Influence Of Downward Gaze On Ocular Aberrations Biology Essay

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Changes in corneal optics associated with near task have been studied after downward gaze. Pressure from the eyelids has been implicated as an influential factor that may alters the shape of the corneal surface in downward gaze (Collins, Kloevekorn-Norgall, Buehren, Voetz & Lingelbach, 2005, Shaw, Collins, Davis & Carney, 2009). In downward gaze, the narrower palpebral aperture induces a 'wave-like' horizontal band of distortion parallel to the eyelid margin, thus resulting changes in astigmatism and higher order aberrations of the eye (Buehren, Collins & Carney, 2003a, Collins, Buehren, Bece & Voetz, 2006a). However, these all changes have been evaluated using conventional methods following a short term (Shaw, Collins, Davis & Carney, 2008) or long term (Buehren et al., 2003a) near task that performed in downward gaze. In addition to the eyelid pressure, there might be some other factors like gravitational force, extraocular muscle pressure that may also alter optics of the human eye during downward gaze. Therefore, ocular aberrations need to be measured in downward gaze for a better understanding of optical characteristics of the human eye during near task.

Accommodation is a most common physiological change of the eye that occurs during near task or reading. During accommodation, changes occur in the shape (particularly the anterior surface), in thickness and the refractive index of the crystalline lens (Brown, 1973, Dubbelman, Van der Heijde & Weeber, 2005, Jones, Atchison & Pope, 2007, Kasthurirangan, Markwell, Atchison & Pope, 2008). Some recent studies (Pierscionek, Popiolek-Masajada & Kasprzak, 2001, Yasuda, Yamaguchi & Ohkoshi, 2003, Yasuda, Yamaguchi & Ohkoshi, 2004) reported changes in corneal curvature with accommodation. In contrast, other studies (Buehren, Collins, Loughridge, Carney & Iskander, 2003b, Read, Buehren & Collins, 2007) took into account the cyclotorsion of the eye due to accommodation and they have found no evidence of corneal changes with accommodation. It is therefore, likely that the corneal changes observed by the other studies were because they did not consider the torsional changes associated with accommodation.

Changes in the crystalline lens during accommodation cause a dramatic change of ocular aberrations. Several studies have shown that spherical aberration shifts from positive sign to negative sign with accommodation (Atchison, Collins, Wildsoet, Christensen & Waterworth, 1995, Collins, Wildsoet & Atchison, 1995, Hazel, Cox & Strang, 2003, He, Burns & Marcos, 2000, Ivanoff, 1956, Koomen, Tousey & Scolnik, 1949, Ninomiya, Fujikado, Kuroda, Maeda, Tano, Oshika, Hirohara & Mihashi, 2002). The changes of spherical aberration with accommodation are probably related to changes in the asphericity or refractive index distribution of the crystalline lens as it alters with accommodation. Cheng et al(Cheng, Barnett, Vilupuru, Marsack, Kasthurirangan, Applegate & Roorda, 2004) have recently conducted the most comprehensive population based study where they found a systematic decrease in spherical aberration with accommodation increase. In addition to the spherical aberration, astigmatism and third order coma also gradually increased with accommodation.

In recent years, much attention has been focused on the relationship between accommodation and myopia development (Abbott, Schmid & Strang, 1998, Gilmartin & Bullimore, 1991, Gwiazda, Bauer, Thorn & Held, 1995). A number of different experimental paradigms has illustrated that an insufficient accommodative response (lag of accommodation) is noticeably greater in myopes than emmetropes (Bullimore, Gilmartin & Royston, 1992, Gwiazda, Thorn, Bauer & Held, 1993, He, Gwiazda, Thorn, Held & Vera-Diaz, 2005). As a development of this idea, it has been postulated that a greater level of accommodative lag, which results in hyperopic image blur during near task, might have a causative role in myopia progression (Flitcroft, 1998, Hung & Ciuffreda, 2000). There is also a considerable speculation that an increased level of higher order aberrations during accommodation introduces retinal image blur which may alter ocular growth (Buehren, Collins & Carney, 2005, Collins, Buehren & Iskander, 2006b). Nevertheless, there is little evidence (Collins et al., 1995, He et al., 2005) for a greater level of higher order aberrations in myopes than emmetropes during accommodation, so further investigations are needed. Most previous studies of accommodation and aberrations have measured wavefront aberrations monocularly in primary gaze. This does not replicate the natural conditions of the human eyes while under taking near work, which typically involves accommodation, convergence and downward gaze. Moreover, natural pupil size has to be taken into account during wavefront analysis. As On one hand, pupil size decreases linearly with accommodative response (Buehren & Collins, 2006, Gambra, Sawides, Dorronsoro & Marcos, 2009). On other hand, magnitude of the higher order wavefront errors is directly proportional to the pupil size (Howland, 2002, Liang & Williams, 1997, Thibos, Hong, Bradley & Cheng, 2002). It is therefore, more relevant to analyse the wavefront aberrations based on an individual's natural pupil diameter to imitate the actual visual performance of the eye during near task. This will allow the optical characteristics of the entire pupil size to be analysed and can potentially take account of pupil related differences that may exist between myopes and emmetropes.

