Vision Res. Vol. 32, No. 7, pp. I21 I-1218, 1992 Printed in Great Britain. All rights reserved
Copyright
0042-6989/92 $5.00 + 0.00 (c’ 1992 Pergamon Press Ltd
Effect of Light Scatter on the Pattern Reversal Visual Evoked Response: Comparison with Psychophysical Results* HITOMT TETSUKA,_F GUANG-JI WANG,?
OSAMU TATSUO
KATSUMI,I_$§ HIROSEt$l
Received 26 June 1991; in revised form
13 November
ANTHONY
J. MORANDI,q
SOICHI
TETSUKA,_F
1991
The effect of light scatter on the pattern reversal visual evoked response (PVER) was studied in 6 normal subjects. The results were compared with contrast visual acuity, contrast sensitivity function, and glare disability. Light scatter was induced by translucent acrylic sheets. Visual acuity measured with the low-contrast charts decreased significantly (P < 0.0001) even with a small degree of light scatter. Contrast sensitivity decreased with a small degree of light scatter especially for high spatial frequencies. PVER amplitudes decreased especially at the smaller checks with its peak shifted to larger checks. PVER was equally sensitive to light scatter compared to psychophysical tests. Contrast sensitivity function
Glare disability
Light scatter
INTRODUCTION Since its introduction more than two decades ago (Spekreijse, 1966) the pattern reversal visual evoked response (PVER) has become clinically important and is used in ophthalmological, neurological, and psychological research. Many investigators have studied various factors that influence the PVER to help establish standard recording conditions (Arden, Bodis-Wollner, Halliday, Jeffreys, Kulikowski, Spekreijse & Regan, 1977) and to assist the interpretation of the results obtained from the patients with visual disturbances. Factors of the pattern stimulus that influence the PVER include the temporal and spatial frequencies of the stimulus, its contrast, the stimulus field size, and the mean luminance. Some electrophysiologists analyze the shape of the PVER amplitude-check size function curve (Regan, 1980) determining not only the smallest check size that produces the observable PVER (critical check size) but also the overall shape of the amplitude-check size function curve, which in normal adults has an inverted-U shape so that the largest amplitude is produced by intermediate check sizes. It has been reported that the shape of the PVER amplitudecheck size function curve changes in many pathological
Pattern reversal VER
Visual acuity
conditions, e.g. functional amblyopia (Levi & Harwerth, 1978). Opacities of the ocular media, such as cornea1 nebula, edema and scars, cells and flare in the anterior chamber, cataract, and the cells in the vitreous make the light scatter in different manners and may decrease retinal image contrast (Miller & Benedek, 1973). Patients with early cataract often complain of increased glare, but visual acuities measured with the usual high-contrast optotypes may show little or no decrease. However, in such patients other psychophysical tests such as contrast sensitivity function (CSF) or an estimate of glare disability may become abnormal, since discrimination of low-contrast objects is impaired (Hess & Woo, 1978; Hess & Carney, 1979; Abrahamsson & Sjbstrand, 1986). Van der Berg and Boltjes (1988) studied the effect of stray light induced by frosted glass on pattern reversal electroretinogram (PERG) and found considerable reduction of PERG amplitudes especially in small check sizes. To the best of our knowledge, the effect of light scatter, an important parameter in visual function, never has been studied systematically with the PVER. This study reports how the PVER is affected by artificially induced light scatter and the results were compared with psychophysical methods, such as visual acuity measurements in different contrast charts, CSF, and glare testing.
*Presented in part at the 1990 meeting of the Association for Research in Vision and Ophthalmology, Sarasota, Fla, U.S.A. tschepens Eye Research Institute, 20 Staniford Street, Boston, MA 02114, U.S.A. $Department of Ophthalmology, Harvard Medical School, Boston. Mass., U.S.A. §To whom reprint requests should be addressed at Schepens Eye Research Institute. SRetina Associates, Boston, Mass., U.S.A.
SUBJECTS AND METHODS Subjects
Six ophthalmologically normal subjects (three males, three females) ranging in age from 25 to 42 yr (mean 33.0) participated in this investigation. Moderate refractive 1211
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NUMBER OF ACRYLIC SHEETS
FIGURE 1. The effect of the translucent acrylic sheets used to produce light scatter on the overall mean luminance level. The abscissa shows the number of layers of acrylic sheets, the ordinate, the relative mean luminance. The log mean luminance decreases linearly with the number of acrylic sheets used. With 10 layers, the fall of mean luminance compared with 0 layers is about 37.1% or 0.2 log unit.
