Monocular Accommodation Research Articles

The Effect of Accommodation Stimulus on Pupil Size and Accommodation Response in Young Myopes Corrected with Soft Contact Lens

Purpose: We investigated the correlation between changes in pupil size with accommodation stimuli and accommodation response after refractive correction by using soft contact lenses in myopes. Methods: Participants were young myopes (N=50, 100 eyes; 23.15 ±1.70 years; age range, 20~28 years) and refractive errors were corrected by using soft contact lenses. Pupil sizes were measured with a pupillometer at the modes of scotopic, low and high mesopic condition (light off, 0.3 and 3 lx) at 3.5 m and 25 cm in the 100 lx illuminance condition. Accommodation amplitude was measured with the push-up test, and the accommodation response was measured by using an open field autorefractometer. The changes of pupil size with the accommodation stimulus were analyzed by paired t-test and the correlation between the amount of changes in pupil size with the accommodation stimulus and accommodation amplitude and accommodation response were analyzed by Pearson correlation and the significance level was p<0.05. Results: Pupil size was significantly increased by accommodation stimulus in uncorrected refractive error in myopia (p<0.05), and significantly decreased after refractive correction with soft contact lenses (p<0.05). After refractive correction, the larger the miosis caused by accommodation stimulus, the smaller the binocular and monocular accommodation responses (p<0.05). However, the percentage of participants shown no difference in pupil size in the lighting condition were 35~37%, and shown mydriasis was 7~11% and the accommodation response of the group not shown miosis was significantly larger than that of the miosis group (p<0.05). Conclusions: The pupil size in myopes corrected with contact lenses decreased by accommodation stimulus, and the accommodation response was decreased. However, in the participants not shown miosis, accommodation response was more than that of miosis group. This study will be cornerstones to understanding of the asthenopia during near works in the contact lens wearers and the visual function of the multifocal contact lens wearers.

Independent development of refraction, accommodation and convergence over two years in primary school children

Abstract Purpose: To examine whether age-related changes in refraction covary with changes in parameters describing accommodation and convergence over time in a group of Danish school children. Methods: Two-hundred and seventy-eight unselected Danish school children (7.1-13.9 years) without manifest strabismus were studied at baseline and after two years. Each examination included measurement of distance visual acuity, unilateral cover test, non-cycloplegic autorefraction, monocular amplitude of accommodation, monocular accommodation facility, near phoria, near point of convergence, and positive and negative fusional vergence. The changes in the measured parameters were studied over time, followed by the testing for dependencies in the development of these parameters. Results: After two years there was a significant increase in distance visual acuity (p=0.04), monocular accommodation facility (p&lt;0.001), near point exophoria (p=0.04), near point of convergence (p=0.01), and fusional range (p&lt;0.001), a significant reduction in hyperopia (p=0.01) and monocular amplitude of accommodation (p&lt;0.001), while the midpoint of the fusional range changed significantly towards convergence (p&lt;0.001). The increase in near point of convergence correlated significantly with the decrease in monocular amplitude of accommodation (p&lt;0.001) and the convergent shift in the midpoint of the fusional range was significantly correlated with a larger fusional range (p&lt;0.001). The changes in all other parameters were independent. Conclusions: The development of refraction, accommodation, and convergence parameters in school children over two years are independent. This may be due to separate processes regulating development, and suggests that therapeutic intervention may be performed on each of the parameters individually without a derived effect on the other parameters.

Dynamic imaging of accommodation by swept-source anterior segment optical coherence tomography

