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The investigation of the visualperformance of the contact lens wearer
The Investigation of the Visual Performance of the Contact Lens Wearer Jonathan S. Pointer, BSc, FBCO, B. Gilmartin, PhD, BSc, FBCO and J. R. Larke, PhD, BSc, FBCO
Jonathan Pointer is an Ophthalmic Optician engaged in research at Aston University, Birmingham.
Abstract The paper describes the assessment of a television system devised to investigate the visual performance of persons wearing an optical aid. It is suggested that a similar device could find application in hydrophilic contact lens practice.
Introduction Despite the technological advances which have produced physiologically acceptable hydrophilic contact lenses, there still remain associated with this form of optical correction certain unpredictable factors; these include variable residual refractive errors (Grosvenor, 1972; Sarver, 1972) and subclinical epithelial oedema (Hess and Garner, 1977). Such effects, too subtle to be detected with the conventional letter chart, are nevertheless disruptive to vision, and the recording of visual acuity (i.e. the Snellen fraction) is only a limited measure of visual performance (Applegate and Massof, 1975). The present authors feel that aspects such as the quality and stability of visual correction which the optical aid provides must be considered. Rather than defining the limit of resolution (i.e. visual acuity), assessment of the quality of vision for object sizes within the resolution limit should be undertaken. A n obvious approach is via contrast sensitivity threshold measurements (Schade, 1956; Campbell and Green, 1965), and indeed this has been attempted in a contact lens context (Applegate and Massof, 1975; Rosenblum and Leach, 1975; Woo and Hess, 1979). The less well known technique of border enhancement (a special case of Mach-band formation) has also been suggested (Remole, 1977) as a
Journal of the British Contact Lens Association
means of quantifying contact lens performance. However, in the visual environment of contact lenses, where poor quality and instability is a consequence of lens movement and flexure, such tests as these - - including the use of Arden's (1978; 1979) book of contrast sensitivity gratings-- are considered to be of limited application in resolving what is essentially an optical rather than a neural problem. Reverting to more clinically accepted techniques, consideration of the visual performance and illumination studies of Weston (1945) suggested the possibility of using a test procedure along the lines of Landolt's broken ring. After some initial trials it was found that the presentation of such test targets on a television screen facilitated a more accurate means of assessment (Larke and Pearson, 1977). In the resulting system, to be discussed here, single target presentation was utilised with a threshold technique of constant stimuli, and from the results obtained it has been found possible to specify visual performance in terms of threshold acuity and variability of vision.
Apparatus The television system and the factors which affect its operation have been described more fully elsewhere (Pointer, Gilmartin and Larke, 1980). To summarize, an electronically generated angular version of Landolt's broken ring test figure was displayed on a monochrome television screen (Figure 1). This test figure was randomly presented within a central viewing area over a specific range of sizes (Table 1), with a choice of instrumental settings for contrast level, figure/ground relationship and presentation/ interval times. (Continued on page 158)
155
intentionally designed to be a flexible piece of equipment. I. The first phase of the work, after initial proving trials, was undertaken with two groups of subjects who wore their best spectacle prescription. The one group (n = 21) covered the acuity range 6/3 - - 6/7.5, whilst a second group (n = 10) all achieved 6/4. The mass of data collected from these two groups under various operational conditions was corrected for guessing by a method after Prince and Fry (1956; 1958) and was then subjected to 3 - factor analysis of variance. The results of this analysis are summarized in Table 2. Table 2
The overall F-Ratios obtained for the three main effect variables when analysis is undertaken on data corrected for guessing n = 21 : 6/3 - - 6/7.5 'COLOUR' (Black/ White)
Figure 1
The television system, with the subject's hand control box in the foreground. The contrast level is set at 55%.
Method of Operation The subject indicated the position of the break in the target, viewed at 9.7 metres, by pressing one of the four buttons on a hand control box (Figure 1). Instructions on the experimental procedure were given to each subject with the aid of a cassette tape recorder. The subject was instructed to respond promptly to each target, guessing being allowed in the event of any uncertainty, but it was stressed that only one response to each target was permitted.
