- Email: [email protected]
The influence of eye movement on soft contactlens visual performance
Contact Lens and Anterior Eye, Vol. 21, No. 2, pp. 61-66, 1998 Printed in Great Britain
© 1998 British Contact Lens Association
THE INFLUENCE OF EYE MOVEMENT ON SOFT CONTACT LENS VISUAL PERFORMANCE Jonathan
S. P o i n t e r *
(Received 6 October 1997" in revised form 29January 1998)
Absl]'act-- An apparently undocumented aspect of the correction provided by soft hydrophilic contact lenses, either at the fitting stage or in wear, is the influence of eye movement on visual performance. Using a televised visual discrimination task both spectacle (control) and soft contact lens (experimental) monocular visual performance was assessed in a group of human subjects (N=5) under conditions of stationary fixation and subsequent to a defined 15 deg ocular excursion. For this subject group, analysis of variance revealed no statistically significant difference (P> 0.9) between visual performance under the control and any of the experimental conditions: the group psychometric functions were similar in all cases, regardless of the direction - or absence - of a deliberate eye movement. It is concluded that even in new and inexperienced soft contact lens wearers, provided that the lens fit is optimum, eye movement does not have a significant influence on the visual performance of the eye/lens system. KEY WORDS: e y e m o v e m e n t , s a c c a d e , soft h y d r o p h i l i c c o n t a c t lens, television, visual p e r f o r m a n c e .
Introduction here appears to be an absence of information on the influence of eye movement on soft (hydrogel) contact lens visual performance. A brief lens decentration associated with a saccade-induced on-eye lens movement could momentarily alter vision. This effect might prove to be problematical for the copy-typist, motorist or sportsperson, for example, where repeated eye movements are being made. Equipment has been described ~which facilitates the unobtrusive measurement of eye movement to a very high degree of spatial and temporal accuracy during rigid contact lens wear. However the measurement of ocular movement in association with soft contact lens (SCL) wear has proven rather more problematical. Of course the contact lens practitioner will be familiar with the direct visualisation of a sequela of ocular version, namely contact lens 'lag', although it should be noted that the extent of the predictive value of this phenomenon in assessing SCL fit has recently been questioned. 2 The intuitive assumption of the clinician would be that provided an optimum SCL fit could be achieved on a given eye, any disruption to the vision achieved by the eye/lens system resulting from naturally-occurring saccadic eye movement would be minimal. From a consideration of the dynamics of the lower lid, for example, a slight translocation of a contact lens on the eye is to be expected as the line of sight moves down, as when changing from distant to near fixation? Indeed such an effect is capitalised upon in alternating-vision bifocal contact lens wear. For a SCL this on-eye translation has recently been shown to be small: ~ while the influence on the visual result of a change in gaze angle was not specifically addressed in that study4, previous work by other authors 5 could not relate degree of SCL translation to a systematic variation in visual performance. The existence of a test procedure with an
T
*PhD, FCOptom, Member of BCLA.
established capability of accurately quantifying visual performance with a refractive aid6,7 should permit an evaluation of this point. This same semi-automated TVbased system has facilitated a study of eyeblink activity during adaptation to SCL wear? In addition, the system's novel configuration of 'broken ring' test stimulus9 appears to eliminate guessing bias associated with single optotype vision assessment. 1° Consequently, using this televised visual discrimination task, the assessment of SCL visual performance in conjunction with a specific eye movement versus the control conditions of stationary fixation and spectacle lens performance should indicate any influence of eye movement. Methods Subjects
Five volunteer male subjects in good general and ocular health participated in this study: group mean age=24.24± 2.53 years. All subjects were naive observers, being habitual wearers of spectacles to correct moderate myopia: at the spectacle plane, group prescription (RE)=-4.15+1.07 DS with -0.45_+0.27 DC. No subject had been permitted any prior contact with the TV system and none admitted any previous experience of contact lens wear. The author achieved a satisfactory fit to the dominant eye (alone) of all subjects with lathe-cut daily wear pHEMA contact lenses. Material designation Filcon l a ( 3 8 ) : poly[2-hydroxyethyl methacrylate] with an equilibrium composition 38.6% water at 20°C, q u o t e d Dk (at 20°C) 8 × 1 0 ll/(at 35°C) 10 x 10 11 cm2mlO2cm3 sec2mmHg. The dominant eye alone of each subject was used for data collection: using sighting tests this was shown consistently to be the right eye of all subjects. A monocular Landolt acuity of 6/4.5 (20/15) was recorded with spectacle or hydrogel contact lenses in each case. 61
JONATHAN S. POINTER
Equipment and Test Procedure The operational procedures and proven clinical reliability of the semi-automated TV system used for the quantification of visual performance with optical aids (spectacles or contact lenses) have previously been reported? An angular version of the LandoR-type broken ring test stimulus 9 (see Figure 1) was generated and displayed on a monochrome TV monitor. The CRT window was of 4 × 3 unit dimensions, but to facilitate the generation of test stimuli of the requisite reduced size the system was arranged such that the raster scan (not interlaced) filled only one-half of the total available screen area, this being the central portion of the window. The redundant part of the screen and a substantial area around it was masked with an evenlyilluminated uniform light grey field: the geometric centre of the screen aperture in this mask was horizontally and vertically aligned with the plane of the subject's test (right) eye. The screen phosphor was the 'P4' variety and space-averaged screen luminance was in the region of 45 cd m 2. Stimulus position within the active screen area and stimulus orientation (break at top, bottom, left or right) were both randomised at each presentation by means of a fast-counting (gated) electronic circuit. The electronics which governed all aspects of stimulus presentation and monitored a subject's response were contained in a ventilated instrument cabinet; aside from a small cooling fan, operation of the equipment was completely silent, providing no auditory cues to stimulus presentation or test response. The dimensions of the test stimuli adhered to the conventional optotype figure:detail ratio of 5:1. Stimulus size varied within a repeating cycle of ten presentations (Table 1) covering the range 0.354.15 rain arc at the test distance. Each stimulus was presented as a dark figure on a light ground at 35% contrast. All measurements were made under photopic conditions. Ambient illumination in the test room was provided by two parallel ceiling-mounted 65/80 watt 'Industrial White 35' 1.5 m fluorescent tubes in a
Table 1. Physical details of the test stimuli. Angular subtense of Stimulus no. stimulus detail in cycle (min arc) 0 and 8 1 and 9 4 5 2 3 6 7
0.35 0.53 0.62 1.15 1.32 2.03 2.74 4.15
Snellen equivalent at test distance 6/2.10 3.18 3.72 6.90 7.92 12.18 16.44 24.90
(20/7.00) (10.60) ( 12.40) (23.00) ( 26.40) (40.60) ( 54.80) ( 83.00)
Each broken ring stimulus presented as a dark figure on a light ground at -0.35 contrast; contrast, C=(Lm~-Lmin)/(Lm~+ Lmin), where L=-luminance.
