Criteria For Monocular Acuity Deficit in Infancy and Early Childhood

Transcription

1 Investigative Ophthalmology & Visual Science, Vol. 29, No. 4, April 1988 Copyright Association for Research in Vision and Ophthalmology Criteria For Monocular Acuity Deficit in Infancy and Early Childhood Eileen E. Dirch*f and Linda A. Hale* Criteria for judging preferential-looking and operant monocular grating acuity test results in pediatric patients are usually based on the distribution of monocular or binocular grating acuities, interocular differences in grating acuities, or test-retest differences obtained from normal populations. In order to compare the sensitivity and specificity of each criterion, normative data were obtained from infants and young children ranging in age from birth to years with common stimuli and staircase procedure. The sensitivity and specificity of each derived criterion were evaluated in two groups of pediatric patients with unilateral eye disorders. Specificity was high for all criteria, ranging from.9 to.99. However, monocular and binocular grating acuity norms showed low sensitivity to monocular grating acuity deficit, primarily due to high intersubject variability in the normal population. Intersubject variability was lower for interocular grating acuity differences and for test-retest differences, leading to higher sensitivity of these criteria for monocular grating acuity deficit. Invest Ophthalmol Vis Sci 29: , 1988 Preferential-looking and operant procedures have recently been used to detect pediatric eye disorders, 1 " to study the natural history of grating acuity development in pediatric eye disorders, 26 " 9 and to monitor the efficacy of various treatment regimens in pre-verbal children. 14 " 12 These studies have used at least four different criteria for judging whether monocular grating acuity is within the normal range: (1) comparison of monocular grating acuity with binocular grating acuities of an age-matched normal populatj on i.2,4..7.i2. ^) comparison of monocular grating acuity with monocular grating acuities of an agematched normal population ; (3) comparison of the interocular grating acuity difference with test-retest differences in grating acuity of an age-matched normal or patient population 8-9 ; and (4) comparison of the interocular grating acuity difference with the interocular grating acuity differences of an agematched normal population Attempts to compare the sensitivity and specificity of the alternate criteria for monocular acuity deficit are complicated by several factors. Preferential-look- From the *Vision Research Center, Retina Foundation of the Southwest, and the tdepartment of Ophthalmology, University of Texas Health Science Center, Dallas, Texas. Supported in part by grants from the National Institutes of Health (EY-236), the Hoblitzelle Foundation, and Mr. and Mrs. David Strittmatter. Submitted for publication: December 12, 1986; accepted October 2, Reprint requests: Eileen E. Birch, PhD, Retina Foundation of the Southwest, Presbyterian Medical Center, 823 Walnut Hill Lane, Suite 414, Dallas, TX ing and operant grating acuities obtained within a given study and age group may span a range of 2 to 4 octaves and an even larger range among the many laboratories using different procedures and/or stimuli for the same age group. 13 " 19 The variability of interocular differences and of test-retest differences in the same age group tend to be lower and more consistent among laboratories. 2 " 22 The application of each criterion to different patient populations confounds the relative merits of each criterion with the characteristics of the population. The degree to which independent assessments of visual function were available to evaluate validity also varied from study to study. The present study was designed to assess developmental trends in monocular and binocular grating acuity, test-retest reliability and interocular differences in grating acuity in a normal population aged - years. Normative ranges for each measurement were established and the sensitivity and specificity of each of the criteria was evaluated for two groups of pediatric patients with unilateral eye disorders, one group with clinically low vision in the affected eye and a second group with clinically normal vision in the affected eye. These two groups were comparable in age and frequency of ophthalmic examinations and allowed for direct assessment of both the false negative (first group) and false positive (second group) rates. Materials and Methods Subjects Four hundred and seventy-five normal healthy children were contacted through the assistance of 636

2 No. 4 CRITERIA FOR ACUITY DEFICIT IN INFANCY / Dirch and Hole 637 nursery staff at a local medical center. All participants in the normal sample were born within 3 weeks of 4 weeks gestation and were assigned to age groups on the basis of post-term age. Children with any known systemic or ocular abnormality or who were at risk for eye disorders by family history were excluded from the normal sample. In order to assess the sensitivity and specificity of each criterion for monocular acuity deficit, two groups of pediatric patients (aged -6 months; mean = 17.7 months, s.d. = 13.9 months) were selected because they had a uniocular eye disorder, were otherwise healthy, and were judged to either have normal vision in the affected eye or moderate to deep visual impairment in the affected eye (Table 1). Each patient had an independent eye examination by a pediatric ophthalmologist, including cycloplegic refraction, motility testing and indirect