Experimental Procedure and Results W o r k to date has largely been concerned with optimizing the operational variables in what was Table 1 Physical Dimensions and Angular Subtense of the Test Figures DIMENSION 1
CLINICAL EQUIVALENTS 2
Figure No. in Cycle
Figure Size
Gap Size
Visual Angle (mins. of arc)
Snellen Notation
0, 8 1,9 4 5 2 3 6 7
5.00 7.50 9.75 14.75 19.75 29.25 39.75 59.75
1.00 1.50 1.75 3.25 3.75 5.75 7.75 11.75
0.35 0.53 0.62 1.15 1.32 2.03 2.74 4.15
6/2.12 3.18 3.71 6.89 7.95 12.19 16.43 24.91
1Dimensions (ram) as measured on television screen. 2Clinical equivalents when viewed at 9.7m.
158
F1,20
=
0.0414
n = 10 : 6/4 FI,9
1.9676
P>0.05
CONTRAST
F5,100
= 387.1964
F5.45
= 315.8112
P<0.01
SIZE
F9,180
= 515.8566
F9,81
= 782.2865
P
A posteriori testing via Tukey's~(1953) multiple comparison of means and inspection of graphicallyrepresented data established an optimum contrast level as setting number three of the six available. This corresponded to a contrast level of 35%. Over experimental runs at this level a maximal number of discrete data points were consistently produced. In a usual run comprising ten cycles of ten figures per cycle, the test figures corresponding to these data points were, in ascending angular subtense, 0/8, 1/9, 4, 5, 2, 3, 6, 7 (Table 1). II. A further 6/4 group of subjects (n = 5) demonstrated the high level of scoring repeatability with the apparatus (Table 3). Subsequent t-testing (twotailed) of this data suggested the non-significance of both learning and fatigue effects in the data. III. To simulate more usual wearing conditions it is essential to incorporate eye movement in any hydrophilic contact lens assessment. A fourth group of subjects (n = 5) were each fitted with a standard H y d r o n Europe soft lens to their right eye only. Correctable vision was recorded as 6/4.5 on the L a n d o r chart in each case. Over a randomised s e q u e n c e - o f nine weekly sessions, after twenty minutes of lens wear, each subject undertook a monocular series of five runs. Before the presentation of the target, an audible cuesounded and the subject made an ocular excursion of 15° from one of eight symmetrically-arranged peripheral fixation points back to the viewing aperture. A t the ninth visit a multiple excursion around the four obliquely placed fixation points was executed before a response was made.,
Journal of the British Contact Lens Association
(i)
Table 3 Repeatability of subject scoring: Ten repeated runs at 9 a.m. and 4 p.m. on same day (analysis undertaken on data corrected for guessing) SUBJECT
CORRELATION (Pearson coefficient) SIGNIFICANCE
(ii)
GROUP
1
2
3
4
5
n
0.988
0.982
0.978
0.973
0.985
0.998
P
P<0.01
P<0.III
P<0.01
P<0.01
P<0.01
efficient r.
5:6/4
Experience of operation and subsequent analysis has shown that an individual's results (i.e. threshold acuity and variability of vision) should be taken as the basis for assessing his performance with alternative optical aids (Figure 2).
The results of subsequent 2-factor analysis of variance on the corrected scores are summarized in Table 4. Size of target is confirmed (see I) as a consistently statistically significant variable, but eye movements from any one of the eight fixation points, or even a multiple excursion, yield similar results.
Conclusion The usefulness of this type of apparatus in examining Table 4 The overall F-Ratios obtained for the two main effect variables when analysis is undertaken on data corrected for guessing
Discussion A sigmoid curve represents the results of each run undertaken with the system. From a consideration of the regression line through those points lying within the central portion of the curve, visual performance may be specified in terms of:--
Figure 2
COMPARATIVE PERFORMANCE
The threshold acuity, usually at the 50% level; and the variability of vision, from a consideration of the gradient of the line, the standard error of estimate in its determination, and the Pearson co-
n = 5 : 6/4.5 FIXATION POINT
F8,32
=
0.5810
P>0.05
SiZE
F9,36
= 112.4234
P<0.01
MEASURES
OF
VISUAL
l B
.= O
i
Presentation time ............ 2.5s Interval time ............... 2.0s Contrast setting .......... 35% T.V. stimulus .............. B-on-W
== L= O
Subject: W.S V / A (FI.E.): 6 / 4 . 5 15 ° oblique eye movement
e-
50C~Othreshold acuity (~w) SPECS
•
SOFT LENSES
*;I # 2 ~
6/'6.164 6 / 5 " 6 0 0 ~ / 6 . 6 7 9
I
I
1.15 1"32
(3)
(+)(,)
Visual angle: rains of arc
(Test figure no.)
