Accommodation dynamics I. Range nonlinearity

BCLA CLEAR presbyopia: Mechanism and optics

With over a billion adults worldwide currently affected, presbyopia remains a ubiquitous, global problem. Despite over a century of study, the precise mechanism of ocular accommodation and presbyopia progression remains a topic of debate. Accordingly, this narrative review outlines the lenticular and extralenticular components of accommodation together with the impact of age on the accommodative apparatus, neural control of accommodation, models of accommodation, the impact of presbyopia on retinal image quality, and both historic and contemporary theories of presbyopia.

Simultaneous Measurement of Objective and Subjective Accommodation in Response to Step Stimulation

Purpose

This study aimed to evaluate differences in objective and subjective accommodation dynamically and simultaneously.

Methods

Thirty-four pre-presbyopic healthy volunteers (mean age ± SD, 41.0 ± 3.2 years) participated in this study. Initially, the reaction time for detecting a change in the target was measured at near. Dynamic accommodation was then monocularly recorded using an open-view Shack-Hartmann aberrometer and compared with the amplitude and velocity of subjective accommodation.

Results

The objective amplitude of accommodation (0.97 ± 0.32 diopter [D]) was significantly greater than the subjective amplitude of accommodation (0.62 ± 0.43 D; P < 0.001). The accommodative velocity was significantly faster for the "before the accommodation" response time (0.47 ± 0.38 D/s) than the "after the accommodation" response time (0.21 ± 0.22 D/s; P = 0.007).

Conclusions

The human eye under the monocular condition quickly adjusts to the focal plane to clearly archive the nearby object, and the focal plane thereafter is slowly and accurately adjusted to the visual target after visual recognition.

Monocular accommodation response to random defocus changes induced by a tuneable lens

Accommodation of the human eye relies on multiple factors and visual cues that include object size, monochromatic and chromatic aberrations, and vergence. Yet, even in monocular conditions, accommodation corrects for defocus. Studies of eye growth in chicks have addressed whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. However, findings have not been unambiguous and questions remain. Here, we report on monocular accommodation studies of emmetropic and myopic human subjects to clarify whether foveal vision drives accommodation in the correct direction by removing out-of-focus blur potentially before relying on other cues. Subjects viewed monocularly a green target at 1-meter distance while being presented with a random sequence of negative defocus step changes induced by a pupil-conjugated current-driven tuneable lens. The natural pupil was constricted by a pupil-conjugated motorized iris using three different diameters and target brightness was set with a liquid crystal variable attenuator. A Hartmann-Shack wavefront sensor with an infrared beacon captured real-time changes of defocus and Zernike polynomial coefficients up to 4th radial order. We find that the young adult eye accommodates reliably in the correct direction but with a latency of 300-700 ms. The findings are discussed in relation to an absorption model of light in outer segments that breaks the defocus symmetry and thus may serve as a plausible guide for accommodation and emmetropization.

Amplitude, Latency, and Peak Velocity in Accommodation and Disaccommodation Dynamics

The aim of this work was to ascertain whether there are differences in amplitude, latency, and peak velocity of accommodation and disaccommodation responses when different analysis strategies are used to compute them, such as fitting different functions to the responses or for smoothing them prior to computing the parameters. Accommodation and disaccommodation responses from four subjects to pulse changes in demand were recorded by means of aberrometry. Three different strategies were followed to analyze such responses: fitting an exponential function to the experimental data; fitting a Boltzmann sigmoid function to the data; and smoothing the data. Amplitude, latency, and peak velocity of the responses were extracted. Significant differences were found between the peak velocity in accommodation computed by fitting an exponential function and smoothing the experimental data (mean difference 2.36 D/s). Regarding disaccommodation, significant differences were found between latency and peak velocity, calculated with the two same strategies (mean difference of 0.15 s and −3.56 D/s, resp.). The strategy used to analyze accommodation and disaccommodation responses seems to affect the parameters that describe accommodation and disaccommodation dynamics. These results highlight the importance of choosing the most adequate analysis strategy in each individual to obtain the parameters that characterize accommodation and disaccommodation dynamics.

