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BINOCULAR VISUAL ACUITY OF NEONATES: THE ACUITY CARD PROCEDURE

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Page 1: BINOCULAR VISUAL ACUITY OF NEONATES: THE ACUITY CARD PROCEDURE

BINOCULAR VISUAL ACUITY OF NEONATES: THE ACUITY CARD PROCEDURE

Velma Dobson Terry L . Schwartz Dorothy J . Sandstrom Lisa Michel

Over the past 10 years, the forced-choice preferential looking (FPL) procedure (Teller et al. 1974, Teller 1979) has been one of the primary methods used for the behavioral assessment of visual acuity of infants (see Dobson and Teller 1978, Boothe et al. 1985, for reviews). In FPL acuity assessment the infant is shown a series of trials in which a patterned target, typically a black-and-white square-wave grating, is paired with a second target of equal space-average luminance. On each trial an adult, who is unaware of whether the grating is on the left or right, watches the infant’s fixation pattern and makes a forced-choice judgment concerning the location of the grating. Typically, the infant is tested with 20 trials of each of four or five grating spatial frequencies, and acuity is estimated as the spatial frequency that results in a score of 75 per cent correct judgements by the observer.

Recently, van Hof-van Duin and Mohn (1986) have shown that the FPL procedure can be used to assess visual acuity of neonates. However, other researchers have found FPL difficult to use with neonates because often they do not remain awake long enough for the large number of trials required. Baraldi et al. (1981), for example, reported that an FPL acuity estimate for a neonate, based on only two spatial frequencies, could take as long as

three hours. Also, those authors’ acuity results were significantly poorer than those reported by researchers who used less time-consuming procedures for be- havioral assessment of acuity of neonates (see Table I).

Early studies used one of two techniques for behavioral acuity assess- ment of neonates-either preferential looking (PL) (Fantz 1965, Miranda 1970) or optokinetic nystagmus (OKN) (Gorman w

E et al. 1957, 1959; Dayton et al. 1964). However, both techniques have limitations. In the PL procedure, each N

infant is tested with only a few (less than 10) trials, and the results are combined across infants to determine the average 5 acuity of infants as a group. Therefore this procedure does not provide acuity estimates for individual infants. In contrast, with the OKN procedure it is possible to estimate the acuity of individual infants. However, it is difficult

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to avoid artifacts in the large gratings required for OKN, and such artifacts may $ result in spuriously good acuity estimates s (Banks and Salapatek 1981). -

Recently two variants of the PL 5 procedure have been developed which % allow rapid acuity assessment of 2 individual infants. In both, an adult B

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observer watches an infant’s looking behavior in response to gratings of different 199

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spatial frequencies and then makes a subjective judgment concerning the highest spatial frequency grating that the infant can resolve. Using one of these variants, Dubowitz et al. (1980) and Morante et al. (1982) showed fullterm neonates !h-, 1/- and %-inch black-and- white stripes at a distance of 12 inches (corresponding to spatial frequencies of approximately 0.21, 0.42 and 0.84 cycle/deg). Based on an adult observer’s rating of the infant’s preference for and fixation on each target, a decision was made as to which targets were visible to the infant. All neonates were judged to see the 0.21 and 0.42 cycle/deg gratings and over 50 per cent were judged to see the 0.84 cycle/deg grating. The technique used by Dubowitz et al. and Morante et al. has not been validated against the FPL procedure, but it is encouraging that their acuity results are similar to those obtained for neonates by Fantz (1 965) and Miranda (1970), using the PL procedure (see Table I).

The second variant of the PL procedure that allows rapid, subjective acuity assessment of infants is the acuity card procedure (McDonald et al. 1985). In this procedure an adult observer presents the infant with gray cards containing black- and-white grating targets of various spatial frequencies. The observer’s task is to determine, from the infant’s looking patterns, which card contains the highest spatial frequency grating that the infant can resolve. The procedure has been validated against the FPL procedure for acuity assessment of infants and children between one and 36 months of age, and the results indicate good agreement between both the means and the standard deviations of the acuity values obtained with the two procedures (Dobson et al. 1985; McDonald et al. 1985, 1986a, b; Teller et at. 1986).