anomalies in three of the six subjects were optically corrected to emmetropia at the time of each testing. Both the electrophysiological and psychophysical tests were performed monocularly using the dominant eye, which was determined by a hole-in-card method. Testing procedures were fully explained to all subjects and informed consent was obtained from each subject before testing. methods Changes in light scatter of images. Light scatter was induced artificially by placing translucent acrylic sheets 12 mm from the cornea. The degree of light scatter was altered by increasing the number of sheets from 0 (no sheets) to 6 in one-increment steps, 8 and 10. The acrylic sheets were placed together in hand-held trial frames. Figure 1 shows the mean luminance level measured with a photometer placed in the same position as the subject’s eye in line with the center of the television monitor that was used for PVER testing. The log mean luminance falls linearly as the degree of light scatter increases. With 10 sheets, the mean luminance level dropped about 37.1% or 0.2 log unit, indicating a 0.02 log unit fall per sheet. Fs~ch~~h~sic~~ tests. Visual acuity was measured under normal room light with the Vacate-Contest Visual Acuity Charts (VCVAC) developed by Wang and Pomerantzeff (1991). The VCVAC consists of four charts: standard, high-contrast (900/), intermediatecontrast (15%), low-contrast (2.5%), and reverse polarity. In the last, the contrast is the same as that of the standard, high-contrast chart, but instead of black optotypes on a white background, the white letters are displayed on the black background. The mean luminance is constant for all charts. There are two E optotypes on thirteen different visual acuity levels ranging from 20/12 to 20/200 with a testing distance of 10 ft (3.05 m). The level of visual acuity is arranged in a geometric progression so that three acuity levels are equivalent to 1.0 octave. The directions of the E optotypes were randomized. CSF was measured in normal room light (50 ft-L) with the Vision Contrast Test System@ (VCTS) 6500 (V&tech,
Dayton, Ohio) developed by Ginsburg ( 1984). The test consists of five rows of 3 in. dia patches corresponding to spatial frequencies of 1.5, 3, 6, 12 and I8 cjdeg. Glare disability was measured with the apparatus developed by Wolf (1960~. The visual task consisted of identifying the positions of gaps in Landolt rings (14.5 min of arc) arranged in three circles, 4. 7, and 10 at the center of a circular glare source that subtended a visual angle of 2” at a testing distance of 71 cm. The targets were located on a translucent screen with a luminance variable (combinations of two sets of neutral density filters) between 0.00184 and 21.38 mL in 40 logarithmic steps. With a bright light facing the subject. the tester increased the background illumination of the Landolt rings until the gaps were identified correctly. The target screen luminance necessary to identify the gaps was determined for five levels of glare luminances. The glare sensitivity value exceeding 3 was considered as abnormal. ~~ectru~h~~io~~g~cff~ test. Steady-state PVER recordings were performed in ambient light. Silver disc electrodes were used. The active electrode was placed at 0, and the reference electrode at P, (standard lo-20 convention). Ground electrodes were placed on both earlobes. The stimulus display was a 17 in., high-resolution television monitor. The overall stimulus field size was 20 x 20 cm, subtending a visual angle of IO x IO‘ at the distance of 120 cm. The mean luminance level was 50 cd/ m*, and the contrast ratio was kept constant at 30%. A small square of black tape was placed in the center of the screen to serve as the fixation target. For testing parameters, the PVER recordings were obtained with five check sizes, 10,20,40, 80, and 160 min of arc. The temporal frequency of the stimulus pattern was 12 reversals/see or 6 cjsec, and the analysis time 5OOmsec. Fifty responses were fed into an averaging computer after amplification through the filter setting of 70 Hz (high-cut) and 1.0 Hz (low-cut). The PVER recording started from the largest check size, 160 min of arc and decreased by 1.0 octave steps to the smallest, 10 min of arc in all subjects. The PVER amplitude-check size function curve was obtained by plotting the amplitudes against five different check sizes in log adjusted expression for each number of acrylic sheets. Since we used the steady-state stimulation to record PVER, the implicit time was not analyzed in this study. In both the psychophysical and the electrophysiological examinations, testing was performed with and without acrylic sheets. In this study, the degree of light scatter was expressed with the number of acrylic sheets applied. For example, when two sheets were used, it was designated as “two” layers. Statistical analysis. To analyze the results on visual acuity or PVER amplitudes, the data can be viewed as arising from a factorial experiment in a randomized block design, where the acuity chart or check size and light scatter are the factors and the 6 subjects are the blocks. For the analysis of visual acuity two questions were addressed: (1) for each visual acuity chart, at what light
LIGHT
SCATTER
AND
scatter level does the visual acuity differ significantly from visual acuity with no light scatter; and (2) are there differences in visual acuity between the charts at each level of light scatter? In these analyses, the logs of visual acuity are used to produce an approximately linear relationship with light scatter level. To answer the first question, the data for each visual acuity chart are viewed as arising from a randomized block design with the light scatter the factor of interest. If a significant F-test indicates differences in visual acuities for the different levels of light scatter, then Dunnett’s (1955) multiple comparison procedure can be used to compare the mean visual acuities at each level of light scatter to the mean visual acuities when there is no scatter with an
FIGURE
2. Photographs
PATTERN
REVERSAL
VER
1213
overall level of significance of 0.05. The second question can be answered by viewing the data for each level of light scatter as arising from a randomized block design with visual acuity measured with four charts the factor of interest. If a significant F-test indicates differences in visual acuities for the different acuity charts, then the multiple comparison method can be used to identify the chart for which the visual acuity differs. For the analysis of PVER amplitudes, two questions also were addressed: (1) is the relationship between light scatter and PVER amplitude the same for all five checks sizes; and (2) for each check size, at what light scatter level does the PVER amplitude differ significantly from PVER amplitude with no light scatter? To answer the
taken through four different conditions of artificially induced light scatter. The number each photograph indicates the number of layers of acrylic sheets.