Therefore, to address these issues, we measured ocular aberrations in both primary and downward gaze (25 degrees) positions, with binocular vision, and various levels of accommodation (0 D, 2.5 D and 5 D) using a custom-built Shack-Hartmann (COAS-HD, Wavefront Sciences, USA) aberrometer system as a function of time.

Methods

Subjects

Twenty six young adult subjects (12 emmetropes and 14 myopes) aged between 19 and 30 years (mean age 25 years) were recruited for this study. Our myopic subjects (n=14) exhibited different ethnic backgrounds, being either Caucasian (n=7) or East Asian (n=7). All subjects were free of any significant ocular diseases and had no history of eye surgery. Approval was obtained from the university human research ethics committee prior to the commencement of the study. Subjects were treated in accordance with the declaration of Helsinki. All subjects had best corrected visual acuity of logMAR 0.00 or better in both eyes. Mean spherical equivalent of myopic subjects was -3.26 ± 1.60 DS and the emmetropic subjects was -0.29 ± 0.36 DS. Myopes were recruiting who have had at least 0.5 DS progression of myopia within the past 2 years. Progression data was obtained from the subject's eye care practitioner if the details were not known by the subject. None of the subjects had anisometropia greater than 1.00 DS, or astigmatism greater than 1.50 DC.

Experimental design

A modified Shack-Hartmann wavefront sensor (SHWS) was used to measure ocular aberrations in primary gaze and downward gaze (25 degrees) with a binocular fixation on a free space high contrast Maltese cross target with and without accommodation. The optical layout of the modified SHWS is shown in Figure 1.The working principle, measurement technique and validation of this modified system are explained in detail elsewhere (R).

Multiple measurements of wavefront aberrations (4X25 frames) from each subject's left eye were taken during five different test conditions (no accommodation primary gaze, no accommodation downward gaze, 2.5 D accommodation primary gaze, 2.5 D accommodation downward gaze and 5.0 D accommodation downward gaze) using the modified SHWS.

The order of testing conditions was randomized (Latin square design) to avoid the potential for systematic bias in the results.

Data collection procedures

Correction of refractive error during testing

Subjects were given their full distance refractive error correction (sphero-cylinder) during each of the five experimental testing conditions. The vertex distance of the trial lenses was considered to determine the appropriate refractive error correction. The refractive error correction was placed in the form of trial lenses mounted in the optical path to the fixation target (Maltese cross) and TV monitor, but outside the measurement beam path to the wavefront sensor. This was necessary, since the spectacle magnification of the refractive error correction may slightly alter the apparent size of entrance pupil diameter used by the wavefront sensor to calculate the wavefront aberrations of the eye, if the correction was placed in the beam path between the eye and wavefront sensor.

Wavefront measurement procedure

For each wavefront measurement, the subject was asked to look at the external Maltese cross target with both eyes. The chin rest position was then adjusted (vertically and horizontally) until the centre of the cross target coincided with the centre of the SHWS fixation target. Therefore, the centre of the SHWS target and the centre of the Maltese cross were coaxial with the visual axis of the subject's tested left eye (OS). This alignment ensured that we were measuring the on-axis aberrations of the subject's left eye. The subject was asked to keep the Maltese cross target "as clear as possible". The subject was asked to blink and then to view the fixation target while 25 frames of wavefront measures were acquired with the modified SHWS (approximately 2.5 secs of recording). The subject was asked to blink again and the measurement of 25 frames were repeated (total of 4 measures x 25 frames = 100 frames in total). During any wavefront measurement where the subject had blinked or reported "losing focus", the measurement was repeated. The subject was instructed to keep their eyelids in the natural position during wavefront measurements (i.e. "not to open their eyes wide"). This was done so as to ensure that the natural influence of the eyelids on the eye was maintained during the wavefront measurements. The intention of the measurement procedure was to capture the optical characteristics of the eye in their natural state during primary and down gaze, both with and without accommodation.

