Adaptation of tonic accommodation to sustained visual tasks in emmetropia and late-onset myopia

AC/A ratios in myopic and emmetropic Hong Kong children and the effect of timolol§

PURPOSE

Caucasian children with myopia have elevated response accommodative vergence to accommodation (AC/A) ratios. The purpose of this study was twofold: to determine if response AC/A ratios vary with refractive error and with myopic progression rate in Hong Kong Chinese children, and to determine the effect of beta-adrenergic antagonism with topical timolol application on AC/A ratios.

METHODS

Thirty children aged eight to 12 years participated in the study. All refractive errors were corrected with spectacle lenses. Accommodative responses were measured using a Shin-Nippon autorefractor and concurrent changes in vergence were assessed using a vertical prism and a Howell-Dwyer card at three metres and 0.33 metre. Accommodative demand was altered using plus or minus two dioptre lenses and lens- and distance-induced response AC/A ratios were calculated. Measurements were repeated 30 minutes after the instillation of topical timolol maleate (0.5 per cent).

RESULTS

AC/A ratios appeared higher in progressing myopic children but the difference was not statistically significant. Timolol application reduced accommodative convergence (AC) in the stable myopes (reduction = -3 +/- 1.14 prism dioptres) but not in the emmetropes (0.69 +/- 0.96 prism dioptres) or progressing myopes (0.16 +/- 0.43 prism dioptres) and this difference between refractive groups was statistically significant (F(2, 27) = 3.766; P = 0.036). However, timolol did not produce a significant change in the accommodative response to positive or negative lenses or response AC/A ratios.

CONCLUSIONS

We did not find that AC/A ratios in myopic Chinese children were elevated and therefore, it is unlikely that elevated AC/A ratios are responsible for the high levels of myopia that occur in Hong Kong. The finding that timolol reduced AC in the stable myopes suggests that the autonomic control of accommodative convergence in these children may be different from that in emmetropic children and those with progressing myopia.

Opposing effects of atropine and timolol on the color and luminance emmetropization mechanisms in chicks

This study analyzed the luminance and color emmetropization response in chicks treated with the nonselective parasympathetic antagonist atropine and the sympathetic β-receptor blocker timolol. Chicks were binocularly exposed (8h/day) for 4days to one of three illumination conditions: 2Hz sinusoidal luminance flicker, 2Hz sinusoidal blue/yellow color flicker, or steady light (mean 680lux). Atropine experiments involved monocular daily injections of either 20μl of atropine (18nmol) or 20μl of phosphate-buffered saline. Timolol experiments involved monocular daily applications of 2 drops of 0.5% timolol or 2 drops of distilled H2O. Changes in the experimental eye were compared with those in the fellow eye after correction for the effects of saline/water treatments. Atropine caused a reduction in axial length with both luminance flicker (-0.078±0.021mm) and color flicker (-0.054±0.017mm), and a reduction in vitreous chamber depth with luminance flicker (-0.095±0.023mm), evoking a hyperopic shift in refraction (3.40±1.77D). Timolol produced an increase in axial length with luminance flicker (0.045±0.030mm) and a myopic shift in refraction (-4.07±0.92D), while color flicker caused a significant decrease in axial length (-0.046±0.017mm) that was associated with choroidal thinning (-0.046±0.015mm). The opposing effects on growth and refraction seen with atropine and timolol suggest a balancing mechanism between the parasympathetic and β-receptor mediated sympathetic system through stimulation of the retina with luminance and color contrast.

Near work-induced transient myopia in Indian subjects

PURPOSE

The aim was to determine the characteristics of near work-induced transient myopia (NITM) in asymptomatic Indian subjects and the influence of target size and contrast.

METHODS

Two studies were conducted: First, 24 myopes and 24 emmetropes viewed four targets (N8 and N12 with 50 and 90 per cent contrasts) placed at 0.2 m for five minutes. The refractive status was assessed objectively, before and after carrying out the near task, with the Grand Seiko WAM-5500 open-field autorefractor under monocular viewing conditions. Second, a different group of 24 myopes and 24 emmetropes viewed a N12 target with 90 per cent contrast for 60 minutes with pre- and post-refractive state measurements repeated as above. NITM was defined as the difference between pre-task and post-task distance refraction.