62
matt white unit. Although not specifically assessed during the course of this study, given the stability of the ambient illumination and the uniformity of the subject group (young adult male myopes) any variation in pupil diameter would have been negligible and should have exerted no influence on the results obtained. The psychophysical test procedure was arranged such that the subject was presented with a randomised sequence of single, variously-sized, four-alternative orientation angular broken ring stimuli on the monochrome TV monitor. At each trial the subject was required to indicate the position of the break in the stimulus (top/bottom/left/right) by pressing only one of four buttons on a hand-held response box, guessing being required in the event of uncertainty. Monocular subjective responses were recorded over five successive runs, each individual run comprising 100 trials (i.e., ten repetitions of the ten-stimulus cycle): an estimate of the level of visual performance (acuity equivalent to 50%correct threshold resolution) was obtained subsequently by linear regression analysis of these psychometric data as corrected for guessing. 6 Immediately prior to the presentation of each stimulus the subject responded to an auditory tone by changing his resting fixation from a point offset 15 deg from the centre of the TV screen back to the screen itself (Figure 1A). It has been claimed11 that 99% of all human eye movements are within 15 deg of the primary position; further it has been stated 12 that most naturally occurring human saccades have magnitudes of 15 deg or less. Consequently in an attempt to simulate natural viewing conditions the subject was required to make an eye movement of this angular extent while undertaking the televised visual discrimination task: although centripetal eye movements are faster than centrifugal movements of the same angular displacement13, the differential would be negligible for an ocular excursion of this magnitude. Following the auditory cue, the maximum duration of stimulus presentation on the TV monitor screen could be 2.92 s. However a previous study~has shown that subjects take less than 40% of this time to make their saccade and register a response: typically ~-1.1 s for the smaller stimuli, decreasing to ~-0.75 s for the larger stimuli. The response (buttonpress) cleared the screen and initiated a 2.65 s interstimulus interval (ISI), during which time the subject refixated the specified eccentric fixation point and awaited the next auditory cue. The preferred choice of direction for this 15 deg ocular movement was unclear. Consequently the opportunity was taken to investigate specific movements in each of eight directions of gaze (Figure 1B): in the four cardinal meridians, with reflxation directly downwards designated '1', through '3' 0nwards-from-theright, as viewed from the subject's perspective), '5' (directly-upwards) and '7' 0nwards-from-the-left); also in the four major oblique meridians, down-and-to-left designated '2', through '4' (up-and-to-left), '6' (up-andto-right) and '8' (down-and-to-right).
THE INFLUENCE OF EYE MOVEMENT ON SONYCONTACT LENS VISUALPERFORMANCE
A
stimulus
~i~
presentation
15 deg saccade
RESPONSE
B
(.4) Schematic representation of the test sequence and a depiction of the angular form of the broken ring test stimulus. (B) Schematic illustration (not to scale) indicating (from the subject's perspective) the location of the eight alternative fixation points around a 15 deg eccentric annulus centred on the stimulus presentation area. F i g u r e 1.
It should be noted that eyeblink activity was specifically not assessed in this present study: earlier work 8 using this equipment in conjunction with the undertaking of an oblique 15 deg saccade (designated '6' above) had found little influence of eyeblinks on visual performance. Blinking during observation of the stimulus was uniformly almost zero: the majority of blink activity occurred early in the ISI, being most intense following the response to a small stimulus. The apparently 'ballistic' nature of saccade-and-response subsequent to the sounding of the auditory cue suggests that blinking is not a significant factor in the undertaking of this psychophysical task. Data Collection Aside from an initial screening visit and subsequent attendance for an optometric examination with SCL fitting to the dominant eye, all subjects made a series of