Journal of the British Contact Lens Association
159
the visual performance of hydrophilic contact lens wearing subjects is currently under investigation, with particular reference to the fitting and subsequent performance of astigmatic eyes.
Acknowledgements This work was supported by a research grant awarded to The University of Aston in Birmingham by Hydron Europe, Farnborough, Hampshire, Great Britain. Address for further correspondence: J. S. Pointer, Dept. Ophthalmic Optics, Aston University, Birmingham B4 7ET
References 1. Applegate, R. A., and Massof, R. W. (1975): Changes in the contrast sensitivity function induced by contact lens wear, Am. J. Optom. and Physiol. Optics, 52 (12): 840 - 846. 2. Arden, G. B. (1978): The importance of measuring contrast sensitivity in cases of visual disturbance, Br. J. Ophthal., 62 (4): 198 - 209. 3. Arden, G. B. (1979): Measuring contrast sensitivity with gratings: A new simple technique for the early diagnosis of retinal and neurological disease, J. Am. Optom. Ass.,'50(1): 35 - 39. 4. Campbell, F. W., and Green, D. G. (1965): Optical and retinal factors affecting visual resolution, J. Physiol., 181(3): 576 - 593. 5. Grosvenor, T. (1972): Visual acuity, astigmatism, and soft contact lenses, Am. J. Optom. and Arch. Am. Acad. Optom, 49(5): 407 - 412. 6. Hess, R. F., and Garner, L. F. (1977): The effect of corneal oedema on visual function, Invest. Ophthal. Vis. Sci., 16 (1): 5 - 13. 7. Larke, J. R., and Pearson, A. S. (1977): An apparatus for the visual examination of the contact lens wearing subject, Trans. New
160
Zealand Contact Lens Soc., 36 - 38. 8. Pointer, J. S., Gilmartin, B., and Larke, J. R. (1980): A device to assess visual performance with optical aids. In Press. 9. Prince, J. H., and Fry, G. A. (1956): The effect of errors of refraction on visual acuity, Am. J. Optom. and Arch. Am. Acad. Optom., 33(7): 353 - 373. 10. Prince, J. H. and Fry, G. A. (1958): Correction for the guessing bias in the Landolt ring test, Am. J. Optom. and Arch. Am. Acad. Optom., 35(3): 134- 141. 11. Remole, A. (1977): Border enhancement as an index of contact lens performance, Am. J. Optom. and Physiol. Optics, 54(3): 153 - 159. 12. Rosenblum, W. M., and Leach, N. E. (1975): The subjective quality (S.Q.F.) of Bausch and Lomb Softens ~ , Am. J. Optom. and Physiol. Optics, 52(10): 658 7 622. 13. Sarver, M. D. (1972): Vision with hydrophilic contact lenses, J. Am. Optom. Ass., 43(3): 316 - 320. 14. Schade, O. H. (1956): Optical and photoelectric analog of the eye, J. Optical Soc. Am., 46(9): 721 - 739. 15. Tukey, R. W. (1953): The Problem of Multiple Comparisons. Privately circulated monograph, Princeton University, U.S.A. Cited in Ryan, T. A. (1959): Multiple comparisons in psychological research - - Appendix: Turkey's method of multiple comparisons, Psychol. Bull., 56(1): 26 - 47. 16. Weston, H. C. (1945): The Relation between Illumination and Visual EfficiencY-- The Effect of Brightness Contrast, M.R.C. Industrial Health Research Board, Report No. 87: H.M.S.O., London. 17. Woo, G. C. S., and Hess, R. F. (1979): Contrast sensitivity function and soft contact lenses, Int. Contact Lens Clinic, 6 (4): 171 - 176.