accompanying
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first question, the light scatter is treated as a continuous variable in the design. If the interaction between check sizes and light scatter represents a significant contribution to overall variability in the PVER amplitudes. then the slope of the line relating PVER amplitudes to fight scatter differs for different check sizes. Pairwise comparisons between the check sizes can be examined by repeating this analysis on the data from two check sizes at a time, with the level of significance adjusted to reflect the multiple test. To answer the second question. the analysis was analogous to that used for the analysis of visual acuity, with the five check sizes replacing the four levels of contrast, RESULTS Figure 2 shows a series of photographs taken through varying numbers of acrylic sheets and illustrates gradual image deterioration with increasing light scatter caused by the sheets. With 2 layers, the edge of the object lost its sharpness. With 3 layers, the image is further degraded, and with 5 and 8 tayers, the image shows further degradation. Psychophysical tests Visual acuity. Figure 3 shows the average visual acuity changes from six subjects tested with varying numbers of acrylic sheets measured with VCVAC. Meas visual acuity without light scatter was 1.07 (+0.10) min of arc with the 90% contrast chart and 1.03 (& 0.25) rnin of arc with the reverse polarity chart. Visual acuity measured at 15 and 2.5% ccmtrast with no light scatter was 1.07 (L-0.16) and 1.80 (kO.34) tin of arc, respectively. To show more clearly the difference in each visual acuity chart in small light scatter ranges, the graph was plotted in the loga~t~ic scale in Fig. 3. With two layers, the visual acuity measured with the standard 90% contrast
-
chart was 1.30 (kO.35) and I. 17 (+0.08) min of arc with the reverse polarity chart. The acuities measured with the 15 and 2.5% contrast charts were 2.13 ( & 0.6 1) and 3. I 7 (kO.93) min of arc, respectively. The decrease of visual acuity compared with no light scatter was 0.37 octave at 90%, 1.00 octave at 15%, and 0.89 octave at 2.5%. The reverse polarity chart shows that the visual acuity was 0.15 octave better than the standard 90% contrast chart. For each chart, there were significant differences in the logs of visual acuity at the different levels of light scatter (P < 0.0~1 in each case). Dunnett’s multiple comparison procedure showed that the Logs of visual acuity measured with the 90% contrast chart showed a significant decrease with 4 layers when compared to no light scatter. With the 2.5 and 15% contrast charts, compared to no light scatter, a significant decrease was observed with two layers. Visual acuity measured with the reverse polarity chart showed a significant decrease with 4 layers, which was the same as the 90% contrast chart. Comparing the visual acuities at each level of light scatter with 1 layer, the visual acuity measured with the low-contrast chart (2.5%) was signi~cantl~~ lower than the acuities measured with the other charts (charts l-3). With two layers, the visual acuity measured with the intermediate-contrast chart (15%) was significantly lower than those measured with high-contrast charts. The visual a&ties measured with the reverse polarity chart showed higher acuities than the 90% contrast chart at alt levels of light scatter, but the difference was not significant . CSF. With no light scatter, the CSF showed the highest contrast sensitivity at 6.0 c/deg. When moderate light scatter is introduced, the contrast sensitivity decreased, especially in the intermediate to high spatial frequency range (Fig. 4). When the light scatter increased, Number
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FIGURE 3, Visual acuity changes with varyiog l&t& titter with four difkrent charts (n = 61, The abscissa shows the nwnber of kryiic sheets, and the ordistate, the visual acuity in min of arc in i~~a~t~i~ scale. *Indites the first sign&ant difference compared to no acrylic sheet in each chart analyzed by an analysis of variance (ANOVA) test.
1.5
3.0
6.0
12 18
30
SPATUG_ FREOUEHCY (CPK?)
FIGURE 4. Changes
of contrast
sensitivity
function
with varying
light scatter by acrylic sheets (n = 6). The abscissa shows the spatial frequency
in cjdeg, and the ordinate,
the contrast sensitivity.
LIGHT SCATTER AND PATTERN
REVERSAL
1215
VER
disability was elevated by 5.40 + 1.66, which is considered to be abnormal in our laboratory, although this change was not statistically significant. A significant change (40.05 rl: 23.70, F = 18.29, P -C 0.05) was observed with 6 layers. In both the CSF and glare tests, measurements with 10 layers were attempted, but no reliable results could be measured. Electrophysiological test 0
1
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4
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NUMBER OF ACRYLIC SHEETS
FIGURE 5. The effect of light scatter on glare disability. Relative glare disability without light scatter is plotted as 1.0 against various numbers of acrylic sheets with mean + 1 SD (n = 6). The abscissa shows the number of sheets, the ordinate the relative glare disability. *Indicates the first significant difference compared to no acrylic sheet analyzed by an analysis of variance (ANOVA) test.
the contrast sensitivity decreased, even in the low spatial frequency range. However, its decrease was always greater in the high-frequency range than the 1ow:These effects resulted in a peak (the spatial frequency showing the highest contrast sensitivity) shift in the CSF curve with a change in the configuration of the CSF curve. The peak seen at 60c/deg without light scatter shifted to 3.0 c/deg with 2 layers, and to 1.5 c/deg with 4 layers. Glare test. Figure 5 illustrates the glare disability with varying degrees of light scatter plotted as a mean obtained from six subjects. With 2 layers, the relative 160 min of arc
The PVER amplitudes decreased as the degree of light scatter increased, especially with a check size of 10 min of arc (Fig. 6). However, PVER was recordable up to 8 layers with a check size of 160 min of arc. Without light scatter, the function curves showed the inverted-U shape in four subjects, and a high-pass filter shape function curve was observed in two subjects. In all subjects, the normal configuration was lost even with a small degree of light scatter, resulting in a flatter PVER amplitudecheck size function curve accompanied by a shift of the peak of its function curve to the larger check sizes. These changes were already observed with I layer in one subject, 2 layers in three subjects, and 3 layers in two subjects. Figure 7 shows the PVER amplitude-check size function curve plotted as the relative value averaged from six subjects, with the largest amplitude in each subject plotted as 1.O. Without light scatter, the function curve showed an inverted-U shape with a peak at 20 min of arc. With 2- 3 layers, the peak shifted to larger 40 minof arc
IO min of arc
FIGURE 6. Samples of the pattern reversal visual evoked response (PVER) recordings with the 160, 40, and IOmin of arc condition in one normal subject. The numbers on the left of each set of PVER tracings denote the number of acrylic sheets.
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HITOMI TETSUKA et al.
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FIGURE 7. The pattern reversal visual evoked response amplitudecheck size function curve plotted as the relative value averaged from six subjects, with the largest amplitude in each subject plotted as 1.0. The abscissa shows the check sizes in min of arc, the ordinate the relative ampIitude.
check sizes, and the marked flattening of the function curve was observed. Figure 8 illustrates the change of the PVER amplitude with various levels of light scatter at each check size. The slopes for each check size are - 1.29, -0.91, -0.68, - 0.3 1, and - 0.21 for 10, 20,40, 80, and 160 min of arc, respectively. The F statistic for testing differences among these slopes was highly significant (P < 0.0001). Pairwise comparisons indicated that the slopes for 80 and 160 min of arc did not differ si~ificantly but were sign~~~ntly smaller than those for 20 and 40min of arc, which did not differ significantly. The slope for 10 min of arc was significantly smaller than 40 min of arc. Although the values of the slopes were higher (- 1.29 vs -0.91) compared with 20 min of arc, the difference was not significant. For each check size, there were significant differences in PVER amplitudes at different levels of light scatter. Dunnett’s multiple comparison procedure showed a significant decrease in the light scatter effect with 2 layers at 10 min of arc. For 20 and 40 min of arc, a significant decrease of the PVER amplitude was observed with 3 layers, for 80 and 160 min of arc, with 4 layers. DISCUSSION The change in visual acuity with contrast has been reported by several researchers. Mainster, Timberlake and Schepens (1981) and Regan and Neima (1983) emphasized the importance of using low contrast in
FIGURE 8. The change of the pattern reversal visual evoked response (PVER) amplitudes with various numbers of acrylic sheets. The PVER amplitudes without light scatter in each check size are plotted as 1.O (n = 6). *Indicates the first significant difference compared to no acrylic sheet in each check size analyzed by an analysis of variance (ANOVA) test.