fourteen data collection visits (each of c.60 min duration) over a 10 week period. The first two visits were control visits, with no auditory tone and no defined eye movement, the subject viewing the TV monitor screen steadily throughout each run: visit 'Specs 0' (week 1) was undertaken with best-sphere monocular spectacle correction in the form of trial case lenses, visit 'SCL 0' (week 2) wearing the monocular soft lens. Visits 'SCL 1' through 'SCL 8' (over weeks 3-10) were experimental visits, undertaken wearing the monocular soft lens and making a defined 15 deg (refixation) eye movement before each televised stimulus presentation, with the direction of the ocular excursion for each subject at each of the eight visits being randomised. The spacing of the visits at weekly intervals was with the specific intention of minimising any possibility of adaptation to the SCL: allowing time for insertion and settling of the lens on the eye prior to data collection, each subject would wear his single SCL for no more than 50 rain at each visit (the author retained the lenses for autoclaving between visits). Visits 'Specs 1', '2', '6' and '7' (additional fortnightly attendances during weeks 3, 5, 7 and 9) were undertaken wearing best-sphere spectacle lenses and making a vertical ('1'), oblique ('2', '6') or horizontal ('7') 15 deg saccade, to allow collection of data to serve as a control for the 'SCL and saccade' data. As with the SCL visits, the choice of direction for the refixatory eye movement for each subject at each of these four spectacle-wearing visits was randomised. Subjects were paid for their participation in this 10 week study on completion of data collection at their final visit. Results The series of raw data generated by each of the five subjects were corrected for guessing 6 and, for each subject at each of the fourteen visits, the mean result (per cent-correct versus angular subtense of stimulus detail) over the five sequential runs was calculated. Twofactor (subject, test condition) analysis of variance (ANOVA) indicated only a borderline statistically significant (SS) difference (P<0.05) between mean results across subjects (N=5), and no SS difference (P> 0.9) across test conditions (N=14: with spectacles 1 stationary/4 saccades; with soft lens - 1 stationary/8 saccades), and no SS interaction (P=I.0). Not surprisingly angular subtense of stimulus detail was a highly SS variable (one-way ANOVA: P < 0.0001), the broken ring stimuli of larger angular subtense in the test cycle promoting relatively higher (per cent-correct) scores than the stimuli of smaller subtense (see Figures 2 and 3). The covariance of visual discrimination ability and size of stimulus detail, at each test condition, is summarised in Figures 2 and 3: these illustrate the series of group (N=5) psychometric functions obtained with either a spectacle or soft lens correction and no eye movement ('0'), with a spectacle correction and four directions of 15 deg saccade ('1', '2', '6', '7') and with a 63
S. P O I N T E R
JONATHAN
1O0
/ i Specs
6
Specs 7
I
..:
..-
/
/
0.:
•/
e //
J
/
7
/
/ /
50 /
/
/ /
Q/
/
/
•./
/
/
~| ,
0.5
1
1.5
2.5
2
i
.=i}
S
i ,
0.5
i
1
ANGULAR
O0
i
2
1,5
2,5
SUBTENSE
0.5
1
1.5
OF
STIMULUS
2,5
2
• i . J . 1.5 2 2.5
1
0.5
DETAIL
(min
•
0.5
,
i ,
J
1
1.5
2
,
2.5
arc)
Figure 2. Control results. Psychometric functions (group data, N=5) obtained wearing best-sphere spectacle lens correction: %-correct response (data corrected for guessing) versus angular subtense of the break in the ring stimulus (min arc). 'Specs O' material obtained with no eye movement, other material generated when a specific 15 deg saccade in the vertical ('1'), oblique ('2" '6') or horizontal ('7') directions was incorporated into the test procedure. Linear regression fit (dashed line) to each data set is virtually identical (r >~0.93; r2 >~0.86; P < 0.001); the small vertical arrow above the abscissa in each frame indicates the group mean threshold resolution value at the 50%-correct response level (see Table 2). Evidently performance on the televised visual discrimination task was not influenced by the undertaking - or direction - of a specific eye movement when spectacle lenses were worn.
100
/ •
![ - ~ " - ]
_.."
•/
/
? O/
•/
•/ /
/
•/ / / %
50
:
/
?
/
/
/
/0
:.|
/• /| ]0O
''
I
,
I
i
I
+
,
I
SCL 4 } i
-
i
.
R
.
i
,
i
0"
.:
0. /
:
/
.: ?
/
?
/
i
,
i
2
_',
•
2.5
ANGULAR
0.5
I¢
:-
i
-| 1.5
/
/
?
,
,
/
e"
1
i
• /
0
,
,
-•
/
/
i
i
e/
/
//
0.5
,
I
/ /
.:
0
r
7
SCL
F
/
,
•
•
:
50
,
:
J
.:0 :• .#
..... 1
1.5
i
2
SUBTENSE
2.5
-"0
,
i
,
i 1.5
,
,
2
0.5
1
OF
STIMULUS
/ • .~
.
2.5
0.S
1
DETAIL
.
,
.
i
1.5
2
(min
,
2.5
0.5
1
1.5
2
2.5
arc)
Figure 3. Experimental results. Psychometric functions (group data, N:5) obtained wearing soft contact lens correction. 'SCL O' data obtained with stationary fixation, results in the other eight frames generated following a specific 15 deg saccade in the vertical ('1" '5'), horizontal ('3" '7) or one of the four major oblique directions ('2" '4" '6" '8'). Linear regression fit (dashed line) is virtually identical (r >>,0.94; r2 >10.88; P < 0.001), not only across each of the nine experimental data sets but also in comparison to the control data shown in Figure 2. Mean 50%-correct threshold resolution values are indicated above the abscissa in each frame. Evidently neither the type of refractive correction worn (spectacles versus SCL) nor the test condition (stationary versus defined eye movement) affected performance on this visual discrimination task.
