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Optimal temporal frequencies in oscillatory movement hyperacuity measurements of visual function in cataract patients
Copyright
RESEARCH
Ophthal. Physiol. Opt. Vol. 15. No. 1, pp. 49-52, 1995 0 1995 Elsevier Science Ltd for British Cokx of Ootometrists Printed in Great Britain. .kl righis reserved 0275-5408/95 $10.00 + 0.00
NOTE
timal temporal frequencies in oscillatory movement hyperacuity measurements of visual function in cataract patients Marlk Hurst”‘, ’ Department ‘The Medical UK
Russell
Watkins
of Optometry, University School, Clinical Sciences
*2 and Terry
Buckingham?’
of Bradford, Bradford, West Yorkshire BD7 Building, Leicester Royal Infirmary, Leicester
1 DP; and LE2 7LX,
Summary Hyperacuity tasks have been suggested for the assessment of potential visual function in the presence of cataracts. To test this suggestion, hyperacuity thresholds for an oscillating bar were measured in 30 subjects with idiopathic cataract and in 24 age-matched normals over a range of oscillation frequencies. Each subject’s cataract was categorized using the Oxford Clinic al Cataract Classification and Grading System. Cataract was found to have a significant effec: on thresholds, although a differential morphological effect on thresholds was equivocal. Thresholds at higher temporal frequencies were significantly raised when compared to the normal group. The main conclusion to be drawn from this study is that motion hyperacuity threslolds appear unaffected by cataract at low oscillation frequencies and should be used in preference to higher frequencies in the assessment of such patients. Ophtnal.
Physiol.
Opt.
1995,
15, 49-52
Cataract can be defined as a loss of the normal transparency of the crystalline lens. Clinically, a cataract is present when the loss of transparency results in a symptomatic visual deficir. The visual deficit is mainly due to light scatter by the lens producing a veiling luminance which is superimposed on the retinal image. This has the effect of reducing image contrast. This reduction may be over all spatial frequencies or selectively at medium and high spatia1 frequencies’. The decrease in visual performance caused by cataract has been documented by authors too numerous to list. However, the assessment of visual function behind a cataract has recently become increasingly important. In the past, such information has been used to aid the diagnosis and management of the patient with cataract and a suspected coexisting retinal/neural disorder. More recently, primarily because of improved instrumentation, it has beern put to
more subtle uses. First, it may now be possible to predict potential postoperative visual performance in a quantitative manner2. Second, because of an increase in medicolegal case?, a need has arisen to justify cataract surgery. Third, trials of anticataract agents are on the increase. For longitudinal studies of this type it is imperative that some type of retinal and neural assessment is carried out to isolate any reduction in visual performance that is not due to advancing cataract4. Despite the development of new techniques, current methods are not without their problem?. A relatively new group of tests uses the remarkable sensitivity of the visual system in accurately locating objects in the visual field; Westheimer6 coined the term ‘hyperacuity’ to describe this ability. Williams and his colleagues’ were the first to argue strongly that some types of hyperacuity measurements could be used to assess vision behind media opacities and indeed could produce superior results to those obtained by interferometry, for example. The basis upon which this statement was made, was primarily on the strength of the resistance of ‘two dot’ vernier measurements to optical blur and also on the fact that hyperacuity is highly dependent on retinal stimulus location, i.e. the highest thresholds are achieved when the fovea1 area is
*MBCQ tFBC0 Receivd: 6 December 1993 Revised form: 15 July 1994
49
50
Ophthal.
Physiol.