RESULTS

In the first study, myopes demonstrated an initial post-task myopic shift of 0.21 D, whereas emmetropes demonstrated a small hyperopic shift of 0.07 D (p < 0.001). The myopes demonstrated a decay time constant of 6.07 seconds. There was no effect of target size or contrast on the magnitude of the NITM or the decay time constant (p > 0.05). In the second study, myopes showed a NITM of 0.31 D, which was significantly greater than emmetropes (p < 0.001). The myopes demonstrated a decay time constant of 8.16 seconds.

CONCLUSION

The magnitude of the NITM was higher in myopes compared to emmetropes for both five minute and 60 minute viewing time. The NITM decayed slightly faster than that found in previous literature for some other ethnic groups. Potential reasons for these findings are discussed.

The Effect of 3D Visual Simulator on Children's Visual Acuity - A Pilot Study Comparing Two Different Modalities

PURPOSE

To evaluate the efficacy of two non-surgical interventions of vision improvement in children.

METHODS

A prospective, randomized, pilot study to compare fogging method and the use of head mounted 3D display. Subjects were children, between 5 to 15 years old, with normal best corrected visual acuity (BCVA) and up to -3D myopia. Subjects played a video game as near point work, and received one of the two methods of treatments. Measurements of uncorrected far visual acuity (UCVA), refraction with autorefractometer, and subjective accommodative amplitude were taken 3 times, at the baseline, after the near work, and after the treatment.

RESULTS

Both methods applied after near work, improved UCVA. Head mounted 3D display group showed significant improvement in UCVA and resulted in better UCVA than baseline. Fogging group showed improvement in subjective accommodative amplitude. While 3D display group did not show change in the refraction, fogging group's myopic refraction showed significant increase indicating the eyes showed myopic change of eyes after near work and treatment.

DISCUSSION

Despite our lack of clear knowledge in the mechanisms, both methods improved UCVA after the treatments. The improvement in UCVA was not correlated to measured refraction values.

CONCLUSION

UCVA after near work can be improved by repeating near and distant accommodation by fogging and 3D image viewing, although at the different degrees. Further investigation on mechanisms of improvements and their clinical significance are warranted.

Association of vitamin D receptor gene start codon (Fok1) polymorphism with high myopia

Background:

High myopia caused primarily due to abnormal emmetropization and excessive axial ocular elongation is associated with sight-threatening ocular pathology. Muscular dysfunction of ocular ciliary muscles due to altered intracellular calcium levels can result in defective mechanotransduction of the eye and retinal defocus. The vitamin D3 receptor (VDR; a intracellular hormone receptor) is known to mediate calcium homeostasis, influencing the development of myopia.

Materials and Methods:

In the present study, a total of 206 high myopia, 98 low myopia and 250 control samples were analyzed for VDR gene Fok1 (exon 2 start codon) polymorphism using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique.

Results:

High myopia patients revealed decrease in the frequency of ff homozygotes (8.3%) as compared to control group (14.0%), with a corresponding increase in frequency of FF homozygotes (68.9% in high myopia vs. 62.8% in controls). The frequency of f allele carriers (Ff and ff) was increased in females of high myopia (35.6%) and low myopia cases (45.4%). Elevated frequency of f allele was found only in early age at onset cases of high myopia (0.227) and later age at onset (10–20 years) cases of low myopia (0.273) as well as in low myopia cases with parental consanguinity (0.458) (P 0.035; χ² = 6.692*).

Conclusion:

The results suggest that VDR gene might not be playing a direct role in the development of myopia, but might contribute indirectly to the risk conferred by mechanical stress factors or growth/development related factors through its role in calcium homeostasis and regulation of ciliary muscle function.

Accommodation steps, target spatial frequency and refractive error

PURPOSE

Errors in the accommodation response of myopes have been reported in many studies although questions remain about the exact differences in accommodation steps when compared with emmetropic individuals.