order to detect early or subtle abnormalities in the macula and optic nerve. Vision measured with high contrast in such patients is often normal, but at the lower contrast can be significantly much more reduced than in normal individuals. Neumann, McCarthy, Steedle, Sanders and Raasnan (1988) reported that 70% of cataract patients showed a decrease of two lines of Snellen acuity in an outdoor environment. In the present study, the low (2.5%) and intermediate (15%) contrast charts were more sensitive in detecting the effect of a moderate degree of light scatter on vision than was the standard high (90”/0) contrast chart. In addition, with the reverse polarity chart, light scatter had the smallest effect, possibly because less light was reflected from the surface of that chart. These results suggest that use of high-contrast optotypes may fail to detect visual disturbances due to slight opacities of the ocular media. Vision measured with the reverse polarity chart is least affected by light scattering. This is in good accord with the fact that some patients with low vision can read better when the black print on white paper is reversed to white print on black background. Previous investigations of CSF have reported changes in the shape of the CSF curve caused by various visual dysfunctions such as amblyopia (Levi & Harwerth, 1978; Sawada, 1983) and cornea1 edema (Hess & Carney, 1979). Even with slight cornea1 edema, a decrease of contrast sensitivity to high frequencies was reported. With severe edema, losses were found at all frequencies. Also, in cataract patients, a loss of contrast sensitivity at high and inte~ediate spatial frequency ranges has been reported (Hess & Woo, 1978; Abrahamsson & SjBstrand, 1986). Artificially creating light scatter is not a simple task. Our method of inducing light scatter by placing
LIGHT
SCATTER
AND
PATTERN
acrylic sheets over the eye may mimic certain pathological conditions such as cataract or cornea1 opacity, but obviously the conditions are not identical. However, our study results generally agree with those of clinical studies. The CSF showed changes even with a small degree of light scatter. The fall in contrast sensitivity was more marked at high spatial frequency ranges. For glare testing, there is a marked increase of glare disability even with a moderate amount of light scatter (l-2 layers). Since the size of the Landolt rings is clearly above the acuity threshold for our subjects, impaired visual discrimination is apparently due to the increased light scatter introduced by layers of acrylic sheets. In summary, in the psychophysical tests, a small degree of light scatter, i.e. with 2 layers, resulted in decreased visual performance, especially for low contrast, higher spatial frequency tasks, and the glare disability was found even with minimal light scatter. As for the PVER results, moderate light scatter (2 layers) induced the loss of normal configurations on the PVER amplitude-check size function curve and also led to the significant decrease of the PVER amplitude especially at 10 and 20 min of arc. At this level of light scatter, the visual acuity measured with high-contrast optotypes was still maintained at the 20/25 level. These results agree with those obtained by Van der Berg and Boltjes (1988) using the PERG. Regarding the effect of light scatter on the PVER, in a study similar to ours done by Mitsuyu and Zimmer (1984) using the Bangerter occlusive patches, they reported that translucent adhesives may better simulate ocular media opacity than defocusing. Several studies were performed on the effect of defocus on the PVER amplitudes in which the effect on the smaller check sizes was more marked than on the larger sizes (Van Lith, Van Marie, Bartl & VijfwinkelBruininga, 1978; Adachi-Usami, 1979; Kakisu & Runne, 1984; Katsumi, Hirose, Sakaue, Mehta & Rosenstein, 1990). Although defocusing image and light scatter are different optical phenomena, since we were not able to analyze the point or line spread function of the images, a clear distinction between defocus and light scatter was not possible, especially when the number of acrylic sheets was increased. Regarding PVER and the psychophysical test, Katsumi, Tanino and Hirose (1985) compared the CSF obtained with the PVER and those obtained psychophysically, and reported that they paralleled in most spatial frequency ranges. The loss of normal configuration of the PVER amplitude-check size function curve, observed in light scatter, with the smaller check sizes more greatly affected, agreed well with those psychophysical studies. The present study suggests caution in interpreting PVER, which may be used as a clinical tool for detecting macula and optic nerve diseases, especially in elderly subjects with nuclear sclerosis of the lens that does not decrease the vision measured by conventional highcontrast vision charts. An increase of glare disability (Wolf & Gardiner, 1965) deterioration of contrast sensitivity (Owsley, Sekular & Siemsen, 1983) and PVER (Adachi. 1989) with age have been reported.
REVERSAL
1217
VER
The PVER can be a useful, adjunctive clinical test to assess visual function, and the proper diagnosis is possible only after all factors that alter the normal PVER are considered. Light scatter is one of important factors. Although our model creating light scatter may not be directly applicable to all pathological conditions, we found that the PVER with intermediate contrast is equally sensitive even to a small contrast of light scatter when compared with the intermediate- or low-contrast visual acuity charts, CSF, and glare testing. We believe that PVER may be a useful objective tool to analyze optical glare effect in the ophthalmology clinic, although clinical studies with large number of patients are required. REFERENCES Abrahamsson. sensitivity
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Acknowledgements-We
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(1978). Visual acuity and checkerboard potentials with defocusing lenses. Documenta Ophthalmologica Proceeding Series, 13, 13~IO. Wang, J.-G. & Pomerantzeff, 0. (1991). A new set of variable-contrast visual acuity charts. Optometry and Vision Science, 68, 34 40. Wolf, E. (1960). Glare and age. Archives af @phthalmology. 64.
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appreciate the continued support offered by Charles L. Schepens, M.D., and Oleg Pomerantzeff, Dpl. Eng. We thank Rebecca B. Rosenstein, Ph.D., for her great help in the statistical analysis. Elizabeth W. Larson and Charlene J. Skladzien provided technical assistance. Lynda Charters helped in the preparation of this manuscript. This study was supported by grants from the Hasbro Children’s Foundation (Pawtucket, RI.) and the Kimmelman Foundation (New York, N.Y.).
