The Effect of Artificial Tears on Visual Performance in

The Effect of Artificial Tears on Visual Performance in Normal Subjects Wearing Contact Lenses

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WILLIAM H. RIDDER 111, OD, PhD, FAA0 and MAN TOMLINSON, PhD, DSc, MCOptorn, F r U O

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ABSTRACT: Purpose. Disruption of the anterior refracting surface of the eye reduces visual performance. I n the case of a contact lens wearer, this surface is the prelens tear film. This study determined whether artificial tears stabilize the tear film i n contact lens wearers and maintained optimal visual performance for a prolonged period. Methods. Five normal subjects all i n good general and ocular health and adapted to contact lens wear were fitted with Acuvue lenses. A temporal, two-alternative, forced-choice paradigm was used t o measure contrast sensitivity. The stimuli were vertically oriented sine-wave gratings (between 0.5 and 14 cpd) presented for 16.67 ms. The stimuli were presented at t w o different times after blink detection: 2 s after blink detection (i.e., before tear layer break-up) or 4 s after tear film break-up. Three conditions were investigated at 4 s after tear layer break-up: (1) without artificial tears added, (2) with Clerz2 (Ciba Vision) instilled, and (3) w i t h Sensitive Eyes (Bausch & Lomb) applied. The artificial tears were instilled at 10-min intervals during the experimental run. Results. High spatial frequency contrast sensitivity was found to be reduced after tear film break-up and was not enhanced by either tear solution. Conclusions. This study indicates that there is no effect of Clerz2 or Sensitive Eyes i n maintaining or improving visual performance beyond the normal prelens tear film break-up time i n normal subjects wearing Acuvue lenses. (Optom Vis Sci 2003;80:826-831)

Key Words: tear layer break-up, vision, c o n t a c t lens, drying, artificial tears

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acients who wear contacc lenses often repon chat [heir vision with [he contacc lens fluccuaces when they blink Previous studies have suggested char this fluctuation may be che result of blink-induced contacc lens m ~ v e m e n c . ' -T~ h e movement of che contact lens causes a prismatic shift in the image of [he world on the retina. This movement then decreases visual sensitivity. This effect has been observed wich spherical sofc concacc lenses, roric soft contacc lenses, and gas-permeable contact lenses. Wich well-ficcing spherical soft contacc Icnrs, chis rnovrmcnr-induced rcduccion in visual sen5itiviry laced about 100 ms afcer a blink 'Iypically, chc longer chc lens took ro stabilize afcer the blink, the longer chis dccrcuc in visual scnsitivicy WAS obscrvcd. T h e dccrcasc in visual scn>itivicy i then followed I>y a pcriotl of optimum visual pcrfi~rrnancc. Oillcr u)rltacc lcn5 ~xlricn~s con~plaint~rarthc-ir vision gradually ':ides : ~ srhcy view :I 5rilrllllus. I5lilllting rend> co inrl~roveviiiorl IiJr . I ] C ~ C j>:icj,ncs, 13li11k-induced contact I c r r 5 movcrricr1c coultl not :lccollrlc fi)r ~llisdccrc:~st!irl visu:J rcnsicivicy. (;orli:rct lcn5 wc:lr resrlJlr ill :I reduction of' L ~ I)TCICI~S C Lcxr lillll lil)i , 1 llldllt: ct ;~l.~%)l)scrvcd an i~lcrc;~>c i l l visu:~lacui~yand cor)rr:i\c 5cl15jiivi~y i l l dry-cyt! j):~~icll~s will1 ~ I ~ I I L ~ c: ~j ~I Ci r ~ ~ kcr;jii~,>:i!j,~ cli;~~ ;lstilici:ll crar drllirli~tr:i~iorl. ' ~ ' J ) L J > , LIIC I)urjlcjsc l ~ f t j l icx~)Cr*, " 0

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Subjects Five r)hscrvcrs wcrc u.scd f t ~ rthis psychophysial studY.All subjects were > 1 F! years old. 'lhcy had (;/(; or bcttcr cc~rrcctcdvision. Ocular health was normal for all subjects. The subjccts were adapted contact lens wearers and were fit with an Acuvue lens. Subjuu were cxcludcd from thc study if [hey wcrc unable to adapt to Acuvue lens wear. Furthermore, if [he subject had a known allergy or developed an allergy to [he artificial tears used in this projrct, they were excluded from the study. An announcemenr was made to the college community and volunteers were enrolled. The subjrru who took part in this project were monetarily cornpensated for their time and efforr. Informed consent was obtained from all subjects afcer the testing procedure was explained to them. The procedures complied wirh the tenets pur forrh in the Declaration of Helsinki and were approved by the Institutional Review Board at the Southern California College of Optorneuy.