Journal of the British Contact Lens Association
Jonathan Pointer is an Ophthalmic Optician engaged in research at Aston University, Birmingham.
Abstract The paper describes the assessment of a television system devised to investigate the visual performance of persons wearing an optical aid. It is suggested that a similar device could find application in hydrophilic contact lens practice.
Introduction Despite the technological advances which have produced physiologically acceptable hydrophilic contact lenses, there still remain associated with this form of optical correction certain unpredictable factors; these include variable residual refractive errors (Grosvenor, 1972; Sarver, 1972) and subclinical epithelial oedema (Hess and Garner, 1977). Such effects, too subtle to be detected with the conventional letter chart, are nevertheless disruptive to vision, and the recording of visual acuity (i.e. the Snellen fraction) is only a limited measure of visual performance (Applegate and Massof, 1975). The present authors feel that aspects such as the quality and stability of visual correction which the optical aid provides must be considered. Rather than defining the limit of resolution (i.e. visual acuity), assessment of the quality of vision for object sizes within the resolution limit should be undertaken. A n obvious approach is via contrast sensitivity threshold measurements (Schade, 1956; Campbell and Green, 1965), and indeed this has been attempted in a contact lens context (Applegate and Massof, 1975; Rosenblum and Leach, 1975; Woo and Hess, 1979). The less well known technique of border enhancement (a special case of Mach-band formation) has also been suggested (Remole, 1977) as a
Journal of the British Contact Lens Association
means of quantifying contact lens performance. However, in the visual environment of contact lenses, where poor quality and instability is a consequence of lens movement and flexure, such tests as these - - including the use of Arden's (1978; 1979) book of contrast sensitivity gratings-- are considered to be of limited application in resolving what is essentially an optical rather than a neural problem. Reverting to more clinically accepted techniques, consideration of the visual performance and illumination studies of Weston (1945) suggested the possibility of using a test procedure along the lines of Landolt's broken ring. After some initial trials it was found that the presentation of such test targets on a television screen facilitated a more accurate means of assessment (Larke and Pearson, 1977). In the resulting system, to be discussed here, single target presentation was utilised with a threshold technique of constant stimuli, and from the results obtained it has been found possible to specify visual performance in terms of threshold acuity and variability of vision.
Apparatus The television system and the factors which affect its operation have been described more fully elsewhere (Pointer, Gilmartin and Larke, 1980). To summarize, an electronically generated angular version of Landolt's broken ring test figure was displayed on a monochrome television screen (Figure 1). This test figure was randomly presented within a central viewing area over a specific range of sizes (Table 1), with a choice of instrumental settings for contrast level, figure/ground relationship and presentation/ interval times. (Continued on page 158)
155
intentionally designed to be a flexible piece of equipment. I. The first phase of the work, after initial proving trials, was undertaken with two groups of subjects who wore their best spectacle prescription. The one group (n = 21) covered the acuity range 6/3 - - 6/7.5, whilst a second group (n = 10) all achieved 6/4. The mass of data collected from these two groups under various operational conditions was corrected for guessing by a method after Prince and Fry (1956; 1958) and was then subjected to 3 - factor analysis of variance. The results of this analysis are summarized in Table 2. Table 2
The overall F-Ratios obtained for the three main effect variables when analysis is undertaken on data corrected for guessing n = 21 : 6/3 - - 6/7.5 'COLOUR' (Black/ White)
Figure 1
The television system, with the subject's hand control box in the foreground. The contrast level is set at 55%.
Method of Operation The subject indicated the position of the break in the target, viewed at 9.7 metres, by pressing one of the four buttons on a hand control box (Figure 1). Instructions on the experimental procedure were given to each subject with the aid of a cassette tape recorder. The subject was instructed to respond promptly to each target, guessing being allowed in the event of any uncertainty, but it was stressed that only one response to each target was permitted.