Constant viewing of distant target (watching TV) binocularly in primary gaze for 10 mins was considered as a control task to standardize the ocular parameters prior to the each test condition. Then, we measured ocular aberrations (OS) in primary gaze using modified SHWS with binocular vision at the end of the 10 mins "control task". During wavefront measurements the fixation was controlled by pausing the TV and asking the subject to fixate on a high contrast Maltese cross target displayed on the TV screen. The different test conditions are:

no-accommodation primary gaze condition: In this condition, subject watched TV

binocularly at 5 m distance in primary gaze for 10 mins duration. After 0mins, 5 mins and 10 mins from the starting time (watching TV) we took the measurements of ocular aberrations using modified SHWS with binocular vision and with fixation on the free space target (Maltese cross at 5m). To observe the recovery of the optical characteristics of the eye following the each test condition, the subject looked in primary gaze and watched TV as a control task for duration of 10 mins. We measured ocular aberrations to monitor the recovery at time 0 mins, 5 mins and 10 mins in the recovery period. The subject remained in the headrest throughout the every testing session that includes control task, test condition and recovery condition.

no-accommodation downward gaze condition: We provided the subject a break of 10

mins after completion of each testing condition. Afterwards subjects again performed the control task (10 mins distance viewing of TV in primary gaze) before beginning the next test condition. During this condition, ocular aberrations were measured for downward gaze (25 degrees) while the subject watched TV in downward gaze at a 5 m distance. The same measurement protocol was followed during the test condition and in the recovery period as in the previous condition.

2.5D accommodation primary gaze: In the next condition, the subject's task

involved near work with a 2.5 D accommodation stimulus level (watching movie on the iPod screen) in primary gaze. In all of the accommodation conditions, target distance was adjusted for spectacle lens effectivity for each subject. Mutti et al's (Mutti, Jones, Moeschberger & Zadnik, 2000) thin lens formula was used to determine the spectacle effectivity power for each subject so that the accommodative stimulus was constant for all subjects. Otherwise a similar protocol was followed as per the other conditions.

2.5D accommodation downward gaze : In the fourth condition, the subject watched

movie on iPod screen in 25 degree downward gaze with a 2.5 D accommodation level. A similar protocol was followed as previous condition (2.5D accommodation primary gaze) except that the measurements of ocular aberrations using modified SHWS were taken at 25 degree downward gaze rather measuring in primary gaze.

5D accommodation downward gaze: we repeated the same protocol of fourth

condition (2.5D accommodation downward gaze) with a 5 D stimulus to observe how the level of accommodation interacts with wavefront aberrations in downward gaze.

Stimulus conditions

The TV monitor used at a 5 m viewing distance was a high definition integrated plasma with horizontal and vertical dimensions of 92 cm (visual angle of 10.66 degrees) and 55 cm (visual angle of 6.33 degrees). The screen luminance for a plasma test screen condition was 20 cd /m 2. We maintained the same level of luminance of the test screen during tasks and measurements by adjusting the contrast and brightness of the TV. We measured the level of luminance of the test screen using J16 digital photometer (Tektronix).

The iPod monitor used at a 40 cm viewing distance was a LCD screen with horizontal and

vertical dimensions of 5.4 cm and 7.5 cm. This subtended a visual angle of 7.78 degrees and 10.87 degrees (similar to the visual angle subtended by the TV monitor for far viewing). The screen luminance for an LCD test screen condition was 20 cd/m 2. We maintained approximately the same level of luminance of the test screen during tasks and measurements. The room illuminance for all testing conditions was set to mesopic levels of 1 lux, measured using Fx-200 Illuminometer Light meter (watt Stopper) for this purpose.

