Visual Acuity Screening of Children 6 Months to 3 Years of Age

Transcription

1 Visual Acuity Screeig of Childre Moths to Years of Age Velma Dobso,* Deborah Salem,t D. Luisa Mayer, Cythia Moss, ad S. Lawso Sebris The operat preferetial lookig () procedure allows a behavioral estimate of visual acuity to be obtaied from childre mo to yr of age. I cliical settigs, there is ofte too little time available to obtai a acuity estimate with the stadard procedure. The goal of this study was to idetify specific spatial frequecies, termed diagostic gratig frequecies, that could eable the techique to be used as a screeig procedure uder coditios where completio of acuity estimatio was ot possible. Oe hudred eighty presumptively ormal childre,, 2, 8, 24, 0, ad mo of age, were each tested with up to 20 trials of a potetial diagostic gratig frequecy to determie the highest spatial frequecy gratig that could be resolved by 90% of childre at each age. For all ages except 8 mo, there existed a spatial frequecy that produced uiformly high performace withi the age group; this spatial frequecy was separated by oe-half to oe octave from a higher spatial frequecy that more tha 0% of childre at that age failed to detect. These results suggest that at all ages except 8 mo, it should be possible to use the procedure as a visio screeig tool by testig idividual childre with the diagostic gratig frequecy appropriate for their age. Ivest Ophthalmol Vis Sci 2:057-0, 985 The course of visual acuity developmet i ormal ifats less tha mo of age has bee well described with preferetial lookig (PL) ad forced-choice preferetial lookig () procedures (reviewed by Dobso ad Teller ). With the additio of operat reiforcemet to the preferetial lookig procedure, data o the acuity developmet of ormal childre i the -mo to -yr age rage have also become available. 2 " 4 Thus, ormative preferetial lookig acuity data are available over a wide age rage ad ca be used for compariso with preferetial lookig acuity data from ifats ad youg childre who have suspected visio problems. Ufortuately, however, the biomial variability iheret i these discrete-trial procedures produces acuity estimates that are severely limited i accuracy uless the child is tested with a large umber of trials. 5 Because the costraits preset i may From the Departmet of Psychology, Uiversity of Washigto, Seattle, Washigto. Supported by grats from the Childre's Eye Care Foudatio (RF-980-), the Uiversity of Washigto Graduate School Research Fud (-248), ad the NIH (EY-0258) to Velma Dobso ad a Gatzert Fellowship to D. Luisa Mayer. Curret addresses: Departmets of Psychiatry ad Psychology, Uiversity of Pittsburgh,* Pittsburgh, Pesylvaia; Departmets of Psychology, Uiversity of Illiois,! Champaig-Urbaa, Illiois, ad Brow Uiversity, Providece, Rhode Islad; Departmet of Ophthalmology, Harvard Medical School ad Childre's Hospital, Bosto, Massachusetts.! Submitted for publicatio: October, 984. Reprit requests: Velma Dobso, PhD, Departmet of Psychology, Uiversity of Pittsburgh, Pittsburgh, PA 520. cliical situatios do ot allow time for a large umber of trials, the use of stadard PL procedures for acuity estimatio may be restricted to the research laboratory. Several years ago we developed a modified PL procedure, the diagostic gratig procedure, for acuity screeig of ifats betwee ad 4 mo of age. I this procedure, each ifat is tested with eough trials to give a statistically meaigful result, but the trials are cocetrated at oe spatial frequecy, that which has bee empirically determied to be the highest spatial frequecy that ca be resolved by early all ormal ifats of a give age i a PL test. The procedure does ot produce a actual acuity estimate, but it does provide a rapid screeig method to determie whether or ot a ifat's acuity is withi the ormal rage. To determie the diagostic gratig frequecies for youg ifats, we tested -, 2-, -, ad 4-moth-old ifats with several potetial diagostic gratig frequecies. Based o sequetial biomial probabilities, we cocluded (P < 0.05) that a ifat could see a gratig if the observer i the procedure made a correct judgmet o five of the first five, ie of the first 0, of the first 5, or 5 of 20 trials with that gratig. For all ages, there existed oe spatial frequecy that could be resolved by at least 90% of ifats ad a adjacet spatial frequecy, oe-half to oe octave higher, that could be resolved by less tha 70% of ifats tested. Based o these results, we selected gratigs of 0.75,.,.5, ad.5 cycles/deg as the 057