ophthalmoscopy. Fourteen of the patients had mild unilateral ptosis, mild unilateral lid hemangioma or a small unilateral polar cataract. On clinical exam, each of this group of patients were judged to have central, steady fixation with each eye, orthophoria and no significant refractive error or fundus abnormalities. The second group of patients was composed of 2 unilaterally aphakic infants and children who had a history of dense congenital unilateral cataract with late surgery and/or poor compliance with lens wear and/or occlusion therapy. On clinical exam, each of these patients was judged to have poor to fair fixation ability with the affected eye, tropia or phoria, and/or significant fundus abnormalities. All aphakic patients wore appropriate optical correction during grating acuity tests. Informed consent was obtained from a parent prior to testing. Apparatus and Procedures For children aged - months, grating acuity was assessed by a forced-choice preferential-looking procedure. The infant was held over the parent's shoulder or on the parent's lap at eye level with two Polacoat Lenscreens (3M, St. Paul, MN) ( diameter; 36 center-to-center separation) which were located in a dark room cm away. Two red lightemitting diodes located midway between the screens wereflashedbetween trials to center the infant's gaze. High contrast (>9%) black and white vertical gratings and paired gray fields were rear-projected onto the screens by means of Kodak slide projectors. The gratings and gray fields were photometrically matched in space-averaged luminance (2. log cd/m 2 ). On each trial the infant was confronted with a grating vs. gray field stimulus pair. The observer, without knowledge of the position of the grating, made a forced-choice judgement about which side the infant preferred. No feedback was given to the Table 1. Patient population Unilateral visual disorder Clinically normal vision in affected eye Mild ptosis Hemangioma Anterior polar cataract Clinically visually impaired in affected eye Dense congenital unilateral cataractt Total N Total* tests* 3 * Seven children were tested at two different ages and five children were tested at three different ages. t These children had late surgery (>6 months), poor compliance with occlusion therapy, poor compliance with contact lens wear and/or fundus abnormalities. observer. On each trial, the grating appeared on a randomly chosen side according to stimulus sequences described by Fellows. 23 Gratings ranging from.38 to 48 c/deg were presented in a 2-down-lup staircase. In the staircase, two consecutive correct responses at a spatial frequency caused the staircase to progress to the next finer spatial frequency ("2- down") while a single incorrect response caused the staircase to reverse to the next broader spatial frequency ("1-up"). Low spatial frequency trials were added at random intervals (two to six trials) to assist in maintaining the infant's attention near threshold. Responses to these stimuli did not affect the progression of the staircase. Ten staircase reversals were obtained and grating acuity was defined as the average of the last eight reversal points. This staircase provides an estimate of threshold at 7.7% correct. 24 In the event of an incorrect response at the lowest spatial frequency (.38 c/deg), this error was counted as a reversal point and another pair of trials at the same spatial frequency was presented. Monocular testing was conducted with an adhesive orthoptic eye patch. For grating acuity assessment in toddlers and young children, an operant procedure was employed. ' 2 Two circular Polacoat Lenscreens (. diameter; 36 center-to-center separation) were mounted in a large black panel. The panel was angled about the midline of the child's head so that the midpoint of each screen was located at 2 cm distance. An aperture was located at the upper edge of each screen so that small pieces of cereal (Cheerios) could be delivered onto either of two ledges located below the screens. The entire stimulus panel could be raised or lowered so that stimuli were always presented at eye level. The child was seated in a high chair and a horizontal tray 14 cm below the center of the screens (chest level for a young child) established a testing distance of 2 cm. The stimuli were black and white, high contrast vertical square-wave gratings paired

3 68 INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / April 1988 Vol. 29 Table 2. Normal population Nominal age group Mean age N # Incomplete Mean # sessions* Mean # trials^ Mean # iesis% > Total * Children aged - months were scheduled for a single session only. If the child failed to complete two tests during a session, they were listed as incomplete. Children aged 13-6 months were asked to schedule return sessions when necessary to complete training and testing. The table lists the mean number of sessions required to complete at least one test. Children who did not complete at least one test by either the third or fourth session were listed as incomplete. t Mean number of trials per completed staircase. % Mean number of completed staircases per session (excluding the 33 children who were too sleepy or fussy to complete a single staircase and 9 children who participated in the initial feasibility study who were only tested once binocularly). with equal luminance grayfields (mean luminance of gratings and grays = 2.4 log cd/m 2 ). During testing, the stimuli were the only source of illumination in the room. Children were initially trained to point to the window containing a low frequency grating (.38 c/deg) rather than the window