64
THE INFLUENCEOF EYEMOVEMENTON SOFTCONTACTLENSVISUALPERFORMANCE Table 2. Individual subject's and the combined group (N=5) threshold acuity results for each test condition. Test condition
Individual results 50%-correct threshold resolution rain arc Subject i 2 3 4
Group (mean) results. N=5 50%-correct threshold resolution Equivalent acuity level min arc (95% confidence limits) 6/ 20/
5
Specs '0' T '2' '6' '7'
1.070 0.846 1.072 1.088 1.052
0.757 0.639 0.729 0.762 0.893
0.989 0.902 0.959 0.857 0.885
0.741 0.994 0.773 0.910 0.987
0.983 0.947 0.999 0.750 0.747
0.908 0.866 0.906 0.873 0.913
(0.777 to (0.745 to (0.776 to (0.753 to (0.812 to
1.039) 0.987) 1.036) 0.993) 1.014)
5.448 5.194 5.438 5.240 5.477
18.160 17.312 18.128 17.468 18.256
SCL '0' '1' '2' '3' '4' '5' '6' '7' '8'
1.034 1.051 1.048 1.192 1.029 1.017 1.034 1.037 1.038
0.826 0.794 0.810 0.908 0.707 0.795 0.897 0.734 0.740
0.891 0.855 1.106 1.092 0.996 1.142 1.114 1.003 1.138
0.991 0.846 0.860 0.852 0.833 1.044 0.991 0.877 0.753
0.929 0.906 0.875 0.711 0.988 0.826 0.935 0.852 0.869
0.934 0.890 0.940 0.951 0.911 0.965 0.994 0.901 0.908
(0.862 to (0.804 to (0.827 to (0.783 to (0.791 to (0.835 to (0.920 to (0.794 to (0.754 to
1.006) 0.976) 1.053) 1.119) 1.031) 1.095) 1.068) 1.008) 1.062)
5.605 5.342 5.639 5.706 5.464 5.789 5.965 5.404 5.446
18.684 17.808 18.796 19.020 18.212 19.296 19.884 18.012 18.152
SCL and eight directions of ocular excursion ('1' through '8'). Evidently regardless of test condition visual performance, as assessed by this televised visual discrimination task, remained invariate: across all fourteen frames of Figures 2 and 3 linear regression fits are seen to have similar gradients and intercepts, with r~> 0.93, r ~~>0.86 and P < 0.001. Visual performance with best-sphere spectacle correction before the dominant eye was closely matched by that with an optimally-fitted SCL u n d e r conditions of stationary fixation, which performance was neither depleted nor improved during spectacle or SCL wear by the incorporation into the test procedure of a 15 deg horizontal, vertical or oblique saccade. The clinical acuity level (calculated on the basis of the 50% -correct threshold resolution value) ~- 6/ 5 (20/17) for each set of group data. Individual and group (N=5) threshold resolution results over the fourteen test conditions are summarised in Table 2: the small vertical arrow above the abscissa in each frame of Figures 2 and 3 indicates the mean (group: N=5) threshold resolution value (rain arc) at the 50%correct response level.
the eye/lens system was intentionally mobile. The results of this concise study indicate that (in the short term at leas0 when wearing an optimally-fitted hydrogel contact lens a 15 deg saccadic eye movement in either the horizontal, vertical or oblique direction has a negligible influence on visual performance (taking performance at stationary fixation as baseline or in comparison with spectacle lens performance similarly incorporating a 15 deg saccade). The confirmation of this point has implications for clinical contact lens practice since, with regard to assessments made within the confines of the consulting room, the incorporation of a defined eye movement might be assumed to reflect more closely 'usual' conditions of wear. Consequently, the establishment of no deleterious visual effects arising from an eye movement at the fitting stage, in conjunction with the routine checks to determine optimum SCL fit, might be of particular significance when assessing the patient who drives extensively, who plays ball or racquet sports, or who works for much of the day as a copy-typist or interacts with a visual display terminal. 15
Discussion
Address for Correspondence
In clinical practice a consideration in defining optimum contact lens fit on an individual eye is the level of best (usually Snellen) visual acuity determined (as with spectacle lenses during routine refraction) at a contrast ~ - - 1.00 (dark letter on a light ground). T h e material presented here, obtained at contrast = - 0 . 3 5 , is in line with earlier results recorded with this televised test systemr: viz, the visual performance achieved with an eye wearing an optimally-fitted SCL can match that obtained with a spectacle correction. (Elsewhere it has been reported 14 that an individual's contrast sensitivity function recorded with spectacles or with a SCL displays no clinically significant difference.) This 'no difference' outcome for visual performance at stationary fixation can now be extended to conditions where
Dr Jonathan S. Pointer, 4a Market Square, Higham Ferrers, Northamptonshire NN10 8BP, UIC REFERENCES 1 Gilman, ILG. Eye and contact lens movement. Am. J. Optom. PhysioL Opt., 59, 602-610 (1982). z Young, G. Evaluation of soft contact lens fitting characteristics. Optom. Vis. Sci., 73, 247-254 (1996). z Borish, I.M. and Perrigin, D. Relative movement of lower lid and line of sight from distant to near fixation.Am. J. Optom. Physiol. Opt., 64, 881-887 (1987). 4 Chateau, N., de Brabander, J., Bouchard, F. and Molenaar, H. Infrared pupillometryin presbyopes fitted with soft contact lenses. Optom. Iris. Sci., 73, 733-741 (1996). 5 Erickson, P. and Robboy, M. Performance characteristics of a hydrophilic concentric bifocal contact lens. Am. J. Optom. Physiol. Opt., 62, 702-708 (1985). 65
JONATHANS. POINTER 6 Pointer, J.S., Gilmartin, B. and Larke, J.R. A device to assess visual performance with optical aids. Am. J. Optom. Physiol. Opt., 58, 408-413 (1981). 7 Pointer, J.S., Gilmartin, B. and karke, J.R. Visual performance with soft hydrophilic contact lenses. Am. J. Optom. Physiol. Opt., 62, 694-701 (1985). 8 Pointer, J.S. Eyeblink activity with hydrophilic contact lenses: a concise longitudinal study. Acta Ophthal., 66, 498-504 (1988). 9 Pointer, J.S., Gilmartin, B. and Larke, J.R. The evolution of the broken ring visual acuity test figure. J. Am. Optom. Assoc., 51, 741-745 (1980). lo Pointer, J.S. Towards the elimination of guessing bias in "Landolt acuity testing. Am. J. Optom. Physiol. Opt., 63, 813-818 (1986).
66
i i Lancaster, W.B. Fifty years' experience in ocular motility (Part I).
Am. J. Ophthalmol., 24, 485-496 (1941). 12 Bahill, A.T., Adler, D. and Stark, L. Most naturally occurring human saccades have magnitudes of 15 degrees or less. Invest. Ophthalmol. Vis. Sci., 14, 468-469 (1975). 13 Brockhurst, R.J. and Lion, K.S. Analysis of ocular movements by means of an electrical method. Arch. Ophthalmol., 46, 311-314 (1951).
14 Nowozyckyj, A., Carney, L.G. and Eft'on, N. Effect of hydrogel lens wear on contrast sensitivity. Am. J. Optom. Physiol. Opt., 65, 263271 (1988). 15 Gasson, A. Visual display units and contact lenses. Contact Lens J., 11, (1) 13-16 (1983).