To study the accommodation process in normal eyes using a commercially available clinical system based on swept-source anterior segment optical coherence tomography (AS-OCT). Ophthalmology Department, University of Parma, Italy. Evaluation of diagnostic technology. Right eyes were analyzed using swept-source AS-OCT (Casia SS-1000). The optical vergence of the internal coaxial fixation target was adjusted during imaging to obtain monocular accommodation stimuli with different amplitudes (0, 3.0, 6.0, and 9.0 diopters [D]). Overlapping of real and conjugate OCT images enabled imaging of all the anterior segment optical surfaces in a single frame. Central corneal thickness (CCT), anterior chamber depth (ACD), and lens thickness were extracted from the OCT scans acquired at different static accommodation stimulus amplitudes. The crystalline lens was analyzed dynamically during accommodation and disaccommodation by acquiring sequential OCT images of the anterior segment at a rate of 8 frames per second. The lens thickness was extracted from the temporal sequence of OCT images and plotted as a function of time. The study analyzed 14 eyes of 14 subjects aged 18 to 46 years. During accommodation, the decrease in the ACD was statistically significant (P < .05), as were the increase in the lens thickness (P < .001) and the slight movement forward of the lens central point (P < .01). The CCT and anterior chamber width measurements did not change statistically significantly during accommodation. The lens thickness at 0 D was positively correlated with age (P < .01). High-resolution real-time imaging and biometry of the accommodating anterior segment can be effectively performed using a commercially available swept-source AS-OCT clinical device. No author has a financial or proprietary interest in any material or method mentioned.

Clinical test responses to different orthoptic exercise regimes in typical young adults

PurposeThe relative efficiency of different eye exercise regimes is unclear, and in particular the influences of practice, placebo and the amount of effort required are rarely considered. This study measured conventional clinical measures following different regimes in typical young adults.MethodsA total of 156 asymptomatic young adults were directed to carry out eye exercises three times daily for 2 weeks. Exercises were directed at improving blur responses (accommodation), disparity responses (convergence), both in a naturalistic relationship, convergence in excess of accommodation, accommodation in excess of convergence, and a placebo regime. They were compared to two control groups, neither of which were given exercises, but the second of which were asked to make maximum effort during the second testing.ResultsInstruction set and participant effort were more effective than many exercises. Convergence exercises independent of accommodation were the most effective treatment, followed by accommodation exercises, and both regimes resulted in changes in both vergence and accommodation test responses. Exercises targeting convergence and accommodation working together were less effective than those where they were separated. Accommodation measures were prone to large instruction/effort effects and monocular accommodation facility was subject to large practice effects.ConclusionsSeparating convergence and accommodation exercises seemed more effective than exercising both systems concurrently and suggests that stimulation of accommodation and convergence may act in an additive fashion to aid responses. Instruction/effort effects are large and should be carefully controlled if claims for the efficacy of any exercise regime are to be made.

Independence between age-related changes in refraction, accommodation and convergence in primary school children

The parameters which describe how refraction, accommodation and convergence develop during childhood are well understood, but the possible interdependencies between these agerelated changes are not known. Hence it is unknown whether refractive development is associated with development of accommodation and convergence. Four-hundred and fifty-two unselected 7-13 year old school children were subjected to autorefraction and unilateral cover test, and measurement of visual acuity, amplitude of accommodation, monocular accommodation facility, near phoria, near point of convergence, and positive and negative fusional vergence using subjective techniques. Linear regression was used to study the correlation of these parameters with age, followed by study of correlations between age-dependent parameters. There was a significant reduction in hyperopia (0.13 D/year, 95% CI [-0.20, -0.06]), a significant increase in the monocular accommodation facility (0:48 cpm/year, 95% CI [0.18, 0.79]), and a non-significant shift towards convergence of the midpoint of the fusional range (0.43 ΔD/year, 95% CI [0.12, 0.74]) with age. Age-dependent variables were not significantly correlated. Autorefraction was not correlated with any other variable. None of the other parameters showed any significant correlations with age. Age-related changes in refraction, monocular accommodation facility and midpoint of fusional range were independent. This indicates that the development of these parameters involves separate processes, and suggests that therapeutic intervention may be performed on these parameters individually without a derived effect on the other age-related parameters.

Vergence and accommodation to multiple-image-plane stereoscopic displays: “real world” responses with practical image-plane separations?

