Documenta Ophthalmologica 80: 143-155, 1992. 1992 Kluwer Academic Publishers. Printed in the Netherlands.
9
Steady-state a c c o m m o d a t i o n and ocular biometry in late-onset m y o p i a
MARK A. BULLIMORE, 1 BERNARD GILMARTIN 2 & JONATHAN M. ROYSTON 2 1University of California, School of Optometry, Berkeley, CA 94720, USA; 2Ophthalmic and Physiological Optics Research Group, Department of Vision Sciences, Aston University, Aston Triangle, Birmingham, B4 7ET, UK Accepted 27 January 1992
Key words: Axial elongation, biometry, late-onset myopia, mental effort, near vision, ocular accommodation Abstract. The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at - 1 , - 3 and - 5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.
Introduction Whereas it is clear that in the majority of individuals refractive error stabilizes around the age of 15 years [1], a small but significant number of young adults exhibit myopic changes after this age (see [2] for a comprehensive review) either as an increase in myopia in existing myopes or as an onset of myopia in individuals previously emmetropic or hyperopic. The latter form is commonly referred to as late-onset myopia, although the terms late myopia or adult-onset myopia are also appropriate. For the purpose of the present study late-onset myopia is defined as that which develops after the age of 15 years. Typically, late-onset myopia is less severe than myopia which develops during childhood, seldom exceeds -2.00 D and affects a smaller proportion of individuals than juvenile-onset myopia [3]. The development of late-onset myopia is inconvenient to those individuals affected
144 and may limit their career development as in the case of pilots [4]. Adult myopic changes may also limit the efficacy of previous or planned refractive surgical procedures. Although many questions pertaining to late-onset myopia remain unanswered, there is strong evidence to suggest that it may be environmentally induced since the prevalence appears to be significantly higher in individuals attending college. Several studies have found that 10-20% of low hyperopes and emmetropes who enter college/military academies are likely to become myopic by the age of 25 [2]. Conversely, Goss et al. [5] found that less than 3% of myopic patients from a general practice population could be classified as late-onset myopes. Shotwell [6] found that the amount of adult myopic progression was proportional to the time that individuals spent reading. The hypothesis that late-onset myopia is environmentally induced has been further supported by recent accommodation studies. Indeed, if accommodation plays a role in the development of late-onset myopia, then late-onset myopes should exhibit accommodative characteristics which are significantly different from other refractive groups. Results of recent investigations into amplitude of accommodation [7], the accommodation stimulus/ response function [8] and tonic accommodation (TA) [9-11] would appear to support this proposal. In the present study we first sought to evaluate further the possible role of accommodation in the development of late-onset myopia by comparing the steady-state accommodative response in late-onset myopes and emmetropes. We assessed the accommodative response for stimuli located at three different stimulus vergences and also determined the influence of mental effort on the accommodative response for these two refractive groups. In an associated paper [12] we have demonstrated that at low levels of accommodative demand the imposition of mental effort induces a parasympathetically-mediated increase in accommodation for emmetropic subjects. At higher levels of accommodative demand, however, the majority of the experimental group demonstrated a sympathetically mediated decrease in accommodation in the presence of mental effort. Furthermore, a previous study has demonstrated significant differences in the effects of mental effort on TA between emmetropes and late-onset myopes [9]. It is also unclear whether late-onset myopia develops due to changes in the refractive or axial components of the eye. It is well established that ocular components attain adult values by the age of 13-15 years: corneal curvature changes very little after the age of 3 years [13], anterior chamber depth and lens thickness reach adult levels by the age of 15 years [13-15] and axial length ceases to increase after the age of 13 years [13, 16, 17]. Goldschmidt [18] speculated that late-onset myopia was most likely to be lenticular in origin while Goss et al. [5] found that the progression of pre-existing myopia in adulthood was related in an increase in corneal curvature. More recently Adams [19] reported that axial elongation accoun-
145 ted for all of his own late-onset myopia. To date the most comprehensive data have been produced by McBrien and Millodot [20] who studied 30 late-onset myopes and 30 emmetropes and proposed that the ultimate cause of late-onset myopia is vitreous chamber elongation. They found no difference in corneal curvature between the two groups although the late-onset myopes had significantly deeper anterior chambers and thinner crystalline lenses. In the present study we complemented our accommodation measurements by comparing ocular biometric data in late-onset myopes and emmetropes. In order to facilitate further comparison we also present biometric data for a group of early-onset myopes.
