PerceptmlandMotor Skills, 1992, 75, 1179-1185. O Perceptual and Motor Skills 1992
VISUAL ACUITY AND STEREOACUITY AMONG MENTALLY RETARDED CHILDREN ' J. E. LETOURNEAU, C. BEAULNE, L. DUPLESSIS ~ c o l d'opiomttrie e Uniuersitt? de Montrkl Summary.-To evaluate the reliability and the validity of Landolt Rings and of the Frisby Test as measures of visual acuity and stereoacujty, respectively, the visual acuity of 30 mentally retarded children was measured with Landolt Rings shown as games, the Sjogren Test, the Dot Visual Acuity Test and stereoacuity with the Frisby Test. Subjects were tested 3 times over a period of 3 weeks to measure the reliability of the tests. No s~gnificantdiFference was observed among these tests. The validity of the Landolt k n g s was measured by a correlation of .55 for the highest IogMAR values of the Sjogren Test with the highest logMAR values of the Landolt Fhgs. Visual acuity was systematically lower on the Dot Visual Acuity Test. The Frisby Test was not reliable among a group of 16 normal children who improved systematically over 3 weeks.
Accurate measurement of children's visual acuity has been a subject of controversy since Worth's first attempts with the Ivory Ball Test in 1903 (Duke-Elder, 1962, p. 374). New tests have been introduced regularly since then (Fern & Manny, 1986); the reliability and validity of these tests are not equally well documented. Grating visual acuity has often been measured but Snellen equivalent acuity tests are still the standard for measurement (Campbell & Green, 1965; Jenkins, Prager, Mazow, Allen, & Russek, 1983). Richman, Petito, and Cron (1984) have adapted the Landolt Rngs Test (the Broken Wheel Acuity Test); however, the Allen Figure Test yielded more responses than the Broken Wheel Acuity Test among 3- to 5-yr.-old children (Schmidt, 1991). Assessment of visual acuity among mentally retarded children raises additional problems. Limited capacity to understand instructions, short attention span, and illiteracy are among the main difficulties. These children are usually classified into four categories. Tesring visual acuity is more important among educable mentally retarded children because they are expected to reach autonomy. The present purpose was to measure the reliability and the validi t y of the Frisby stereoacuity test and of modified testing conditions with the Landolt Rings Test.
METHOD The subjects were 18 boys and 12 girls, aged 12 to 16 years (M: 13.9 yr.) and attending a special school for educable mentally retarded children. Their average IQ of 66 on the WISC was equivalent to eight years of mental 'Requests for reprints should be addressed to J. E. Letourneau, ~ c o l ed'optometrie, UniversitC de Montrtal, PO Box 6128, Montdal, Qutbec, Canada H3C 3J7.
1180
J. E. LETOURNEAU, ET AL.
age; they had been tested during the last two years. They had no history of visual deficiencies (other than refractive errors) that could be expected to reduce visual acuity. They were tested binocularly once a week over a period of three weeks. They wore their spectacles. The sequence was always identical using the Dot Visual Acuity Test, Frisby, Sjogren, and Landolt Rings tests. Testing at the child's eye level was completed by a single trained clinician, in a room with standard classroom illumination (35 ft-cd 10). Targets were shown separately so that no crowding effect would exist. Institutional approval of the investigation and informed consent of the subjects were obtained. A group of 7 boys and 9 girls, aged 7 to 9 years (M: 8.0 yr.) were selected among normal children in an elementary school. The purpose was to evaluate the reliability of the Frisby test. The children's mental age was equivalent to that of the mentally retarded children. The Dot Visual Acuity Test was administered according to the authors' recommendations (Kirschen, Rosenbaum, & Ballard, 1983). The child's task was to find a black dot randomly placed on the 7-cm diameter white search field and touch accurately the dot with a finger from a distance of 10 inches. When the dot was detected, the black mask was moved to the next smallest dot and the task repeated. A descending method of limits was used to approximate the threshold dot size. When the first incorrect response was recorded, the preceding larger dot size was displayed again. If this was identified correctly, the smaller dot was presented