Copyright
0042-6989/92 $5.00 + 0.00 (c’ 1992 Pergamon Press Ltd
Effect of Light Scatter on the Pattern Reversal Visual Evoked Response: Comparison with Psychophysical Results* HITOMT TETSUKA,_F GUANG-JI WANG,?
OSAMU TATSUO
KATSUMI,I_$§ HIROSEt$l
Received 26 June 1991; in revised form
13 November
ANTHONY
J. MORANDI,q
SOICHI
TETSUKA,_F
1991
The effect of light scatter on the pattern reversal visual evoked response (PVER) was studied in 6 normal subjects. The results were compared with contrast visual acuity, contrast sensitivity function, and glare disability. Light scatter was induced by translucent acrylic sheets. Visual acuity measured with the low-contrast charts decreased significantly (P < 0.0001) even with a small degree of light scatter. Contrast sensitivity decreased with a small degree of light scatter especially for high spatial frequencies. PVER amplitudes decreased especially at the smaller checks with its peak shifted to larger checks. PVER was equally sensitive to light scatter compared to psychophysical tests. Contrast sensitivity function
Glare disability
Light scatter
INTRODUCTION Since its introduction more than two decades ago (Spekreijse, 1966) the pattern reversal visual evoked response (PVER) has become clinically important and is used in ophthalmological, neurological, and psychological research. Many investigators have studied various factors that influence the PVER to help establish standard recording conditions (Arden, Bodis-Wollner, Halliday, Jeffreys, Kulikowski, Spekreijse & Regan, 1977) and to assist the interpretation of the results obtained from the patients with visual disturbances. Factors of the pattern stimulus that influence the PVER include the temporal and spatial frequencies of the stimulus, its contrast, the stimulus field size, and the mean luminance. Some electrophysiologists analyze the shape of the PVER amplitude-check size function curve (Regan, 1980) determining not only the smallest check size that produces the observable PVER (critical check size) but also the overall shape of the amplitude-check size function curve, which in normal adults has an inverted-U shape so that the largest amplitude is produced by intermediate check sizes. It has been reported that the shape of the PVER amplitudecheck size function curve changes in many pathological
Pattern reversal VER
Visual acuity
conditions, e.g. functional amblyopia (Levi & Harwerth, 1978). Opacities of the ocular media, such as cornea1 nebula, edema and scars, cells and flare in the anterior chamber, cataract, and the cells in the vitreous make the light scatter in different manners and may decrease retinal image contrast (Miller & Benedek, 1973). Patients with early cataract often complain of increased glare, but visual acuities measured with the usual high-contrast optotypes may show little or no decrease. However, in such patients other psychophysical tests such as contrast sensitivity function (CSF) or an estimate of glare disability may become abnormal, since discrimination of low-contrast objects is impaired (Hess & Woo, 1978; Hess & Carney, 1979; Abrahamsson & Sjbstrand, 1986). Van der Berg and Boltjes (1988) studied the effect of stray light induced by frosted glass on pattern reversal electroretinogram (PERG) and found considerable reduction of PERG amplitudes especially in small check sizes. To the best of our knowledge, the effect of light scatter, an important parameter in visual function, never has been studied systematically with the PVER. This study reports how the PVER is affected by artificially induced light scatter and the results were compared with psychophysical methods, such as visual acuity measurements in different contrast charts, CSF, and glare testing.
*Presented in part at the 1990 meeting of the Association for Research in Vision and Ophthalmology, Sarasota, Fla, U.S.A. tschepens Eye Research Institute, 20 Staniford Street, Boston, MA 02114, U.S.A. $Department of Ophthalmology, Harvard Medical School, Boston. Mass., U.S.A. §To whom reprint requests should be addressed at Schepens Eye Research Institute. SRetina Associates, Boston, Mass., U.S.A.
SUBJECTS AND METHODS Subjects
Six ophthalmologically normal subjects (three males, three females) ranging in age from 25 to 42 yr (mean 33.0) participated in this investigation. Moderate refractive 1211
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FIGURE 1. The effect of the translucent acrylic sheets used to produce light scatter on the overall mean luminance level. The abscissa shows the number of layers of acrylic sheets, the ordinate, the relative mean luminance. The log mean luminance decreases linearly with the number of acrylic sheets used. With 10 layers, the fall of mean luminance compared with 0 layers is about 37.1% or 0.2 log unit.