Materials and methods
Accommodation study Fourteen male emmetropes and fourteen male late-onset myopes aged between 19 and 23 (mean age = 20.9 years) participated in this part of the study. Subjects' refractive errors were determined prior to the experiment by distance subjective refraction. While cycloplegic refractions were not performed, pseudomyopia and latent hyperopia had been excluded in previous examinations. All subjects could achieve corrected visual acuities of 6/5 or better and had normal binocular vision. The emmetropic subjects had refractive errors between piano and +0.50 dioptres mean-sphere (with not more than 0.25 dioptres of astigmatism). The myopic subjects had refractive errors between -0.50 and -3.50 dioptres mean-sphere (mean: -1.74 D; s.d.: 0.91 D) and their mean age of onset was 16.7 years (s.d.: 1.8). Myopic subjects were corrected with ultrathin soft contact lenses which were inserted at least twenty minutes before any measurements were taken to allow adequate adaptation. All measurements of accommodation were made using a modified Canon R-1 autorefractor, a commercially available infra-red optometer [21-23]. The instrument is capable of producing a complete record of an eye's refractive state at approximately one second intervals, to an accuracy of 0.12 D and allows the subject an unrestricted binocular view by means of a semi-silvered mirror. Subject alignment is monitored and maintained with reference to the image of the pupil on an infra-red video monitor. The instrument had been calibrated previously [24] and all readings in the present study were taken from the left eye under binocular viewing conditions. Natural pupils were used throughout the study. Measurements were made for stimuli at three vergences namely - 1 , - 3 and - 5 dioptres. Stimulus vergence was varied by placing real targets 100, 33 and 20 cm from the subjects' eyes. Targets consisted of a five-by-five matrix of black single-digit numbers on a white background (luminance:
146 25 cd/m2). Targets were scaled for each stimulus distance such that the digits subtended 0.4 degrees at the eye and the total angular subtense of the matrix was 6.5 x 6.5 degrees. Accommodative responses were measured for two conditions which differed by the level of mental effort required, as described in an associated paper [12]. The low-level task (i.e. the passive condition) required the subject to read the array of numbers to himself. The higher level of mental effort (i.e. the active condition) involved the subjects adding the numbers in rows or columns of five. Each experimental condition yielded around one hundred estimates of the accommodative response. It is important to note that the visual nature of the task, the targets and the changing of fixation, were identical for the two conditions. Consequently it is assumed any difference in accommodative response between the two conditions is solely attributable to the amount of mental effort exerted. Each experimental session consisted, therefore, of six sets of measurements, i.e. two mental conditions at each of the three stimulus vergences. The order in which these measurements were taken was randomised in order to minimize the influence of tonic adaptation [25] or accommodation fatigue [26]. A two minute rest period was allowed between each measurement and the subject was permitted to gaze freely around the laboratory.
Ocular biometry study Biometric measurements were made on 24 emmetropes (mean age: 21.7 years; s.d.: 3.16), and 14 late-onset myopes (mean age: 21.1 years; s.d.: 2.43). The emmetropic subjects had refractive errors between -0.50 and +0.50 dioptres mean-sphere. The late-onset myopes had refractive errors between -0.50 and - 3.25 dioptres mean-sphere (mean: -2.18 D; s.d.: 1.05 D) and their mean age-of-onset was 17.0 years (s.d.: 0.8). In order to facilitate further comparison a group of 14 early-onset myopes (mean age: 21.5 years; s.d.: 2.56) was also included in this part of the study. The early-onset myopes had all developed myopia prior to their 15th birthday and their mean age-of-onset was 10.7 years (s.d.: 2.2). Since males have a tendency to have longer axial lengths than females, each refractive group consisted of 50% males and 50% females. All measurements were made on the right eye only. The early-onset myopes had refractive errors between -2.75 and -16.25 dioptres mean-sphere (mean: -6.18 D; s.d.: 3.45 D). Subjects' refractive errors were determined prior to the experiment by distance subjective refraction. All subjects could achieve corrected visual acuities of 6/5 or better and had normal binocular vision. Corneal curvature was measured using a Zeiss keratometer. Three readings were taken for the horizontal and vertical meridians, from which the mean radius of corneal curvature was calculated. Axial ultrasonic measurements of the eye were made using a Storz Omega Compu-Scan Biometric Rule, equipped with a 10 MHz focused transducer.