containing an equal luminance gray. Gratings appeared randomly on the left or right screen. When the child made a correct choice, a food reward was immediately delivered to the ledge below the grating. If the child made an incorrect choice, no food was delivered and the next stimulus pair was presented. Each child was required to reach a criterion of six consecutive correct choices in the training program in order to progress to the grating acuity test. In the grating acuity test, the child was presented with gratings ranging from 2.2 c/deg up to a maximum of 48 c/deg in approximate half- Table 3. Grating acuity protocols Nominal age group Total RE/LE RE/RE 12 BINOC/ BINOC 12 RE/ BINOC _ BINOC* only * Data were obtained in an initial feasibility study where only a single binocular test was attempted (n = 9), in sessions where children would not complete a second test (n = 16), or when parent or child refused monocular occlusion (n = 21). 132 octave steps using the same staircase procedure employed with infants. 24 Acuity was defined as the average of the last eight reversal points. In the event of two correct responses at the highest spatial frequency (48 c/deg), this was counted as a reversal point and another pair of trials at the same spatial frequency were presented. Monocular testing was conducted with either an adhesive orthoptic eye patch or a black plastic patch with an elastic band (whichever the child preferred). The number of normal children participating in each age group is given in Table 2 along with the number of children who failed to complete testing. Each session consisted of at least one pair of grating tests: (1) right eye and left eye; (2) right eye and repeat right eye; (3) binocular and repeat binocular; or (4) right eye and binocular. None of the infants aged - months participated in condition 4 (right eye and binocular) since an extensive series of these tests has recently been presented. 21 The pair of tests conducted first was randomly chosen. When cooperation permitted, additional pairs of tests were included in the session. When multiple tests were conducted in the same session, the observer or tester was blind to the acuity result until all tests were completed. The number of children who failed to complete testing, the average number of sessions required to complete testing, the average number of trials per test and the average number of tests per session are shown in Table 2. The number of children participating in each test is given in Table 3. Some children in the older age groups (>13 months) were able to complete more than two tests in a single session. For children aged 1- months, a single 3 min session was scheduled. For children aged 13-6 months, a 1 hr session was

4 No. 4 CRITERIA FOR ACUITY DEFICIT IN INFANCY / Birch ond Hole 639 scheduled and, in a few cases, return sessions were scheduled in order to complete training and testing. Any one staircase test had to be completed within a single session or the data were discarded. For the calculation of monocular grating acuity means and variances, data from one eye of each child were used. To quantify interocular grating acuity differences and test-retest reliability data in a similar format, a difference score (in octaves) was computed for each pair of test results (right eye grating acuity - left eye grating acuity, right eye grating acuity - retest right eye grating acuity, or binocular grating acuity - retest binocular grating acuity). Mean difference scores were computed using the raw difference scores in order to examine practice and fatigue effects, ie, whether the first or second test in the pair of tests administered consistently resulted in higher grating acuity. The sign of each difference score was then discarded and mean difference scores were computed using the absolute values of the raw difference scores in order to determine the average difference between tests regardless of the order in which they were administered. The same procedure was used in comparing monocular versus binocular grating acuity except that the difference scores were computed so that positive values always indicated higher binocular grating acuity and negative values always indicated higher monocular grating acuity. Results Mean grating acuity obtained under monocular and binocular viewing conditions for each age group is shown in Figure 1. Grating acuity improved from approximately 1.8 c/deg (2/33) at -2 months to approximately 32 c/deg (2/19) at 49-6 months. Nine of the children in the -2 month age group (8.9%) had one or more "reversals" of a staircase within the lowest spatial frequency available and this "floor effect" may have slightly elevated the mean grating acuity of this age group. The data from these children were not excluded since this would have resulted in an even greater artificial elevation of the mean. Five of the children in the 49-6 month age group (31.3%) and one child in the month age group (4%) had one or more pairs of correct responses at the highest spatial frequency available and this "ceiling effect" may have slightly lowered the mean grating acuity of this age group. Data from these children were not excluded because, over all trials at 48 c/deg, no child averaged above 7% correct. During months 6-6 there was a slight but consistent superiority of binocular grating acuity over monocular grating acuity (mean difference =.12 octaves). For children aged 13-6 months who completed both a right o TO ^ o o> 1 1-?.7.38 r 1 9 o Q Q -- O Post-term age (months) n 8 monocular binocular ^ Fig. 1. Mean monocular ( ) and binocular (O) grating