'rhc subjccr vicwcd the srimulu\ r n ~ ~ n o c u l aon r l ~a 'l'cktronix 608 oscilloscc~pca t 1 m with thc test cyc. 'lhcy wcrc stat~ili~xd in a was n~onitoredwith an chin and forehead rest. The nc~nvicwir~gcyc Eye Trac eyc monitor to dctcrminc when blinks oc~urrcd.

Visual Stimulus A C R T image synrhesizer (Innisfree, Picasso, Cambridge, MA) produced [he visual stimulur, a vertically-oriented, sine-wave graring (16.67-ms srimulus duration, square-wave onset and offset), on the Tektronix 608 moniror. The phosphor decay rare of the moniror is approximately 1 ms for a 90% contrast, square-wave grating.23 A computer controlled the C R T image synthesizer. The srimulus screen subtended angles of6.0° vertically by 8.0" horizontally at a 1.O-rn viewing distance and had a mean screen luminance of 22.3 cdlm2. Surrounding the screen was a back-projected white board (13" by 26") matched in luminance to the oscilloscope. The room luminance was approximately 1 cdlm'.

Tear Break-Up Measurement

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Before contrast sensitiviry was measured, che time to tear break-up was determined. This was accomplished with a Tearscopel'lus (Eaglevision) mounted on a slit lamp. The subject was wearing an Acume contact lens durin,o&s measurement. The rear break-up rime &-as taken as the time from the last blink until the TearscopePlus image became distorted within the subject's pupil border. This indicated break-up over the subject's optical axis. This measurement was made \vithour an a d c i a l tear placed in the eye. The mean t m break-up dmes ( r S D ) for the subjects were 6.5 5 1.14 s (N = 78) for subjecr 1; 6.7 ? 1.17 s (N = 47) for subjecr 3; 6.7 I 1.96 s (N = 36) for subject 3: 6.4 t 1.12 s (N = 56) for subject 4; and 7.7 t 1.S1 s (N = 63) for subjecr 5. The total number of tear bredi-up measurements (i.e., N) for the different subiecrs are not the sanme bemuse nor all subjects had the sanme nunmkr of dam collzcrion sessions. The number of sessions and the order of condirions 6or each subject 3 r t displ~!.ed in Table 1. The rear brr.~k-up rimes were deterntined before each esperinmenral session to he certain that [here \\.;ISno l a ~ day-to-day e \.ariation.

TAGLE 1. The number ui data collection session5 attcndtxd by each Subjtu-t

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Technique The subject viax-ed the srimulus under monocular conditions. Five spadal frequencies were used (0.5, 1.0, 4.0, 10.0, and 14.0 cpd). Sdmulus conuasr is defined as C = [(L,, - L-)/(L,, t L-)], where L, and L, are the maximum and minimum luminances of the sine-wave gradng. A temporal, nvo-alternadve, forced-choice paradiap combined with a staircase procedure was us&. Trials were inidated when che experimenter determined that i n center ~ of the oscdloscope. The subject the subjecr \vas f ~ ~ a tthe then initiated the trial by blinking. The stimulus was presented at one of nvo times afrer the blink. In the first experiment, the sdmulus k v a s presented : !s h e r blink detection. Typically tear break-up rakes several seconds. so this experiment determined contmst sensiri\i?- before rear break-up. Funhemmore. this time \vas chosen to avoid lens nmovrnmenr-induced loss in visu~1perfornlmce. This \\-aour collrrol condition. The data from the next experiments \Yere conmp;tred ro rhis. I n our second series of r~perinlents.contrast sensiriviry \\.as deter~llinrd