147 This fully computerised ultrasound unit automatically takes 512 readings within 0.5 seconds, thus providing a large number of readings and at the same time minimizing the effects of eye movement. Any possible effects of eye movement may be examined by reference to the standard deviation values of the axial length readings. The manufacturer's recommendation that any mean value whose standard deviation exceeds 0.1 mm should be rejected was adhered to throughout the study. A footswitch is provided with the instrument which initiates the measurement procedure. Continuous pressure on the switch allowed up to 32 blocks of readings to be made and stored automatically in the instrument's memory. Prior to any readings being taken, the tip of the ultrasound probe was sterilised and the subject's corneas were anaesthetised using 0.4% oxybuprocaine HC1. A chin-rest and brow-bar were employed to minimize head movements and a collimated target apparatus incorporating the subjects refractive error was placed before the non-test eye in order to keep fixation and accommodation steady. The probe was then placed on the cornea and adjusted carefully according to the tone emitted by the instrument until alignment was achieved. The probe was kept as steady as possible until at least ten readings were obtained. If less than ten of these readings failed to fulfill the acceptance criterion further readings were taken.
Results
Accommodation study Each experimental session yielded six mean values of the accommodative response for each subject. The group means of each of these values for both the late-onset myopes and the emmetropes are given in Table 1. For the passive condition the late-onset myopes exert less accommodation than the emmetropes, the difference being greatest for the - 5 D stimulus. There is, however, a much smaller difference between the groups for the active condition. The effect of the imposition of mental effort can be assessed by subtracting the accommodative response for the passive condition from that for the active condition. This change in accommodation, termed the cognitiveinduced shift, was calculated for each subject and the mean values are shown, for each refractive group, in Fig. 1. It can be seen that for the emmetropic group the imposition of mental effort induces a significant increase in mean accommodative response for the - 1 D stimulus, a response approximately equivalent to the passive condition at - 3 D and a small reduction in response at - 5 D, as found in an associated paper [12]. For the myopic group, however, mental effort induces an increase in accommodative responses for all stimulus locations. The finding for the
148 +0.15
g ,it c,q e-i w O
+0.10
/
+0.05
_z iJ,i
_> t-
0
O 9w
-0.05
N
EMMETROPES(N = 14)
B
LATE-ONSET MYOPES (N = 14)
i,,u -0.10
=
,
,
1.00
3.00
5.00
STIMULUS
VERGENCE
(-D)
Fig. I. Mean cognitive induced shifts in accommodation for each stimulus vergence and for each refractive group. Error bars represent one standard error of the mean.
myopic group is comparable to that for emmetropes who show no evidence of sympathetic inhibition [12].
Statistical analyses. Three-factor analyses of variance [refractive group (myopes vs. emmetropes), stimulus vergence and task (passive vs. active)] were carried out for the values of accommodative response shown in Table 1. Analyses revealed significant effects for vergence (F = 4566.33; p < 0.001; d.f. = 2, 156) and refractive group (F = 4.97; p < 0.05; d.f. = 1,156). Such analyses take no account, however, of the within-subject design of this experiment. Three-factor analyses of variance (stimulus vergence, task and Table 1. Mean accommodative response values (in dioptres) for each refractive group and each experimental condition. The mean cognitive induced shifts are also given. Figures in brackets represent standard errors of the mean Stimulus vergence
Emmetropes (N = 14)
Late-onset myopes (N = 14)
-1.00
Passive Active Shift
1.06 (0.04) 1.14 (0.05) +0.09 (0.02)
1.00 (0.05) 1.08 (0.05) +0.08 (0.02)
-3.00
Passive Active Shift
2.70 (0.04) 2.69 (0.04) -0.01 (0.01)
2.62 (0.05) 2.69 (0.05) +0.07 (0.02)
-5.00
Passive Active Shift
4.40 (0.05) 4.37 (0.06) -0.03 (0.02)
4.27 (0.06) 4.34 (0.07) +0.07 (0.02)
149 subject) were, therefore, carried out for the values of accommodative response for each refractive group. F-ratios for main treatment effects and second-order interactions were computed using denominators that took account of the interactive effects with respect to subjects. For example, F-ratios for task effects were calculated by dividing the mean square for task by the mean square for the task/subject interaction [27]. For the emmetropic group significant effects were indicated for subjects ( F = 45.41; p < 0.001; d.f. = 13, 78) and vergence (F = 2744.01; p < 0.001; d.f. = 2, 78). The task/vergence interaction was also significant ( F = 9.97; p
9
Steady-state a c c o m m o d a t i o n and ocular biometry in late-onset m y o p i a
MARK A. BULLIMORE, 1 BERNARD GILMARTIN 2 & JONATHAN M. ROYSTON 2 1University of California, School of Optometry, Berkeley, CA 94720, USA; 2Ophthalmic and Physiological Optics Research Group, Department of Vision Sciences, Aston University, Aston Triangle, Birmingham, B4 7ET, UK Accepted 27 January 1992
Key words: Axial elongation, biometry, late-onset myopia, mental effort, near vision, ocular accommodation Abstract. The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at - 1 , - 3 and - 5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.