Quantification of the ciliary muscle and crystalline lens interaction during accommodation with synchronous OCT imaging

Two SD-OCT systems and a dual channel accommodation target were combined and precisely synchronized to simultaneously image the anterior segment and the ciliary muscle during dynamic accommodation. The imaging system simultaneously generates two synchronized OCT image sequences of the anterior segment and ciliary muscle with an imaging speed of 13 frames per second. The system was used to acquire OCT image sequences of a non-presbyopic and a pre-presbyopic subject accommodating in response to step changes in vergence. The image sequences were processed to extract dynamic morphological data from the crystalline lens and the ciliary muscle. The synchronization between the OCT systems allowed the precise correlation of anatomical changes occurring in the crystalline lens and ciliary muscle at identical time points during accommodation. To describe the dynamic interaction between the crystalline lens and ciliary muscle, we introduce accommodation state diagrams that display the relation between anatomical changes occurring in the accommodating crystalline lens and ciliary muscle.

Dynamics of the near response under natural viewing conditions with an open-view sensor

We have studied the temporal dynamics of the near response (accommodation, convergence and pupil constriction) in healthy subjects when accommodation was performed under natural binocular and monocular viewing conditions. A binocular open-view multi-sensor based on an invisible infrared Hartmann-Shack sensor was used for non-invasive measurements of both eyes simultaneously in real time at 25Hz. Response times for each process under different conditions were measured. The accommodative responses for binocular vision were faster than for monocular conditions. When one eye was blocked, accommodation and convergence were triggered simultaneously and synchronized, despite the fact that no retinal disparity was available. We found that upon the onset of the near target, the unblocked eye rapidly changes its line of sight to fix it on the stimulus while the blocked eye moves in the same direction, producing the equivalent to a saccade, but then converges to the (blocked) target in synchrony with accommodation. This open-view instrument could be further used for additional experiments with other tasks and conditions.

Changes in accommodative responses with multifocal contact lenses: a pilot study

PURPOSE

To evaluate induced changes in pupil and accommodative response for different accommodative stimuli with three different multifocal contact lenses (CLs).

METHODS

Accommodative and pupil responses with three aspheric multifocal CLs of simultaneous focus center near (PureVision Low Add, PureVision High Add and Focus Progressives) for two accommodative stimuli of -2.5 and -4.00 D were recorded in 10 young subjects. Accommodative response, peak velocity, and time constant of accommodation and pupil constriction were assessed. The measurements were evaluated in all the participants with distance-single vision CLs and with the three multifocal CLs. The monocular measurements were performed in a random sequence for each participant. All parameters were measured with a Hartmann-Shack aberrometer (IRX-3; Imagine Eyes, Orsay, France).

RESULTS

Differences were not found in accommodation response for both accommodative stimuli studied between the single vision lens and the three different multifocal CLs (p > 0.05 for both stimuli). For the 2.5 D stimulus, significant differences were not found in peak velocity and time constant between the single vision lens and the three different multifocal CLs (p > 0.05 for all situations); in amplitude of pupil constriction, differences were only found for the situation with PureVision High Add, where the value was lower than the single vision lens (p = 0.015). For the 4.00 D stimulus, differences with single vision lens in peak of velocity and time constant were only found with PureVision Low Add, where the peak velocity value was higher and the time constant was lower (p = 0.024 and p = 0.032 for peak of velocity and time constant, respectively); for amplitude of pupil constriction differences were not found (p > 0.05).

CONCLUSIONS

Data obtained in this pilot study suggest that in young observers, the multifocal CLs studied do not induce large changes in accommodative system compared with the single vision lens.

The sensitivity of the 2- to 4-month-old human infant accommodation system

PURPOSE

The goal of this study was to compare objectively the sensitivity of the accommodation system in human infants and adults under binocular and monocular viewing conditions.

METHODS

Full-term infants from 2 to 4 months of age and pre-presbyopic adults were presented with a high-contrast cartoon stimulus moving sinusoidally in diopters around a mean position of 2 D (50 cm). Three stimulus amplitudes were used in one trial (0.25, 0.50, and 0.75 D), with unpredictable stimulus motion during each amplitude change. Eccentric photorefraction was used to record accommodative responses at 25 Hz. The stimulus was made monocular by placing an infrared filter over the right eye, to block visible light but pass the near-infrared wavelength of the photorefractor and allow responses to be recorded from both eyes.