2 058 INVESTIGATIVE OPHTHALMOLOGY VISUAL SCIENCE / August 985 Vol. 2 diagostic gratig frequecies for -, 2-, -, ad 4- moth-olds, respectively. The purpose of the preset study was to exted the diagostic gratig acuity screeig procedure by determiig the diagostic gratig frequecies appropriate for childre i the -mo to -yr age rage. The PL procedure used was operat preferetial lookig (), which has bee show to be a effective method for acuity estimatio i this age rage. 2 ' Overall Strategy Materials ad Methods The goal of the study was to determie for each age group the highest spatial frequecy gratig that could be resolved biocularly by at least 90% of childre (i.e., the diagostic gratig for each age). Based o previous research, a potetial diagostic gratig frequecy (the target gratig frequecy) was selected for each age group. The target gratig frequecies were 4., 4., 9.,9., 8.7, ad 8.7 cycles/ deg for childre aged, 2, 8, 24, 0, ad mo, respectively. Our strategy was to fid the appropriate diagostic gratig frequecy by a process of approximatio. Iitially, five childre i each age group were tested with the target gratig frequecy. Based o these results, a secod gratig frequecy was added for each age group, ad childre tested subsequetly i each age group were assiged radomly to be tested with oe of the two gratig frequecies. The secod frequecy was oe-half octave higher i spatial frequecy if at least four of the five childre tested at that age had performed sigificatly above chace o the target frequecy, ad oe-half octave lower i spatial frequecy if fewer tha four of the five childre had performed sigificatly above chace o the target frequecy. For two age groups, 8 ad 24 mo, a third spatial frequecy had to be added because o either of the first two spatial frequecies tested at those ages did ie of 0 childre perform sigificatly above chace o the test gratig. Subjects Subjects were solicited by letters set to parets listed i the birth aoucemets sectio of local ewspapers. Parets were paid $5 per sessio ad reimbursed for parkig or bus fare. Subjects were 80 childre who were bor withi ±0 days of their due date, ad were tested withi ±9 days of, 2, 8, 24, 0 or mo postatal. Accordig to their parets' reports, they had o developmetal abormalities ad o kow visual defects. The refractive error of each subject was screeed with isotropic photorefractio. 7 Noe of the subjects showed more tha 2 diopters (D) of myopia o the horizotal meridia (the relevat meridia for the focus of vertical gratigs). Te subjects showed more tha D of oblique-axis astigmatism, ad, therefore, their data were ot icluded i the preset report. Apparatus The apparatus has bee described previously. 2 ' It cosisted of a gray cardboard scree (Crescet #5) with a 4-mm cetral peephole. Two 9-deg stimulus holes were located with their ceters 8 deg to the right ad to the left of the ceter of the peephole. Stimulus gratigs were iserted i a masoite wheel attached to the back of the scree. By rotatig the wheel, the spatial frequecy ad leftright positio of the gratig could be chaged quickly betwee trials. A rectagular cardboard shutter, 5 X cm with a opeig i frot of the peephole, covered the stimulus positios betwee trials. A series of light emittig diodes (LEDs) attached to the shutter was used to ceter the child's gaze at the begiig of each trial. A cardboard shield suspeded 0.5 cm i frot of the scree preveted the perso holdig the child from seeig the positio of the gratig. Two smoked plexiglas reiforcemet boxes were located approximately 80 cm from the subject's eyes, oe to the left ad oe to the right of the scree. Each box cotaied a aimated toy that was activated whe the observer correctly idicated the locatio of the gratig, ad at the observer's discretio durig the traiig phase of the procedure. The stimuli were vertical black-ad-white squarewave gratigs prited o high cotrast photographic paper (Itergraphics Precisio Phototoolig Services; Kirklad, WA). O each trial, oe test gratig ad oe cotrol gratig were preseted. The cotrol gratig was subthreshold for adults (8. cycles/deg). The cotrast of the gratigs raged from 82% to 84%, ad they differed i space-average lumiace by o more tha 0.02 log uit from oe aother ad by o more tha 0.07 log uit from the lumiace of the scree (.2 log cd/m 2 ). Procedure After the procedure was explaied, iformed coset was obtaied from the paret. The the child was seated aloe or o the paret's lap with his or her eyes 54 ± cm from the peephole. A adult observer was seated behid the scree ad viewed the child's face through the peephole. A adult experimeter, seated beside the observer, rotated the stimulus wheel to positio the stimuli, set the reiforcemet device so that a correct judgmet by the observer