METHODS

The characteristics of the accommodation step response to large (4/1D) and small (3/2D) steps in targets with low (0.5 cpd), mid (4 cpd) and high (16 cpd) spatial frequency (SF) information was measured in myopes (MYOs) and emmetropes (EMMs).

RESULTS

In terms of step size, the larger steps showed a greater response in the 4 cpd condition than the 0.5 and 16 cpd conditions and an improved percentage correct response in the 4 cpd compared to the 16 cpd steps. In small step conditions target SF had less effect upon the magnitude of the response. In terms of refractive group differences, MYOs had a lower proportion of correct accommodation responses compared to EMMs during the small steps only, however, when correct steps were performed there were no differences in the characteristics of both large and small step responses between MYOs and EMMs.

CONCLUSIONS

These findings suggest that MYOs have some difficulty interpreting small changes in defocus to initiate or possibly fine tune a small accommodation response, however, when a correct accommodation step response is made, the MYOs accommodation plant responds in a similar manner to EMMs.

Characteristics of accommodative behavior during sustained reading in emmetropes and myopes

Accommodation has long been suspected to be involved in the development of myopia because near work, particularly reading, is known to be a risk factor. In this study, we measured several dynamic characteristics of accommodative behavior during extended periods of reading under close-to-natural conditions in 20 young emmetropic and stable myopic subjects. Accommodative responses, errors, and variability (including power spectrum analysis) were analyzed and related to accommodative demand and subject refractive error. All accommodative behaviors showed large inter-subject variability at all of the reading demands. Accommodative lags and variability significantly increased with closer demands for all subjects (ANOVA, p<0.05). Myopes had significantly greater variability in their accommodation responses compared to emmetropes (ANOVA, p<0.05) and had larger accommodative lags at further reading distances (unpaired t test p<0.05). Power spectrum analysis showed a significant increase in the power of accommodative microfluctuations with closer demands (ANOVA, p<0.05) and with increasing myopia at the closest reading demand (ANOVA, p<0.01). The difference in the stability of the accommodative behavior between individuals with different refractive states suggests a possible relationship between variability in accommodation and the development of myopia.

The lens of the eye as a focusing device and its response to stress

The continued peripheral growth of the lens, resulting in the concentration of older tissue toward the center, has the important optical consequence of producing a lens of variable refractive index. An approach consisting of the projection of fine laser beams through excised lenses in physiological solution has been used for in vitro study of lens optical quality. By varying the separation of the incident beams and/or the wavelength characteristics of the laser used, lens refractive properties and relative transparency may be examined. In the review provided, these optical properties are correlated to lens suture anatomy, lens mitochondrial morphology and function and the function of lens heat shock proteins. In addition, lens spherical aberration is evaluated as a function of accommodation. This work can be highlighted as follows: Mammalian lens suture morphology has a direct impact on lens optical function and, while suture structure of mammalian and avian lenses are very different, they both show an age-related deterioration in morphology and focusing ability. The distribution and appearance of mitochondria of the lens epithelium and superficial fiber cells are similar in all vertebrates. Lens mitochondrial integrity is correlated to lens focusing ability, suggesting a correlation between lens optical properties and lens metabolic function. The induction of cold cataract measured optically in cultured mammalian lenses is enhanced by thermal (heat) shock and this effect is prevented by inhibiting heat shock protein production. Finally, lens accommodative function can be studied by measuring lens refractive change using a physiological model involving an intact accommodative apparatus.

The effect of common reductions in letter size and contrast on accommodation responses in young adult myopes and emmetropes

PURPOSE

Accommodation errors during reading and the subsequent near work-induced transient myopia (NITM) that occurs have been implicated in the development and progression of myopia. This study investigated the effects of two letter variables, size and contrast, on accommodation accuracy during the near task and on NITM and its subsequent decay. These were varied so as to mimic what might occur when students photocopy and reduce reading materials.