Data analysis

The presence of relay lens system in our modified SHWS inverted the wavefront in primary gaze and there was an additional rotation of the wavefront due to the hot mirror rotation in downward gaze. Therefore we had to rotate the wavefront (Lundstrom & Unsbo, 2007) to its original position using custom written software. In downward gaze, the eye rotation was also associated with cyclotorsion which may introduce an artifact in the changes of wavefront aberrations. Therefore, in order to correct any cyclotorsion in downward gaze, the iris images from the wavefront sensor in primary and downward gaze were analysed for each subject using custom written software (Matlab based).

Orthogonal refractive components (M, J0 and J45) were determined from wavefront aberrations (Robert Iskander, Davis, Collins & Franklin, 2007) using custom written software (Matlab based) along with conventional Zernike polynomials up to 8th radial order using the method recommended by the Optical society of America (Thibos, Applegate, Schwiegerling & Webb, 2000).

To assess the influence of downward gaze alone on the ocular aberrations, wavefront was fitted with Zernike polynomials for fixed 5.0 mm pupil diameter at no-accommodation conditions (primary and downward gaze) and for 3.0 mm pupil diameter at 2.5D accommodation conditions (primary and downward gaze). The 5.0 mm and 3.0 mm fixed entrance pupils were selected because they were smaller than the minimum diameter of natural pupil size for all subjects during no-accommodation and accommodation conditions respectively. To determine the effect of accommodation on the aberrations in downward gaze, wavefront was analysed for the full individual natural pupil size at various accommodative levels (0D, 2.5D and 5D).

Statistical analysis was performed using SPSS (version 17.0) software. Repeated

measures MANOVA was performed to assess the significance level of wavefront changes in the various conditions (within-subjects factors) including primary gaze versus downward gaze, with accommodation versus without accommodation and the effects of the time within the task and recovery periods. The between subjects factors were refractive error group and ethnicity of the myopes.

Results

Primary gaze versus downward gaze

No-accommodation condition

Analysis of ocular wavefront has showed a number of significantly changed Zernike coefficients in downward gaze. The mean changes of refractive components from baseline (for 5.0 mm pupil diameter) in primary gaze and downward gaze over 10 mins duration are presented in figure 1 and table 1. Change in mean spherical equivalent (M) showed a myopic shift and primary astigmatism (J0) shifted in against-the-rule (ATR) direction in downward gaze, whereas no significant variations observed in the refractive components in primary gaze (figure 1). Repeated measures MANOVA revealed a significant influence of gaze position for the changes in M (p=0.001) and J0 (p=0.001) components (table 1). There was also significant gaze by time (p=.040), gaze by refractive error (p=0.012) and gaze by time by refractive error (p=0.029) interactions for the changes in J0. Pairwise comparisons, revealed that in the downward gaze, there was significant difference between refractive error groups in changes in primary astigmatism J0 (p= ?) and secondary astigmatism J45 (p=?) in downward gaze at 10 mins task period from baseline measurement. The myopic group showed a greater ATR astigmatic shift (mean change 0.10 ± 0.005 D from baseline) compared to the emmetropic subjects (mean change 0.01± 0.007 D from baseline) after 10 mins task in downward gaze. No significant variation was observed in spherical component or astigmatism (p<0.05) during the periods of recovery compared to the baseline measurement, indicating an immediate recovery in the refractive status of the eye from downward gaze to primary gaze.

Of the higher order aberrations, the terms that changed significantly in downward gaze compared to the primary gaze were vertical trefoil C(3,-3) [p<0.001], vertical coma C(3,-1) [p=0.031], secondary astigmatism 0° C(4,2) [p=0.012], tetrafoil C(4,4) [p=0.009], secondary vertical coma C(5,-1) [p=0.002], vertical pentafoil C(5,-5) [p=0.003], secondary spherical astigmatism C(6,0) [p=0.001]. There was a trend for the vertical coma C(3,-1) to shift in the positive direction, whereas the vertical trefoil C(3,-3) shifted in the negative direction. Similar to the previous study (R), we also found a significant correlation (R2 = 0.701, p<0.001) between the changes of these two coefficients [C(3,-1) and C(3,-3)] in downward gaze. There was also a gaze by time interaction for higher order coefficients such as C(3,-3) [p=0.048], C(3,-1) [p=0.044], C(4,-2) [p=0.032], C(4,4) [p=0.044] and C(5,-5) [p=0.031]. Change in secondary astigmatism 45° C(4,-2) in downward gaze from baseline was significantly greater in myopes than emmetropes at 5 min task (p=?) and 10 min task (p=?).