again. This process continued until two consecutive errors were made on the same size dot. The threshold was the smallest dot identified correctly twice. The standard acuity measure was the Sjogren test presented at 10 feet (Sjogren, 1939). Its primary advantage is that of being understood by illiterate children. The test was made of black silhouettes of a wide open hand on a white background. Size variation corresponded to different visual acuities. The child had to show the position of the fingers (up, right, left, or down). The Sjogren test was chosen because it is an attempt to increase interest and improve testability of preschool children. The optotypes (fingers) are more meaningful than the Tumble E test and should increase the number of pertinent answers among mentally retarded children. The Snellen equivalent of the symbols was changed a few years ago and the new values of the Sjogren test were used here. Visual acuity was expressed as the logarithm of the Minimum Angle of Resolution (IogMAR) and was determined using a method of interpolation that gave credit for every optotype read correctly (Kitchin & Bailey, 1981). Westheimer (1979) has shown that the IogMAR scale of acuity closely approximates an interval scale and so is a more appropriate scale for the application of parametric statistics. For example, each hand in a row had an assigned value of logMAR 0.02. A Snellen acuity of 619-2 is
*
VISUAL ACUITY AND STEREOACUITY
1181
the equivalent of 0.22 logMAR. The shortcomings of the Snellen chart have been discussed elsewhere (Lovie-Kitchin, 1988, 1989; Wild & Hussey, 1985). The Landolt Rings were described to the child as small garages. The child held a small toy (a bicycle) in hls hand. H e had to find the garage door to park his bicycle on the first trial. All of the children readily understood and enjoyed participating in this task. The Landolt Rings are equivalent to Snellen values of visual acuity. Scores were transformed in logMAR notation. The test was presented at 20 feet. The acuity equivalents are given in Table 1. TABLE 1 SNELL-STERLING A N D SNELLEN ACUITYEQUIVALENTS Snell-Sterling %
Snellen
LogMAR
The Frisby stereotest is made up of three plates of 1 mm, 3 mm, and 6 mm thick. O n the surface of each plate were four squares representing random dot-like patterns. One of the four squares displayed a 3-cm diameter circle missing printed on the reverse of the plate. When the plate was presented on a uniform white background, children saw four squares of random dots, but only those able to discriminate the disparity created by the thickness of the plate could see the circle. At 40 cm the disparities of the three plates were 340, 170, and 55 seconds of arc. A wide variety of disparities could be created by changing the test distance. The words "circle," "balloon," or "ball" were used to explain the test. In addition, the examiner allowed some time to the child to develop stereopsis. When the child found the right answer at a given disparity, the plate was rotated and used again until three consecutive right answers were recorded; then the test distance or the plate was changed to a more difficult task. The 6-mm plate was first shown three times at 40 cm. Then, with each right answer, the distance was increased by steps of 10 cm to a maximum of 80 crn. Subsequently, a thinner plate was used. The stereothreshold (seconds of arc) was then recorded as the smallest disparity the child could see. The test was recorded three times (at one-week intervals) in the two groups of subjects.
J. E. LETOURNEAU, ET AL.
1182
RESULTS Reliability An analysis of variance showed that no difference was observed among means on the three trials (logMAR values) when subjects were measured on the Sjogren test (F,,2, = 1.32, p = 0.27) and with the Landolt Rings test (F,,,, = 2.35, p = 0.11). These tests were reliable (Table 2 ) . Since the intervals between the nine targets used in the Dot Visual Acuity Test were unequal, it was considered inappropriate to use parametric inferential statistical tests. The percentage of adequate responses was used as an estimate of the reliabihty of the test. Analysis shows that 42% of the subjects had identical results over the three trials and that 97% had identical results over two trials. The reliability of the Dot Visual Acuity Test had already been shown among normal children (Letourneau & Giroux, 1988).