Conventional stereoscopic displays present images on a single focal plane. The resulting mismatch between the stimuli to the eyes' focusing response (accommodation) and to convergence causes fatigue and poor stereo performance. One solution is to distribute image intensity across a number of widely spaced image planes-a technique referred to as depth filtering. Previously, we found this elicits accurate, continuous monocular accommodation responses with image-plane separations as large as 1.1 Diopters (D, the reciprocal of distance in meters), suggesting that a small number of image planes could eliminate vergence-accommodation conflicts over a large range of simulated distances. Evidence exists, however, of systematic differences between accommodation responses to binocular and monocular stimuli when the stimulus to accommodation is degraded, or at an incorrect distance. We examined the minimum image-plane spacing required for accurate accommodation to binocular depth-filtered images. We compared accommodation and vergence responses to changes in depth specified by depth filtering, using image-plane separations of 0.6 to 1.2 D, and equivalent real stimuli. Accommodation responses to real and depth-filtered stimuli were equivalent for image-plane separations of ∼ 0.6 to 0.9 D, but differed thereafter. We conclude that depth filtering can be used to precisely match accommodation and vergence demand in a practical stereoscopic display.

Effect of heterophoria type and myopia on accommodative and vergence responses during sustained near activity in children

The influence of phoria-type and myopia on changes to vergence and accommodation during prolonged near-task was examined in 53 children. Participants were classified into phoria and refractive categories based on near phoria and cycloplegic refraction respectively. Measures of near phoria, binocular (BA) and monocular accommodation (MA) were obtained before and during a 20min task when children binocularly fixated a high-contrast target at 33cm through best corrective lenses. Vergence adaptation and accommodative adaptation were quantified using changes to near phoria and tonic accommodation respectively. The direction and magnitude of vergence adaptation was modified by the phoria-type (p<0.001). Emmetropic exophores displayed convergent (less exo than baseline) adaptation while esophores showed divergent shifts (less eso than baseline) in phoria upon prolonged fixation. Myopic children also followed a similar pattern but showed greater divergent (or less convergent) shift (p<0.001) in vergence adaptation for all phoria categories compared to emmetropes. Phoria-type also influenced the pattern of BA vs. MA (p<0.001) such that exophores showed BA>MA while esophores showed MA>BA in both refractive groups. Accommodative adaptation was higher in myopes (p=0.010) but did not demonstrate a significant effect of phoria (p=0.4). The influence of phoria-type on vergence adaptation and the pattern of BA vs. MA relates primarily to the varying fusional vergence demands created by the direction of phoria. The greater divergent (or less convergent) shift in vergence adaptation seen in myopes (compared to emmetropes) could be attributed to their higher accommodative adaptation. Nevertheless, the adaptive patterns observed in myopic children do not appear to explain their high response AV/A ratios identified as a risk factor for myopia development.

Evidence that convergence rather than accommodation controls intermittent distance exotropia

This study considered whether vergence drives accommodation or accommodation drives vergence during the control of distance exotropia for near fixation. High accommodative convergence to accommodation (AC/A) ratios are often used to explain this control, but the role of convergence to drive accommodation (the CA/C relationship) is rarely considered. Atypical CA/C characteristics could equally, or better, explain common clinical findings. Nineteen distance exotropes, aged 4-11 years, were compared while controlling their deviation with 27 non-exotropic controls aged 5-9 years. Simultaneous vergence and accommodation responses were measured to a range of targets incorporating different combinations of blur, disparity and looming cues at four fixation distances between 2 m and 33 cm. Stimulus and response AC/A and CA/C ratios were calculated. Accommodation responses for near targets (p = 0.017) and response gains (p = 0.026) were greater in the exotropes than in the controls. Despite higher clinical stimulus AC/A ratios, the distance exotropes showed lower laboratory response AC/A ratios (p = 0.02), but significantly higher CA/C ratios (p = 0.02). All the exotropes, whether the angle changed most with lenses ('controlled by accommodation') or on occlusion ('controlled by fusion'), used binocular disparity not blur as their main cue to target distance. Increased vergence demand to control intermittent distance exotropia for near also drives significantly more accommodation. Minus lens therapy is more likely to act by correcting overaccommodation driven by controlling convergence, rather than by inducing blur-driven vergence. The use of convergence as a major drive to accommodation explains many clinical characteristics of distance exotropia, including apparently high near stimulus AC/A ratios.