acuity for 397 infants and children aged -6 months. Each mean is plotted at the center of the nominal age group. The solid line corresponds to the grating acuity above which 99% of the assumed normal distribution of monocular grating acuities obtained in each age group occurred. The dashed line corresponds to the grating acuity above which 99% of the assumed normal distribution of binocular grating acuities obtained in each age group occurred. The arrows indicate means which may have been slightly elevated or depressed by "floor" and "ceiling" effects described in the text. eye and binocular test during the same session (n = 74), binocular grating acuity averaged.13 octaves higher than right eye grating acuity, which was not significantly greater than zero (t 73 =.33, n.s.). The distribution of grating acuities within each age group did not significantly deviate from a normal distribution for either monocular or binocular testing (Kolmogorov-Smirnov tests, 26 ). On the assumption of the best-fitting normal distribution for each age group, the minimum grating acuity above which 99% of normal grating acuities would be expected were calculated for monocular and binocular testing and are shown in Figure 1 as the solid and dashed lines, respectively. There were no significant practice or fatigue effects, ie, none of the means of the raw difference scores differed significantly from zero by t-tests. Mean interocular differences in grating acuity for each age group are shown in Figure 2A. With the exception of months to, mean interocular grating acuity differences ranged from.17 to.39 octaves. The mean interocular differences were.82 at to 2 months and. at 3 to months. Under the assumption of the best-fitting normal distribution for each age group, the solid line in Figure 2A shows the maximum interocular grating acuity difference below which 99% of the normal population would be expected to occur. Test-retest reliability data for monocular and binocular grating acuity are shown in Figure 2B. Mean monocular test-retest reliability showed no trend with age and ranged from.22 to.44 octaves. The correlation between monocular test and retest results was

5 64 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / April 1988 Vol. 29 > O o O «/> 2 ) o O L B I Post term age (months) Fig. 2. (A) Mean interocular grating acuity differences ( ) for 161 infants and children. The solid line corresponds to the interocular difference below which 99% of the assumed normal distribution of interocular differences in each age group occurred. (B) Mean test-retest grating acuity differences of 178 infants and children for monocular ( ) and binocular (O) testing. The solid line corresponds to the test-retest difference below which 99% of the assumed normal distribution of monocular test-retest differences in each age group occurred. The dashed line corresponds to the testretest difference below which 99% of the assumed normal distribution of binocular test-retest differences in each age group occurred. high (r =.93, P < 1). Mean binocular test-retest reliability was also fairly constant across age groups and ranged from. to.63 octaves. Binocular test and retest results were significantly correlated (r =.9, P < 1). Under the assumption of the best-fitting normal distribution for each age group, the solid and dashed lines in Figure 2B show maximum test-retest differences below which 99% normal test-retest differences would be expected to occur in monocular and binocular testing conditions. These boundaries which characterize 99% of the normal population correspond to the criteria which have been employed previously for the evaluation of monocular grating acuity deficit in pediatric patients. Grating acuities from each patient were compared to the normal distributions of monocular and of binocular grating acuities. Interocular differences in grating acuity for each patient were compared to the normal distributions of interocular differences, to monocular test-retest differences, and to binocular test-retest differences. The usefulness of each criterion in detecting monocular grating acuity deficit in the two groups of pediatric patients is summarized in Table 4. Four outcomes were possible: (1) abnormal grating acuity or abnormal interocular difference in the presence of visual impairment (true positive); (2) normal grating acuity or normal interocular difference in the absence of visual impairment (true negative); (3) abnormal grating acuity or abnormal interocular difference in the absence of visual impairment (false positive); or (4) normal grating acuity or normal interocular difference in the presence of visual impairment (false negative). The sensitivity of each criterion was calculated as the proportion of visually impaired eyes which had abnormal grating acuity or abnormal interocular difference. The specificity of each criterion was calculated as the proportion of normal vision eyes which had normal grating acuity or normal interocular difference. The positive predictive value was calculated as the proportion of eyes with abnor- Table 4. Positive predictive value of grating acuity in the assessment of monocular acuity deficit Grating acuity criteria Monocular Binocular Interocular difference Difference criteria Monocular test-retest Binocular test-retest True positive True negative False positive False negative Sensitivity Specificity PPVf 17(1%)* 72 (6%) 3 (3%) 18(16%) (16%) 71 (6%) 4 (4%) 17(1%) 3 (27%) 74 (67%) 1(1%) (%) 31 (28%) 73 (66%) 2 (2%) 4 (4%) 27 (2%) 73 (66%) 2 (2%) 8 (7%) * Percent of total (n = 1 tests). t Positive predictive value.