© 1998 British Contact Lens Association
THE INFLUENCE OF EYE MOVEMENT ON SOFT CONTACT LENS VISUAL PERFORMANCE Jonathan
S. P o i n t e r *
(Received 6 October 1997" in revised form 29January 1998)
Absl]'act-- An apparently undocumented aspect of the correction provided by soft hydrophilic contact lenses, either at the fitting stage or in wear, is the influence of eye movement on visual performance. Using a televised visual discrimination task both spectacle (control) and soft contact lens (experimental) monocular visual performance was assessed in a group of human subjects (N=5) under conditions of stationary fixation and subsequent to a defined 15 deg ocular excursion. For this subject group, analysis of variance revealed no statistically significant difference (P> 0.9) between visual performance under the control and any of the experimental conditions: the group psychometric functions were similar in all cases, regardless of the direction - or absence - of a deliberate eye movement. It is concluded that even in new and inexperienced soft contact lens wearers, provided that the lens fit is optimum, eye movement does not have a significant influence on the visual performance of the eye/lens system. KEY WORDS: e y e m o v e m e n t , s a c c a d e , soft h y d r o p h i l i c c o n t a c t lens, television, visual p e r f o r m a n c e .
Introduction here appears to be an absence of information on the influence of eye movement on soft (hydrogel) contact lens visual performance. A brief lens decentration associated with a saccade-induced on-eye lens movement could momentarily alter vision. This effect might prove to be problematical for the copy-typist, motorist or sportsperson, for example, where repeated eye movements are being made. Equipment has been described ~which facilitates the unobtrusive measurement of eye movement to a very high degree of spatial and temporal accuracy during rigid contact lens wear. However the measurement of ocular movement in association with soft contact lens (SCL) wear has proven rather more problematical. Of course the contact lens practitioner will be familiar with the direct visualisation of a sequela of ocular version, namely contact lens 'lag', although it should be noted that the extent of the predictive value of this phenomenon in assessing SCL fit has recently been questioned. 2 The intuitive assumption of the clinician would be that provided an optimum SCL fit could be achieved on a given eye, any disruption to the vision achieved by the eye/lens system resulting from naturally-occurring saccadic eye movement would be minimal. From a consideration of the dynamics of the lower lid, for example, a slight translocation of a contact lens on the eye is to be expected as the line of sight moves down, as when changing from distant to near fixation? Indeed such an effect is capitalised upon in alternating-vision bifocal contact lens wear. For a SCL this on-eye translation has recently been shown to be small: ~ while the influence on the visual result of a change in gaze angle was not specifically addressed in that study4, previous work by other authors 5 could not relate degree of SCL translation to a systematic variation in visual performance. The existence of a test procedure with an
T
*PhD, FCOptom, Member of BCLA.
established capability of accurately quantifying visual performance with a refractive aid6,7 should permit an evaluation of this point. This same semi-automated TVbased system has facilitated a study of eyeblink activity during adaptation to SCL wear? In addition, the system's novel configuration of 'broken ring' test stimulus9 appears to eliminate guessing bias associated with single optotype vision assessment. 1° Consequently, using this televised visual discrimination task, the assessment of SCL visual performance in conjunction with a specific eye movement versus the control conditions of stationary fixation and spectacle lens performance should indicate any influence of eye movement. Methods Subjects
Five volunteer male subjects in good general and ocular health participated in this study: group mean age=24.24± 2.53 years. All subjects were naive observers, being habitual wearers of spectacles to correct moderate myopia: at the spectacle plane, group prescription (RE)=-4.15+1.07 DS with -0.45_+0.27 DC. No subject had been permitted any prior contact with the TV system and none admitted any previous experience of contact lens wear. The author achieved a satisfactory fit to the dominant eye (alone) of all subjects with lathe-cut daily wear pHEMA contact lenses. Material designation Filcon l a ( 3 8 ) : poly[2-hydroxyethyl methacrylate] with an equilibrium composition 38.6% water at 20°C, q u o t e d Dk (at 20°C) 8 × 1 0 ll/(at 35°C) 10 x 10 11 cm2mlO2cm3 sec2mmHg. The dominant eye alone of each subject was used for data collection: using sighting tests this was shown consistently to be the right eye of all subjects. A monocular Landolt acuity of 6/4.5 (20/15) was recorded with spectacle or hydrogel contact lenses in each case. 61
JONATHAN S. POINTER
Equipment and Test Procedure The operational procedures and proven clinical reliability of the semi-automated TV system used for the quantification of visual performance with optical aids (spectacles or contact lenses) have previously been reported? An angular version of the LandoR-type broken ring test stimulus 9 (see Figure 1) was generated and displayed on a monochrome TV monitor. The CRT window was of 4 × 3 unit dimensions, but to facilitate the generation of test stimuli of the requisite reduced size the system was arranged such that the raster scan (not interlaced) filled only one-half of the total available screen area, this being the central portion of the window. The redundant part of the screen and a substantial area around it was masked with an evenlyilluminated uniform light grey field: the geometric centre of the screen aperture in this mask was horizontally and vertically aligned with the plane of the subject's test (right) eye. The screen phosphor was the 'P4' variety and space-averaged screen luminance was in the region of 45 cd m 2. Stimulus position within the active screen area and stimulus orientation (break at top, bottom, left or right) were both randomised at each presentation by means of a fast-counting (gated) electronic circuit. The electronics which governed all aspects of stimulus presentation and monitored a subject's response were contained in a ventilated instrument cabinet; aside from a small cooling fan, operation of the equipment was completely silent, providing no auditory cues to stimulus presentation or test response. The dimensions of the test stimuli adhered to the conventional optotype figure:detail ratio of 5:1. Stimulus size varied within a repeating cycle of ten presentations (Table 1) covering the range 0.354.15 rain arc at the test distance. Each stimulus was presented as a dark figure on a light ground at 35% contrast. All measurements were made under photopic conditions. Ambient illumination in the test room was provided by two parallel ceiling-mounted 65/80 watt 'Industrial White 35' 1.5 m fluorescent tubes in a
Table 1. Physical details of the test stimuli. Angular subtense of Stimulus no. stimulus detail in cycle (min arc) 0 and 8 1 and 9 4 5 2 3 6 7
0.35 0.53 0.62 1.15 1.32 2.03 2.74 4.15
Snellen equivalent at test distance 6/2.10 3.18 3.72 6.90 7.92 12.18 16.44 24.90
(20/7.00) (10.60) ( 12.40) (23.00) ( 26.40) (40.60) ( 54.80) ( 83.00)
Each broken ring stimulus presented as a dark figure on a light ground at -0.35 contrast; contrast, C=(Lm~-Lmin)/(Lm~+ Lmin), where L=-luminance.