anomalies in three of the six subjects were optically corrected to emmetropia at the time of each testing. Both the electrophysiological and psychophysical tests were performed monocularly using the dominant eye, which was determined by a hole-in-card method. Testing procedures were fully explained to all subjects and informed consent was obtained from each subject before testing. methods Changes in light scatter of images. Light scatter was induced artificially by placing translucent acrylic sheets 12 mm from the cornea. The degree of light scatter was altered by increasing the number of sheets from 0 (no sheets) to 6 in one-increment steps, 8 and 10. The acrylic sheets were placed together in hand-held trial frames. Figure 1 shows the mean luminance level measured with a photometer placed in the same position as the subject’s eye in line with the center of the television monitor that was used for PVER testing. The log mean luminance falls linearly as the degree of light scatter increases. With 10 sheets, the mean luminance level dropped about 37.1% or 0.2 log unit, indicating a 0.02 log unit fall per sheet. Fs~ch~~h~sic~~ tests. Visual acuity was measured under normal room light with the Vacate-Contest Visual Acuity Charts (VCVAC) developed by Wang and Pomerantzeff (1991). The VCVAC consists of four charts: standard, high-contrast (900/), intermediatecontrast (15%), low-contrast (2.5%), and reverse polarity. In the last, the contrast is the same as that of the standard, high-contrast chart, but instead of black optotypes on a white background, the white letters are displayed on the black background. The mean luminance is constant for all charts. There are two E optotypes on thirteen different visual acuity levels ranging from 20/12 to 20/200 with a testing distance of 10 ft (3.05 m). The level of visual acuity is arranged in a geometric progression so that three acuity levels are equivalent to 1.0 octave. The directions of the E optotypes were randomized. CSF was measured in normal room light (50 ft-L) with the Vision Contrast Test System@ (VCTS) 6500 (V&tech,
Dayton, Ohio) developed by Ginsburg ( 1984). The test consists of five rows of 3 in. dia patches corresponding to spatial frequencies of 1.5, 3, 6, 12 and I8 cjdeg. Glare disability was measured with the apparatus developed by Wolf (1960~. The visual task consisted of identifying the positions of gaps in Landolt rings (14.5 min of arc) arranged in three circles, 4. 7, and 10 at the center of a circular glare source that subtended a visual angle of 2” at a testing distance of 71 cm. The targets were located on a translucent screen with a luminance variable (combinations of two sets of neutral density filters) between 0.00184 and 21.38 mL in 40 logarithmic steps. With a bright light facing the subject. the tester increased the background illumination of the Landolt rings until the gaps were identified correctly. The target screen luminance necessary to identify the gaps was determined for five levels of glare luminances. The glare sensitivity value exceeding 3 was considered as abnormal. ~~ectru~h~~io~~g~cff~ test. Steady-state PVER recordings were performed in ambient light. Silver disc electrodes were used. The active electrode was placed at 0, and the reference electrode at P, (standard lo-20 convention). Ground electrodes were placed on both earlobes. The stimulus display was a 17 in., high-resolution television monitor. The overall stimulus field size was 20 x 20 cm, subtending a visual angle of IO x IO‘ at the distance of 120 cm. The mean luminance level was 50 cd/ m*, and the contrast ratio was kept constant at 30%. A small square of black tape was placed in the center of the screen to serve as the fixation target. For testing parameters, the PVER recordings were obtained with five check sizes, 10,20,40, 80, and 160 min of arc. The temporal frequency of the stimulus pattern was 12 reversals/see or 6 cjsec, and the analysis time 5OOmsec. Fifty responses were fed into an averaging computer after amplification through the filter setting of 70 Hz (high-cut) and 1.0 Hz (low-cut). The PVER recording started from the largest check size, 160 min of arc and decreased by 1.0 octave steps to the smallest, 10 min of arc in all subjects. The PVER amplitude-check size function curve was obtained by plotting the amplitudes against five different check sizes in log adjusted expression for each number of acrylic sheets. Since we used the steady-state stimulation to record PVER, the implicit time was not analyzed in this study. In both the psychophysical and the electrophysiological examinations, testing was performed with and without acrylic sheets. In this study, the degree of light scatter was expressed with the number of acrylic sheets applied. For example, when two sheets were used, it was designated as “two” layers. Statistical analysis. To analyze the results on visual acuity or PVER amplitudes, the data can be viewed as arising from a factorial experiment in a randomized block design, where the acuity chart or check size and light scatter are the factors and the 6 subjects are the blocks. For the analysis of visual acuity two questions were addressed: (1) for each visual acuity chart, at what light
LIGHT
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scatter level does the visual acuity differ significantly from visual acuity with no light scatter; and (2) are there differences in visual acuity between the charts at each level of light scatter? In these analyses, the logs of visual acuity are used to produce an approximately linear relationship with light scatter level. To answer the first question, the data for each visual acuity chart are viewed as arising from a randomized block design with the light scatter the factor of interest. If a significant F-test indicates differences in visual acuities for the different levels of light scatter, then Dunnett’s (1955) multiple comparison procedure can be used to compare the mean visual acuities at each level of light scatter to the mean visual acuities when there is no scatter with an
FIGURE
2. Photographs
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overall level of significance of 0.05. The second question can be answered by viewing the data for each level of light scatter as arising from a randomized block design with visual acuity measured with four charts the factor of interest. If a significant F-test indicates differences in visual acuities for the different acuity charts, then the multiple comparison method can be used to identify the chart for which the visual acuity differs. For the analysis of PVER amplitudes, two questions also were addressed: (1) is the relationship between light scatter and PVER amplitude the same for all five checks sizes; and (2) for each check size, at what light scatter level does the PVER amplitude differ significantly from PVER amplitude with no light scatter? To answer the
taken through four different conditions of artificially induced light scatter. The number each photograph indicates the number of layers of acrylic sheets.