Three-dimensional imaging with monocular cues using holographic stereography

Two quantitative criteria are derived to evaluate monocular cues in holographic stereograms. We find that the reconstruction has correct monocular cues when the whole scene is located in a so-called "monocular cues area" with compatible monocular and binocular cues. In contrast, incorrect monocular cues appear when the scene is in the other two areas, namely, the "visible multi-imaging area" and the "lacking information area." A pupil-function integral imaging algorithm is developed to simulate monocular observation, and a holographic printing system is set up to fabricate full-parallax holographic stereograms. Both simulations and experiments agree with the criteria.

Potential signal to accommodation from the Stiles–Crawford effect and ocular monochromatic aberrations

The purpose of this study is to determine if cues within the blurred retinal image due to the Stiles–Crawford (SC) effect and the eye's monochromatic aberrations can drive accommodation with a small pupil (3 mm) that is typical of bright photopic conditions. The foveal, psychophysical SC function (17 min arc) and ocular monochromatic aberrations were measured in 21 visually normal adults. The retinal image of a 10.2 min arc disc was simulated for spherical defocus levels of −1 D, 0 D and +1 D in each of four conditions consisting of combinations of the presence or absence of the individual SC function and monochromatic aberrations with a 3 mm pupil. Accommodation was recorded in 11 participants as each viewed the simulations through a 0.75 mm pinhole. The SC effect alone did not provide a significant cue to accommodation. Monochromatic aberrations provided a statistically significant but rather small cue to monocular accommodation.

Accommodative Fluctuations, Lens Tension, and Ciliary Body Thickness in Children

To investigate the relationship among microfluctuations in accommodation, resting tension on the crystalline lens, ciliary body thickness, and refractive error in children. Subjects were 49 children, aged 8 to 15 years. Subjects wore habitual correction over their left eye and an infrared filter over the right eye during accommodative measurements. Monocular accommodation was measured continuously for two, 30-second periods using a PowerRef I at a sampling rate of 25 Hz while subjects viewed a high-contrast target at 0.25 m. The high (1.0 to 2.3 Hz) and low- (0 to 0.6 Hz) frequency components of the power spectrum from a fast Fourier transform of the accommodative response were used in analysis. Resting tension on the crystalline lens was assessed by measuring the amplitude of the oscillations of the crystalline lens after a rightward 20 degrees saccadic eye movement. Ciliary body thickness was measured 2 mm posterior to the scleral spur from images obtained with a Zeiss Visante optical coherence tomography (OCT). Cycloplegic spherical equivalent refractive error was obtained with the Grand Seiko autorefractor. The mean +/- SD spherical equivalent refractive error was -1.00 D +/- 2.25 (range, -6.00 D to +3.44 D). Greater power in the log of the high-frequency component of accommodative microfluctuations was associated with thinner ciliary bodies (p = 0.03) and lower ages (p = 0.0004). More hyperopic refractive errors with greater power in the high-frequency component (p = 0.0005) and the low-frequency component (p = 0.02). No statistically significant relationship was found for the low-frequency component or root mean square of accommodative microfluctuations and refractive error. High-frequency microfluctuations of accommodation appear to be suppressed with thicker ciliary bodies. These variations in accommodation need to be observed in a longitudinal study to better assess the functional significance of their relationship to ciliary body size and refractive error.

Binocular Adaptation to +2 D Lenses in Myopic and Emmetropic Children

To compare vergence adaptation to +2 D addition lenses in myopic and emmetropic children and to evaluate the influence of the accommodative-vergence crosslink (AC/A ratio) on this adaptation. Nine myopic and 11 emmetropic children fixated a near target at a distance of 33 cm. Measures of binocular and monocular accommodation and phoria were obtained during a 20-min near task with and without +2 D lenses. Response AC/A ratios were determined from the experimental results. Vergence adaptation was quantified by the magnitude of reduction in phoria and the percentage of completeness (PC, return of adapted phoria to habitual level) after the near task. Myopic children showed significantly higher AC/A ratios, which led to greater lens-induced exophoria and a greater demand for vergence adaptation. Both refractive groups showed significant vergence adaptation; however, myopes exhibited significantly reduced (p < 0.01) magnitudes compared with emmetropes (myopes = 3.95 +/- 0.15 Delta; emmetropes = 4.41 +/- 0.08 Delta). The mean PC was also significantly (p < 0.001) reduced in myopes (61.02 +/- 1.57) compared with emmetropes (76.6 +/- 2.10). There was a significant correlation between magnitude of adaptation and AC/A in both the refractive groups; however, myopes consistently showed reduced magnitudes compared to emmetropes. AC/A ratio influenced PC in emmetropic but not myopic children. In the accommodation system, +2 D lenses eliminated the accommodative lags observed in myopic children during natural viewing conditions. These lenses resulted in a small over-focus (-0.24 +/- 0.27 D) at the onset of near work, which decreased during sustained viewing through the near add. Myopic children demonstrate reduced magnitude and completeness of vergence adaptation to +2 D lenses. The magnitude of vergence adaptation varied with AC/A in both refractive groups; however, the presence of myopia differentiated the amount of adaptation for all AC/A ratios. Conversely, the degree of completeness appears to be primarily associated with the type of refractive error.