Experimental Procedure and Results W o r k to date has largely been concerned with optimizing the operational variables in what was Table 1 Physical Dimensions and Angular Subtense of the Test Figures DIMENSION 1
CLINICAL EQUIVALENTS 2
Figure No. in Cycle
Figure Size
Gap Size
Visual Angle (mins. of arc)
Snellen Notation
0, 8 1,9 4 5 2 3 6 7
5.00 7.50 9.75 14.75 19.75 29.25 39.75 59.75
1.00 1.50 1.75 3.25 3.75 5.75 7.75 11.75
0.35 0.53 0.62 1.15 1.32 2.03 2.74 4.15
6/2.12 3.18 3.71 6.89 7.95 12.19 16.43 24.91
1Dimensions (ram) as measured on television screen. 2Clinical equivalents when viewed at 9.7m.
158
F1,20
=
0.0414
n = 10 : 6/4 FI,9
1.9676
P>0.05
CONTRAST
F5,100
= 387.1964
F5.45
= 315.8112
P<0.01
SIZE
F9,180
= 515.8566
F9,81
= 782.2865
P
A posteriori testing via Tukey's~(1953) multiple comparison of means and inspection of graphicallyrepresented data established an optimum contrast level as setting number three of the six available. This corresponded to a contrast level of 35%. Over experimental runs at this level a maximal number of discrete data points were consistently produced. In a usual run comprising ten cycles of ten figures per cycle, the test figures corresponding to these data points were, in ascending angular subtense, 0/8, 1/9, 4, 5, 2, 3, 6, 7 (Table 1). II. A further 6/4 group of subjects (n = 5) demonstrated the high level of scoring repeatability with the apparatus (Table 3). Subsequent t-testing (twotailed) of this data suggested the non-significance of both learning and fatigue effects in the data. III. To simulate more usual wearing conditions it is essential to incorporate eye movement in any hydrophilic contact lens assessment. A fourth group of subjects (n = 5) were each fitted with a standard H y d r o n Europe soft lens to their right eye only. Correctable vision was recorded as 6/4.5 on the L a n d o r chart in each case. Over a randomised s e q u e n c e - o f nine weekly sessions, after twenty minutes of lens wear, each subject undertook a monocular series of five runs. Before the presentation of the target, an audible cuesounded and the subject made an ocular excursion of 15° from one of eight symmetrically-arranged peripheral fixation points back to the viewing aperture. A t the ninth visit a multiple excursion around the four obliquely placed fixation points was executed before a response was made.,
Journal of the British Contact Lens Association
(i)
Table 3 Repeatability of subject scoring: Ten repeated runs at 9 a.m. and 4 p.m. on same day (analysis undertaken on data corrected for guessing) SUBJECT
CORRELATION (Pearson coefficient) SIGNIFICANCE
(ii)
GROUP
1
2
3
4
5
n
0.988
0.982
0.978
0.973
0.985
0.998
P
P<0.01
P<0.III
P<0.01
P<0.01
P<0.01
efficient r.
5:6/4
Experience of operation and subsequent analysis has shown that an individual's results (i.e. threshold acuity and variability of vision) should be taken as the basis for assessing his performance with alternative optical aids (Figure 2).
The results of subsequent 2-factor analysis of variance on the corrected scores are summarized in Table 4. Size of target is confirmed (see I) as a consistently statistically significant variable, but eye movements from any one of the eight fixation points, or even a multiple excursion, yield similar results.
Conclusion The usefulness of this type of apparatus in examining Table 4 The overall F-Ratios obtained for the two main effect variables when analysis is undertaken on data corrected for guessing
Discussion A sigmoid curve represents the results of each run undertaken with the system. From a consideration of the regression line through those points lying within the central portion of the curve, visual performance may be specified in terms of:--
Figure 2
COMPARATIVE PERFORMANCE
The threshold acuity, usually at the 50% level; and the variability of vision, from a consideration of the gradient of the line, the standard error of estimate in its determination, and the Pearson co-
n = 5 : 6/4.5 FIXATION POINT
F8,32
=
0.5810
P>0.05
SiZE
F9,36
= 112.4234
P<0.01
MEASURES
OF
VISUAL
l B
.= O
i
Presentation time ............ 2.5s Interval time ............... 2.0s Contrast setting .......... 35% T.V. stimulus .............. B-on-W
== L= O
Subject: W.S V / A (FI.E.): 6 / 4 . 5 15 ° oblique eye movement
e-
50C~Othreshold acuity (~w) SPECS
•
SOFT LENSES
*;I # 2 ~
6/'6.164 6 / 5 " 6 0 0 ~ / 6 . 6 7 9
I
I
1.15 1"32
(3)
(+)(,)
Visual angle: rains of arc
(Test figure no.)