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4 s iilicr I I I C . I \ , C ~ I , . I ~ \I ~ ; \ I I l~rc\.tk-~~p ti111c( ~ l c ~ c ~ ~ ~p~.cvit~t~sly iii~~cIi~ik ti~irc,i111dthis ~ l i s ~ ~ I i,lil ~1:;;. ~ c ~1 .l Sp.iti.11 ~rcqt~c~ic\is p l o ~ t c ~ oril tlic l i ~ r i ~ ~ t ~ ~ c ; ~ l \\'ill Oe T C ( C ' C I . ~ Cto~ 3s tllc P S ~ ~ C ~ ~ I ~ I C I I ~~ 1B; lI r n . First, tli~+ Colirr;tst .this, ; I I ~ L cc)ntr.lst ~ s c ~ ~ s i t i ~isi t plottfit y o11 rlie vcrtic.tI .\xis. Tlic sclisitivity \:IS c\crci-~iii~icct \\~irl~out ;irtifici,~ltc;trs :~d[ sensitivip-, p e k sparid hequenq-, high spatial and thus etlect the subiect's visual performance over time. C l e d frequenq slops, and lo\\- spatid frequency slope)." The curves contains hydro~~ethylcellulose and a polosamer 406, which mod- drab-n through the d a t ~are the b u r fit based on this function. ifies the wetting characteristics of the solution and increases its Visual acui? is excmpolated horn these functions by cxmding the viscosity. Sensitive Eyes is a salt solution with a lower viscosity. cun-e to the s axis. The extrapolaced acuities for the C l e d and With the nvo-alternative forced-choice task, nvo time intends Bausch fi Lornb Sensin\ e Eyes data were lower than the condition were presented to rhe subject (on nvo separate blinks). but the without mificial tears (no arrificial tears after tear la!-er break-up, stimulus was only in one interval. During a single run, the subjea 33.8 cpd; Clem', 21.7 cpd; Bausch fi Lomb Sensitive Eyes, 34.6 was required to correcrly idenrib the time interval that contained cpd). the stimulus. Stimulus conuast wvas increased by 0.1 log unit for Fig. 2 displays the average contrast sensitivity dam for all of the incorrect responses and decreased by 0.1 log unit for nx-o consec- subjects. The grouped data display rhe same general uend as seen utive correct responses. The subject made the responses with h e in Fig. 1. The conuasr ssnsitiviry is decreased when h e tear layer computer mouse. The procedure continued for 11 reversals of breaks up. Additionally, this effect increases with increasing spatial sdrnulus contrast, and the last 9 reversals were averaged to give a frequency. threshold and standard deviation. Initially, an ANOVA was used to determine wherher &ere was An Eye Trac Model 200 monitored the blink of the nonviewing a significant difference bemeen any of rhe test conditions (open eye. The Eye Trac was connected to the computer to relay the and filled symbols). The .XiVOVA ~ndicatedthat h e r e was a sigstarting time of the trial. Processing time by the eye monitor and the computer was determined by feeding the eye monitor signal into a Tekrronix model 5441 dual-channel storage oscilloscope. The signal to the Tektronix 608 oscilloscope from the computer was fed into the second channel of the storage - oscilloscope. The time difference berween the two signals was taken as the processing time. An average of 10 trials gave a processing rime of20.2 ms (SD, 8.52). Thus, the stimulus was presented approximately 20 ms after the signal for stimulus presentation was sent from the computer.