There was no statistically significant difference (p<0.05) observed between the changes of the wavefront aberrations in Asian and Caucasian myopes in downward gaze.

For a clinical interpretation of the wavefront aberrations change in downward gaze, the group mean refractive power maps were determined over 5.0 mm pupil after 10 mins task in primary gaze and 25 degrees downward gaze (Figure 2). Although the group mean sphero-cylindrical change (difference map in figure 2) for the 25 degrees downward gaze compared to primary gaze was smaller, individually there were often clinically significant changes. The mean refractive power difference map highlights the horizontal band like distortion parallel to the upper eye lid that is most likely to be associated with biomechanical forces due to eye lid pressure on the corneal surface in downward gaze (Figure 2).

During accommodation (2.5 D stimulus)

Primary spherical aberration C(4,0) was found to be more negative (p=0.004) with accommodation in downward gaze compared to primary gaze (mean differences of -0.003 µm, -0.008 µm and -0.013 µm at 0, 5 and 10 mins of the task period respectively) and secondary spherical aberration C(6,0) had more positive (p=0.02 ) shift with accommodation in downward gaze compared to primary gaze (mean differences of -0.001 µm, 0.002 µm and 0.005 µm at 0, 5 and 10 mins of the task period respectively). The level of the variation in changes of both primary and secondary spherical aberration between primary and downward gaze systematically increased [p=0.04 (gaze by time)] over 10 mins near task. With 3.0 mm analysis diameter, the mean sphero-cylindrical difference between primary gaze and downward gaze was equivalent to 0.33 DS/-0.06 DC X 176 (RMS 0.35 D) after 10 mins task with accommodation, reaching clinical significance (Figure 3). This finding indicates that the accommodation responses with a similar stimulus may vary between primary and downward gaze, thus highlighting the complex relationship between the nature of ocular aberrations and the visual task.

Effect of accommodation on aberrations in downward gaze

Similar to the gaze angle, the accommodation was also a significant factor affecting wavefront aberrations. Figure 4 illustrates the changes of mean refractive power (M) [4a] and primary astigmatism (J0) [4b] with three different levels of accommodation (0D, 2.5 D and 5.0 D) in downward gaze with natural pupil size. Results of repeated measures MANOVA revealed a significant influence of accommodation stimulus (p<0.001) and time (p<0.001) on the changes in mean refractive power (M), indicative of a reduction in accommodative lag over time (Figure 4a). The mean change in astigmatism revealed a tendency for an increase in with-the-rule (WTR) direction with accommodation (Figure 4b) that had a significant interaction with accommodation stimulus (P<0.001).

Higher order wavefront data exhibited that vertical trefoil C(3,3) [p=0.008], horizontal coma C(3,1) [p<0.001], secondary astigmatism C(4,2) [p=0.001] and primary spherical aberration C(4,0) [p=0.005] all changed significantly with increased accommodation level in downward gaze with natural pupil size (Figure 5).

There was also a significant interaction between accommodation and refractive error group for the changes in C(3,3) [p=0.03], C(4,0) [p=0.03] and C(6,0) [p=0.04],indicating different pattern of change in the myopes and emmetropes for various level of accommodation. Pairwise comparisons revealed that myopes had greater level of vertical trefoil C(3,-3) compared to emmetropes [mean difference 0.10±0.04 µm] after 10 mins task with 2.5 D accommodation in downward gaze.

The magnitude of higher order RMS (HORMS) increased (p<0.001) with both accommodation stimuli (2.5 D and 5.0 D) compared to no-accommodation condition (0 D) in downward gaze for both myopes and emmetropes (Table 2). However accommodation with 2.5 D stimulus had a greater level of HORMS than 5.0D stimulus for both refractive error groups. Pupillary constriction with increased accommodation may be responsible for this distribution of higher order aberrations under natural viewing condition. Mean higher order RMS was significantly greater (p=0.04) in myopes than emmetropes for 5.0 D accommodative response.