METHODS

Based on their refractive errors, young adult subjects (18-25 years) were classified into three groups: emmetropes (n = 19), stable myopes (n = 17), and progressing myopes (n = 17). Three print sizes (N4, N6, and N8) and two print contrasts (90% and 60%) were used to give six different reading targets. Targets were presented in random order at 25 cm (4 D demand) and the text read for comprehension for 3 minutes. For each target, accommodation accuracy and NITM and its decay were measured using the free space Shin-Nippon SRW-5000 autorefractor.

RESULTS

When data for all subjects were pooled, there was a significant effect of letter size (p = 0.030) but not contrast (p = 0.898) on accommodation accuracy; however, differences were small and unlikely to be clinically relevant. NITM (p = 0.033) and its decay (p = 0.012) also varied with letter size. NITM was greater and decay longer for larger letters. We found no effect of refractive error group on accommodation accuracy. In contrast, there was a significant difference in the magnitude of NITM and its decay for emmetropic and myopic subjects (although no effect of progression status); myopes had larger NITM values and longer decay times to baseline than emmetropes (NITM myopes: 0.37 +/- 0.14D vs. emmetropes: 0.19 +/- 0.17 D, p = 0.005; decay time myopes: 15.12 +/- 6.58 seconds vs. emmetropes 7.10 +/- 4.82 seconds, p = 0.0045). The differences in NITM and its decay between the two refractive groups were of similar magnitude for all six combinations of letter size and contrast.

CONCLUSIONS

Our data do not support the suggestion that common reductions in letter size or contrast of reading material (as might occur for photocopied reading materials) cause greater accommodation inaccuracy and greater near work-induced adaptation effects that would exacerbate myopia development in young adults.

Accommodation functions: co-dependency and relationship to refractive error

We assessed the extent to which different accommodative functions are correlated and whether accommodative functions predict the refractive error or the progression of myopia over a 12 month period in 64 young adults (30 myopes and 34 non-myopes). The functions were: amplitude of accommodation; monocular and binocular accommodative facility (6 m and 40 cm); monocular and binocular accommodative response to target distance; AC/A and CA/C ratios, tonic accommodation (dark focus and pinhole), accommodative hysteresis, and nearwork-induced transient myopia. Within groups of related accommodative functions (such as facility measures or open-loop measures) measurements on individuals were generally significantly correlated, however correlations between functions from different groups were generally not significant. Although accommodative amplitude and pinhole (open loop) accommodation were significantly different in myopes than in non-myopes, these functions were unrelated to myopia progression. Facility of accommodation and accommodative lag was independent predictors of myopia progression.

The effect of a beta-adrenoceptor antagonist on accommodative adaptation in Hong Kong children

PURPOSE

Increased susceptibility to nearwork-induced accommodative adaptation has been suggested as a risk factor for myopia development. We investigated whether accommodative adaptation may explain in part the high prevalence of myopia in Hong Kong children and examined the effect of beta-antagonism with topical timolol maleate on accommodative adaptation.

METHODS

Thirty children (10 emmetropes and 20 myopes) aged between 8 and 12 years were recruited. Tonic accommodation was measured before and after 5 min of video game-playing using an open-field Shin-Nippon autorefractor. Measurements were repeated 30 min after timolol instillation.

RESULTS

Children with progressing myopia demonstrated accommodative adaptation following the near task, whereas stable myopes showed counter-adaptive, hyperopic accommodative changes. Timolol increased the magnitude of accommodative adaptation in the stable myopes but had little effect on responses of the progressing myopes or emmetropes.

CONCLUSIONS

Neuropharmacological modulation of the accommodative system may have a possible etiological role in the progression of myopia.

Reduction of asthenopia related to accommodative relaxation by means of far point stimuli

PURPOSE

To conduct an experimental investigation of the effect of accommodative relaxation using far point shift stimuli for the reduction of asthenopia.

METHODS

Twenty-two female students accommodated to a far point shift stimuli during a 2-min period immediately after a 15-min sustained task on a three-dimensional display. Before and after the trial, their accommodative step response and symptoms were assessed. The far point shift stimuli in the optical system, which were presented on a refractometer, were created by moving the target scenery images from far to near, lineally centred about the far point position of each eye. During 2 min of fixating on the far point shift stimuli, changes in refraction were recorded in the same eye.