Introduction Whereas it is clear that in the majority of individuals refractive error stabilizes around the age of 15 years [1], a small but significant number of young adults exhibit myopic changes after this age (see [2] for a comprehensive review) either as an increase in myopia in existing myopes or as an onset of myopia in individuals previously emmetropic or hyperopic. The latter form is commonly referred to as late-onset myopia, although the terms late myopia or adult-onset myopia are also appropriate. For the purpose of the present study late-onset myopia is defined as that which develops after the age of 15 years. Typically, late-onset myopia is less severe than myopia which develops during childhood, seldom exceeds -2.00 D and affects a smaller proportion of individuals than juvenile-onset myopia [3]. The development of late-onset myopia is inconvenient to those individuals affected
144 and may limit their career development as in the case of pilots [4]. Adult myopic changes may also limit the efficacy of previous or planned refractive surgical procedures. Although many questions pertaining to late-onset myopia remain unanswered, there is strong evidence to suggest that it may be environmentally induced since the prevalence appears to be significantly higher in individuals attending college. Several studies have found that 10-20% of low hyperopes and emmetropes who enter college/military academies are likely to become myopic by the age of 25 [2]. Conversely, Goss et al. [5] found that less than 3% of myopic patients from a general practice population could be classified as late-onset myopes. Shotwell [6] found that the amount of adult myopic progression was proportional to the time that individuals spent reading. The hypothesis that late-onset myopia is environmentally induced has been further supported by recent accommodation studies. Indeed, if accommodation plays a role in the development of late-onset myopia, then late-onset myopes should exhibit accommodative characteristics which are significantly different from other refractive groups. Results of recent investigations into amplitude of accommodation [7], the accommodation stimulus/ response function [8] and tonic accommodation (TA) [9-11] would appear to support this proposal. In the present study we first sought to evaluate further the possible role of accommodation in the development of late-onset myopia by comparing the steady-state accommodative response in late-onset myopes and emmetropes. We assessed the accommodative response for stimuli located at three different stimulus vergences and also determined the influence of mental effort on the accommodative response for these two refractive groups. In an associated paper [12] we have demonstrated that at low levels of accommodative demand the imposition of mental effort induces a parasympathetically-mediated increase in accommodation for emmetropic subjects. At higher levels of accommodative demand, however, the majority of the experimental group demonstrated a sympathetically mediated decrease in accommodation in the presence of mental effort. Furthermore, a previous study has demonstrated significant differences in the effects of mental effort on TA between emmetropes and late-onset myopes [9]. It is also unclear whether late-onset myopia develops due to changes in the refractive or axial components of the eye. It is well established that ocular components attain adult values by the age of 13-15 years: corneal curvature changes very little after the age of 3 years [13], anterior chamber depth and lens thickness reach adult levels by the age of 15 years [13-15] and axial length ceases to increase after the age of 13 years [13, 16, 17]. Goldschmidt [18] speculated that late-onset myopia was most likely to be lenticular in origin while Goss et al. [5] found that the progression of pre-existing myopia in adulthood was related in an increase in corneal curvature. More recently Adams [19] reported that axial elongation accoun-
145 ted for all of his own late-onset myopia. To date the most comprehensive data have been produced by McBrien and Millodot [20] who studied 30 late-onset myopes and 30 emmetropes and proposed that the ultimate cause of late-onset myopia is vitreous chamber elongation. They found no difference in corneal curvature between the two groups although the late-onset myopes had significantly deeper anterior chambers and thinner crystalline lenses. In the present study we complemented our accommodation measurements by comparing ocular biometric data in late-onset myopes and emmetropes. In order to facilitate further comparison we also present biometric data for a group of early-onset myopes.