RESULTS

Fourier analysis was used to determine the accommodative response at the frequency of the stimulus. Significant signal-to-noise ratios indicated that, on average, the 2- to 4-month-old infants generated an accommodative response to at least the 0.75 D amplitude monocular stimulus and the 0.75 and 0.50 D binocular stimuli. Adults responded to the 0.25 D amplitude both binocularly and monocularly.

CONCLUSIONS

In infants 2 to 4 months of age, the developing visual system compensates for small changes in defocus relative to the typical amounts of hyperopic refractive error found at that age.

Age-related changes in accommodative dynamics from preschool to adulthood

PURPOSE

To study variations in dynamic measures of accommodation and disaccommodation with age in subjects ranging from preschool to adulthood.

METHODS

Accommodative responses to a step stimulus cartoon movie alternating from distance to near were recorded with a dynamic infrared photorefractor. Subjects viewed at least three stimulus cycles of far and near for four near stimulus demands (2, 3, 4, and 5 D). Latencies, peak velocities, and the magnitude of accommodative microfluctuations were calculated from the responses and compared in 41 subjects from 3 to 38 years of age.

RESULTS

Mean accommodative and disaccommodative latencies decreased linearly with age. The magnitude of accommodative microfluctuations during sustained near accommodation had a significant quadratic relationship to age, with subjects in the first decade of life having the largest fluctuations and subjects in the third decade of life having the smallest for all stimulus demands. Accommodative peak velocities were fastest in subjects in the first two decades of life, compared with subjects in the third and fourth decades; however, disaccommodative peak velocities showed no significant age differences.

CONCLUSIONS

Age-related changes in dynamics occur in accommodative and disaccommodative latencies, accommodative peak velocities, and accommodative microfluctuations, all of which decrease with increasing age from preschool to adulthood. Disaccommodative peak velocities showed no change with age.

Dynamic accommodation responses following adaptation to defocus

PURPOSE

Adaptation to defocus is known to influence the subjective sensitivity to blur in both emmetropes and myopes. Blur is a major contributing factor in the closed-loop dynamic accommodation response. Previous investigations have examined the magnitude of the accommodation response following blur adaptation. We have investigated whether a period of blur adaptation influences the dynamic accommodation response to step and sinusoidal changes in target vergence.

METHOD

Eighteen subjects (six emmetropes, six early onset myopes, and six late onset myopes) underwent 30 min of adaptation to 0.00 D (control), +1.00 D or +3.00 D myopic defocus. Following this adaptation period, accommodation responses to a 2.00 D step change and 2.00 D sinusoidal change (0.2 Hz) in target vergence were recorded continuously using an autorefractor.

RESULTS

Adaptation to defocus failed to influence accommodation latency times, but did influence response times to a step change in target vergence. Adaptation to both +1.00 and +3.00 D induced significant increases in response times (p = 0.002 and p = 0.012, respectively) and adaptation to +3.00 D increased the change in accommodation response magnitude (p = 0.014) for a 2.00 D step change in demand. Blur adaptation also significantly increased the peak-to-peak phase lag for accommodation responses to a sinusoidally oscillating target, although failed to influence the accommodation gain. These changes in accommodative response were equivalent across all refractive groups.

CONCLUSION

Adaptation to a degraded stimulus causes an increased level of accommodation for dynamic targets moving towards an observer and increases response times and phase lags. It is suggested that the contrast constancy theory may explain these changes in dynamic behavior.

Human infants' accommodation responses to dynamic stimuli

PURPOSE

A young infant's environment routinely consists of moving objects. The dynamics of the infant accommodative system are almost unknown and yet have a large impact on habitual retinal image quality and visual experience. The goal of this study was to record infants' dynamic accommodative responses to stimuli moving at a range of velocities.

METHODS

Binocular accommodative responses were recorded at 25 Hz. Data from infants 8 to 20 weeks of age and pre-presbyopic adults were analyzed. A high-contrast image of a clown was moved between 20- and 50-cm viewing distances at four velocities (a step, 50 cm/s, 20 cm/s, and 5 cm/s).

RESULTS

Most infants who had clear responses were able to initiate their response within a second of stimulus onset. The infants were able to discriminate the different stimulus velocities and to adjust their response velocities and durations in an appropriate fashion.

CONCLUSIONS

The data indicate that by the third postnatal month infants are able to respond with latencies within a factor of two of adults' and that there is little immaturity in the motor capabilities of the accommodative system compared with the sensory visual system at the same age.