Table 2 shows the effect of accommodation on pupil size as a function of time in myopes and emmetropes. Not surprisingly, for both myopes and emmetropes, there was a systematic reduction (constriction) in pupil size with an increased level of accommodation (a two-factor repeated measures ANOVA p<0.001) over 10 mins task. For a moderate level of accommodation (2.5 D), there was not any noticeable difference observed between the pupil size in myopes and emmetropes. However, for a higher level of accommodation (5.0 D), myopes showed a bigger pupil diameter compared to emmetropes (mean differences 0.59 ± 0.41 mm, 0.77±0.54 mm and 0.63±0.44 mm at 0 min, 5 mins and 10 mins respectively) over 10 mins near task (Figure 5). Although these differences were not statistically significant, overall bigger pupil diameter in myopes may be responsible for a greater level of higher order aberrations compared to emmetropes during near task.

Discussion

Influence of downward gaze angle on wavefront aberrations

We found that significant wavefront changes can occur in downward gaze in comparison with primary gaze with no-accommodation condition. The mean astigmatism shift was in the direction of ATR astigmatism. This finding is consistent with the results of previous studies (Buehren et al., 2003a, Collins et al., 2006a, Shaw et al., 2008) that have showed substantial changes in the topographical and optical characteristics of cornea after reading. Therefore, it is likely that changes in astigmatism of the eye in downward gaze would be associated with eyelid pressure on corneal surface. We also found that changes in astigmatism were greater as a function of time. There is evidence that the magnitude of eyelid pressure induced corneal changes increase with the length of time spent reading(Collins et al., 2005).

Downward gaze induced changes in astigmatism (primary, oblique and secondary) after 10 mins of visual task (no- accommodation) were greater in the myopic group compared with the emmetropic group. In the previous study, palpebral aperture was found to be significantly smaller in myopes (7.12 ± 1.00 mm) than emmetropes (8.09 ± 1.56 mm) in reading gaze for a younger population. The differences in wavefront aberrations between myopes and emmetropes in downward gaze with no-accommodation are likely to be associated with the disparity in eyelid morphometry in refractive error groups in downward gaze. We did not find any significant difference in changes of wavefront aberration in downward gaze between Caucasian myopes and Asian myopes and therefore the effect of ethnicity should not have confounded the results of this study.

The mean refractive power of the eye was shifted in myopic direction in downward gaze with no-accommodation condition. Whilst the change that we have observed in mean spherical refractive power was small in magnitude, it was consistently observed in both myopic and emmetropic subjects, was highly statistically significant. Myopic shift of the eye in downward gaze is probably the results of zonular slackening under the action of gravity that allows the crystalline lens to move forward towards cornea, although this assumption needs to be confirmed by measuring the changes of crystalline lens and ciliary body in downward gaze. In contrast, Shaw et al (Shaw et al., 2008) found a hyperopic shift of mean corneal refractive power after short term reading in 20 degrees downward gaze. This suggests that different mechanisms may be responsible for the changes in corneal and internal optics of the eye in downward gaze. However data from previous work are not directly comparable with our study as they have measured corneal changes in primary gaze rather in downward gaze. Therefore, it is necessary to measure changes in corneal optics in downward gaze.

Previous studies (Buehren et al., 2003a, Shaw et al., 2008) reported that narrower palpebral aperture during reading may lead to an increase in corneal higher order aberrations. In comparison with those studies of cornea, we found numerous changes in ocular higher order aberrations in downward gaze. The changes in vertical trefoil and vertical coma in both magnitude and direction are consistent with previously reported results of corneal aberrations following reading (Buehren et al., 2003a). Other than coma and trefoil, there were also significant changes in secondary astigmatism, tetrafoil, secondary coma, pentafoil and secondary spherical aberration in downward gaze. These changes may be related to the change in internal optics of the eye such as crystalline lens tilt or changes of crystalline lens shape in downward gaze due to the variation in zonular tension or extra ocular muscle force on the eye in downward gaze. An increased level of higher order RMS in downward gaze could play a role in refractive error development through degradation of retinal image quality.

In agreement with previous studies (R), we also noticed a band shape like distortion parallel to the upper eyelid after 10 mins task (Figure). The magnitude of gaze angle has a significant impact on the induced corneal distortion (shaw eyelid pressure,2009, chaw cornal refractive). The most study related reading angle reported to be between 24 degrees and 26 degrees (R), which is similar to the 25 degrees downward gaze condition in this study.