In the original acuity card study (McDonald et al. 1985), cards with two stimulus configurations were used: (a) a one-aperture configuration in which the grating was located behind a single centrally placed circular opening, and (b) a two-aperture configuration, similar to that used in FPL, in which one peripherally located opening contained the grating and the other contained a gray

stimulus of equal space-average luminance. No differences in the acuity results obtained with the two stimulus configurations were found for four-, eight-. and 16-week-olds and six-month- olds. However, observers expressed a preference for the one-aperture cards when testing four-week-olds and for the two-aperture cards when testing older infants (McDonald et al. 1985). Based on these results, Brown and Yamamoto (1986) used a set of one-aperture acuity cards to measure visual acuity of fullterm and preterm newborns. Their results for fullterm newborns agreed well with previous PL results (see Table I).

The goals of the present study were: (1) to add to the existing acuity card data by testing healthy fullterm neonates; and (2) to determine whether two-aperture cards are as effective as one-aperture cards for acuity assessment of neonates. If two- aperture cards were found to be effective, then a researcher or clinician interested in acuity assessment of infants would need only one set of cards to be able to assess children over the entire age-range between birth and 36 months.

Method Subjects and observers Subjects were 40 healthy fullterm newborn infants (mean age 63 hours, range 16 to 120 hours) born within f 11 days of due date, with birthweights between 2720 and 4500g. Data from an additional 23 infants were eliminated from the study. Of these 23 infants, 15 fell asleep, 12 during acuity testing and three during refractive error screening. A further seven were eliminated because refractive error screening*, using isotropic photorefraction (Howland et al. 1983), indicated significant astigmatism (three), hyperopia (three) or myopia (one), as judged by three adult raters who had no knowledge of the individual infants’ acuity results. Finally, *Green et al. (1980) have calculated that optical factors, such as refractive error, have little effect on a neonate’s acuity. Nevertheless, we felt it would be prudent to exclude the data of any neonates with unusual refractive errors. Therefore isotropic photorefraction was used to screen neonates in the study. Calibration difficulties prevented quantification of infants’ refractive errors. However, qualitative comparison of the results for neonates in the study was possible.

Page 3: BINOCULAR VISUAL ACUITY OF NEONATES: THE ACUITY CARD PROCEDURE

TABLE 1 Visual acuity values reported for neonates $

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Acuity 0; Mean SD N

P N Technique (cy/deg)t (octaves) m Study

Gorman et a/.

Gorman et a / .

Dayton et a/.

Fantz (1965)

Miranda (1970)

(1957)

(1959)

(1964)

Dubowitz et a/. ( 1980)

Baraldi et a/. (1981)

Morante et a/. ( 1982)

Van Hof-van Duin and Mohn (1986)

Brown and

Dobson et a/.

Dobson ef a/.

Yamamoto (1986)

(1985)

(1985)

100

100

I8

19

27

31

15

89

109

30

20

20

Optokinetic nystagmus

Optokinetic nystagmus

Optokinetic nystagmus

Preferential looking

Preferential looking

Subjective preferential looking

Forced-choice preferential looking

Subjective preferential looking

Forced-choice preferential looking

One-aperture acuity cards

One-aperture acuity cards

Two-aperture acuity cards

0.90 -

1 . 3 -

2.6 0.67

0*63* -

0.45 -

1 . 1 1 0.72

0-70 0.39

0.95 0.73

1.05 0.54

tBased on the widely used metric that 6/6 Snellen is equivalent to resolution of a 30 cy/deg grating, these values can be converted to Snellen notation as follows: (a) Snellen numerator = 6, (b) Snellen denominator = 6 x (30; acuity in cy/deg). Thus an acuity of 1 cy/deg can be equated with a Snellen acuity of 6/180. *Because limited numbers of spatial frequencies were used in these studies, these means are lower-bounds estimates of acuity.

data from one infant were excluded because of photorefractive equipment malfunction.

The four authors served as observers. One (v.D.) had extensive experience with the acuity card procedure. The other observers were a graduate student (D.J.s.) and an undergraduate (L.M.), both of whom had observed infants in preferential looking studies, and a post- doctoral fellow (T.L.s.), who had little previous experience in observing infants. These observers had little difficulty learning the acuity card procedure; all three felt comfortable with the procedure

after testing three to five practice subjects. In the present study, each observer tested 10 infants, five with each stimulus configuration.

Acuity cards The acuity cards measured 28 x 58cm and were constructed of gray cardboard. The one-aperture cards contained a centrally located 9-3cm (14.5 deg) diameter circular opening, behind which was positioned a photographically produced high-contrast (82 to 84 per cent) black- and-white square-wave grating. A 1 -5mm peephole was located in the center of each 201

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Fig. 1. Acuity card assessment of a fullterm neonate, using two-aperture cards.

grating, through which the infant’s visual acuity was observed.

