s5 P I P a. t rn ra f h a S t c e c t f c U 4 1 S 5 1 A, 5 - L E H ( a C.. v1 na t n i9 di n lh a s5 P I P a I t rn r A f. h a S t c e c t f U 4 6 S 5 3 A, 6 - L E H a Ma.. J J n ( nc L.. d 1a dcr i 9 t J h t O S f eh pau 6 el f 1tmr c L A a i E B. J nw g ( v A. n. d. ph1a e f t mh p r p ae t w rf a n i
7 Clured verlays and lenses can imprve reading fluency: A. Lightstne et al. 285 difficulties. Ophthalmic and Physilgical Optics 15 (5), Menacker, S. J., Bretn, M. E., Bretn, M. L., Radcliffe, J. and Gale, G. A. (1993). D tinted lenses imprve the reading perfrmance f dyslexic children? Archives f OphthabnZgy 111, Neale, M. D The Neale Analysis f Reading Ability. 2nd edn. Macmillan Educatin, Lndn. Tthaker, L. (1991). Multiple Cmparisns fr Researchers. Sage, Califrnia. Tyrrell, R., Hlland, K., Dennis, D. and Wilkins, A. J. (1995). Clured verlays, visual discmfrt, visual search and classrm reading. Jurnal f Research in Reading 18 (l), Wilkins, A. J. (1993). A System fr Precisin Ophthalmic Tinting, Cerium Visual Technlgies, Tenterden, Kent. Wilkins, A. J. (1994). Overlays fr classrm and ptmetric use. Ophthalmic and Physilgical Optics 14, Wilkins, A. J. (1997). A system fr precisin phthalmic tinting and its rle in the treatment f visual stress. ed. C. Dickinsn, I. Murray and D. Carden. In: Jhn Daltn s Clur Visin Legacy, Taylr Francis, Lndn. Wilkins, A. J., Evans, B. J. W., Brwn, J. A., Busby, A. E., Wingfield, A. E., Jeanes, R. J. and Bald, J. (1994). Dublemasked placeb-cntrlled trial f precisin spectral filters in children wh use clured verlays. Ophthalmic and Physilgical Optics 14 (4), Wilkins, A. J., Jeanes, R. J., Pumfrey, P. D. and Laskier, M. (1996). Rate f Reading Test: its reliability, and its validity in the assessment f the effects f clured verlays. Ophthalmic and Physilgical Optics 16, Zeki, S. (1983a). Clur cding in the cerebral crtex: the respnses f wavelength-selective and clur-cded cells in mnkey visual crtex t changes in wavelength cmpsitin. Neurscience 9, Zeki, S. (1983b). Clur cding in the cerebral crtex: the reactin f cells in mnkey visual crtex t wavelengths and clurs. Neurscience 9,