Evaluation of the monocular depth cue in 3D displays

Binocular disparity and monocular depth information are the principal functions of ideal 3D displays. 3D display systems such as stereoscopic or multi-view, super multi-view (SMV), and multi-focus (MF) displays were considered for the testing of the satisfaction level with the monocular accommodation of three different depths of 3D object points. The numerical simulation and experimental results show that the MF 3D display gives a monocular depth cue. In addition, the experimental results of the monocular MF 3D display show clear monocular focus on four different depths. Therefore, we can apply the MF 3D display to monocular 3D displays.

Age‐related changes in static accommodation and accommodative miosis

An attempt was made to explore the validity of the Hess-Gullstrand and Duane-Fincham models of presbyopia development, on the assumption that accommodative miosis could be used as an indicator of ciliary muscle effort. Monocular accommodation response and pupil size were measured as a function of accommodation demand over the range 0-4 D, in 48 normal subjects with ages between 17 and 56 years. The slope of the response/stimulus curve was found to decrease only slowly with age up to about 35 years and then to decline more rapidly. Accommodative miosis per dioptre of accommodation response did not change systematically with age up to about 35 years, this being apparently more in accord with the Hess-Gullstrand model. However, accommodative miosis varied very widely between younger subjects of similar age and accommodative amplitude (from zero to around 1 mm per dioptre of accommodation response for subjects in their twenties). It is concluded that miosis does not necessarily accompany accommodation and that its magnitude is not related in any simple general way to ciliary muscle contraction. Hence it cannot be used to support or refute particular theories of presbyopia.

Age-Related changes in ocular aberrations with accommodation

This study investigates the changes in aberrations with monocular accommodation as a function of age. Second-order and higher order wavefront aberrations and pupil size were measured as a function of accommodation demand over the range of 0-4 D in the right eyes of 47 normal subjects with ages between 17 and 56 years. Higher order ocular Zernike aberrations were analyzed for the natural pupil size in terms of their equivalent defocus and were also determined for fixed pupil diameters of 4.5 mm in the unaccommodated eyes and 2.5 mm in the accommodating eyes. With relaxed accommodation (0 D accommodation stimulus), the major change with age was in the value of C4(0), which increased in positive value over the age range studied, although the total higher order RMS wavefront aberration did not increase. When the data were analyzed for natural pupils, spherical aberration was again found to change systematically in the positive direction with age. The equivalent defocus of total higher order RMS error for natural pupils showed no significant correlation with age (p > .05). With active accommodation, spherical aberration was found to decrease and become negative as the accommodative response increased in the younger subjects (<40 years). Near-zero spherical aberration was found at accommodation levels of about 0.50 D in the youngest subjects (<20 years) and at around 2-3 D in subjects between 20 and 39 years. In the older subjects (>40 years), the spherical aberration showed only small changes, some of which were positive, within the limited amplitude of accommodation available. Other higher order aberrations and the RMS of higher order aberrations did not appear to change systematically with accommodation, except in the oldest subjects. The change with age in the relationship between aberration and accommodation is interpreted in terms of the changing gradients of refractive index and surface curvatures of the crystalline lens.