The two-aperture cards contained two 9- 3cm (1 3 - 5 deg) circular openings with their centers located 1 1 * 3cm (17 - 5 deg) to the left and right of a 4mm central peephole. A grating (identical to the gratings used in the one-aperture cards) was located behind one aperture of each card and gray cardboard was placed behind the second aperture.

Gratings ranged in spatial frequency from 0 - 16 to 11.2 cycleddeg for the one- aperture cards, and from 0.18 to 12.3 cycleddeg for the two-aperture cards, in approximately half-octave* steps. The slight differences in spatial frequencies between the two types of card was because the gratings were peripheral in the two- aperture cards, and therefore at a greater distance from the infant’s eyes than the central gratings of the one-aperture cards. The cards were front-illuminated by the room lights. Luminance of the cards during testing ranged from 11.7 to

*An octave is a halving or doubling of spatial frequency, e.g. from 1 - 5 to 3 cyclesldeg.

21*9cd/m2, as measured with a Sunset Unittic light-meter calibrated to an SEI photometer. Luminance of the gratings differed from the luminance of the gray surround by less than 1 per cent.

Procedure Testing was conducted in two rooms adjacent to two of the fullterm nurseries at Magee-Womens Hospital, Pittsburgh. Informed consent was obtained from a parent after the procedure had been explained. The infant was held 1 -5m from the photorefraction camera and the three photographs of the infant’s eyes required for isotropic photorefraction were taken. The infant was then tested with the acuity cards, being held by an adult experimenter 36 f 3cm from the position at which the acuity cards were presented (Fig. 1). No apparatus, such as that used by McDonald et al. (1985) to minimize distractions during testing of older infants, was employed because neonates were not distracted by other objects in the room.

For acuity testing, the observer had available a subset of 10 one-aperture or 10

Page 5: BINOCULAR VISUAL ACUITY OF NEONATES: THE ACUITY CARD PROCEDURE

two-aperture cards. The subset used for each infant was chosen at random from seven possible subsets of each card type. The purpose in using different subsets was to keep the observer unaware of the absolute spatial frequencies in the subset of cards used in each test. The highest spatial frequency subset was made up of gratings ranging from 0.49 (0.53) to 1 1 - 2 (12.3) cycleddeg for the one-aperture (two-aperture) cards, in approximately half-octave steps, and the lowest spatial frequency subset was made up of gratings from 0.16 (0.18) to 1.40 (1 .53) cycleddeg for the one-aperture (two- aperture) cards. The intermediate subsets differed from one another by a half- octave in the spatial frequencies of the highest and lowest cards in the subset, with the exception of the three lowest spatial frequency subsets, all of which contained one or more cards with the lowest spatial frequency grating available. For testing, the cards in the subset to be used were numbered one to 10 (from lowest to highest spatial frequency) and placed face-down on a table beside the observer. Thus the observer knew the relative size of the gratings but was blind to the absolute spatial frequency of each grating and to the location of the grating (for the two-aperture cards). The observer’s task was to determine from the infant’s looking behavior which card contained the highest spatial frequency grating seen by the infant.

Initially the observer presented the infant with a low spatial frequency (approximately 0 .2 cycle/deg) grating to determine the infant’s looking pattern to a visible grating. When the observer felt comfortable with the infant’s looking pattern, she began presenting the cards in the test subset. Typically an observer would begin with a lower-frequency grating, number three or four in the subset, determine whether the infant’s looking behavior suggested that he or she could see the grating, and then proceed sequentially to higher or lower spatial frequencies, as dictated by the infant’s responses. For two-aperture cards, the observer could ask one of the other adults present whether the grating was located on the left or right, but only after she had presented the grating at least once on each

side. Observers were not required to present all cards in the subset. Testing was terminated when the observer was ready to record a decision concerning which grating was just above threshold for the infant. With one-aperture cards, the observer’s decision was usually based on the infant’s ability to refixate the central grating position of each card after the card had been moved to the left or right. With the two-aperture cards, the decision was based largely on the differential fixation shown to the two sides of the card by the infant as he or she was rotated from side to side in front of the card.

The duration of each test was timed, beginning when the initial, low spatial frequency card was presented and ending when the observer was ready to record a judgement concerning the infant’s acuity threshold.