6 No. 4 CRITERIA FOR ACUITY DEFICIT IN INFANCY / Dirch ond Hole 641 mal grating acuity or interocular difference which were visually impaired. 27 For eyes which were judged clinically to have normal vision (both eyes of children with mild unilateral ptosis, hemangioma, or polar cataract and the phakic eyes of unilateral aphakic patients), monocular and binocular grating acuity criteria produced false positive rates of 3% and 4%, respectively. These included three normal eyes (fellow eyes of eyes with polar cataract or unilateral cataract) and the affected eye of one child with mild lid hemangioma. For the same eyes, interocular difference and test-retest criteria produced false positive rates of 1% to 2% (one normal eye of a patient with polar cataract in the fellow eye and one eye affected by mild lid hemangioma). For eyes which were judged clinically to be visually impaired, grating acuity criteria produced false negative rates of 1% to 16% while interocular difference criteria produced false negative rates ranging from 4% to 7%. False negatives did not predominate in younger age groups, but were approximately equally distributed among all age groups. While the specificity of all criteria was high, ranging from.9 to.99, the sensitivity of interocular difference criteria ( ) was greater than the sensitivity of grating acuity criteria (.49-.1). Discussion The monocular and binocular grating acuity data presented here show a rapid improvement in grating acuity between the -2 month and 3- month age groups from approximately 1.8 to.9 c/deg (a 1.7 octave change) followed by a more gradual pace thereafter (averaging less than a.2 octave change per 3 months). While mean test-retest differences were fairly constant across age, interocular grating acuity differences were highest in the two youngest age groups and low and constant during months 6-6. Similar grating acuity, interocular difference, and test-retest data have been obtained in our laboratory using the same stimuli with a constant stimulus procedure for monocular and binocular testing of infants 621 and binocular testing of young children. 142 The grating acuity data also agree well with data from other laboratories (eg, ). Interocular grating acuity differences exceeding one octave have been found in other populations of young normal infants. 4 ' 2 Interocular grating acuity differences of less than one octave were reported in a population in which 14 of the 16 children were over 7 months of age. 9 The determination of each of the criteria for monocular acuity deficit from the data from a single large normative population and the application of these criteria to well-defined patient groups permits evaluation of the relative sensitivity and specificity of each criterion. Comparison of patient test results with either monocular or binocular grating acuity norms had high specificity but poor sensitivity. Low sensitivity was most likely due to the range of individual differences present in the normal population and the resultant overlap between normal and patient populations. Specifically, in the present study, the range of grating acuities within which 98% of the normal population occurs spans an average of 3.3 octaves for the preferential-looking age groups and an average of 2 octaves for the operant age groups. These ranges are similar to those reported by other laboratories. 13,16,17,2 Higher sensitivity was obtained when the interocular grating acuity difference was compared with the normal range of interocular differences or with the normal range of test-retest differences. This was primarily due to the lower variability of normal interocular and test-retest differences as compared with the range of normal grating acuities. In the preferentiallooking age groups, the range within which 98% of the normal population occurred spanned an average of 1. octaves for interocular grating acuity differences, 1 octaves for monocular test-retest differences and 1.2 octave for binocular test-retest differences. Variability was even lower in the operant age groups, spanning.9 octave,.9 octave and.7 octave, respectively. Little difference in sensitivity and specificity was found among the three difference criteria. This was primarily due to the approximate equality of the three difference criteria over the age range of the patient population (3 to 6 months) and, therefore, it made little practical difference which criterion was chosen. The criteria did not agree in the -2 month age group, where interocular grating acuity difference criteria exceed test-retest criteria by more than 1 octave. For example, if monocular and binocular test-retest criteria were applied to the interocular grating acuity differences obtained from normals in the present study, 38% of -2 month-old normals would appear as false positives (interocular difference significantly greater than test-retest difference). Use of clinical findings as the "gold standard" for comparison with grating acuity data may have resulted in different sensitivity and specificity values than would have been obtained with other "gold standards," such as Snellen acuity. Other authors have reported comparisons between grating acuity and Snellen acuity in verbal pediatric populations (eg, 29 ). However, the present study was designed to assess the utility of grating acuity criteria for the determination of monocular acuity deficit in the popu-