Opt. 1995 15: No 1
stimulated’. Unfortunately, the distance between dots in a ‘two dot test’ does influence measurements. Moreover, optimal thresholds are obtained at a specific separation for a particular standard of visual acuity (VA) which an individual achieves. Thresholds therefore need to be determined at a number of gap sizes before an evaluation of visual function can be made. Oscillatory movement displacement thresholds (OMDTs) also produce thresholds in the hyperacuity range, under certain conditions. The OMDT can be defined as the smallest amplitude of displacement that elicits the perception of movement for a given temporal frequency of oscillation. The range over which OMDTs are usually determined is 1-15 Hz. The oscillation is usually sinusoidal but can be square-wave or triangular-wave. Square waveforms tend to give lower while triangular give higher thresholds than sinusoidal waveforms. OMDTs are thought to be mediated primarily by the magnocellular pathway of the visual system’. It has been suggested that OMDT be applied clinically to assess visual function behind opaque media”. This suggestion arose from the observation that, like other hyperacuities, optical degradation by external blurring lenses and external scattering sources scarcely affect OMDT”. Initially, these results were obtained in circumstances quite different to those which occur in cataractous patients, in whom changes in retinal illumination coexist with a change in image quality. For instance, the artificial image degradation was induced using homogeneous filtering and few cataracts are homogeneous. In addition, cataract severity varies and different morphologies of cataract affect the retinal image in different ways. Nuclear cataracts both absorb and scatter incident light. Cortical and posterior subcapsular cataracts tend to absorb little incident light; their destructive effects on vision predominantly arise from light scatter within the eye. These effects will lower retinal illumination and reduce contrast sensitivity to differing degrees and are likely to be different from those effects induced by artificial means. Later papers”,‘3 presented data on actual cataract patients. Whitaker and Elliott’3 performed a number of other experiments to determine the optimum configuration for the OMDT technique. They concluded that a temporal frequency of 2 Hz should be used because of the effects of experimentally induced contrast attenuation on younger observers on the detection of high oscillation frequency (16 Hz). They also concluded that stationary reference position was largely immaterial and placed the flanking lines a total angular distance of 120min arc apart with the oscillating bar centrally placed. This type of hyperacuity has already been used in the presence of various conditions which affect the visual system, such as amblyopiai4, chronic open angle glaucoma and ocular hypertension’5, optic neuritis16, age-related maculopathy” and diabetic retinopathy18. The aim of this study was, in the first instance, to assess
the effects of oscillation frequency on thresholds obtained from patients with cataract. Second, we hoped to reveal whether cataract morphology had any differential effects on thresholds. This knowledge would allow further refinement of the technique for its use in assessing visual function behind cataract. Subjects Thirty subjects with age-related idiopathic cataracts were recruited from an anticataract drug trial based at the Bradford Royal Infirmary. Informed consent was obtained in each case. There were 12 subjects with cortical cataracts, 12 had posterior subcapsular cataracts and the remaining 6 had nuclear cataract as classified by the Oxford Clinical Cataract Classification and Grading System”. LogMAR scores ranged from +0.44 to -0.14. The mean logMAR score for each group of cataract patients is shown in Figure I. These subjects had the advantage of being very carefully monitored for any retinal or neural problem during the drug trial. We recruited 24 age-matched subjects as a control group. These subjects underwent a full ophthalmic examination and were included in the study if they met the following strict criteria: 1. 2. 3. 4. 5. 6. 7.
logMAR score 0.0 or better (see Figure 1); no lens opacity in the undilated pupil area; no history of previous eye disease; no current eye disease or amblyopia in either eye; no systemic disease considered likely to affect results; intraocular pressure < 22 mmHg; refractive error < +6.00D spherical/< f2.00D cylindrical.
The ages of this control group ranged from 51 to 75 years LogMAR 0.4
1 I
0.3 -
-
0.2 -
0.1 ................................ ................................. 0.0 . -0.1
!
::::,,,::::::.::.::.:::::::::::::: ::.: ::::::~.::::.,::::::::::::: .....:............... ............ ::::.:::::::::::.::::::::::::::::, ................................. ::::~::::::::::::.::.:::.::::::::. ......................... ...... .............................. :::::.:::::::::::::.:::::::::::::, ................................. Norms
C0rt
:::.:::.::::.::::::::::::.:::::::: ................................. :::::::.::::.:::_:::::::::::::::: ::::::::::::::::::::::::::.:::::: :::::::.::::~::::::::::::.:::::::ii .................................. :::::::.:::::::::::_:::::.::::::,: :::::::::::::::::::::.:::.::::::.: .................................. z::::::::::.:::::::::::::::::::: ::::::::::::::..::.::.:.. .:::::,: :::i::::::::::::::::::::::::::_::_ ..:::.:::::::::.:::::.::. :::::::: ~:::::::::::::::::::...... ::::::.:::::: .................... ...... ........................ ........ ::::::::::: ::::.::.::::::.:::,::: .:::::::::::.::::::-:::::::::::: ::::::::::: ~:::~::_:.:::::: ............................ :::_::::~_~_~__.~_:i~.~~..~~~~. ii ................... ............ :::::.:::::::::::::::::::::::::x: ......................... ........ ::::~:::::~:::::::i:>::::::.:: ~:::::::::::::::::::..:::.:.:::::: ::.:::::::::::::::::::ii:::::::::: ::::::::::::.:::::::.::::::.:::::: :::::::::::::::::::::::::::_i:_ii ................................:: ............................ .:::::::::::::_:::_:.:::::..:::::: ................................. .i::::ii:_::::_:::__.:::::..:::::: ................................. .................................. ................... ::::::::::::: ............. :::::::::::::::::::i:.::.:::::..:: .::.:::::::::::::i.: ::::::::: ::~::::~:::: ...... ........... .... ........ .................... .... :::::: .... ..... ........................ ::::::::::::.::.:::::i: :.::i .:: ................................
PSC
NS
Figure 1. LogMAR scores of each group of subject: Norms, age-matched normal group; PSC, posterior subcapsular cataract group; Cort, cortical cataract group and NS, nuclear cataract group.