Results Preliminary analyses An analysis of variance (ANOVA) was used to examine the effects of card type (one-aperture vs. two-aperture) and observer on acuity estimates. The results indicated no effect of card type (F 1 , 35 = 0.32, NS) or observer (F 3, 35 = 0.40, NS). Data were then collapsed across card type and observer, and ANOVAS were run to examine the effect of type of anesthetic at birth (spinal, epidural or localhone) and the effect of the card subset used during testing. There was no effect of anesthetic (F 2, 37 = 1.67, NS) on acuity results. There was a significant effect of card subset (F 6, 33=2-99, p<0.05). However, a Bonferoni t-test indicated that among the seven subsets of cards, the only pairwise comparison that produced a significant difference was between card subsets two and five, with subset two (composed of gratings between approximately 0.3 and 7 cyclesldeg) showing better acuity scores than card subset five (composed of gratings between approximately 0 .2 and 3 cyclesldeg). The fact that the most widely differing card subset pairs (subsets one vs. seven, one vs. six and two vs. six) did not produce significantly different acuity results indicates that the effect of card subset was not general enough to have had a substantial effect on the data.

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ONE-APERTURE CARDS

a TWO-APERTURE CARDS

0.4 0.5 0 .7 -1.0- 1.6 2.0

Fig. 2. Frequency distributions of acuity estimates for one- (left column) and two- (right column) aperture cards. Open and closed arrows below abscissa indicate mean acuity values of 0.95 cycle/deg and 1.05 cycles/deg obtained with one- aperture and two-aperture cards. respectively.

SPATIAL FREOUENCV (CVCLES/DEG)

Acuity results Acuity estimates for the 40 neonates are shown in Figure 2. The standard deviations of the one-aperture and two- aperture acuity estimates were 0.73 and 0.54 octave, respectively. Test durations averaged 7.8 minutes (SD 3.1) for the one-aperture cards and 8.3 minutes (SD 3.0) for the two-aperture cards. 51 of the 63 infants in the study completed both photorefraction screening and acuity card testing, a success rate of 81 per cent.

Observers’ preferences Although no significant differences in acuity results were found for one- vs. two- aperture acuity cards, all four observers preferred the two-aperture cards, primarily because, by rotating the cards through 180°, it was possible to compare the infant’s response when the grating changed left-right location.

Discussion The results of the present study confirm the finding of Brown and Yamamoto (1986) that the grating acuity card procedure can be used to estimate visual acuity in fullterm newborn infants in a hospital setting. They also indicate it to be a robust procedure which is unaffected by stimulus configuration.

As shown in Table I, the mean acuity values obtained in the present study are within an octave of those reported in most

of the previous behavioral studies of acuity in neonates, and are virtually identical to the recent FPL results of van Hof-van Duin and Mohn (1986). The two studies with substantially different results were the OKN study of Dayton et al. (1964), in which the unusually good acuities obtained probably were caused by low spatial frequency artifacts in the stimuli used (Banks and Salapatek 1981), and the FPL study of Baraldi et al. (1981), in which many neonates did not stay awake and alert long enough to complete the entire test procedure.

The standard deviations of 0.73 and 0.54 octave found for the one- and two- aperture cards, respectively, are within the range of standard deviations reported previously for neonates (see Table I). They are also within the range of standard deviations reported for grating acuity card (0.65 to 1 . 1 octave) (McDonald et al. 1985, 1986b) and FPL (0.45 to 0.89 octave) (Allen 1979) tests of binocular acuity in four- and eight-week-old infants. Thus the distribution of acuity scores shown by young infants appears to be similar, regardless of the technique used.

Our success rate of 8 1 per cent is similar to the 83 per cent success rate of Brown and Yamamoto (1986) for healthy fullterm neonates, but lower than the 87 to 100 per cent success rates in studies of four- and eight-week-olds tested in the laboratory (McDonald et al. 1985, 19863). Our test times are also longer than those reported by McDonald and colleagues, probably because of the difficulty in working with sleepy infants.

These results suggest that a researcher or clinician interested in acuity assessment of infants and young children can use one piece of equipment (the two-aperture acuity cards) to evaluate children from birth onward. Further studies of high-risk neonates are needed to determine whether screening such infants with the acuity card procedure will yield information that is predictive of later acuity deficits.

Accepted for publication 21st May 1986.