3 No. ACUITY SCREENING OF CHILDREN / Dobso er ol. 059 would activate the appropriate reiforcer, ad recorded the observer's judgmets. The procedure cosisted of three phases: traiig, criterio, ad the diagostic gratig test phase. Iitially, the observer attempted to trai the child to look at or poit to a low spatial frequecy gratig to activate the reiforcer. The specific low frequecy gratigs for each age were.,., 2.2, 2.2, 4., ad 4. cycles/deg for -, 2-, 8-, 24-, 0-, ad - moth-olds, respectively. Durig traiig the shield was raised to allow the paret to see the stimulus positios ad help the child lear the task. The observer was also aware of the positio of the gratig ad told the experimeter where to place the gratig o each trial. Traiig trials were repeated o each side ad alterated from side to side, as appropriate, to couteract the idividual child's atural respose tedecies (alteratio, side preferece, etc). Traiig cotiued util the observer judged that the child was showig a cosistet respose (lookig, headturig, or poitig) to the gratig from trial to trial. The ext phase of testig, the criterio phase, was a objective test of whether or ot the child was respodig reliably to the low frequecy gratig used durig traiig. Durig this phase the observer was uaware of the positio of the gratig o each trial ad the shield was lowered so that the paret could ot see the stimulus positios. The right-left positio of the gratig was varied accordig to a pseudoradom order from trial to trial ad the observer pressed a butto to idicate whether she judged the stimulus to be o the left or right o each trial. If the observer pressed the correct butto, the aimated toy o the same side as the gratig was activated. Testig cotiued util oe of the followig criteria was met: six of the first six trials, eight of the first ie trials, 0 of the first 2 trials, or seve i a row correct. The cumulative probability of meetig oe of the first three criteria by chace is less tha The criterio of seve i a row correct was icluded to allow for additioal traiig to occur i those cases where the observer had misjudged the timig of the move from traiig to the criterio phase. Whe the observer met oe of the four criteria described above, the test phase bega. As i the criterio phase, both observer ad paret were uaware of the left-right positio of the gratig, the locatio of the gratig was varied from trial to trial accordig to a predetermied pseudoradom order, ad the observer's task was to use the child's respose as a idicator of gratig locatio. I this phase, however, a potetial diagostic gratig was used as the test gratig. Testig cotiued util the observer judged the locatio of the potetial diagostic gratig correctly o six of the first six, ie of the first 0, of the first 5, or 5 of 20 trials, or util the child became ucooperative. The probability of meetig oe of the four cutoff criteria is less tha 0.05 (sequetial biomial probabilities). After each icorrect respose, a trial with the low frequecy gratig used durig the traiig ad criterio phases was iserted. If a icorrect respose was give to the low frequecy gratig, oe to two more trials with that gratig were preseted, ad the observer was required to get at least oe of these trials correct to allow testig to cotiue. Also, if the total umber of icorrect resposes to the low frequecy gratig reached three at ay time durig the test phase, testig was discotiued. Thus, the ability of the observer to judge the locatio of the gratig o trials with the low frequecy gratig was used to idicate whether poor performace o the potetial diagostic gratig was due to the iability of the child to see the gratig or to a geeral declie i the child's performace i the test situatio. Diagostic Gratigs Results For each child the percets correct scored by the observer o the potetial diagostic gratig ad o the low spatial frequecy gratig are show i Figure. Percet correct o the low frequecy gratig was calculated by combiig the observer's percet correct durig the criterio phase ad the observer's percet correct o trials i which the low frequecy gratig was preseted durig the test phase. The oe 8- moth-old ad two 24-moth-olds who showed performace below 75% o the low spatial frequecy gratig all showed performace above 75% o the potetial diagostic gratigs with which they were tested. I our previous study, we foud that for all ages tested there existed oe spatial frequecy that () could be resolved by at least 90% of ifats tested; ad (2) was separated by oe-half to oe octave from aother higher spatial frequecy that could be resolved by less tha 70% of ifats tested. Examiatio of Figure shows that this was also true for all but oe of the ages tested i the preset study. For -, 2-, 0-, ad -moth-olds, the spatial frequecy resolved by 90% of childre tested was separated by oe-half octave from the spatial frequecy resolved by less tha 70% of childre tested, while for 24-moth-olds, the two spatial frequecies that met these criteria were separated by oe octave. For 8-moth-olds, there was o spatial frequecy resolved by 90% of childre tested, ot eve the 4. cycles/deg gratig that was resolved by 95% of the - ad 2-motholds tested with that spatial frequecy.