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Data Analysis Contrast scnsitivicy functions wcre produced from the individual and group dara. Visual acuity was determined from che contrast sensitivity function. Fivc diffcrcnr analysis of variance (ASOVA) 1 4 0I i 10 tests wcre run to dccerrninc whether chc expcrirncr~talconditions Ior1 SPATIAL PRE0I:ENCY (CI'l)) affccred corltr:L\t scnsitiviry. All the ANOVA ccsts wcre :I rcpca~cdme;Ljuro design wich cwo fcrorj (cr\t c o ~ ~ d i r i ;orl ~ ~spacial d frcquuicy) Initially, an ANOVA WLS pcrfi)rmcd or^ :ill LIIC d ; l ~ : ~FIGURE 1 . I LO dcrcrrninc whctl~crsignificat,~diffcrcnccs cxi,rcd. 'l'h~ri,:III rcy)rc-,tyl! fJ;rt,j ot)l;lifli!d 2 S dflf!r t ) l i r ~ k(c.or~lrc~l ( l : ~ t ~Fill~+cl ) , (,jrf,li5 rqjrc!ANOVA wu used co dcrcrn~incwllrrher rJ~cuJrltr:ur x n 4 ~ i v i r ~j l . r , ~( ~ , ) I ; I ~t~t.jini!il 4 s .jftc!r ti!dr l.~yl:r I~r(?ak-up. Fill~:li~\k olisct (ilrlcs,Jiti~rctrt;rrtitiL.i,\l tc:~rs,or ~ l i l \ > ~ ~Z I' ItI\\t~ . I Zlens ~ nl.ltcri.ils to ~ I e ( c r t l ~ i\VII~IIIC'T t~c tllis is ;I glol>.~l~ ~ l t c ~ i r ~ l~h~~i1?ec.~t~sc tc n ~ ~ ~ ,111i cc>tlt.~ct . ICIISCS ll.~vchcctt sIto\vn to Il.i\.c sul\sr.~r\ti.\llythc santc dctri~rlcnt;llcli;,cts em tcar pll!aiolop, catr.;rl~ol.~tio~l of thc d;it.r fronl this stuJy olr tllc ACU~ c l Itlay bc possil>lc. c-\cl~lirionall\-, a liz~tionwith ditYcrcnt artificial tears. 'Tlrus. sonre sitbjccts III.I!. l ~ s \ ~ cv11e lens to ~ t l l c lr ~ ~ J n >lc~lscs I;trgcr suhjcct s;lnrple sire may he used to scc \vhcrl~crtltcrc is ;III in~provcn\cntin tc;w sr.~biliz;ition\vitl~one type of :irtificial tc:tr and not atrorhcr. T h e group dnci fc)r contr;ist sensitivity m.y then intcrsubject ciri;ibilin- in tllc response to artificial te;w supplcnlcnclinically dry-e!-c si~bjcctssl~ouldbe cs3ulincxl l n ~ s kthe itrdividu31 effects. If wc mkc an acuity ch:ulge of 1 log- ration. And t~rr;~lly, hIAR linc as sipnificant, we see thsr bur of our subjects had no to detcrminc \\?hcther the obscrvntions n~ncichcrc in cont.1ct lens signifiant change in visual performance with the artificiil te;irs. wearers holds for other conditions. For the fifth subject, the acuity decreased with both of the arrificikd t a r s . Thus, we ;ue unable to determine whether different subjecrs ACKNOWLEDGMENTS respond differently to various artificial rears. PIID. T I I ~ nt~nzt-n'pr A third possibility concerns the mte ofartificial rear supplemen- The swtirtizzl.~lw!wis rrlapcrjSrmedqv RIU/DcLJ~~A, at t/~c2002 .inrerie'z~r.ic:zAen~-y of Opron:.~. l t posrcrji~t~~zr t~d tation. It may be that a 10-min administration interval is too long w m p r ~ ~ ~ in Sari D i p , C i . to observe the major benetits from artificial tears. Even though the R ~ L - c ~Fcbnwy L ' c ~ 7. 0 0 3 : rcokion recrir,edJunc 10, 2003. retention time for most artificial tear solutions does exceed 1 0 rnin," it is possible that the retention time does not equate to tear stabilization time. Thus, a greater drop frequency may result in a REFERENCES visual improvement that we did not observe. 1. Ridder X'H 111, Tomlinson h Blink induced, temporal variations in Thus, we were not able ro corroborate the report in the literamre conrrasr sensitivity. Inr Conram Lens Clinics 1991;1 5 2 31-7. 2. Tomlinson A, Ridder WH 111. Effccr of lens movement on vision ofan improvement in contact lens visual performance with the use with RGP conracr lenses. J Br Contact Lens Xssoc 1992: 15:25-9. ofartificial tears." W e chose the artificial tear solutions C l c d and 3. Watanabe RK, Ridder WH 111, Tomlinson A. \%ual performance of Sensitive Eyes because they are recommended for use during conthree disposable soir conracr lenses. Inr Conracr Lens Clinics 1993: -. tact lens wear as rewettins drops. T h e Clerzl solution contains 20:106-12. hydroxyethylcellulose and a p o l o m e r 4 0 6 (surfactant), which 4. Ridder WH 111, Tomlinson A, Watanabe RK. Visual cffecu of moremodifies the wetting characteristics of the solution and increases its ment of coric soft lenses. Proceeding of the 9th Asian Pacific Optoviscosity and probably i u residence rime in the eye.27 U n f o m metric Conference, Hong KO% 1993:13%. nately, these additives did not appear to affect the visual perfor5. Tornlinson A, Ridder WH 111,Waranabe R Blink-induced variarions mance compared with the preserved and buffered salt solution of in visual performance with roric soft contact lenses. Oprorn Vii Sci the ~ensitivkEyes solution.'~tis possible that solutions specifically 1994:71:545-9. formulated as d y e y e tear supplements, which differ from contact 6. Ruben M,Guillon hi. Conracr Lens Practice. London: Chap81 Hall, 1994. lens rewetting drops, may improve visual performance. These dry7. Tomlinson A, CedarsraffTH. Tear evaporation from the human eye: eye solutions normally contain agenu other than hydroxyethylcelthe effects of contacr lens wear. J Br Contact Lens Assoc 1952;j: o r carboxymethylcellulose (e.g., hydr~xylpro~ylmethylcellulose 141-50. lulose, which are not approved for contact lens use) to improve the 8. Golding TR, Efron N, Brennan N h Sok lens lubricants and prelens wertabiliry of the ocular surface. These additives are believed to tear film stability. Optom Vis Sci 1990;67:461-5. provide a longer residence time o n the ocular surface and may then 9. Young G, Efron N. Characteristics of rhe pre-lens rear film during improve visual performance.2"29 hydrogel contact lrns wear. Ophthalmic Physiol Opt 199 1 ;I 1:53-8. There are several studies that report improvements in the visual 10. Timberlake GT, Doane MG, Bertera JH. Short-term, low-conrras performance of patients wirh dry-eye disease with the use ofartifivisual acuity reduction associated wirh in vivo conracr lens drying, cia1 tear s o l ~ t i o n s . ' " ~ ~y''~ This may be a result of the different Optom Vis Sci 1992;69:75540. benefiu provided to a dry-eye parient by the use of artificial tcar I I. Tuct R, Bradley A, Regley C, Thibo, LY.Oprical and impact of [car break-up in human eyes. Invot Ophrhalrnol Vi, Sci 2000;41: rolutions compared with those for the contact lens wearer. In dry 4 1 17-23. eye, the arrificial tear solution may provide the majority of the A. 'l'hr effect of' tcar fjlm i n w t prccorneal rear film and thus improve vi,ion, whcrca5 in the 12. 'l'hai L C , Riddcr WH 111, 'l'on~linr~n 1)rc~k-upon uJnrrx\r xn5itiviry. Invc>tOl)hrhalrnol Vis Sci 2001;/,2: contact lrns wearer, the additional solution is merely supplcmenctll S506. ro a complete tear fil~il.'J'he contact lens rewetting [car solurion I 3, 'I'II:I~ I,(:, 'I-OIII~~IIW~I A, ItidJrr Wl i, (;~IJII:ICC le115( I r y i r :~A~I I ~v i s , ~ u l : ~ p p c a nun:lble ro ovcrcolne the disruptivc cffcct, of contact lenscs j>crfi~rn]:~~~ce: the visio~~ cYck wit11 u , r ~ t ~I ~r~ I X \ ,O ~ I I C JVi, I ~ I' x i -- and r u s t ~ i rchr ~ existir~gt a r f;l111. 200'2;70:3H 1-8. 111 u,riclusion, thcsr: rcsults confirm prcviou5 ol)scrv;itionr, 14. ' l ' \ ~ i I xLN, ~ ~ I I I I I I ~X+ l%r:~~lley A, I%cy,Icy c;, Ihcrior:~tii~r~ of' rctir~d which indicacc [hilt rcxr laycr I>rrnlcatises :I d w r c u e in ujrltr;L,t in~:~gc qlrdity dllc UJ t~rclklll)of' llrc u~rrlc:ll(car filrrr. Irlvc%tOphsr~~\irivity fi)r l ~ i g h.sj>:~rial ~ ~ C ~ I J C I I~CI Y I L I ~ I I I ; ~L I ~I ~C~II]II I ) , ~ I J C ~ ~ L J ~ ~ J l a j l ~ j Vi* ~ j j Sci I !N'A/,O;S544, 15. 1 fil~~clr:~tigh N, Wright A, ' l ' l ~ i l )I.,~ ~lkglcy ~ (1, l5r;1dIt:~A , Wilr,~](;, ~CC~CA>CS ;ic~lity.'I'IIc results :11so d c l n o r l s t r : ~t11:1t ~ ~ :~rrificialtc:lr