Acknowledgements We thank Dr. Robert Guthrie, Chief of Pediatrics; Ms. Gay Sims and Ms. Marilyn Pollard, Head

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Nurses; and the nursing staff of the 3800 and 2800 wards of Magee-Womens Hospital for their enthusiastic support of our research. We also thank the mothers and infants who participated in the study, and acknowledge the valuable comments provided by MaryAlice McDonald and Drs. Angela Brown and Davida Y. Teller on an earlier draft of the manuscript. This research was supported by the Magee-Womens Hospital Research Fund and by NIH grants R 0 1 EY05804, T32 AM07458, and DOE-OSE G008200344.

Authors’ Appointments *Velma Dobson, Terry L. Schwartzt, Dorothy J. Sandstrom, Lisa Michel; Departments of Psychiatry, Psychology and Medicine, University of Pittsburgh, Pittsburgh, PA. tCurrent address: Department of Ophthalmology, West Virginia University, Morgantown, WV.

*Correspondence to first author at Department of Psychology, 462 Langley Hall, University of Pittsburgh, Pittsburgh, P A 15260.

SUMMARY The acuity card procedure is a variant of the forced-choice preferential looking technique. In this study, four observers each tested 10 infants (all healthy fullterm neonates) with acuity cards containing either a one-aperture or a two-aperture stimulus configuration. For both card types the mean test time was eight minutes. No effect on acuity results was found between the two different configurations. The mean acuity values were within one actave of those reported in most previous behavioral studies of the acuity of neonates. The authors consider that the acuity card procedure provides a rapid, accurate technique for assessment of healthy newborn infants in a hospital setting.

RESUME Acuite visuelle binoculaire chez le nouveau-ne: le procede de la carte d’acuitd Le procede de la carte d’acuite est une variante de la technique de regards preferentiels A choix force. Dans cette etude quatre observateurs examinkrent chacun 10 nourrissons (tous nouveau-nes a terme en bonne sante) avec des cartes d’acuite contenant une configuration de stimulus B une ou les deux types de cartes le temps moyen du test etait de huit minutes. Aucun effet sur les resultats d’acuite n’est apparu lie A I’usage de l’une ou I’autre des configurations. Les valeurs d’acuite moyenne etaient dans une octave de celles rapportees dans la plupart des etudes comportementales anterieures de I’acuite des nouveau-nes. Les auteurs pensent que le procede de la carte d’acuitk fournit une technique rapide et precise pour I’examen des nouveau-nes en bonne sante dans un milieu hospitalier.

ZUSAMMENFASSUNG Binokulare Sehscharfe bei Neugeborenen: Untersuchung mit der Acuity Card Die Untersuchung mit der Acuity Card ist eine Abwandlung der forcierten Preferential Looking Methode. In dieser Studie haben je vier Untersucher 10 Kinder (alles gesunde reife Neugeborene) mit Acuity Cards untersucht, die uber eine oder zwei Offnungen stimulierten. Fur beide Testkarten betrug die mittlere Untersuchungszeit acht Minuten. Die Ergebnisse fur die Sehscharfe waren bei beiden Testkarten gleich. Die mittleren Werte fur die Sehscharfe lagen innerhalb einer Oktave der meisten bisher veroffentlichten Verhaltensstudien mit Sehscharfenbestimmung bei Neugeborenen. Die Autoren sind der Meinung, daR die Untersuchung mit der Acuity Card eine schnelle, genaue Methode ist, um gesunde Neugeborene in einer Klinik zu beurteilen.

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RESUMEN Agudeza visual binocular en 10s recien nacidos: aplicacion de una tarjeta de agudeza El procedimiento de la tarjeta de agudeza es una variante de la tecnica de la visi6n preferencial por seleccion forzada. En el presente estudio cuatro observadores exploraron cada uno 10 lactantes (todos nacidos a tkrmino y sanos) con tarjetas de agudeza que contenian una abertura o dos aferturas como estimulo configurado. Para ambos tipos de tarjeta el tiempo promedio de la prueba fue de ocho minutos. No se ha116 ningun efecto sobre la agudeza entre 10s dos tipos de configuraciones. Los valore promedio de agudeza eran dentro de una octava de 10s publicados en la mayoria de 10s estudios previos de comportamiento de la agudeza en recien nacidos. Los autores consideran que la tarjeta de agudeza proporciona una tecnica rapida y depurada para la evaluacibn de lactantes recien nacidos sanos en un ambiente hospitalario.

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