Journal of the British Contact Lens Association
159
the visual performance of hydrophilic contact lens wearing subjects is currently under investigation, with particular reference to the fitting and subsequent performance of astigmatic eyes.
Acknowledgements This work was supported by a research grant awarded to The University of Aston in Birmingham by Hydron Europe, Farnborough, Hampshire, Great Britain. Address for further correspondence: J. S. Pointer, Dept. Ophthalmic Optics, Aston University, Birmingham B4 7ET
References 1. Applegate, R. A., and Massof, R. W. (1975): Changes in the contrast sensitivity function induced by contact lens wear, Am. J. Optom. and Physiol. Optics, 52 (12): 840 - 846. 2. Arden, G. B. (1978): The importance of measuring contrast sensitivity in cases of visual disturbance, Br. J. Ophthal., 62 (4): 198 - 209. 3. Arden, G. B. (1979): Measuring contrast sensitivity with gratings: A new simple technique for the early diagnosis of retinal and neurological disease, J. Am. Optom. Ass.,'50(1): 35 - 39. 4. Campbell, F. W., and Green, D. G. (1965): Optical and retinal factors affecting visual resolution, J. Physiol., 181(3): 576 - 593. 5. Grosvenor, T. (1972): Visual acuity, astigmatism, and soft contact lenses, Am. J. Optom. and Arch. Am. Acad. Optom, 49(5): 407 - 412. 6. Hess, R. F., and Garner, L. F. (1977): The effect of corneal oedema on visual function, Invest. Ophthal. Vis. Sci., 16 (1): 5 - 13. 7. Larke, J. R., and Pearson, A. S. (1977): An apparatus for the visual examination of the contact lens wearing subject, Trans. New
160
Zealand Contact Lens Soc., 36 - 38. 8. Pointer, J. S., Gilmartin, B., and Larke, J. R. (1980): A device to assess visual performance with optical aids. In Press. 9. Prince, J. H., and Fry, G. A. (1956): The effect of errors of refraction on visual acuity, Am. J. Optom. and Arch. Am. Acad. Optom., 33(7): 353 - 373. 10. Prince, J. H. and Fry, G. A. (1958): Correction for the guessing bias in the Landolt ring test, Am. J. Optom. and Arch. Am. Acad. Optom., 35(3): 134- 141. 11. Remole, A. (1977): Border enhancement as an index of contact lens performance, Am. J. Optom. and Physiol. Optics, 54(3): 153 - 159. 12. Rosenblum, W. M., and Leach, N. E. (1975): The subjective quality (S.Q.F.) of Bausch and Lomb Softens ~ , Am. J. Optom. and Physiol. Optics, 52(10): 658 7 622. 13. Sarver, M. D. (1972): Vision with hydrophilic contact lenses, J. Am. Optom. Ass., 43(3): 316 - 320. 14. Schade, O. H. (1956): Optical and photoelectric analog of the eye, J. Optical Soc. Am., 46(9): 721 - 739. 15. Tukey, R. W. (1953): The Problem of Multiple Comparisons. Privately circulated monograph, Princeton University, U.S.A. Cited in Ryan, T. A. (1959): Multiple comparisons in psychological research - - Appendix: Turkey's method of multiple comparisons, Psychol. Bull., 56(1): 26 - 47. 16. Weston, H. C. (1945): The Relation between Illumination and Visual EfficiencY-- The Effect of Brightness Contrast, M.R.C. Industrial Health Research Board, Report No. 87: H.M.S.O., London. 17. Woo, G. C. S., and Hess, R. F. (1979): Contrast sensitivity function and soft contact lenses, Int. Contact Lens Clinic, 6 (4): 171 - 176.
Journal of the British Contact Lens Association