Materials and methods
Accommodation study Fourteen male emmetropes and fourteen male late-onset myopes aged between 19 and 23 (mean age = 20.9 years) participated in this part of the study. Subjects' refractive errors were determined prior to the experiment by distance subjective refraction. While cycloplegic refractions were not performed, pseudomyopia and latent hyperopia had been excluded in previous examinations. All subjects could achieve corrected visual acuities of 6/5 or better and had normal binocular vision. The emmetropic subjects had refractive errors between piano and +0.50 dioptres mean-sphere (with not more than 0.25 dioptres of astigmatism). The myopic subjects had refractive errors between -0.50 and -3.50 dioptres mean-sphere (mean: -1.74 D; s.d.: 0.91 D) and their mean age of onset was 16.7 years (s.d.: 1.8). Myopic subjects were corrected with ultrathin soft contact lenses which were inserted at least twenty minutes before any measurements were taken to allow adequate adaptation. All measurements of accommodation were made using a modified Canon R-1 autorefractor, a commercially available infra-red optometer [21-23]. The instrument is capable of producing a complete record of an eye's refractive state at approximately one second intervals, to an accuracy of 0.12 D and allows the subject an unrestricted binocular view by means of a semi-silvered mirror. Subject alignment is monitored and maintained with reference to the image of the pupil on an infra-red video monitor. The instrument had been calibrated previously [24] and all readings in the present study were taken from the left eye under binocular viewing conditions. Natural pupils were used throughout the study. Measurements were made for stimuli at three vergences namely - 1 , - 3 and - 5 dioptres. Stimulus vergence was varied by placing real targets 100, 33 and 20 cm from the subjects' eyes. Targets consisted of a five-by-five matrix of black single-digit numbers on a white background (luminance:
146 25 cd/m2). Targets were scaled for each stimulus distance such that the digits subtended 0.4 degrees at the eye and the total angular subtense of the matrix was 6.5 x 6.5 degrees. Accommodative responses were measured for two conditions which differed by the level of mental effort required, as described in an associated paper [12]. The low-level task (i.e. the passive condition) required the subject to read the array of numbers to himself. The higher level of mental effort (i.e. the active condition) involved the subjects adding the numbers in rows or columns of five. Each experimental condition yielded around one hundred estimates of the accommodative response. It is important to note that the visual nature of the task, the targets and the changing of fixation, were identical for the two conditions. Consequently it is assumed any difference in accommodative response between the two conditions is solely attributable to the amount of mental effort exerted. Each experimental session consisted, therefore, of six sets of measurements, i.e. two mental conditions at each of the three stimulus vergences. The order in which these measurements were taken was randomised in order to minimize the influence of tonic adaptation [25] or accommodation fatigue [26]. A two minute rest period was allowed between each measurement and the subject was permitted to gaze freely around the laboratory.
Ocular biometry study Biometric measurements were made on 24 emmetropes (mean age: 21.7 years; s.d.: 3.16), and 14 late-onset myopes (mean age: 21.1 years; s.d.: 2.43). The emmetropic subjects had refractive errors between -0.50 and +0.50 dioptres mean-sphere. The late-onset myopes had refractive errors between -0.50 and - 3.25 dioptres mean-sphere (mean: -2.18 D; s.d.: 1.05 D) and their mean age-of-onset was 17.0 years (s.d.: 0.8). In order to facilitate further comparison a group of 14 early-onset myopes (mean age: 21.5 years; s.d.: 2.56) was also included in this part of the study. The early-onset myopes had all developed myopia prior to their 15th birthday and their mean age-of-onset was 10.7 years (s.d.: 2.2). Since males have a tendency to have longer axial lengths than females, each refractive group consisted of 50% males and 50% females. All measurements were made on the right eye only. The early-onset myopes had refractive errors between -2.75 and -16.25 dioptres mean-sphere (mean: -6.18 D; s.d.: 3.45 D). Subjects' refractive errors were determined prior to the experiment by distance subjective refraction. All subjects could achieve corrected visual acuities of 6/5 or better and had normal binocular vision. Corneal curvature was measured using a Zeiss keratometer. Three readings were taken for the horizontal and vertical meridians, from which the mean radius of corneal curvature was calculated. Axial ultrasonic measurements of the eye were made using a Storz Omega Compu-Scan Biometric Rule, equipped with a 10 MHz focused transducer.