OMDT
and those of the cataract group from 53 to 83 years. A non-parametric Mann-Whitney U-test showed that the ages of the groups were not significantly different. OMDTs were determined in the dominant eyes (determined by the sighting method) of these individuals. Apparatus
and methods
The apparatus used in this study has been described elsewhere16. Briefly, a target that was able to undergo controlled degrees of horizontal oscillation was backproj’: cted onto a diffusing screen. Also back-projected onto the ,screen was a pair of stationary references positioned such that they were equidistant either side of the target. The stationary references and the target subtended 40min arc vertically by 10min arc horizontally at a viewing distance of 6 m and had a luminance of 500cd m-‘. This produced a Michelson contrast of 95 %. Refractive corrections were worn and the test was carried out monocularly with natural pupils. Displacement thresholds for temporal frequencies of oscil’ation of 1, 4, 7, 10 and 13Hz were determined using. the psychophysical method of limits. Each threshold deter!inination used a set of three ascending and three descending runs. This method is very rapid, taking a total of atout seven and a half minutes per eye. Results A x’ test for normality indicated that the OMDT data may be cc nsidered to be normally distributed and so parametric statistics were used. Fipure 2 shows the mean OMDTs for the three groups of cataract subjects and the age-matched normal group plotted agair.st temporal frequency.
lo+, Cl
I 5
, 10
I 15 Temporal
Frequency
(Hz)
Figure 2. Oscillatory movement displacement thresholds plotted on a log scale against temporal frequency: 0, normal group; W, cortical cataract group; 0, posterior subcapsular cataract group; A, nuclear cataract group.
in cataract:
M. Hurst
et al.
51
The logMAR scores at all temporal frequencies used in this study were significantly correlated with OMDTs (4 Hz, r = 0.40, P < 0.005; 7Hz, Y = 0.44, P < 0.001; lOHz, r = 0.41, P < 0.005; 13Hz, r = 0.33, P < 0.05) with the exception of 1 Hz (r = 0.10, n.s.). This provides further support for the notion that thresholds achieved using higher temporal frequencies are limited by the same optical factors which limit visual acuity. A between group, repeated measures, analysis of variance revealed that OMDTs were significantly affected by the presence of cataract (F3.50= 3.38, P < 0.05) and in particular by nuclear cataract (post hoc Scheffe test, P < 0.05). OMDTs significantly increased as temporal frequency increased (F4,2,,0= 68.61, P < 0.000001). There was also a significant interaction between cataract type and temporal frequency (F,2.200= 5.58, P < 0.000001) with both the nuclear and posterior subcapsular groups giving significantly increased thresholds compared to normals at 7 Hz (post hoc Scheffe test, P < 0.000001, P < 0.05 respectively). At 10 Hz and 13 Hz, only the thresholds obtained from the group with nuclear cataract were significantly higher than the normals (post hoc Scheffe test, P < 0.005). As logMAR visual acuity was significantly correlated with thresholds obtained at higher temporal frequencies, visual acuity was then used as a covariate in a similar between subject, repeated measures analysis of variance. This revealed no significant difference between thresholds obtained from the normals and cataract groups (F3,50= 0.83, n.s.). It is therefore reasonable to suggest that the significant difference in thresholds found between the nuclear cataract group and normals in the preliminary analysis was probably due to the fact that the nuclear cataract group had the poorest mean acuity (see Figure I). Discussion The main conclusion to be drawn from this study is that OMDTs are unaffected by early cataract at low oscillation frequencies. Since the visual acuities of the patients in this study had logMAR scores ranging from -0.14 to +0.44, no inference can be made regarding the effects of more advanced cataract. The resistance of OMDT to image degradation has been previously demonstrated by both artificial image degradation” and by cataracts’2.‘3. In both of the above studies, a single oscillation frequency of 2Hz was used. In our study, medium and high oscillation frequency thresholds were higher for some cataract morphologies when compared to those for the age-matched control group. Although this was of statistical significance in some cataract morphologies at higher temporal frequencies, reference to Figure 2 demonstrates a trend for thresholds to be raised in all cataract morphology groups as temporal frequency increases. These findings are in broad agreement with those of Whitaker and Elliott13.
52
Ophthal.
Physiol.