Effects of pharmacologically manipulated amplitude and starting point on edinger-westphal-stimulated accommodative dynamics in rhesus monkeys

PURPOSE

The aim of this study was to determine whether pharmacologically manipulated resting refraction, amplitude, and starting point affect accommodative and disaccommodative dynamics in anesthetized adolescent rhesus monkeys.

METHODS

Pilocarpine and atropine were applied topically to manipulate resting refraction, accommodative amplitude, starting point, and end point in two monkeys with permanent electrodes in the Edinger-Westphal nucleus. Accommodation was centrally stimulated with submaximal and maximal current amplitudes. Dynamic accommodative responses were measured with infrared photorefraction before and during the course of action of the drugs. Accommodative and disaccommodative dynamics were analyzed in terms of peak velocity as a function of amplitude, starting point, and end point.

RESULTS

Pilocarpine caused a myopic shift in resting refraction of 11.62 +/- 1.17 D. Centrally stimulated accommodative amplitude was 10.08 +/- 1.15 D before pilocarpine and 0.68 +/- 0.29 D after pilocarpine. Changes were found in accommodative dynamics as a function of starting point and in disaccommodative dynamics as a function of amplitude and end point. Accommodative amplitude was 11.25 +/- 0.18 D before atropine administration and 0.52 +/- 0.11 D after atropine administration. Accommodative dynamics as a function of amplitude were not substantially altered during the course of pilocarpine-induced accommodation or atropine-induced cycloplegia.

CONCLUSIONS

Accommodative response amplitude is reduced with pilocarpine by shifting the eye to a more myopic state and with atropine by cycloplegia. Pharmacologic manipulations showed that accommodative and disaccommodative dynamics in anesthetized monkeys depend on amplitude, starting point, and end point of the response and on the contributions of neural and receptor activity.

Vergence accommodation and monocular closed loop blur accommodation have similar dynamic characteristics

Retinal blur and disparity are two different sensory signals known to cause a change in accommodative response. These inputs have differing neurological correlates that feed into a final common pathway. The purpose of this study was to investigate the dynamic properties of monocular blur driven accommodation and binocular disparity driven vergence-accommodation (VA) in human subjects. The results show that when response amplitudes are matched, blur accommodation and VA share similar dynamic properties.

Application of video-based technology for the simultaneous measurement of accommodation and vergence

Accommodation and vergence are two ocular motor systems that interact during binocular vision. Independent measurement of the response dynamics of each system has been achieved by the application of optometers and eye trackers. However, relatively few devices, typically earlier model optometers, allow the simultaneous assessment of accommodation and vergence. In this study we describe the development and application of a custom designed high-speed digital photorefractor that allows for rapid measures of accommodation (up to 75Hz). In addition the photorefractor was also synchronized with a video-based stereo eye tracker to allow a simultaneous measurement of accommodation and vergence. Analysis of accommodation and vergence could then be conducted offline. The new instrumentation is suitable for investigation of young children and could be potentially used for clinical populations.

Age related changes in the characteristics of the near pupil response

Static and dynamic aspects of the near pupil response were studied in human subjects in the age range when accommodative amplitude steadily declines. Dynamic accommodative and pupillary responses to step stimuli were recorded in 66 subjects (ages: 14-45 years). Exponential fits to data provided amplitude, peak velocity and time constants. Accommodative amplitude decreased linearly with age (p < 0.05). Pupil constriction per diopter of accommodative response increased exponentially with age (p < 0.05). The amplitude of pupil constriction for a 2D stimulus decreased linearly with age (p < 0.05) and for a 5D stimulus did not change with age (p = 0.90). The latency of pupil constriction did not change with age (p = 0.65), while the mean peak velocity decreased linearly with age (p < 0.05). An increase in the amount of pupil constriction per diopter of accommodative response, but not per diopter of stimulus amplitude, suggests that the near effort per se does not increase with age. There is a slight reduction in the speed of near pupil response with age.

Dynamic control of ocular disaccommodation: first and second-order dynamics

Velocity and acceleration characteristics provide valuable information about dynamic control of accommodation. We investigated velocity and acceleration of disaccommodation (near-far focusing) from three starting positions. Peak velocity and peak acceleration of disaccommodation increased with the proximity of starting position however for a given starting position they were invariant of response magnitude. These results suggest that all disaccommodation responses are initiated towards a constant primary destination and are switched mid-flight to attain the desired final position. Large discrepancies between the primary destination and desired final position appear to produce overshoots and oscillations of small responses from proximal starting positions.