62
matt white unit. Although not specifically assessed during the course of this study, given the stability of the ambient illumination and the uniformity of the subject group (young adult male myopes) any variation in pupil diameter would have been negligible and should have exerted no influence on the results obtained. The psychophysical test procedure was arranged such that the subject was presented with a randomised sequence of single, variously-sized, four-alternative orientation angular broken ring stimuli on the monochrome TV monitor. At each trial the subject was required to indicate the position of the break in the stimulus (top/bottom/left/right) by pressing only one of four buttons on a hand-held response box, guessing being required in the event of uncertainty. Monocular subjective responses were recorded over five successive runs, each individual run comprising 100 trials (i.e., ten repetitions of the ten-stimulus cycle): an estimate of the level of visual performance (acuity equivalent to 50%correct threshold resolution) was obtained subsequently by linear regression analysis of these psychometric data as corrected for guessing. 6 Immediately prior to the presentation of each stimulus the subject responded to an auditory tone by changing his resting fixation from a point offset 15 deg from the centre of the TV screen back to the screen itself (Figure 1A). It has been claimed11 that 99% of all human eye movements are within 15 deg of the primary position; further it has been stated 12 that most naturally occurring human saccades have magnitudes of 15 deg or less. Consequently in an attempt to simulate natural viewing conditions the subject was required to make an eye movement of this angular extent while undertaking the televised visual discrimination task: although centripetal eye movements are faster than centrifugal movements of the same angular displacement13, the differential would be negligible for an ocular excursion of this magnitude. Following the auditory cue, the maximum duration of stimulus presentation on the TV monitor screen could be 2.92 s. However a previous study~has shown that subjects take less than 40% of this time to make their saccade and register a response: typically ~-1.1 s for the smaller stimuli, decreasing to ~-0.75 s for the larger stimuli. The response (buttonpress) cleared the screen and initiated a 2.65 s interstimulus interval (ISI), during which time the subject refixated the specified eccentric fixation point and awaited the next auditory cue. The preferred choice of direction for this 15 deg ocular movement was unclear. Consequently the opportunity was taken to investigate specific movements in each of eight directions of gaze (Figure 1B): in the four cardinal meridians, with reflxation directly downwards designated '1', through '3' 0nwards-from-theright, as viewed from the subject's perspective), '5' (directly-upwards) and '7' 0nwards-from-the-left); also in the four major oblique meridians, down-and-to-left designated '2', through '4' (up-and-to-left), '6' (up-andto-right) and '8' (down-and-to-right).
THE INFLUENCE OF EYE MOVEMENT ON SONYCONTACT LENS VISUALPERFORMANCE
A
stimulus
~i~
presentation
15 deg saccade
RESPONSE
B
(.4) Schematic representation of the test sequence and a depiction of the angular form of the broken ring test stimulus. (B) Schematic illustration (not to scale) indicating (from the subject's perspective) the location of the eight alternative fixation points around a 15 deg eccentric annulus centred on the stimulus presentation area. F i g u r e 1.
It should be noted that eyeblink activity was specifically not assessed in this present study: earlier work 8 using this equipment in conjunction with the undertaking of an oblique 15 deg saccade (designated '6' above) had found little influence of eyeblinks on visual performance. Blinking during observation of the stimulus was uniformly almost zero: the majority of blink activity occurred early in the ISI, being most intense following the response to a small stimulus. The apparently 'ballistic' nature of saccade-and-response subsequent to the sounding of the auditory cue suggests that blinking is not a significant factor in the undertaking of this psychophysical task. Data Collection Aside from an initial screening visit and subsequent attendance for an optometric examination with SCL fitting to the dominant eye, all subjects made a series of