Changes in accommodative and vergence responses when viewing through near addition lenses

It has been proposed that near addition lenses reduce the lag of accommodation and may slow myopia progression. In this study, we investigated the differences in accommodative response and near phoria in subjects with and without near addition lenses. Fourteen subjects (7 emmetropic and 7 myopic) participated in the study. Monocular and binocular accommodative responses to a target at 40 cm were measured with and without +2.00 diopter (D) lenses using a Canon R-1 optometer (Canon Europe N.V., Amsterdam, The Netherlands). Near dissociated phoria was measured using 3 testing methods: Maddox rod, cover test, and Von Graefe technique. The differences in accommodative response and near phoria between the 2 viewing conditions with and without the near addition lens were significant ( P < 0.0001). No significant differences were revealed in accommodative response and near phoria between refractive error groups. The average accommodative responses of all subjects were 2.03 ± 0.06 (SE) D (without +2.00 D lens) and 0.60 ± 0.07 D (with +2.00 D lens) under monocular viewing conditions and 2.11 ± 0.06 D (without +2.00 D lenses) and 0.77 ± 0.07 D (with +2.00 D lenses) under binocular viewing conditions. The average near phorias of all subjects were −2.08 ± 0.69 prism diopters (PD, without +2.00 D lenses) and −7.90 ± 0.68 PD (with +2.00 D lenses). We discuss the effect of near addition lenses on the defocus of the retinal image, accommodative response, and near phoria. In addition, we propose that the observed difference between binocular and monocular accommodation, when viewing through near addition lenses, is caused by an increased vergence accommodation.

Accommodation as a function of age and the linearity of the response dynamics

The changes with age in the accommodation responses to dynamic stimuli can reveal useful information on the factors underlying presbyopia development. Analysis of the monocular accommodation responses of 19 normal observers (ages 18–49 years) to stimuli whose vergence varied sinusoidally with time at different temporal frequencies (peak-to-peak stimulus 1.33–2.38 D, at 0.05–1.00 Hz) showed that at all ages both the gain and phase of the response were essentially linear functions of the temporal frequency. Extrapolation of least-squares, regression line fits to the gain data for each subject gave the gain at zero frequency, G 0, and the cut-off frequency, f c, at which the gain fell to zero. G 0 reduced with age but f c remained essentially constant at about 1.7 Hz, up to at least the age of about 40. The magnitude of the response to step stimuli covering the same stimulus range was well correlated with the value of G 0. The linear changes in phase lag with temporal frequency corresponded to simple time delays t d. The time lag varied from close to zero for the youngest subjects to about 0.5 s for the subjects in their late forties. There was substantial variation between the responses of subjects of similar age: those subjects with high values of G 0 also tended to have low values of t d, both effects probably being due to the superior ability of some individuals to predict the sinusoidal changes in the accommodation stimulus. Comparison of theoretical step responses, derived by applying linear theory to the parameters obtained from the results for the sinusoidal stimuli, with the actual responses to unpredictable steps for the same subjects supports the view that prediction effects and other possible factors make linear theory inapplicable to this type of data. The results are discussed in the context of current ideas on the development of presbyopia: it is suggested that the constancy of f c with age is the result of the ciliary body maintaining its efficiency, whereas the fall in G 0 and increase in t d result from increases in lens rigidity.

Monocular accommodation response for totally occluded objects.

To investigate the accuracy and stability of the monocular accommodation response when attention is directed toward a totally occluded distant object. A static infrared optometer was used to measure the monocular accommodative response in 16 participants while they viewed a distant letter target in the presence or absence of an intervening object that totally occluded the distant target from view. Participants were instructed to attend to and focus for the distant target. Participants differed widely in their accommodative responses for a totally occluded distant object, focusing close to the distant object, close to the intervening object, in between the two objects, and beyond the distant object. Some of this variability could be attributed to individual differences in the dark focus. Accommodation was more variable when the distant target was totally occluded by an obstruction at an intermediate distance (50 cm). Many individuals focus poorly in the presence of a visual obstruction that totally occludes the object of regard.

Fatigue reduces tonic accommodation.

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Fatigue reduces tonic accommodation

Ocular accommodation adopts a mean baseline response level of approximately 1.0D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460–469).We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33cm. In a monocular condition, accommodation was stimulated for 3min with lens values alternating from −1.5 to +1.5D at a rate of 0.25Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9prism diopters (PD) base-out to 9PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA.These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.