Characteristics of pupil responses during far-to-near and near-to-far accommodation

The static and dynamic aspects of the pupil response associated with various amplitudes of far-to-near (accommodation) and near-to-far (disaccommodation) focusing responses were evaluated in this study. Dynamic refractive and pupillary changes were measured simultaneously during focusing responses to targets at real distances in 12 young (23-26 years) emmetropic subjects. The targets were presented alternately at far (6 m) and at one of six near positions from 1 D (1 m) to 6 D (16.7 cm) in 1 D steps. The latency, magnitude and peak velocity of pupil changes associated with accommodation and disaccommodation responses were calculated. The latency of refractive changes was shorter than that of pupillary changes. In general, the pupil constricted with accommodation and started dilating while accommodation was still maintained, resulting in reduced pupil dilation with disaccommodation. The magnitude of the pupil response increased linearly with that of accommodation and disaccommodation. The amount of pupil change per dioptre of refractive change was invariant with various amplitudes of refractive change. The peak velocity of pupil constriction was greater than that of pupil dilation for corresponding amplitudes. The pupil response is more closely associated with accommodation than with disaccommodation.

Acceleration characteristics of human ocular accommodation

Position and velocity of accommodation are known to increase with stimulus magnitude, however, little is known about acceleration properties. We investigated three acceleration properties: peak acceleration, time-to-peak acceleration and total duration of acceleration to step changes in defocus. Peak velocity and total duration of acceleration increased with response magnitude. Peak acceleration and time-to-peak acceleration remained independent of response magnitude. Independent first-order and second-order dynamic components of accommodation demonstrate that neural control of accommodation has an initial open-loop component that is independent of response magnitude and a closed-loop component that increases with response magnitude.

Dynamic aspects of accommodation: age and presbyopia

There has been no comprehensive study involving each of the primary dynamic components of accommodation in the same cohort as related to age and presbyopic onset; furthermore, the current findings are equivocal. Dynamic monocular components of accommodation (latency, time constant, peak velocity/amplitude relationship, and microfluctuations) were assessed objectively using an infrared optometer within the linear region of accommodation in 30 visually-normal human subjects aged 21-50 years. The time constant and the peak velocity/amplitude relationship did not change with age. However, latency progressively increased, and microfluctuation amplitude and frequency progressively decreased, with increasing age. The invariance in time constant suggests that the gross biomechanical aspects of the lens and related structures in the remaining linear region are relatively unaffected by age. In contrast, the decrease in microfluctuation activity with age suggests more subtle alterations in the biomechanical aspects of the lens to these very small perturbations, such as a response amplitude non-linearity. With respect to neurologic control, the progressive latency increase suggests a processing delay of the blur input, and this is consistent with age-related changes in reaction time measures. The lack of any age-related changes in the peak velocity/amplitude relationship implies normalcy of central and peripheral neuromotor control, as well as grossly normal first-order lens biomechanics, in this linear response region. The results are consistent with the Hess-Gullstrand theory of presbyopia.

The role of short-wavelength sensitive cones and chromatic aberration in the response to stationary and step accommodation stimuli

The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives "static" accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.

Amplitude dependent accommodative dynamics in humans

Dynamics of accommodation (far-to-near focus) and disaccommodation (near-to-far focus) are described as a function of response amplitude. Accommodative responses to step stimuli of various amplitudes presented in real space were measured in eight 20-30 year old subjects. Responses were fitted with exponential functions to determine amplitude, time constant and peak velocity. Despite the intersubject variability, the results show that time constants of accommodation and peak velocity of disaccommodation increase with amplitude in all subjects. The dynamics of accommodation and disaccommodation are dependent on amplitude, but have different properties in each case.

Dynamics of the accommodation response to abrupt changes in target vergence as a function of age

The dynamic accommodation responses to small, abrupt changes in an accommodation stimulus were studied in two experiments. In the first, responses of 19 subjects with ages distributed between 18 and 49 years were measured for step stimuli of +/- 1.05 D. In the second, responses to small step stimuli (+/- 0.75, +/- 1.75 D) were recorded for a group of six 'young' (mean age 22, range 16-26 years) and six 'old' (mean age 42, range 36-48 years) subjects. In both experiments, the low target vergences always lay within the accommodation range of all subjects. Whether the data were analysed in terms of simple reaction and response times, or in terms of the frequency response, no marked systematic changes with age were found in the dynamics of the response in either of the experiments. It is concluded that, for small stimuli within the amplitude of accommodation, the response dynamics over the adult age range studied remain remarkably constant, even though the amplitude progressively reduces with age as complete presbyopia is approached.