4 00 INVESTIGATIVE OPHTHALMOLOGY VISUAL SCIENCE / August 985 Vol. 2 O LU O \- z LLJ o LLJ CL DC LU > LU C/) CO O 00 m I 80 ' < * u. # * MONTHS m t u. I 8 MONTHS :- T 2 MONTHS 24 MONTHS MONTHS _L L, A " , i t 0 MONTHS SPATIAL FREQUENCY (CYCLES/DEGREE) ** 4Km i * i ^ T T : CHANCE i * i t CHANCE - Fig.. Observer's percet correct as a fuctio of spatial frequecy at, 2, 8, 24, 0, ad mo. Arrows idicate actual spatial frequecies used. The low spatial frequecy gratigs used durig traiig, criterio ad testig at each age, show by the starred arrows, were.,., 2.2, 2.2, 4., ad 4. cycles/deg for -, 2-, 8-, 24-, 0-, ad -moth-olds, respectively. At each age, each child cotributed two data poits to the graph, oe poit at the lowest (starred) spatial frequecy ad oe poit at oe of the higher spatial frequecies. Observer's scores of /, 9/0, /5, or 5/20 were couted as a "pass." The diagostic gratig frequecy (i.e., the highest spatial frequecy o which 90% of childre obtaied a passig score) was estimated to be 4., 4.,.9,., ad. cycles/deg at, 2, 24, 0, ad mo, respectively, as idicated by the ope arrows i the figure. A diagostic gratig frequecy could ot be defied for 8-moth-olds. Based o these results, we estimated the diagostic gratig frequecies to be 4., 4.,.9,., ad. cycles/deg for -, 2-, 24-, 0-, ad -moth-olds, respectively. These values are listed i Table, alog with the previously established diagostic gratig frequecies for - to 4-moth-old ifats tested with the procedure. We were uable to use the results of the 8-moth-olds to estimate a diagostic gratig frequecy for that age, accordig to our criteria for selectio of a diagostic gratig frequecy. Table. Diagostic gratig frequecies Age (mo) Procedure Diagostic gratig (cycles/deg) ?.9.. Success Rates Table 2 shows the umber ad percetage of childre who were tested successfully, ad the umber ad percetage of childre who failed to complete the test durig the iitial traiig period ad durig the test phase with the potetial diagostic gratig. It is clear from the table that the most difficult ages to test were 8 ad 24 mo. It is also clear from compariso of the two rightmost colums i Table 2 that the majority of childre who did ot complete testig at these ages were utestable because they did ot lear the task or perform the task well eough to allow the observer to complete the criterio phase of the procedure. Test Duratio The mea ad stadard deviatio of the time required at each age to complete all phases of testig traiig, criterio, ad testig with the potetial diagostic gratig are show i Table. At all ages, the mea test time averaged less tha 0 mi, ad those childre who passed the test (ie, those childre for whom the observer's performace met oe of the

5 No. 8 ACUITY SCREENING OF CHILDREN / Dobso er ol. 0 Table 2. Success rates Age (mo) Total subjects Complete tests (%) Icomplete durig traiig ad criterio (%) Icomplete after traiig ad criterio (%) (82) (00) (47) (5) (88) (95) () 5 (2) 7 (2) 2 (2) (5) 2 (2) 0 (2) 9 (7) criteria of /, 9/0, /5, or 5/20 trials correct) required less time tha childre who failed (six icorrect resposes by the observer) or did ot complete the test. Discussio The results of the preset study suggest that it is possible at most ages to defie a maximum (diagostic) spatial frequecy that will allow above-chace performace by most childre with presumptively ormal visual acuity tested i the procedure. For the -, 2-, 0-, ad -moth-olds tested i the preset study, the spatial frequecy desigated later as the diagostic gratig frequecy produced uiformly high percets correct i the task, while a spatial frequecy oe-half octave higher produced percets correct o differet from chace i may of the presumptively ormal ifats. Thus, the diagostic gratig frequecy was easy to specify for these four ages. For 24-moth-olds, the desigated diagostic gratig frequecy was separated by oe octave from the spatial frequecy that produced poor performace by at least 0% of ifats at that age, ad the spatial frequecy midway betwee these two gratig frequecies produced above-chace performace by 80% of the 24- moth-olds tested. Thus, the appropriate diagostic gratig frequecy at 24 mo was less well defied tha at, 2, 0, or moths. The somewhat ambiguous performace by 24-moth-olds is probably due to the frequet lack of cooperatio of childre i this age group, as is also reflected i the relatively low success rate for 24-moth-olds show i Table 2. For 8-moth-olds, there was o spatial frequecy for which at least 90% of the group showed abovechace performace. This is almost certaily due to the lack of cooperatio of the 8-moth-olds, sice oly 80% of childre at this age showed above-chace performace at the spatial frequecy (4. cycles/deg) at which 95% of - ad 2-moth-olds produced above-chace performace. As show i Table, the mea time required to complete the procedure raged from 29 mi for 2- moth-olds who failed the potetial diagostic gratig to mi for -moth-olds who passed the potetial diagostic gratig. Sice testig with the potetial diagostic gratig required oly six to 20 trials, the primary cause of the variatio i total test duratio was the amout of time required to teach the child the task required for testig. Therefore, the best way to shorte the procedure would be to reduce the umber of trials prelimiary to testig with the diagostic gratig. Oe of us (Dr. Mayer), i usig for cliical assessmet of visual acuity, has foud that for most ifats ad childre, oly six Table. Total time for testig (traiig, criterio, ad diagostic gratig phases combied) Passed* Failed^ Icomplete Age (mo) Mea (mi) sd Mea (mi) sd Mea (mi) sd Sd: stadard deviatio. * Observer's correct judgmets = /, 9/0, /5, or 5/20. t Observer's icorrect judgmets ^.