Tests Sjiigren Test Landolt Rings
Source
MS
df
F
P
Between Trials Within Trials Between Trials Within Trials
0.00626
2
1.32
0.27
0.00472 0.01387 0.00591
60 2 58
2.35
0.11
An analysis of variance showed no difference among the three trials on the Frjsby test (F,,,, = 2.82) among mentally retarded children; however, a significant difference was found (F,,,, = 5.54, p c 0 . 0 3 ) among the normal children who improved systematically on each trial (Table 3). Normal children learned the Frisby test while mentally retarded children did not. Since the test was not reliable among normal children, it was inappropriate to evaluate the validity of the test. The differences between the two groups were significant (p
VISUAL ACUITY AND STEREOACUITY AMONG MENTALLY RETARDED CHILDREN ' J. E. LETOURNEAU, C. BEAULNE, L. DUPLESSIS ~ c o l d'opiomttrie e Uniuersitt? de Montrkl Summary.-To evaluate the reliability and the validity of Landolt Rings and of the Frisby Test as measures of visual acuity and stereoacujty, respectively, the visual acuity of 30 mentally retarded children was measured with Landolt Rings shown as games, the Sjogren Test, the Dot Visual Acuity Test and stereoacuity with the Frisby Test. Subjects were tested 3 times over a period of 3 weeks to measure the reliability of the tests. No s~gnificantdiFference was observed among these tests. The validity of the Landolt k n g s was measured by a correlation of .55 for the highest IogMAR values of the Sjogren Test with the highest logMAR values of the Landolt Fhgs. Visual acuity was systematically lower on the Dot Visual Acuity Test. The Frisby Test was not reliable among a group of 16 normal children who improved systematically over 3 weeks.
Accurate measurement of children's visual acuity has been a subject of controversy since Worth's first attempts with the Ivory Ball Test in 1903 (Duke-Elder, 1962, p. 374). New tests have been introduced regularly since then (Fern & Manny, 1986); the reliability and validity of these tests are not equally well documented. Grating visual acuity has often been measured but Snellen equivalent acuity tests are still the standard for measurement (Campbell & Green, 1965; Jenkins, Prager, Mazow, Allen, & Russek, 1983). Richman, Petito, and Cron (1984) have adapted the Landolt Rngs Test (the Broken Wheel Acuity Test); however, the Allen Figure Test yielded more responses than the Broken Wheel Acuity Test among 3- to 5-yr.-old children (Schmidt, 1991). Assessment of visual acuity among mentally retarded children raises additional problems. Limited capacity to understand instructions, short attention span, and illiteracy are among the main difficulties. These children are usually classified into four categories. Tesring visual acuity is more important among educable mentally retarded children because they are expected to reach autonomy. The present purpose was to measure the reliability and the validi t y of the Frisby stereoacuity test and of modified testing conditions with the Landolt Rings Test.
METHOD The subjects were 18 boys and 12 girls, aged 12 to 16 years (M: 13.9 yr.) and attending a special school for educable mentally retarded children. Their average IQ of 66 on the WISC was equivalent to eight years of mental 'Requests for reprints should be addressed to J. E. Letourneau, ~ c o l ed'optometrie, UniversitC de Montrtal, PO Box 6128, Montdal, Qutbec, Canada H3C 3J7.
1180
J. E. LETOURNEAU, ET AL.
age; they had been tested during the last two years. They had no history of visual deficiencies (other than refractive errors) that could be expected to reduce visual acuity. They were tested binocularly once a week over a period of three weeks. They wore their spectacles. The sequence was always identical using the Dot Visual Acuity Test, Frisby, Sjogren, and Landolt Rings tests. Testing at the child's eye level was completed by a single trained clinician, in a room with standard classroom illumination (35 ft-cd 10). Targets were shown separately so that no crowding effect would exist. Institutional approval of the investigation and informed consent of the subjects were obtained. A group of 7 boys and 9 girls, aged 7 to 9 years (M: 8.0 yr.) were selected among normal children in an elementary school. The purpose was to evaluate the reliability of the Frisby test. The children's mental age was equivalent to that of the mentally retarded children. The Dot Visual Acuity Test was administered according to the authors' recommendations (Kirschen, Rosenbaum, & Ballard, 1983). The child's task was to find a black dot randomly placed on the 7-cm diameter white search field and touch accurately the dot with a finger from a distance of 10 inches. When the dot was detected, the black mask was moved to the next smallest dot and the task repeated. A descending method of limits was used to approximate the threshold dot size. When the first incorrect response was recorded, the preceding larger dot size was displayed again. If this was identified correctly, the smaller dot was presented again. This process continued until two