fourteen data collection visits (each of c.60 min duration) over a 10 week period. The first two visits were control visits, with no auditory tone and no defined eye movement, the subject viewing the TV monitor screen steadily throughout each run: visit 'Specs 0' (week 1) was undertaken with best-sphere monocular spectacle correction in the form of trial case lenses, visit 'SCL 0' (week 2) wearing the monocular soft lens. Visits 'SCL 1' through 'SCL 8' (over weeks 3-10) were experimental visits, undertaken wearing the monocular soft lens and making a defined 15 deg (refixation) eye movement before each televised stimulus presentation, with the direction of the ocular excursion for each subject at each of the eight visits being randomised. The spacing of the visits at weekly intervals was with the specific intention of minimising any possibility of adaptation to the SCL: allowing time for insertion and settling of the lens on the eye prior to data collection, each subject would wear his single SCL for no more than 50 rain at each visit (the author retained the lenses for autoclaving between visits). Visits 'Specs 1', '2', '6' and '7' (additional fortnightly attendances during weeks 3, 5, 7 and 9) were undertaken wearing best-sphere spectacle lenses and making a vertical ('1'), oblique ('2', '6') or horizontal ('7') 15 deg saccade, to allow collection of data to serve as a control for the 'SCL and saccade' data. As with the SCL visits, the choice of direction for the refixatory eye movement for each subject at each of these four spectacle-wearing visits was randomised. Subjects were paid for their participation in this 10 week study on completion of data collection at their final visit. Results The series of raw data generated by each of the five subjects were corrected for guessing 6 and, for each subject at each of the fourteen visits, the mean result (per cent-correct versus angular subtense of stimulus detail) over the five sequential runs was calculated. Twofactor (subject, test condition) analysis of variance (ANOVA) indicated only a borderline statistically significant (SS) difference (P<0.05) between mean results across subjects (N=5), and no SS difference (P> 0.9) across test conditions (N=14: with spectacles 1 stationary/4 saccades; with soft lens - 1 stationary/8 saccades), and no SS interaction (P=I.0). Not surprisingly angular subtense of stimulus detail was a highly SS variable (one-way ANOVA: P < 0.0001), the broken ring stimuli of larger angular subtense in the test cycle promoting relatively higher (per cent-correct) scores than the stimuli of smaller subtense (see Figures 2 and 3). The covariance of visual discrimination ability and size of stimulus detail, at each test condition, is summarised in Figures 2 and 3: these illustrate the series of group (N=5) psychometric functions obtained with either a spectacle or soft lens correction and no eye movement ('0'), with a spectacle correction and four directions of 15 deg saccade ('1', '2', '6', '7') and with a 63
S. P O I N T E R
JONATHAN
1O0
/ i Specs
6
Specs 7
I
..:
..-
/
/
0.:
•/
e //
J
/
7
/
/ /
50 /
/
/ /
Q/
/
/
•./
/
/
~| ,
0.5
1
1.5
2.5
2
i
.=i}
S
i ,
0.5
i
1
ANGULAR
O0
i
2
1,5
2,5
SUBTENSE
0.5
1
1.5
OF
STIMULUS
2,5
2
• i . J . 1.5 2 2.5
1
0.5
DETAIL
(min
•
0.5
,
i ,
J
1
1.5
2
,
2.5
arc)
Figure 2. Control results. Psychometric functions (group data, N=5) obtained wearing best-sphere spectacle lens correction: %-correct response (data corrected for guessing) versus angular subtense of the break in the ring stimulus (min arc). 'Specs O' material obtained with no eye movement, other material generated when a specific 15 deg saccade in the vertical ('1'), oblique ('2" '6') or horizontal ('7') directions was incorporated into the test procedure. Linear regression fit (dashed line) to each data set is virtually identical (r >~0.93; r2 >~0.86; P < 0.001); the small vertical arrow above the abscissa in each frame indicates the group mean threshold resolution value at the 50%-correct response level (see Table 2). Evidently performance on the televised visual discrimination task was not influenced by the undertaking - or direction - of a specific eye movement when spectacle lenses were worn.
100
/ •
![ - ~ " - ]
_.."
•/
/
? O/
•/
•/ /
/
•/ / / %
50
:
/
?
/
/
/
/0
:.|
/• /| ]0O
''
I
,
I
i
I
+
,
I
SCL 4 } i
-
i
.
R
.
i
,
i
0"
.:
0. /
:
/
.: ?
/
?
/
i
,
i
2
_',
•
2.5
ANGULAR
0.5
I¢
:-
i
-| 1.5
/
/
?
,
,
/
e"
1
i
• /
0
,
,
-•
/
/
i
i
e/
/
//
0.5
,
I
/ /
.:
0
r
7
SCL
F
/
,
•
•
:
50
,
:
J
.:0 :• .#
..... 1
1.5
i
2
SUBTENSE
2.5
-"0
,
i
,
i 1.5
,
,
2
0.5
1
OF
STIMULUS
/ • .~
.
2.5
0.S
1
DETAIL
.
,
.
i
1.5
2
(min
,
2.5
0.5
1
1.5
2
2.5
arc)
Figure 3. Experimental results. Psychometric functions (group data, N:5) obtained wearing soft contact lens correction. 'SCL O' data obtained with stationary fixation, results in the other eight frames generated following a specific 15 deg saccade in the vertical ('1" '5'), horizontal ('3" '7) or one of the four major oblique directions ('2" '4" '6" '8'). Linear regression fit (dashed line) is virtually identical (r >>,0.94; r2 >10.88; P < 0.001), not only across each of the nine experimental data sets but also in comparison to the control data shown in Figure 2. Mean 50%-correct threshold resolution values are indicated above the abscissa in each frame. Evidently neither the type of refractive correction worn (spectacles versus SCL) nor the test condition (stationary versus defined eye movement) affected performance on this visual discrimination task.