Opt. 1995
15: No 1
The choice of temporal frequency thus appears to have implications for the predictive abilities of the technique. If OMDTs are classed as abnormal when more than two standard deviations from the mean of the normal group, a total of four (17%) of the normal group were classed as abnormal, although each of the four were at different temporal frequencies (thus higher than the 2.28% which would be expected to be found if only one temporal frequency were affected in a normally distributed sample). Four (33%) of the cortical cataract group, four (33%) of the posterior subcapsular cataract group and four (66%) of the nuclear cataract group had abnormal OMDTs for at least oozetemporal frequency. For the cataract group as a whole, the least abnormal OMDTs occurred at 1 Hz; two (7%) of the cataract group had abnormal OMDTs at this frequency. Interestingly, these thresholds were derived from the two oldest patients in the study, one of whom had cortical and the other posterior subcapsular cataract. Studies by Elliott et al.” and Watkins” have both established that OMDTs increase with age. The greatest number of abnormal OMDTs was at 1OHz with 11 (37%) of the group being classed as abnormal. This would seem to suggest that low frequencies of oscillation should be used when assessing patients with cataract. In view of the relative resistance of low frequency OMDTs to image degradation, it has been suggested that the task is used to assess visual function in the presence of opaque media’“-“. Although previous studies”-‘“.” have suggested that OMDTs at low temporal frequencies are elevated by most visual pathway disorders, before this technique can be advocated as having a clinical application, the effect on thresholds of more advanced cataract than those investigated in this study need to be evaluated. Acknowledgements Financial support for R. Watkins was provided by the Dolland and Aitchison Group and for M. Hurst by ACRAF UK. We would like to thank Mr N. Strong PhD FRCOphth. for his helpful comments on an earlier draft of this paper. References 1 Hess, R. F. and Woo, G. Vision through cataracts. Invest. Opkthalmol. Visual Sci. 17, 428-435 (1978) 2 Halliday, B. L. and Ross, J. E. Comparison of two interferometers for predicting visual acuity in patients with cataract. Br. J. O@thalmnl. 67, 273-277 (1983) 3 Bettman, J. W. Seven hundred medi-legal cases in ophthalmology. Ophthalmology 97, 1379-1384 (1990)
4 Elliott, D. B., Gilchrist, J., Hurst, M., Pickwell, L. D., Sheridan, M., Weatherill, J. and Whitaker, D. The subjective assessment of cataract. Ophthal. Pkysiol. Opt. 9, 16-19 (1989) 5 Davis, E. T., Sherman, J., Bass, S. J. and Schnider, C. M. Pre-surgical prediction of post-surgical visual function in cataract patients: multivariate statistical analyses of test measures. Clin. Vision Sci. 6, 191-207 (1991) 6 Westheimer, G. Visual acuity and hyperacuity. Invest. Ophtkalmol. 14, 570-572 (1975) 7 Williams, R. A., Enoch, J. M. and Essock, E. A. The resistance of selected hyperacuity configurations to retina1 image degradation. Invest. Ophtkalmol. Visual Sci. 25, 389-399 (1984) 8 Westheimer, G. The spatial grain of the perifoveal visual field. Vision Res. 22, 157-162 (1982) 9 Watkins, R. and Buckingham, T. The influence of stimulus luminance and contrast on hyperacuity thresholds for oscillatory movement. Opktkal. Physiol. Opt. 12, 33-37 (1992) 10 Whitaker, D. and Buckingham, T. Theory and evidence for a clinical hyperacuity test. Opktkal. Pkysiol. Opt. 7, 431-435 (1987) 11 Whitaker, D. and Buckingham, T. Oscillatory movement displacement thresholds: resistance to optical image degradation. Opktkal. Pkysiol. Opt. 7, 121-125 (1987) 12 Whitaker, D. and Deady, J. Prediction of visual function behind cataract using displacement threshold hyperacuity. Opktkal. Pkysiol. Opt. 9, 20-24 (1989) 13 Whitaker, D. and Elliott, D. Towards establishing a clinical displacement threshold technique to evaluate visual function behind cataract. Clin. Vision Sci. 4, 61-69 (1989) 14 Buckingham, T., Watkins, R., Bansal, P. and Bamford, K. Hyperacuity thresholds for oscillatory movement are abnormal in strabismic and anisometropic amblyopes. Opfom. Vision Sci. 68, 351-356 (1991) 15 Watkins, R. and Buckingham, T. Motion perception hyperacuity is abnormal in primary open-angle glaucoma and other hypertension. Invest. Opkthalmol. Vision Sci. (Suppl.) 32, 1103 (1991) 16 Watkins, R., Coker, T. and Buckingham, T. Optic neuritis elevates hyperacuity thresholds for oscillatory movement. Clin. Vision Sci. 6, 457-462 (1991) 17 Watkins, R. Hyperacuity thresholds for oscillatory movement in normal and disordered vision. PhD Thesis, University of Bradford, UK (1991) 18 Watkins, R. and Buckingham, T. Pre-retinopathic central visual dysfunction in diabetes mellitus revealed by a hyperacuity test. Acta Opktkalmol. 70, 659-664 (1992) 19 Sparrow, J. M., Bron, A. J., Brown, N. A. P., Ayliffe, W. and Hill, A. R. The Oxford Clinical Cataract Classification and Grading System. Int. Opktkalmol. 9, 207-225 (1986) 20 Elliott, D. B., Whitaker, D. and Thompson, P. Use of displacement threshold hyperacuity to isolate the neural component of senile visual loss. Appl. Opt. 28, 1914-1918 (1989) 21 Watkins, R. and Buckingham, T. Visual dysfunction in type II diabetic patients revealed by a hyperacuity test. Acta Opktkalmol. 70, 659-664 (1992)
RESEARCH
Ophthal. Physiol. Opt. Vol. 15. No. 1, pp. 49-52, 1995 0 1995 Elsevier Science Ltd for British Cokx of Ootometrists Printed in Great Britain. .kl righis reserved 0275-5408/95 $10.00 + 0.00
NOTE
timal temporal frequencies in oscillatory movement hyperacuity measurements of visual function in cataract patients Marlk Hurst”‘, ’ Department ‘The Medical UK
Russell
Watkins
of Optometry, University School, Clinical Sciences
*2 and Terry
Buckingham?’