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(/~rl~l~;~riw)ll 05 f l ~ ( l r ~ ~ :11ld ~ iSha~k-1 l1 Iartrrlanrl wavcfranl rcnsing I I I C ~ ~ I I Ifor ~ < rrroi~jrorirrgI C ; I ~filrrr },rc;lk,ll), Vis sci 1999; 76f.kJpr71); 1 13, 16. I(icgcr (;. 'f'trc irnporr:lnuof tfrc prwJrncaI tcar film for the qualityof opticjll irrt~gir~g. Ilr J Ophth;rlmc~l1392;76:1 57-8. 17. 'I',is 1994;53:63747. 19. Hjcrrurn KR. Test and symptoms in kcratnconjunctivitis sicca and their u)rrclation. Acta Ophthalmol Scand 1996;74:43641. 20. IB SH, Tseng SC. Rose bengal staining and cytologic characteristics aswciated with lipid tear dcficienq. Am J Ophthalmol 1997;124: 73650. 21. Swan KC. Use of methylcellulose in ophthalmology. Arch O ~ h t h a l mol 1945;33:37&80. 22. Huang FC, Tseng SH, Shih MH, Chen FK. Effect of artificial rears on corneal surface regularity, contrast sensitivity, and glare disability in dry eyes. Ophthalmology 2002;109: 1934-40. 23. Ridder WH 111, Tomlinson A. Suppression of contrast sensiriviry during eyelid blinks. Vision Res 1993;33:1795-802. 24. Hame& RS, Smith EL 111, Crawford ML, von Noorden GK. Be-