accompanying
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first question, the light scatter is treated as a continuous variable in the design. If the interaction between check sizes and light scatter represents a significant contribution to overall variability in the PVER amplitudes. then the slope of the line relating PVER amplitudes to fight scatter differs for different check sizes. Pairwise comparisons between the check sizes can be examined by repeating this analysis on the data from two check sizes at a time, with the level of significance adjusted to reflect the multiple test. To answer the second question. the analysis was analogous to that used for the analysis of visual acuity, with the five check sizes replacing the four levels of contrast, RESULTS Figure 2 shows a series of photographs taken through varying numbers of acrylic sheets and illustrates gradual image deterioration with increasing light scatter caused by the sheets. With 2 layers, the edge of the object lost its sharpness. With 3 layers, the image is further degraded, and with 5 and 8 tayers, the image shows further degradation. Psychophysical tests Visual acuity. Figure 3 shows the average visual acuity changes from six subjects tested with varying numbers of acrylic sheets measured with VCVAC. Meas visual acuity without light scatter was 1.07 (+0.10) min of arc with the 90% contrast chart and 1.03 (& 0.25) rnin of arc with the reverse polarity chart. Visual acuity measured at 15 and 2.5% ccmtrast with no light scatter was 1.07 (L-0.16) and 1.80 (kO.34) tin of arc, respectively. To show more clearly the difference in each visual acuity chart in small light scatter ranges, the graph was plotted in the loga~t~ic scale in Fig. 3. With two layers, the visual acuity measured with the standard 90% contrast
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chart was 1.30 (kO.35) and I. 17 (+0.08) min of arc with the reverse polarity chart. The acuities measured with the 15 and 2.5% contrast charts were 2.13 ( & 0.6 1) and 3. I 7 (kO.93) min of arc, respectively. The decrease of visual acuity compared with no light scatter was 0.37 octave at 90%, 1.00 octave at 15%, and 0.89 octave at 2.5%. The reverse polarity chart shows that the visual acuity was 0.15 octave better than the standard 90% contrast chart. For each chart, there were significant differences in the logs of visual acuity at the different levels of light scatter (P < 0.0~1 in each case). Dunnett’s multiple comparison procedure showed that the Logs of visual acuity measured with the 90% contrast chart showed a significant decrease with 4 layers when compared to no light scatter. With the 2.5 and 15% contrast charts, compared to no light scatter, a significant decrease was observed with two layers. Visual acuity measured with the reverse polarity chart showed a significant decrease with 4 layers, which was the same as the 90% contrast chart. Comparing the visual acuities at each level of light scatter with 1 layer, the visual acuity measured with the low-contrast chart (2.5%) was signi~cantl~~ lower than the acuities measured with the other charts (charts l-3). With two layers, the visual acuity measured with the intermediate-contrast chart (15%) was significantly lower than those measured with high-contrast charts. The visual a&ties measured with the reverse polarity chart showed higher acuities than the 90% contrast chart at alt levels of light scatter, but the difference was not significant . CSF. With no light scatter, the CSF showed the highest contrast sensitivity at 6.0 c/deg. When moderate light scatter is introduced, the contrast sensitivity decreased, especially in the intermediate to high spatial frequency range (Fig. 4). When the light scatter increased, Number
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FIGURE 3, Visual acuity changes with varyiog l&t& titter with four difkrent charts (n = 61, The abscissa shows the nwnber of kryiic sheets, and the ordistate, the visual acuity in min of arc in i~~a~t~i~ scale. *Indites the first sign&ant difference compared to no acrylic sheet in each chart analyzed by an analysis of variance (ANOVA) test.
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FIGURE 4. Changes
of contrast
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with varying
light scatter by acrylic sheets (n = 6). The abscissa shows the spatial frequency
in cjdeg, and the ordinate,
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LIGHT SCATTER AND PATTERN
REVERSAL
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disability was elevated by 5.40 + 1.66, which is considered to be abnormal in our laboratory, although this change was not statistically significant. A significant change (40.05 rl: 23.70, F = 18.29, P -C 0.05) was observed with 6 layers. In both the CSF and glare tests, measurements with 10 layers were attempted, but no reliable results could be measured. Electrophysiological test 0
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NUMBER OF ACRYLIC SHEETS
FIGURE 5. The effect of light scatter on glare disability. Relative glare disability without light scatter is plotted as 1.0 against various numbers of acrylic sheets with mean + 1 SD (n = 6). The abscissa shows the number of sheets, the ordinate the relative glare disability. *Indicates the first significant difference compared to no acrylic sheet analyzed by an analysis of variance (ANOVA) test.
the contrast sensitivity decreased, even in the low spatial frequency range. However, its decrease was always greater in the high-frequency range than the 1ow:These effects resulted in a peak (the spatial frequency showing the highest contrast sensitivity) shift in the CSF curve with a change in the configuration of the CSF curve. The peak seen at 60c/deg without light scatter shifted to 3.0 c/deg with 2 layers, and to 1.5 c/deg with 4 layers. Glare test. Figure 5 illustrates the glare disability with varying degrees of light scatter plotted as a mean obtained from six subjects. With 2 layers, the relative 160 min of arc
The PVER amplitudes decreased as the degree of light scatter increased, especially with a check size of 10 min of arc (Fig. 6). However, PVER was recordable up to 8 layers with a check size of 160 min of arc. Without light scatter, the function curves showed the inverted-U shape in four subjects, and a high-pass filter shape function curve was observed in two subjects. In all subjects, the normal configuration was lost even with a small degree of light scatter, resulting in a flatter PVER amplitudecheck size function curve accompanied by a shift of the peak of its function curve to the larger check sizes. These changes were already observed with I layer in one subject, 2 layers in three subjects, and 3 layers in two subjects. Figure 7 shows the PVER amplitude-check size function curve plotted as the relative value averaged from six subjects, with the largest amplitude in each subject plotted as 1.O. Without light scatter, the function curve showed an inverted-U shape with a peak at 20 min of arc. With 2- 3 layers, the peak shifted to larger 40 minof arc
IO min of arc
FIGURE 6. Samples of the pattern reversal visual evoked response (PVER) recordings with the 160, 40, and IOmin of arc condition in one normal subject. The numbers on the left of each set of PVER tracings denote the number of acrylic sheets.
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FIGURE 7. The pattern reversal visual evoked response amplitudecheck size function curve plotted as the relative value averaged from six subjects, with the largest amplitude in each subject plotted as 1.0. The abscissa shows the check sizes in min of arc, the ordinate the relative ampIitude.
check sizes, and the marked flattening of the function curve was observed. Figure 8 illustrates the change of the PVER amplitude with various levels of light scatter at each check size. The slopes for each check size are - 1.29, -0.91, -0.68, - 0.3 1, and - 0.21 for 10, 20,40, 80, and 160 min of arc, respectively. The F statistic for testing differences among these slopes was highly significant (P < 0.0001). Pairwise comparisons indicated that the slopes for 80 and 160 min of arc did not differ si~ificantly but were sign~~~ntly smaller than those for 20 and 40min of arc, which did not differ significantly. The slope for 10 min of arc was significantly smaller than 40 min of arc. Although the values of the slopes were higher (- 1.29 vs -0.91) compared with 20 min of arc, the difference was not significant. For each check size, there were significant differences in PVER amplitudes at different levels of light scatter. Dunnett’s multiple comparison procedure showed a significant decrease in the light scatter effect with 2 layers at 10 min of arc. For 20 and 40 min of arc, a significant decrease of the PVER amplitude was observed with 3 layers, for 80 and 160 min of arc, with 4 layers. DISCUSSION The change in visual acuity with contrast has been reported by several researchers. Mainster, Timberlake and Schepens (1981) and Regan and Neima (1983) emphasized the importance of using low contrast in
FIGURE 8. The change of the pattern reversal visual evoked response (PVER) amplitudes with various numbers of acrylic sheets. The PVER amplitudes without light scatter in each check size are plotted as 1.O (n = 6). *Indicates the first significant difference compared to no acrylic sheet in each check size analyzed by an analysis of variance (ANOVA) test.