of Bradford, Bradford, West Yorkshire BD7 Building, Leicester Royal Infirmary, Leicester
1 DP; and LE2 7LX,
Summary Hyperacuity tasks have been suggested for the assessment of potential visual function in the presence of cataracts. To test this suggestion, hyperacuity thresholds for an oscillating bar were measured in 30 subjects with idiopathic cataract and in 24 age-matched normals over a range of oscillation frequencies. Each subject’s cataract was categorized using the Oxford Clinic al Cataract Classification and Grading System. Cataract was found to have a significant effec: on thresholds, although a differential morphological effect on thresholds was equivocal. Thresholds at higher temporal frequencies were significantly raised when compared to the normal group. The main conclusion to be drawn from this study is that motion hyperacuity threslolds appear unaffected by cataract at low oscillation frequencies and should be used in preference to higher frequencies in the assessment of such patients. Ophtnal.
Physiol.
Opt.
1995,
15, 49-52
Cataract can be defined as a loss of the normal transparency of the crystalline lens. Clinically, a cataract is present when the loss of transparency results in a symptomatic visual deficir. The visual deficit is mainly due to light scatter by the lens producing a veiling luminance which is superimposed on the retinal image. This has the effect of reducing image contrast. This reduction may be over all spatial frequencies or selectively at medium and high spatia1 frequencies’. The decrease in visual performance caused by cataract has been documented by authors too numerous to list. However, the assessment of visual function behind a cataract has recently become increasingly important. In the past, such information has been used to aid the diagnosis and management of the patient with cataract and a suspected coexisting retinal/neural disorder. More recently, primarily because of improved instrumentation, it has beern put to
more subtle uses. First, it may now be possible to predict potential postoperative visual performance in a quantitative manner2. Second, because of an increase in medicolegal case?, a need has arisen to justify cataract surgery. Third, trials of anticataract agents are on the increase. For longitudinal studies of this type it is imperative that some type of retinal and neural assessment is carried out to isolate any reduction in visual performance that is not due to advancing cataract4. Despite the development of new techniques, current methods are not without their problem?. A relatively new group of tests uses the remarkable sensitivity of the visual system in accurately locating objects in the visual field; Westheimer6 coined the term ‘hyperacuity’ to describe this ability. Williams and his colleagues’ were the first to argue strongly that some types of hyperacuity measurements could be used to assess vision behind media opacities and indeed could produce superior results to those obtained by interferometry, for example. The basis upon which this statement was made, was primarily on the strength of the resistance of ‘two dot’ vernier measurements to optical blur and also on the fact that hyperacuity is highly dependent on retinal stimulus location, i.e. the highest thresholds are achieved when the fovea1 area is
*MBCQ tFBC0 Receivd: 6 December 1993 Revised form: 15 July 1994
49
50
Ophthal.
Physiol.