7 642 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / April 1988 Vol. 29 Table. Positive predictive value of grating acuity criteria obtained with various preferential-looking and operant paradigms. Monocular Grating acuity criteria Binocular Interocular difference Difference criteria Monocular test-retest Binocular test-retest Present study (99% criterion) Present study (9% criterion) Constant 1421 stimulus OPL 17 Acuity cards lation where preferential-looking and operant procedures are most likely to be employed, ie, patients who are too young to cooperate with Snellen acuity tests. In addition, this study was limited to two highly selected patient groups with unilateral eye disorders and clear-cut clinical findings of excellent or poor vision in the affected eye. This choice provided for a straightforward evaluation of false positives and false negatives, but cannot address the sensitivity and specificity in other unilateral eye disorders. While preferential-looking and operant grating acuity procedures developed in various laboratories share many features in common, differences in statistical criterion, mode of stimulus presentation and psychophysical procedure potentially could affect the absolute or relative sensitivity and specificity of various criteria. To examine the effect of choice of statistical criterion, the present patient data were reevaluated using a 9% rather than a 99% criterion. Results are summarized in Table. While the positive predictive value generally decreased (primarily due to higher false positive rates), interocular difference and test-retest difference criteria still performed better than monocular or binocular grating acuity criteria. Also in Table are the results obtained when criteria were based on data obtained with other procedures and/or stimuli. These produce predictive values roughly in keeping with those reported in the present study. Thus, while differences among techniques may introduce some variability into the grating acuity data, there exists general agreement among results using different techniques. The lower variability present in the normal distribution of interocular differences and test-retest differences in the present study provides for greater positive predictive value in the assessment of monocular grating acuity deficit in pediatric patient populations. Visual and many non-visual factors, including behavioral state, physical skills and social skills, may contribute to individual differences in preferential-looking and operant grating acuity. In most cases, many of the non-visual factors are constant during both tests conducted during a single session and, therefore, although they may affect the grating acuity obtained, have relatively little effect on the measurement of interocular differences. The power of the interocular difference criterion is even more apparent in longitudinal studies. For example, in a recent study of grating acuity development following early treatment of congenital unilateral cataract, many patients achieved grating acuities within the normal range with their aphakic eyes by age 3 to years. Nonetheless, fairly constant interocular grating acuity differences of. to 1 octave were noted during years 3 to. While use of an interocular difference criterion is clearly not applicable to all clinical situations (eg, bilateral visual impairment), its use in cases of suspected monocular visual deficit should significantly improve discrimination of eyes with normal grating acuity from eyes with grating acuity deficits. Key words: preferential looking, operant, grating acuity, interocular differences, pediatric patients Acknowledgments Special thanks to the staff of the newborn nursery at Margot Perot Women and Children's Hospital of Presbyterian Medical Center, Dallas, Texas, for their assistance in contacting participants of this study. References 1. Manning K, Fulton A, Hansen R, Mayer D, Petersen R, and Barg D: Preferential looking testing: Application to evaluation of high-risk, prematurely born infants and children. J Pediatr Ophthalmol Strabismus 19:286, Mayer D and Fulton A: Preferential looking grating acuities of infants at risk for amblyopia. Trans Ophthalmol Soc UK 4:93, Mayer D, Fulton A, and Sossen P: Preferential looking acuity of pediatric patients with developmental disabilities. Behav Brain Res :189, Dobson V: Clinical applications of preferential-looking measures of visual acuity. Behav Brain Res :2, Mohindra I, Jacobson S, Zwaan J, and Held R: Psychophysical assessment of visual acuity in infants with visual disorders. Behav Brain Res :1, Birch E and Stager D: Monocular acuity and stereopsis in infantile esotropia. Invest Ophthalmol Vis Sci 26:1624, Jacobson S, Mohindra I, and Held R: Age of onset of ambly-

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