147 This fully computerised ultrasound unit automatically takes 512 readings within 0.5 seconds, thus providing a large number of readings and at the same time minimizing the effects of eye movement. Any possible effects of eye movement may be examined by reference to the standard deviation values of the axial length readings. The manufacturer's recommendation that any mean value whose standard deviation exceeds 0.1 mm should be rejected was adhered to throughout the study. A footswitch is provided with the instrument which initiates the measurement procedure. Continuous pressure on the switch allowed up to 32 blocks of readings to be made and stored automatically in the instrument's memory. Prior to any readings being taken, the tip of the ultrasound probe was sterilised and the subject's corneas were anaesthetised using 0.4% oxybuprocaine HC1. A chin-rest and brow-bar were employed to minimize head movements and a collimated target apparatus incorporating the subjects refractive error was placed before the non-test eye in order to keep fixation and accommodation steady. The probe was then placed on the cornea and adjusted carefully according to the tone emitted by the instrument until alignment was achieved. The probe was kept as steady as possible until at least ten readings were obtained. If less than ten of these readings failed to fulfill the acceptance criterion further readings were taken.
Results
Accommodation study Each experimental session yielded six mean values of the accommodative response for each subject. The group means of each of these values for both the late-onset myopes and the emmetropes are given in Table 1. For the passive condition the late-onset myopes exert less accommodation than the emmetropes, the difference being greatest for the - 5 D stimulus. There is, however, a much smaller difference between the groups for the active condition. The effect of the imposition of mental effort can be assessed by subtracting the accommodative response for the passive condition from that for the active condition. This change in accommodation, termed the cognitiveinduced shift, was calculated for each subject and the mean values are shown, for each refractive group, in Fig. 1. It can be seen that for the emmetropic group the imposition of mental effort induces a significant increase in mean accommodative response for the - 1 D stimulus, a response approximately equivalent to the passive condition at - 3 D and a small reduction in response at - 5 D, as found in an associated paper [12]. For the myopic group, however, mental effort induces an increase in accommodative responses for all stimulus locations. The finding for the
148 +0.15
g ,it c,q e-i w O
+0.10
/
+0.05
_z iJ,i
_> t-
0
O 9w
-0.05
N
EMMETROPES(N = 14)
B
LATE-ONSET MYOPES (N = 14)
i,,u -0.10
=
,
,
1.00
3.00
5.00
STIMULUS
VERGENCE
(-D)
Fig. I. Mean cognitive induced shifts in accommodation for each stimulus vergence and for each refractive group. Error bars represent one standard error of the mean.
myopic group is comparable to that for emmetropes who show no evidence of sympathetic inhibition [12].
Statistical analyses. Three-factor analyses of variance [refractive group (myopes vs. emmetropes), stimulus vergence and task (passive vs. active)] were carried out for the values of accommodative response shown in Table 1. Analyses revealed significant effects for vergence (F = 4566.33; p < 0.001; d.f. = 2, 156) and refractive group (F = 4.97; p < 0.05; d.f. = 1,156). Such analyses take no account, however, of the within-subject design of this experiment. Three-factor analyses of variance (stimulus vergence, task and Table 1. Mean accommodative response values (in dioptres) for each refractive group and each experimental condition. The mean cognitive induced shifts are also given. Figures in brackets represent standard errors of the mean Stimulus vergence
Emmetropes (N = 14)
Late-onset myopes (N = 14)
-1.00
Passive Active Shift
1.06 (0.04) 1.14 (0.05) +0.09 (0.02)
1.00 (0.05) 1.08 (0.05) +0.08 (0.02)
-3.00
Passive Active Shift
2.70 (0.04) 2.69 (0.04) -0.01 (0.01)
2.62 (0.05) 2.69 (0.05) +0.07 (0.02)
-5.00
Passive Active Shift
4.40 (0.05) 4.37 (0.06) -0.03 (0.02)
4.27 (0.06) 4.34 (0.07) +0.07 (0.02)
149 subject) were, therefore, carried out for the values of accommodative response for each refractive group. F-ratios for main treatment effects and second-order interactions were computed using denominators that took account of the interactive effects with respect to subjects. For example, F-ratios for task effects were calculated by dividing the mean square for task by the mean square for the task/subject interaction [27]. For the emmetropic group significant effects were indicated for subjects ( F = 45.41; p < 0.001; d.f. = 13, 78) and vergence (F = 2744.01; p < 0.001; d.f. = 2, 78). The task/vergence interaction was also significant ( F = 9.97; p