Opt. 1995 15: No 1
stimulated’. Unfortunately, the distance between dots in a ‘two dot test’ does influence measurements. Moreover, optimal thresholds are obtained at a specific separation for a particular standard of visual acuity (VA) which an individual achieves. Thresholds therefore need to be determined at a number of gap sizes before an evaluation of visual function can be made. Oscillatory movement displacement thresholds (OMDTs) also produce thresholds in the hyperacuity range, under certain conditions. The OMDT can be defined as the smallest amplitude of displacement that elicits the perception of movement for a given temporal frequency of oscillation. The range over which OMDTs are usually determined is 1-15 Hz. The oscillation is usually sinusoidal but can be square-wave or triangular-wave. Square waveforms tend to give lower while triangular give higher thresholds than sinusoidal waveforms. OMDTs are thought to be mediated primarily by the magnocellular pathway of the visual system’. It has been suggested that OMDT be applied clinically to assess visual function behind opaque media”. This suggestion arose from the observation that, like other hyperacuities, optical degradation by external blurring lenses and external scattering sources scarcely affect OMDT”. Initially, these results were obtained in circumstances quite different to those which occur in cataractous patients, in whom changes in retinal illumination coexist with a change in image quality. For instance, the artificial image degradation was induced using homogeneous filtering and few cataracts are homogeneous. In addition, cataract severity varies and different morphologies of cataract affect the retinal image in different ways. Nuclear cataracts both absorb and scatter incident light. Cortical and posterior subcapsular cataracts tend to absorb little incident light; their destructive effects on vision predominantly arise from light scatter within the eye. These effects will lower retinal illumination and reduce contrast sensitivity to differing degrees and are likely to be different from those effects induced by artificial means. Later papers”,‘3 presented data on actual cataract patients. Whitaker and Elliott’3 performed a number of other experiments to determine the optimum configuration for the OMDT technique. They concluded that a temporal frequency of 2 Hz should be used because of the effects of experimentally induced contrast attenuation on younger observers on the detection of high oscillation frequency (16 Hz). They also concluded that stationary reference position was largely immaterial and placed the flanking lines a total angular distance of 120min arc apart with the oscillating bar centrally placed. This type of hyperacuity has already been used in the presence of various conditions which affect the visual system, such as amblyopiai4, chronic open angle glaucoma and ocular hypertension’5, optic neuritis16, age-related maculopathy” and diabetic retinopathy18. The aim of this study was, in the first instance, to assess
the effects of oscillation frequency on thresholds obtained from patients with cataract. Second, we hoped to reveal whether cataract morphology had any differential effects on thresholds. This knowledge would allow further refinement of the technique for its use in assessing visual function behind cataract. Subjects Thirty subjects with age-related idiopathic cataracts were recruited from an anticataract drug trial based at the Bradford Royal Infirmary. Informed consent was obtained in each case. There were 12 subjects with cortical cataracts, 12 had posterior subcapsular cataracts and the remaining 6 had nuclear cataract as classified by the Oxford Clinical Cataract Classification and Grading System”. LogMAR scores ranged from +0.44 to -0.14. The mean logMAR score for each group of cataract patients is shown in Figure I. These subjects had the advantage of being very carefully monitored for any retinal or neural problem during the drug trial. We recruited 24 age-matched subjects as a control group. These subjects underwent a full ophthalmic examination and were included in the study if they met the following strict criteria: 1. 2. 3. 4. 5. 6. 7.
logMAR score 0.0 or better (see Figure 1); no lens opacity in the undilated pupil area; no history of previous eye disease; no current eye disease or amblyopia in either eye; no systemic disease considered likely to affect results; intraocular pressure < 22 mmHg; refractive error < +6.00D spherical/< f2.00D cylindrical.
The ages of this control group ranged from 51 to 75 years LogMAR 0.4
1 I
0.3 -
-
0.2 -
0.1 ................................ ................................. 0.0 . -0.1
!
::::,,,::::::.::.::.:::::::::::::: ::.: ::::::~.::::.,::::::::::::: .....:............... ............ ::::.:::::::::::.::::::::::::::::, ................................. ::::~::::::::::::.::.:::.::::::::. ......................... ...... .............................. :::::.:::::::::::::.:::::::::::::, ................................. Norms
C0rt
:::.:::.::::.::::::::::::.:::::::: ................................. :::::::.::::.:::_:::::::::::::::: ::::::::::::::::::::::::::.:::::: :::::::.::::~::::::::::::.:::::::ii .................................. :::::::.:::::::::::_:::::.::::::,: :::::::::::::::::::::.:::.::::::.: .................................. z::::::::::.:::::::::::::::::::: ::::::::::::::..::.::.:.. .:::::,: :::i::::::::::::::::::::::::::_::_ ..:::.:::::::::.:::::.::. :::::::: ~:::::::::::::::::::...... ::::::.:::::: .................... ...... ........................ ........ ::::::::::: ::::.::.::::::.:::,::: .:::::::::::.::::::-:::::::::::: ::::::::::: ~:::~::_:.:::::: ............................ :::_::::~_~_~__.~_:i~.~~..~~~~. ii ................... ............ :::::.:::::::::::::::::::::::::x: ......................... ........ ::::~:::::~:::::::i:>::::::.:: ~:::::::::::::::::::..:::.:.:::::: ::.:::::::::::::::::::ii:::::::::: ::::::::::::.:::::::.::::::.:::::: :::::::::::::::::::::::::::_i:_ii ................................:: ............................ .:::::::::::::_:::_:.:::::..:::::: ................................. .i::::ii:_::::_:::__.:::::..:::::: ................................. .................................. ................... ::::::::::::: ............. :::::::::::::::::::i:.::.:::::..:: .::.:::::::::::::i.: ::::::::: ::~::::~:::: ...... ........... .... ........ .................... .... :::::: .... ..... ........................ ::::::::::::.::.:::::i: :.::i .:: ................................
PSC
NS
Figure 1. LogMAR scores of each group of subject: Norms, age-matched normal group; PSC, posterior subcapsular cataract group; Cort, cortical cataract group and NS, nuclear cataract group.