64
THE INFLUENCEOF EYEMOVEMENTON SOFTCONTACTLENSVISUALPERFORMANCE Table 2. Individual subject's and the combined group (N=5) threshold acuity results for each test condition. Test condition
Individual results 50%-correct threshold resolution rain arc Subject i 2 3 4
Group (mean) results. N=5 50%-correct threshold resolution Equivalent acuity level min arc (95% confidence limits) 6/ 20/
5
Specs '0' T '2' '6' '7'
1.070 0.846 1.072 1.088 1.052
0.757 0.639 0.729 0.762 0.893
0.989 0.902 0.959 0.857 0.885
0.741 0.994 0.773 0.910 0.987
0.983 0.947 0.999 0.750 0.747
0.908 0.866 0.906 0.873 0.913
(0.777 to (0.745 to (0.776 to (0.753 to (0.812 to
1.039) 0.987) 1.036) 0.993) 1.014)
5.448 5.194 5.438 5.240 5.477
18.160 17.312 18.128 17.468 18.256
SCL '0' '1' '2' '3' '4' '5' '6' '7' '8'
1.034 1.051 1.048 1.192 1.029 1.017 1.034 1.037 1.038
0.826 0.794 0.810 0.908 0.707 0.795 0.897 0.734 0.740
0.891 0.855 1.106 1.092 0.996 1.142 1.114 1.003 1.138
0.991 0.846 0.860 0.852 0.833 1.044 0.991 0.877 0.753
0.929 0.906 0.875 0.711 0.988 0.826 0.935 0.852 0.869
0.934 0.890 0.940 0.951 0.911 0.965 0.994 0.901 0.908
(0.862 to (0.804 to (0.827 to (0.783 to (0.791 to (0.835 to (0.920 to (0.794 to (0.754 to
1.006) 0.976) 1.053) 1.119) 1.031) 1.095) 1.068) 1.008) 1.062)
5.605 5.342 5.639 5.706 5.464 5.789 5.965 5.404 5.446
18.684 17.808 18.796 19.020 18.212 19.296 19.884 18.012 18.152
SCL and eight directions of ocular excursion ('1' through '8'). Evidently regardless of test condition visual performance, as assessed by this televised visual discrimination task, remained invariate: across all fourteen frames of Figures 2 and 3 linear regression fits are seen to have similar gradients and intercepts, with r~> 0.93, r ~~>0.86 and P < 0.001. Visual performance with best-sphere spectacle correction before the dominant eye was closely matched by that with an optimally-fitted SCL u n d e r conditions of stationary fixation, which performance was neither depleted nor improved during spectacle or SCL wear by the incorporation into the test procedure of a 15 deg horizontal, vertical or oblique saccade. The clinical acuity level (calculated on the basis of the 50% -correct threshold resolution value) ~- 6/ 5 (20/17) for each set of group data. Individual and group (N=5) threshold resolution results over the fourteen test conditions are summarised in Table 2: the small vertical arrow above the abscissa in each frame of Figures 2 and 3 indicates the mean (group: N=5) threshold resolution value (rain arc) at the 50%correct response level.
the eye/lens system was intentionally mobile. The results of this concise study indicate that (in the short term at leas0 when wearing an optimally-fitted hydrogel contact lens a 15 deg saccadic eye movement in either the horizontal, vertical or oblique direction has a negligible influence on visual performance (taking performance at stationary fixation as baseline or in comparison with spectacle lens performance similarly incorporating a 15 deg saccade). The confirmation of this point has implications for clinical contact lens practice since, with regard to assessments made within the confines of the consulting room, the incorporation of a defined eye movement might be assumed to reflect more closely 'usual' conditions of wear. Consequently, the establishment of no deleterious visual effects arising from an eye movement at the fitting stage, in conjunction with the routine checks to determine optimum SCL fit, might be of particular significance when assessing the patient who drives extensively, who plays ball or racquet sports, or who works for much of the day as a copy-typist or interacts with a visual display terminal. 15
Discussion
Address for Correspondence
In clinical practice a consideration in defining optimum contact lens fit on an individual eye is the level of best (usually Snellen) visual acuity determined (as with spectacle lenses during routine refraction) at a contrast ~ - - 1.00 (dark letter on a light ground). T h e material presented here, obtained at contrast = - 0 . 3 5 , is in line with earlier results recorded with this televised test systemr: viz, the visual performance achieved with an eye wearing an optimally-fitted SCL can match that obtained with a spectacle correction. (Elsewhere it has been reported 14 that an individual's contrast sensitivity function recorded with spectacles or with a SCL displays no clinically significant difference.) This 'no difference' outcome for visual performance at stationary fixation can now be extended to conditions where
Dr Jonathan S. Pointer, 4a Market Square, Higham Ferrers, Northamptonshire NN10 8BP, UIC REFERENCES 1 Gilman, ILG. Eye and contact lens movement. Am. J. Optom. PhysioL Opt., 59, 602-610 (1982). z Young, G. Evaluation of soft contact lens fitting characteristics. Optom. Vis. Sci., 73, 247-254 (1996). z Borish, I.M. and Perrigin, D. Relative movement of lower lid and line of sight from distant to near fixation.Am. J. Optom. Physiol. Opt., 64, 881-887 (1987). 4 Chateau, N., de Brabander, J., Bouchard, F. and Molenaar, H. Infrared pupillometryin presbyopes fitted with soft contact lenses. Optom. Iris. Sci., 73, 733-741 (1996). 5 Erickson, P. and Robboy, M. Performance characteristics of a hydrophilic concentric bifocal contact lens. Am. J. Optom. Physiol. Opt., 62, 702-708 (1985). 65
JONATHANS. POINTER 6 Pointer, J.S., Gilmartin, B. and Larke, J.R. A device to assess visual performance with optical aids. Am. J. Optom. Physiol. Opt., 58, 408-413 (1981). 7 Pointer, J.S., Gilmartin, B. and karke, J.R. Visual performance with soft hydrophilic contact lenses. Am. J. Optom. Physiol. Opt., 62, 694-701 (1985). 8 Pointer, J.S. Eyeblink activity with hydrophilic contact lenses: a concise longitudinal study. Acta Ophthal., 66, 498-504 (1988). 9 Pointer, J.S., Gilmartin, B. and Larke, J.R. The evolution of the broken ring visual acuity test figure. J. Am. Optom. Assoc., 51, 741-745 (1980). lo Pointer, J.S. Towards the elimination of guessing bias in "Landolt acuity testing. Am. J. Optom. Physiol. Opt., 63, 813-818 (1986).
66
i i Lancaster, W.B. Fifty years' experience in ocular motility (Part I).
Am. J. Ophthalmol., 24, 485-496 (1941). 12 Bahill, A.T., Adler, D. and Stark, L. Most naturally occurring human saccades have magnitudes of 15 degrees or less. Invest. Ophthalmol. Vis. Sci., 14, 468-469 (1975). 13 Brockhurst, R.J. and Lion, K.S. Analysis of ocular movements by means of an electrical method. Arch. Ophthalmol., 46, 311-314 (1951).
14 Nowozyckyj, A., Carney, L.G. and Eft'on, N. Effect of hydrogel lens wear on contrast sensitivity. Am. J. Optom. Physiol. Opt., 65, 263271 (1988). 15 Gasson, A. Visual display units and contact lenses. Contact Lens J., 11, (1) 13-16 (1983).