The visual symptom of monocular diplopia as a result of post reading corneal irregularity has been reported (Campbell, 1998, Carney, Liubinas & Bowman, 1981, Ford, Davis, Reed, Weaver, Craven & Tyler, 1997, Golnik & Eggenberger, 2001, Knoll, 1976, Mandell, 1966). Knoll (Knoll, 1976) noticed that double images induced by post reading corneal distortion may last for several hours. An another study (Buehren et al., 2003a) found that the time course of recovery required 120 mins to approach pre-reading corneal shape after the 60 mins reading task. However, we observed an immediate recovery of wavefront changes of the eye following visual task in downward gaze. Because the time period of the visual task in this experiment extended only 10 mins, longer periods of task may aggrandize the degree of wavefront changes that occur in downward gaze, and the recovery may take consequential time.

Downward gaze angle also influenced wavefront aberrations of the eye, predominantly spherical aberration, with accommodation. In terms of mean refractive power, downward gaze refraction had significantly a greater spherical power (0.33 D) in the direction of hyperopia compared to primary gaze. This suggests that an additional hyperopic defocus induced by downward gaze may introduce an optical blur at retinal image plane during reading. Henceforth, it could be considered that reading in primary gaze might be a better option than downward gaze.

Influence of accommodation on wavefront aberrations in downward gaze

In comparison with previous studies, a similar pattern observed in changes in spherical refractive power and astigmatism with accommodation in downward gaze. A new finding of this study is that accommodation response increases as a function of time during near task. A possible mechanism could be that level of higher order aberrations decrease over time with a gradual pupillary miosis, thus enhancing the response of accommodation.

Till to date, of the higher order aberrations, spherical aberration and coma are known to be altered with accommodation. In the present study, other than these two coefficients, we also found significant changes in trefoil and secondary astigmatism with accommodation in downward gaze. However, in general, coma and spherical aberrations are the dominant higher order aberrations for most eyes (Howland & Howland, 1977, Liang & Williams, 1997). Spherical aberration moved most consistently in to negative direction with increasing accommodation. A prominent transition observed for horizontal coma which was also increased significantly in to negative direction with accommodation in downward gaze. However, previous studies(Atchison et al., 1995, Cheng et al., 2004, He et al., 2000, Howland & Buettner, 1989) found an inconsistent trend in changes of coma across individual. It seems the direction and magnitude of the change in coma with accommodation is more consistent in downward gaze rather than primary gaze. Recently, Rosales et al (Rosales, Wendt, Marcos & Glasser, 2008) found a significant tilt of crystalline lens of rhesus monkeys around the horizontal axis, which changed at a rate of 0.147±0.25 degree/D. A similar study (Glasser & Kaufman, 1999) was found that gravity influences the movement of crystalline lens during accommodation in monkeys. Therefore, it is reasonable to expect that gravitational effects would increase the tilt of human crystalline lens about a horizontal axis during accommodation in downward gaze, thus resulting dramatic change in horizontal coma. The change in trefoil could be assumed as an effect of zonular fibres that may compress the equator of crystalline lens during accommodation (Wilson, 1993).

Another observation of this study is that pupil size decreases with increasing accommodation (Table 2). As a result of this pupillary miosis, the magnitude of higher aberrations such as coma, trefoil, secondary astigmatism decreased in higher level of accommodation (5.0 D) compared to moderate level of accommodation (2.5 D). This result was expected due to the effect of pupil size on higher order aberrations(Charman, 1991). This result is supported by another population based study (Lopez-Gil, Castejon-Mochon, Benito, Marin, Lo-a-Foe, Marin, Fermigier, Renard, Joyeux, Chateau & Artal, 2002), where they found a smaller amount of higher order RMS with accommodation for the natural pupil than for the fixed 4.0 mm pupil. Smaller than 3.0 mm pupils are also expected to produce larger errors in accommodation by increasing depth of focus and reducing spatial frequency at retinal plane due to the effect of diffraction(Ward & Charman, 1985). Other than two subjects, everyone of this study population had greater than 3.0 mm pupil for all accommodation conditions. It appears unlikely that the higher order aberrations increased in the present study due to the pupillary miosis.

Figure 1. The group mean changes (±SE) of refractive components in primary gaze (blue bar) and in downward gaze (black bar) with no accommodation condition over 10 mins task for fixed 5.0 mm pupil diameter. Sky blue colour bars represent the recovery of the refractive components following task in primary gaze and the ash colour bars represent recovery of the refractive components following task in downward gaze.

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