OMDT
and those of the cataract group from 53 to 83 years. A non-parametric Mann-Whitney U-test showed that the ages of the groups were not significantly different. OMDTs were determined in the dominant eyes (determined by the sighting method) of these individuals. Apparatus
and methods
The apparatus used in this study has been described elsewhere16. Briefly, a target that was able to undergo controlled degrees of horizontal oscillation was backproj’: cted onto a diffusing screen. Also back-projected onto the ,screen was a pair of stationary references positioned such that they were equidistant either side of the target. The stationary references and the target subtended 40min arc vertically by 10min arc horizontally at a viewing distance of 6 m and had a luminance of 500cd m-‘. This produced a Michelson contrast of 95 %. Refractive corrections were worn and the test was carried out monocularly with natural pupils. Displacement thresholds for temporal frequencies of oscil’ation of 1, 4, 7, 10 and 13Hz were determined using. the psychophysical method of limits. Each threshold deter!inination used a set of three ascending and three descending runs. This method is very rapid, taking a total of atout seven and a half minutes per eye. Results A x’ test for normality indicated that the OMDT data may be cc nsidered to be normally distributed and so parametric statistics were used. Fipure 2 shows the mean OMDTs for the three groups of cataract subjects and the age-matched normal group plotted agair.st temporal frequency.
lo+, Cl
I 5
, 10
I 15 Temporal
Frequency
(Hz)
Figure 2. Oscillatory movement displacement thresholds plotted on a log scale against temporal frequency: 0, normal group; W, cortical cataract group; 0, posterior subcapsular cataract group; A, nuclear cataract group.
in cataract:
M. Hurst
et al.
51
The logMAR scores at all temporal frequencies used in this study were significantly correlated with OMDTs (4 Hz, r = 0.40, P < 0.005; 7Hz, Y = 0.44, P < 0.001; lOHz, r = 0.41, P < 0.005; 13Hz, r = 0.33, P < 0.05) with the exception of 1 Hz (r = 0.10, n.s.). This provides further support for the notion that thresholds achieved using higher temporal frequencies are limited by the same optical factors which limit visual acuity. A between group, repeated measures, analysis of variance revealed that OMDTs were significantly affected by the presence of cataract (F3.50= 3.38, P < 0.05) and in particular by nuclear cataract (post hoc Scheffe test, P < 0.05). OMDTs significantly increased as temporal frequency increased (F4,2,,0= 68.61, P < 0.000001). There was also a significant interaction between cataract type and temporal frequency (F,2.200= 5.58, P < 0.000001) with both the nuclear and posterior subcapsular groups giving significantly increased thresholds compared to normals at 7 Hz (post hoc Scheffe test, P < 0.000001, P < 0.05 respectively). At 10 Hz and 13 Hz, only the thresholds obtained from the group with nuclear cataract were significantly higher than the normals (post hoc Scheffe test, P < 0.005). As logMAR visual acuity was significantly correlated with thresholds obtained at higher temporal frequencies, visual acuity was then used as a covariate in a similar between subject, repeated measures analysis of variance. This revealed no significant difference between thresholds obtained from the normals and cataract groups (F3,50= 0.83, n.s.). It is therefore reasonable to suggest that the significant difference in thresholds found between the nuclear cataract group and normals in the preliminary analysis was probably due to the fact that the nuclear cataract group had the poorest mean acuity (see Figure I). Discussion The main conclusion to be drawn from this study is that OMDTs are unaffected by early cataract at low oscillation frequencies. Since the visual acuities of the patients in this study had logMAR scores ranging from -0.14 to +0.44, no inference can be made regarding the effects of more advanced cataract. The resistance of OMDT to image degradation has been previously demonstrated by both artificial image degradation” and by cataracts’2.‘3. In both of the above studies, a single oscillation frequency of 2Hz was used. In our study, medium and high oscillation frequency thresholds were higher for some cataract morphologies when compared to those for the age-matched control group. Although this was of statistical significance in some cataract morphologies at higher temporal frequencies, reference to Figure 2 demonstrates a trend for thresholds to be raised in all cataract morphology groups as temporal frequency increases. These findings are in broad agreement with those of Whitaker and Elliott13.