6 02 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 985 Vol. 2 to eight traiig trials ad o criterio trials are ecessary prior to acuity testig. Now that we have determied diagostic gratig frequecies for -, 2-, 24-, 0- ad -moth-olds we ca ask, how would oe implemet the procedure i a cliical settig? We foresee two types of cliical situatios i which the diagostic gratig procedure would be useful. First is the situatio i which oe is iterested i large-scale screeig, where the purpose is to idetify childre with poorer-tha-ormal visual acuity ad to refer them for more extesive testig. We have ot yet used the diagostic gratig procedure for mass screeig, but prelimiary results with the diagostic gratig procedure suggest that the procedure is potetially useful as a screeig tool. 89 The secod situatio i which the diagostic gratig procedure could be useful is the situatio that exists whe oe does ot kow at the outset of testig whether a child will complete eough trials to produce a acuity estimate with oe of the stadard procedures for estimatig acuity threshold. I this case, the diagostic gratig procedure ca be used sequetially with gratigs of several spatial frequecies to provide, first, a estimate of whether a child is blid, the a decisio cocerig whether a child's acuity is withi the ormal rage, ad, fially, a estimate of acuity that becomes more ad more refied the loger the child cooperates. 0 What is the advatage of the diagostic gratig procedure over other preferetial lookig procedures that have bee used cliically? I additio to the diagostic gratig procedure, three other preferetial lookig procedures have bee used for the assessmet of visual acuity i ifats ad youg childre with visio disorders."" Each of the three uses a differet staircase procedure to produce a acuity estimate. At first glace, these staircase procedures might seem more attractive tha the diagostic gratig procedure to the cliicia because, regardless of how may trials are completed, the staircase procedures produce a sigle umber that gives a estimate of the child's acuity threshold. O the other had, whe small umbers of trials are used, the diagostic gratig procedure produces oly a statemet cocerig whether or ot the child's acuity is withi the ormal rage. Ufortuately, however, the sese of accuracy produced by the results of staircase procedures whe small umbers of trials are used is a false oe. Statistically, the stadard error of estimatio of visual acuity will be larger the smaller the umber of trials used. For example, if the stadard deviatio of the child's psychometric fuctio is oe octave, the a acuity estimate resultig from 20 trials, a commo occurrece with two of the staircase procedures," 2 has a cofidece iterval of about ±.5 octaves at best. This meas, for example, that although the results of a 20-trial staircase procedure may yield a omial acuity estimate of 20/40, i fact, all that the staircase procedure has show is that the child's acuity is betwee 20/50 ad 20/400. O the other had, 20 trials with the diagostic gratig procedure is ofte eough to allow oe to coclude with certaity that a child's acuity is withi the ormal rage.* Thus, the advatage of the diagostic gratig procedure over staircase procedures is that whe it is possible to obtai oly a small umber of trials from a child (eg, due to costraits o a experimeter's time or lack of cooperatio from a child), the diagostic gratig procedure allows oe to coclude whether acuity is withi the ormal rage, whereas staircase procedures oly allow oe to coclude that acuity is somewhere withi a several-octave rage, which may iclude values outside the ormal acuity limits for the child's age. I summary, the goal of the preset study was to idetify sigle gratig frequecies that could be used as idicators of whether or ot a child's performace was withi the ormal rage o a acuity task. For -, 2-, 0-, ad -moth-olds, there was a clearly-defied highest spatial frequecy that could be detected by at least 