RESULTS

While looking at the far point shift stimuli, 10 of 22 subjects had changes in refraction that showed a hypermetropic shift, and the other 12 subjects had changes in refraction that showed a myopic shift. The time taken for the accommodative step response from far to near post-trial in the myopic shift group was markedly prolonged, and the accommodative lag at the far target in the optometer was significantly increased. In the myopic shift group, the symptoms of "eye fatigue", "eye pain", "eye heaviness", and "eye dryness" also increased after the trial. In the hyperopic shift group, however, only the symptom of "eye dryness" increased, with no reduction of accommodation function.

CONCLUSIONS

We suggest that accommodative relaxation by accommodative far point shift stimuli is effective in the reduction of asthenopia.

The autonomic control of accommodation and implications for human myopia development: a review

Prolonged nearwork has long been associated with myopia development, however, there is no well described linking mechanism. One theory suggests that if accommodation accuracy during nearwork is not maintained, the defocused retinal image leads to myopia development. Here we review the findings of research aimed at determining whether the autonomic inputs to the ciliary smooth muscle are involved in this type of environmental myopia. We examine whether an autonomic imbalance could be a precursor to axial elongation and the resulting myopia. Accommodation responses, such as tonic accommodation and nearwork-induced accommodative adaptation, as a function of refractive error, are described in relation to an autonomic imbalance model. The collective results of this research point to anomalous accommodation responses, possibly as a result of underlying anomalous autonomic input to the ciliary muscle, being involved in myopia development and progression.

Open- and closed-loop regressions compared in myopic and emmetropic subjects

Post-near task accommodation regressions have been measured in a number of studies to investigate the association between myopia and nearwork. The slow nature of the regression found when measuring both open- and closed-loop regressions in some subjects has been hypothesised to be associated with myopia development and progression. However, it is not known whether the speed and characteristics of the regression for an individual is the same under open- and closed-loop conditions. In this study we measured post-near task accommodation regression functions under open- and closed-loop conditions for myopic and emmetropic subjects. We compared the responses of each subject under the two conditions and found that some subjects responded differently depending on the condition (i.e. a fast regression under one condition and a slow regression under the other condition). When compared as a group, the myopic subjects showed significantly more prolonged open-loop regressions than closed-loop regressions. In terms of the effect on the post-task accommodation, the myopes appear to be more affected by the measurement conditions than the emmetropes in this study. The results highlight the importance of the type of accommodation stimulus when investigating the accommodation response.

Wavefront aberration and its relationship to the accommodative stimulus-response function in myopic subjects

BACKGROUND

Autorefractors are increasingly used in myopia research because they are convenient tools to investigate aspects of the accommodation response. The degree to which the autorefractor measures are affected by ocular aberrations has been highlighted by studies that have shown changes in aberration levels through different parts of the pupil and with accommodation. We have compared accommodative accuracy as measured with a Shin-Nippon SRW 5000 autorefractor with wavefront error as measured with a Hartmann-Shack wavefront sensor to investigate how factors such as accommodation demand, ocular aberrations, and pupil size can influence autorefractor measures.

METHODS

Accommodation stimulus-response curves were determined (using negative lenses) for 30 young healthy subjects (20 myopic [-0.75 to -6.00 D] and 10 emmetropic). Accommodation levels ranged from 0 to 4 D in 1 D steps. Wavefront aberrations were also determined for the same accommodation levels using a Hartmann-Shack wavefront sensor for both the subjects' natural pupil sizes and for a 2.9-mm pupil.

RESULTS

For all subjects, there was a consistent increase in negative spherical aberration with increases in accommodative stimulus. However, there was no consistent change in paraxial spherocylindrical refractive correction with accommodation stimulus. For the emmetropic subjects, accommodation error as measured with the autorefractor was statistically similar to the total spherocylindrical correction for the eye as estimated by the Hartmann-Shack wavefront sensor, but only for a 2.9-mm pupil (the pupil size utilized by the autorefractor). For the myopic subjects, accommodation error as measured with the autorefractor was statistically similar to the higher-order aberrations, but only when measured for a natural pupil size.