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The choice of temporal frequency thus appears to have implications for the predictive abilities of the technique. If OMDTs are classed as abnormal when more than two standard deviations from the mean of the normal group, a total of four (17%) of the normal group were classed as abnormal, although each of the four were at different temporal frequencies (thus higher than the 2.28% which would be expected to be found if only one temporal frequency were affected in a normally distributed sample). Four (33%) of the cortical cataract group, four (33%) of the posterior subcapsular cataract group and four (66%) of the nuclear cataract group had abnormal OMDTs for at least oozetemporal frequency. For the cataract group as a whole, the least abnormal OMDTs occurred at 1 Hz; two (7%) of the cataract group had abnormal OMDTs at this frequency. Interestingly, these thresholds were derived from the two oldest patients in the study, one of whom had cortical and the other posterior subcapsular cataract. Studies by Elliott et al.” and Watkins” have both established that OMDTs increase with age. The greatest number of abnormal OMDTs was at 1OHz with 11 (37%) of the group being classed as abnormal. This would seem to suggest that low frequencies of oscillation should be used when assessing patients with cataract. In view of the relative resistance of low frequency OMDTs to image degradation, it has been suggested that the task is used to assess visual function in the presence of opaque media’“-“. Although previous studies”-‘“.” have suggested that OMDTs at low temporal frequencies are elevated by most visual pathway disorders, before this technique can be advocated as having a clinical application, the effect on thresholds of more advanced cataract than those investigated in this study need to be evaluated. Acknowledgements Financial support for R. Watkins was provided by the Dolland and Aitchison Group and for M. Hurst by ACRAF UK. We would like to thank Mr N. Strong PhD FRCOphth. for his helpful comments on an earlier draft of this paper. References 1 Hess, R. F. and Woo, G. Vision through cataracts. Invest. Opkthalmol. Visual Sci. 17, 428-435 (1978) 2 Halliday, B. L. and Ross, J. E. Comparison of two interferometers for predicting visual acuity in patients with cataract. Br. J. O@thalmnl. 67, 273-277 (1983) 3 Bettman, J. W. Seven hundred medi-legal cases in ophthalmology. Ophthalmology 97, 1379-1384 (1990)
4 Elliott, D. B., Gilchrist, J., Hurst, M., Pickwell, L. D., Sheridan, M., Weatherill, J. and Whitaker, D. The subjective assessment of cataract. Ophthal. Pkysiol. Opt. 9, 16-19 (1989) 5 Davis, E. T., Sherman, J., Bass, S. J. and Schnider, C. M. Pre-surgical prediction of post-surgical visual function in cataract patients: multivariate statistical analyses of test measures. Clin. Vision Sci. 6, 191-207 (1991) 6 Westheimer, G. Visual acuity and hyperacuity. Invest. Ophtkalmol. 14, 570-572 (1975) 7 Williams, R. A., Enoch, J. M. and Essock, E. A. The resistance of selected hyperacuity configurations to retina1 image degradation. Invest. Ophtkalmol. Visual Sci. 25, 389-399 (1984) 8 Westheimer, G. The spatial grain of the perifoveal visual field. Vision Res. 22, 157-162 (1982) 9 Watkins, R. and Buckingham, T. The influence of stimulus luminance and contrast on hyperacuity thresholds for oscillatory movement. Opktkal. Physiol. Opt. 12, 33-37 (1992) 10 Whitaker, D. and Buckingham, T. Theory and evidence for a clinical hyperacuity test. Opktkal. Pkysiol. Opt. 7, 431-435 (1987) 11 Whitaker, D. and Buckingham, T. Oscillatory movement displacement thresholds: resistance to optical image degradation. Opktkal. Pkysiol. Opt. 7, 121-125 (1987) 12 Whitaker, D. and Deady, J. Prediction of visual function behind cataract using displacement threshold hyperacuity. Opktkal. Pkysiol. Opt. 9, 20-24 (1989) 13 Whitaker, D. and Elliott, D. Towards establishing a clinical displacement threshold technique to evaluate visual function behind cataract. Clin. Vision Sci. 4, 61-69 (1989) 14 Buckingham, T., Watkins, R., Bansal, P. and Bamford, K. Hyperacuity thresholds for oscillatory movement are abnormal in strabismic and anisometropic amblyopes. Opfom. Vision Sci. 68, 351-356 (1991) 15 Watkins, R. and Buckingham, T. Motion perception hyperacuity is abnormal in primary open-angle glaucoma and other hypertension. Invest. Opkthalmol. Vision Sci. (Suppl.) 32, 1103 (1991) 16 Watkins, R., Coker, T. and Buckingham, T. Optic neuritis elevates hyperacuity thresholds for oscillatory movement. Clin. Vision Sci. 6, 457-462 (1991) 17 Watkins, R. Hyperacuity thresholds for oscillatory movement in normal and disordered vision. PhD Thesis, University of Bradford, UK (1991) 18 Watkins, R. and Buckingham, T. Pre-retinopathic central visual dysfunction in diabetes mellitus revealed by a hyperacuity test. Acta Opktkalmol. 70, 659-664 (1992) 19 Sparrow, J. M., Bron, A. J., Brown, N. A. P., Ayliffe, W. and Hill, A. R. The Oxford Clinical Cataract Classification and Grading System. Int. Opktkalmol. 9, 207-225 (1986) 20 Elliott, D. B., Whitaker, D. and Thompson, P. Use of displacement threshold hyperacuity to isolate the neural component of senile visual loss. Appl. Opt. 28, 1914-1918 (1989) 21 Watkins, R. and Buckingham, T. Visual dysfunction in type II diabetic patients revealed by a hyperacuity test. Acta Opktkalmol. 70, 659-664 (1992)