90% of ifats at each of those ages, ad this spatial frequecy was desigated as the diagostic gratig frequecy for each age. For 24- moth-olds, the diagostic gratig frequecy was less well defied, ad for 8-moth-olds it was ot possible to use our specific criteria to desigate a diagostic gratig frequecy. Similarly, the percetage of -, 2-, 0-, ad -moth-olds who completed testig raged from 82% to 00%, while the success rates for 8- ad 24-moth-olds were cosiderably lower, due primarily to the difficulty of teachig childre of this age the respose required i testig, ad to our iability to maitai their iterest i the task. Although obstacles remai that limit the usefuless of the procedure with 8- ad 24-moth-olds, it is clear that use of the diagostic gratig procedure should sigificatly icrease the usefuless of testig. For example, i situatios where time is too limited to obtai the full umber of trials required for acuity estimatio, the diagostic gratig procedure allows a statistically meaigful estimate of whether * To provide a acuity value rather tha simply a statemet that the child's acuity is withi the ormal rage, the child who passes the diagostic gratig appropriate for his or her age ca be assiged the mea acuity value for ormal childre of that same age. This assiged acuity value will be as accurate a estimate of the child's true acuity as the acuity value obtaied from a staircase procedure employig a small umber of trials. 5

7 No. 8 ACUITY SCREENING OF CHILDREN / Dobso er ol. 0 or ot a child's acuity is withi the ormal rage. I additio, i situatios where it is uclear whether a child will remai cooperative through a loger procedure, the diagostic gratig procedure allows the examier to establish first whether the child's acuity is withi the ormal rage, ad the, by testig the child with other spatial frequecies sequetially, accordig to the criteria for pass ad fail used with the diagostic gratigs, to defie the child's actual acuity threshold as accurately as time allows. 0 Key words: acuity screeig, ifats, childre, operat preferetial lookig, gratig acuity Ackowledgmets The authors thak Dr. Davida Teller, for commets o the mauscript; Leslie Mior, for assistace i ruig subjects; ad Marjorie Zachow, for secretarial assistace. Refereces. Dobso V ad Teller DY: Visual acuity i huma ifats: a review ad compariso of behavioral ad electrophysiological studies. Visio Res 8:49, Mayer DL ad Dobso V: Assessmet of visio i youg childre: a ew operat approach yields estimates of acuity. Ivest Ophthalmol Vis Sci 9:5, Mayer DL ad Dobso V: Visual acuity developmet i ifats ad youg childre as assessed by operat preferetial lookig. Visio Res 22:4, Birch EE, Gwiazda J, Bauer JA Jr, Naegele J, ad Held R: Visual acuity ad its meridioal variatios i childre aged 7-0 moths. Visio Res 2:09, Teller DY: Measuremet of visual acuity i huma ad mokey ifats: the iterface betwee laboratory ad cliic. Behav Brai Res 0:5, 98.. Dobso V, Teller DY, Lee CP, ad Wade B: A behavioral method for efficiet screeig of visual acuity i youg ifats. I. Prelimiary laboratory developmet. Ivest Ophthalmol Vis Sci 7:42, Howlad HC, Braddick O, Atkiso J, ad Howlad'B: Optics of photorefractio: orthogoal ad isotropic methods. J Opt Soc Am 7:70, Fulto AB, Maig KA, ad Dobso V: Ifat visio testig by a behavioral method. Ophthalmology 8:4, Sebris SL, Dobso V, ad Hartma EE: Assessmet ad predictio of visual acuity i - to.4-year-old childre bor prior to term. Hum Neurobiol :87, Dobso V: Cliical applicatios of preferetial lookig measures of visual acuity. Behav Brai Res 0:25, 98.. Gwiazda J, Wolfe JM, Brill S, Mohidra I, ad Held R: Quick assessmet of preferetial lookig acuity i ifats. Am J Optom Physiol Opt 57:420, Mayer DL, Fulto AB, ad Hase RM: Preferetial lookig acuity obtaied with a staircase procedure i pediatric patiets. Ivest Ophthalmol Vis Sci 2:58, Atkiso J, Braddick O, ad Pimm-Smith E: "Preferetial lookig" for moocular ad biocular acuity testig of ifats. Br J Ophthalmol :24, 982.