CONCLUSIONS

The relationship between the accommodation accuracy as measured with the autorefractor and the total wavefront aberration as measured with a Hartmann-Shack wavefront sensor is largely influenced by the higher-order (fourth and above) aberration levels. For the emmetropic subjects, the errors measured by the two methods agree when adjusted to measure at similar pupil sizes. For the myopic subjects with similar pupil sizes, however, the Hartmann-Shack wavefront sensor underestimates the accommodation error at higher accommodation levels (2 to 4 D) compared with the autorefractor.

Intraocular pressure in anisometropic children

BACKGROUND

There is evidence that intraocular pressure (IOP) is higher in myopes than in hyperopes or emmetropes, and it has been suggested that myopia may be the result of a high IOP. We studied IOP in the two eyes of anisometropes, thus controlling for nuisance variables affecting IOP measurement.

METHODS

Sixty-seven Chinese children, aged between 8 and 14 years, with anisometropia not <2 D were studied. A Topcon CT-60 noncontact tonometer was used for IOP measurement. Cycloplegia was achieved using two drops of tropicamide 1%, and retinoscopy was performed after residual accommodation had decreased to <2 D. A-scan ultrasonography was carried out using a Storz Alpha II Biometric Ruler.

RESULTS

There were no statistically significant differences in IOP between the less myopic and more myopic eyes.

CONCLUSIONS

Refractive error and axial length differences in anisometropic children are not related to differences in IOP and seem more likely to be due to genetically determined discrepancies in scleral structure, as previously proposed.

Oculomotor functions and late-onset myopia

In order to elucidate the role of optical defocus in the development of late-onset myopia (LOM), we employed both theoretical and experimental approaches. In the theoretical study, which has been reported previously, we suggested a model in which an accommodative sensory operator was added to simulate the sensory part of the accommodation system. Results derived from the model showed that the sensory part might increase the system's threshold to the accommodative error (or defocus) signal. In this study, we measured the perceptual depth-of-focus and the system's threshold to the defocus signal for three refractive error groups: emmetropic (Emm), stable myopic (S.M.), and progressing myopic (P.M.). Results show that there are no significant differences in the perceptual depth-of-focus among the three groups. However, the defocus threshold values of the P.M. group are significantly higher than the values of the other two groups. This result in combination with our previous findings, leads us to suggest that individuals susceptible of developing myopia from sustained near-work have a specific oculomotor risk profile. Although we are still trying to determine the specific sequence of changes among dark-focus, AC/A ratio, the accommodative system's defocus threshold, and refractive error, we are convinced that the changes of oculomotor parameters underlie the type of myopia associated with near-work.

Clinical evaluation of infrared autorefractors for use in contact lens over refraction

We examined the ability of autorefractors to provide an accurate means of contact lens over-refraction measurement. Over-refraction measures, performed using six commercially available automated infrared autorefractors and retinoscopy, were compared with subjective refraction. A total of 40 contact lens wearers (20 soft lens wearers and 20 rigid gas permeable (RGP) lens wearers) participated in the study. All six autorefractors were able to provide over-refraction measures in all subjects. Data analysis revealed differences in autorefractor performance between the two types of contact lens wearer with greater accuracy and lower variability being found in the soft lens wearing subjects compared with the RGP group. The level of agreement between retinoscopy and subjective over-refraction results was lower in soft lens wear and higher in RGP wear than the levels found between the autorefractor and subjective over-refraction measures. We conclude that autorefractors provide a useful means of measuring contact lens over-refraction in soft lens wearers. Over-refraction results should be treated more cautiously in RGP wearers.

The effect of topical beta-adrenoceptor antagonists on accommodation in emmetropia and myopia

Using topical instillation of non-selective and selective beta-adrenoceptor antagonists we examine the proposal that a deficit in inhibitory sympathetic innervation of ciliary smooth muscle may be a specific precursor to the onset and development of late-onset myopia (LOM). Post-task accommodative hysteresis, a possible consequence of such a deficit, was assessed by measuring the time-course of regression of accommodation when open-loop (darkness) conditions were immediately imposed following far and near tasks. For the proposal to be feasible only LOMs should exhibit post-task responses which fail to differentiate the type of beta-antagonist employed. As the overall profile of responses to beta-adrenoceptor antagonism was equivalent for each of the three different refractive groups (emmetropes N = 6; early-onset myopes N = 5; LOMs N = 5) we conclude that a propensity to LOM is not associated with a deficit in sympathetic inhibition.