order to detect early or subtle abnormalities in the macula and optic nerve. Vision measured with high contrast in such patients is often normal, but at the lower contrast can be significantly much more reduced than in normal individuals. Neumann, McCarthy, Steedle, Sanders and Raasnan (1988) reported that 70% of cataract patients showed a decrease of two lines of Snellen acuity in an outdoor environment. In the present study, the low (2.5%) and intermediate (15%) contrast charts were more sensitive in detecting the effect of a moderate degree of light scatter on vision than was the standard high (90”/0) contrast chart. In addition, with the reverse polarity chart, light scatter had the smallest effect, possibly because less light was reflected from the surface of that chart. These results suggest that use of high-contrast optotypes may fail to detect visual disturbances due to slight opacities of the ocular media. Vision measured with the reverse polarity chart is least affected by light scattering. This is in good accord with the fact that some patients with low vision can read better when the black print on white paper is reversed to white print on black background. Previous investigations of CSF have reported changes in the shape of the CSF curve caused by various visual dysfunctions such as amblyopia (Levi & Harwerth, 1978; Sawada, 1983) and cornea1 edema (Hess & Carney, 1979). Even with slight cornea1 edema, a decrease of contrast sensitivity to high frequencies was reported. With severe edema, losses were found at all frequencies. Also, in cataract patients, a loss of contrast sensitivity at high and inte~ediate spatial frequency ranges has been reported (Hess & Woo, 1978; Abrahamsson & SjBstrand, 1986). Artificially creating light scatter is not a simple task. Our method of inducing light scatter by placing
LIGHT
SCATTER
AND
PATTERN
acrylic sheets over the eye may mimic certain pathological conditions such as cataract or cornea1 opacity, but obviously the conditions are not identical. However, our study results generally agree with those of clinical studies. The CSF showed changes even with a small degree of light scatter. The fall in contrast sensitivity was more marked at high spatial frequency ranges. For glare testing, there is a marked increase of glare disability even with a moderate amount of light scatter (l-2 layers). Since the size of the Landolt rings is clearly above the acuity threshold for our subjects, impaired visual discrimination is apparently due to the increased light scatter introduced by layers of acrylic sheets. In summary, in the psychophysical tests, a small degree of light scatter, i.e. with 2 layers, resulted in decreased visual performance, especially for low contrast, higher spatial frequency tasks, and the glare disability was found even with minimal light scatter. As for the PVER results, moderate light scatter (2 layers) induced the loss of normal configurations on the PVER amplitude-check size function curve and also led to the significant decrease of the PVER amplitude especially at 10 and 20 min of arc. At this level of light scatter, the visual acuity measured with high-contrast optotypes was still maintained at the 20/25 level. These results agree with those obtained by Van der Berg and Boltjes (1988) using the PERG. Regarding the effect of light scatter on the PVER, in a study similar to ours done by Mitsuyu and Zimmer (1984) using the Bangerter occlusive patches, they reported that translucent adhesives may better simulate ocular media opacity than defocusing. Several studies were performed on the effect of defocus on the PVER amplitudes in which the effect on the smaller check sizes was more marked than on the larger sizes (Van Lith, Van Marie, Bartl & VijfwinkelBruininga, 1978; Adachi-Usami, 1979; Kakisu & Runne, 1984; Katsumi, Hirose, Sakaue, Mehta & Rosenstein, 1990). Although defocusing image and light scatter are different optical phenomena, since we were not able to analyze the point or line spread function of the images, a clear distinction between defocus and light scatter was not possible, especially when the number of acrylic sheets was increased. Regarding PVER and the psychophysical test, Katsumi, Tanino and Hirose (1985) compared the CSF obtained with the PVER and those obtained psychophysically, and reported that they paralleled in most spatial frequency ranges. The loss of normal configuration of the PVER amplitude-check size function curve, observed in light scatter, with the smaller check sizes more greatly affected, agreed well with those psychophysical studies. The present study suggests caution in interpreting PVER, which may be used as a clinical tool for detecting macula and optic nerve diseases, especially in elderly subjects with nuclear sclerosis of the lens that does not decrease the vision measured by conventional highcontrast vision charts. An increase of glare disability (Wolf & Gardiner, 1965) deterioration of contrast sensitivity (Owsley, Sekular & Siemsen, 1983) and PVER (Adachi. 1989) with age have been reported.
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The PVER can be a useful, adjunctive clinical test to assess visual function, and the proper diagnosis is possible only after all factors that alter the normal PVER are considered. Light scatter is one of important factors. Although our model creating light scatter may not be directly applicable to all pathological conditions, we found that the PVER with intermediate contrast is equally sensitive even to a small contrast of light scatter when compared with the intermediate- or low-contrast visual acuity charts, CSF, and glare testing. We believe that PVER may be a useful objective tool to analyze optical glare effect in the ophthalmology clinic, although clinical studies with large number of patients are required. REFERENCES Abrahamsson. sensitivity
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Acknowledgements-We
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(1978). Visual acuity and checkerboard potentials with defocusing lenses. Documenta Ophthalmologica Proceeding Series, 13, 13~IO. Wang, J.-G. & Pomerantzeff, 0. (1991). A new set of variable-contrast visual acuity charts. Optometry and Vision Science, 68, 34 40. Wolf, E. (1960). Glare and age. Archives af @phthalmology. 64.
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appreciate the continued support offered by Charles L. Schepens, M.D., and Oleg Pomerantzeff, Dpl. Eng. We thank Rebecca B. Rosenstein, Ph.D., for her great help in the statistical analysis. Elizabeth W. Larson and Charlene J. Skladzien provided technical assistance. Lynda Charters helped in the preparation of this manuscript. This study was supported by grants from the Hasbro Children’s Foundation (Pawtucket, RI.) and the Kimmelman Foundation (New York, N.Y.).