25, 26.

27,

28.

29. 30.

havioral stt~tlicsof I ~ J Cacrlcitivc ~)criodeo r cl~vclr,~~r~,rr~r of vir,r;rl functiclnsitl nlcrnkcys.I',cll:~vl5r;lirl f(cc 1900:4 1 ;179- 98, TreesGR,' f o r n l i n ~ nA. Iiffcc! of artificiaI tc:lr u~luti/~ns :rrrtl hllirl~ on tear film evaporation, Optom Vis Sci 1990;67;88(r90. PonsAM, Lorcnte A, Albarran (;, Montct R. Artips JM. C:h:~ractcr. ilarion of ,hc visual pcrformnncc with ~ f tl;cily r wc;tr di\pos;~l,lcU I ~ I racr lcnses. Ophthalmic l'hysiol Opt 1398; 18:40-8. Jaanuc SD.Lubricants and other prcparatiorls for ocular surfacc discaw. In: Barrlctt JD, Jaanus SD, cds. Clinical Ocular J'harrnacolo~y. Boston: Buttcrwonh-Hcincmann; 1995:355-67. Wilson CG. Zhu YP, Fricr M, Rao 15, Gilchrist J', Pcrkins AC. Ocular contact time of a carbomcr gel (GclTcars) in humans. Kr J Ophthalmol 1398;82: 1 131-4. Calonge M. The treatment of dry eye. Surv Ophthalmol 2001; 45(Suppl 2):S227-39. Albarrans C, Pons Ah4, Lorente A, Montes R, Artigas JM. Influence of the tear film on optical quality of the eye. Contact Lens Anterior Eye 1997;20: 129-35.

W~lliarnH. Ridder, I11 Southern California Colkge of Optometry 2575 Yorba Linda Blvd. Fullerton, C4 92831-1699 e-mail: [email protected]