Clinical and laboratory investigations of the relationship of accommodation and convergence function with refractive error. A literature review

Studies of the relationship of clinical and laboratory measures of accommodation and convergence function with refractive error are reviewed. There are inconsistencies in results from study to study presumably due, in part, to methodological differences. However, some basic trends can be outlined. In studies in young adults, accommodation in darkness (dark focus), optical reflex accommodation, and proximally induced accommodation are less in myopes than in emmetropes and hyperopes. It also appears that nearpoint esophoria is associated with higher rates of myopia progression in children. Implications for myopia etiology are discussed.

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Velocity percepts of apparent laser speckle motion modulated by voluntary changes of visual accommodation: real-time, in-vivo measurements of the accommodative response

This study examined negative volitional accommodation under open-loop viewing conditions by means of a coherent laser light pattern (633 nm) whose direction and velocity of motion is determined by the refractive state of the eye. Using the direct scaling procedure of free magnitude estimation, the subjects were required to quantitatively judge velocities of speckles when the retinal image of the subjects' eye was minimally influenced and when the subject was instructed to exert voluntary negative accommodation. Real-time, in-vivo measures of accommodation indicated that the dioptric strength of the crystalline eye lens was reduced under the condition of mental effort, and that the volitional accommodation values ranged between -0.26 and -1.90 D. The amplitude of these responses were noticeably correlated with individual measures of dark focus, so that larger dark focus values were associated with higher amplitudes of the voluntary negative response, and vice versa. By converting the dioptric strength of added optical lenses and the in-vivo measures of the eye lens curvature into angular velocities of laser speckles whose speed changes with focus relative the plane of stationarity, a specification of the proximal stimulus flow was obtained. The psychophysical functions describing the relationship between subjective speed and angular velocity generated either by placing lenses in front of the eye or by voluntary changes in accommodation had approximately the same appearance suggesting that the percepts reported by the subjects were valid and veridical estimates of the velocity of speckle movement.

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia

The characteristics of post-task regression of accommodation to pre-task tonic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within-task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post-task regression occurs in late-onset myopes (LOMs) when compared with emmetropes (EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late-onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post-task regression may have some value in predicting those individuals who may be susceptible to post-task accommodative hysteresis. A pre-requisite for such a predictive value is that for a given individual the variation in inter-trial regression patterns is not significant. The aim of this study is principally to investigate the inter-trial variability of post-task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post-task regressions. A modified Canon R1 infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post-task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subject's baseline TA. Repeatability of post-task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodative adaptation and age of onset of myopia

Tonic accommodation in an empty field and accommodative adaptation, defined as accommodation shift after 2 min viewing of a 4 D near task, were measured using an infrared optometer. Subjects were classified into three types: emmetropes (n = 18), early onset myopes (n = 18) and late onset myopes (n = 15). The relationship between accommodative adaptation and the onset age of myopia were compared. There was no statistical difference in the pre-task tonic accommodation among the three subject groups. However, the accommodative adaptation differed in the three groups during the 5 min post-task period. The average accommodative adaptation of emmetropia, early onset myopia and late onset myopia were 0.66, 0.27 and 1.19 D, respectively. Accommodative adaptation in the late onset myopia group was significantly higher than in the emmetropia group. Furthermore, the accommodative adaptation in the early onset myopia group was significantly lower than in the emmetropia group. Two possible aetiologies of myopia are discussed: one for the group which shows low accommodative adaptation that has no relationship with the age of onset and another for the group which shows high accommodative adaptation that has a relationship to an onset age of 15 years or later. The aetiology is also considered with regard to a potentiating chemical effect on the synapses of the ciliary muscle.