consecutive errors were made on the same size dot. The threshold was the smallest dot identified correctly twice. The standard acuity measure was the Sjogren test presented at 10 feet (Sjogren, 1939). Its primary advantage is that of being understood by illiterate children. The test was made of black silhouettes of a wide open hand on a white background. Size variation corresponded to different visual acuities. The child had to show the position of the fingers (up, right, left, or down). The Sjogren test was chosen because it is an attempt to increase interest and improve testability of preschool children. The optotypes (fingers) are more meaningful than the Tumble E test and should increase the number of pertinent answers among mentally retarded children. The Snellen equivalent of the symbols was changed a few years ago and the new values of the Sjogren test were used here. Visual acuity was expressed as the logarithm of the Minimum Angle of Resolution (IogMAR) and was determined using a method of interpolation that gave credit for every optotype read correctly (Kitchin & Bailey, 1981). Westheimer (1979) has shown that the IogMAR scale of acuity closely approximates an interval scale and so is a more appropriate scale for the application of parametric statistics. For example, each hand in a row had an assigned value of logMAR 0.02. A Snellen acuity of 619-2 is
*
VISUAL ACUITY AND STEREOACUITY
1181
the equivalent of 0.22 logMAR. The shortcomings of the Snellen chart have been discussed elsewhere (Lovie-Kitchin, 1988, 1989; Wild & Hussey, 1985). The Landolt Rings were described to the child as small garages. The child held a small toy (a bicycle) in hls hand. H e had to find the garage door to park his bicycle on the first trial. All of the children readily understood and enjoyed participating in this task. The Landolt Rings are equivalent to Snellen values of visual acuity. Scores were transformed in logMAR notation. The test was presented at 20 feet. The acuity equivalents are given in Table 1. TABLE 1 SNELL-STERLING A N D SNELLEN ACUITYEQUIVALENTS Snell-Sterling %
Snellen
LogMAR
The Frisby stereotest is made up of three plates of 1 mm, 3 mm, and 6 mm thick. O n the surface of each plate were four squares representing random dot-like patterns. One of the four squares displayed a 3-cm diameter circle missing printed on the reverse of the plate. When the plate was presented on a uniform white background, children saw four squares of random dots, but only those able to discriminate the disparity created by the thickness of the plate could see the circle. At 40 cm the disparities of the three plates were 340, 170, and 55 seconds of arc. A wide variety of disparities could be created by changing the test distance. The words "circle," "balloon," or "ball" were used to explain the test. In addition, the examiner allowed some time to the child to develop stereopsis. When the child found the right answer at a given disparity, the plate was rotated and used again until three consecutive right answers were recorded; then the test distance or the plate was changed to a more difficult task. The 6-mm plate was first shown three times at 40 cm. Then, with each right answer, the distance was increased by steps of 10 cm to a maximum of 80 crn. Subsequently, a thinner plate was used. The stereothreshold (seconds of arc) was then recorded as the smallest disparity the child could see. The test was recorded three times (at one-week intervals) in the two groups of subjects.
J. E. LETOURNEAU, ET AL.
1182
RESULTS Reliability An analysis of variance showed that no difference was observed among means on the three trials (logMAR values) when subjects were measured on the Sjogren test (F,,2, = 1.32, p = 0.27) and with the Landolt Rings test (F,,,, = 2.35, p = 0.11). These tests were reliable (Table 2 ) . Since the intervals between the nine targets used in the Dot Visual Acuity Test were unequal, it was considered inappropriate to use parametric inferential statistical tests. The percentage of adequate responses was used as an estimate of the reliabihty of the test. Analysis shows that 42% of the subjects had identical results over the three trials and that 97% had identical results over two trials. The reliability of the Dot Visual Acuity Test had already been shown among normal children (Letourneau & Giroux, 1988).
Tests Sjiigren Test Landolt Rings
Source
MS
df
F
P
Between Trials Within Trials Between Trials Within Trials
0.00626
2
1.32
0.27
0.00472 0.01387 0.00591
60 2 58
2.35
0.11
An analysis of variance showed no difference among the three trials on the Frjsby test (F,,,, = 2.82) among mentally retarded children; however, a significant difference was found (F,,,, = 5.54, p c 0 . 0 3 ) among the normal children who improved systematically on each trial (Table 3). Normal children learned the Frisby test while mentally retarded children did not. Since the test was not reliable among normal children, it was inappropriate to evaluate the validity of the test. The differences between the two groups were significant (p