On the linearity of accommodation dynamics

Available experimental data on the gain and phase lags in the closed-loop accommodation responses to stimuli whose dioptric vergence changes sinusoidally with time are examined. It is shown that both phase lags and gain change almost linearly with the temporal frequency of the stimulus. This implies that the phase lags correspond to a frequency-independent time delay and that, if the system is linear, the impulse response should take a symmetrical (sin x/x)(2) form, with a corresponding standard form of step response. Comparison of this derived step response with the quasi-exponential results found experimentally underlines the limitations of attempting to generalize dynamic accommodation performance from responses obtained with predictable sinusoidally-changing stimuli and the application of a linear model. Consideration of the frequency response which would lead to an exponential step response supports the argument that, although the gain results obtained with sinusoidal stimuli are reasonably representative of those applying with other forms of stimulus, the predictable nature of sinusoidally-changing stimuli makes the derived phase lags inapplicable to non-predictable step or other stimuli. Other limitations in linear models of the response are discussed.

Analysis of accommodation function with ultrasonography

The aim of this study was to present a new method that uses ultrasonography to analyse accommodation function, and to do a preliminary investigation of its possible use in clinical practice. Using the method of continuous ultrasonographic biometry, changes in lens thickness were measured during accommodation. From these measurements response latency and duration were determined. Normal values for latency and duration were obtained by measuring 20 healthy subjects of different ages. Measurements were also performed on three patients with different accommodation disorders: diabetes, Adie's syndrome and third nerve palsy. Normal response latency is 394 ms (+/-46 SD) and independent of are. Normal response duration increases with age from an average 306 ms at 15 years-of-age to an average 954 ms at 55 years-of-age. Normal latency as well as duration appear to have a large interindividual variability. The diabetic patient had a delayed latency but a normal response duration. The patient with Adie's syndrome had a delayed latency and prolonged duration. The patient with third nerve palsy had a normal latency and duration. We conclude that ultrasonographically determine latency and duration give additional information on accommodation function that is more complete and objective than maximum accommodative amplitude alone. The results in our patients suggest that, in selected cases, this information may aid in the diagnosis and management of patient's complaints.

Dynamics of accommodative vergence movements controlled by the dominant and non-dominant eye

Accommodative vergence responses to accommodative stimuli presented at three different amplitudes in a stepwise and a sinusoidal mode to either the dominant or the non-dominant eye were studied. The motor control loop was opened by one eye viewing the target and the other eye covered. Each eye was stimulated monocularly and eye movements were recorded with an infra-red reflection system. Latency and time constant of the accommodative vergence response to step stimulation or the phase of the vergence response to sine wave stimulation, did not vary systematically with stimulus amplitude or eye stimulated. However, the gain of the accommodative vergence movements was highest in the non-dominant eye at a particular stimulus amplitude. This suggested that accommodative vergence angles can vary depending on which eye is leading and driving the accommodative vergence system in a specific fixation situation.

In vivo determination of the biomechanical properties of the component elements of the accommodation mechanism

A method of obtaining in vivo information about the component elements of the accommodation mechanism is described. Changes in axial lens thickness during accommodation are measured continuously by ultrasonographic biometry. The measurements are fitted with a dynamic biomechanical model of the mechanism of accommodation. This gives a time constant that depends both on the direction of accommodation and on the properties of the lens, zonules and choroid. Two ratios can be calculated that give information about the visco-elastic properties of the lens, zonules and choroid. This information is especially useful to evaluate the changes that lead to presbyopia.

Switching control of accommodation: experimental and simulation responses to ramp inputs

The irregular staircase appearance of the accommodation response to a ramp stimulus, slow movement of a target towards the subject's eye, suggests a switching between different states of control. Experiments and model simulations indicate that blur processing by retina and cortex underlies threshold triggering of the switch. The ON state consists of a fairly high gain, closed-loop control system that minimizes error. The OFF state provides for a quasi-open-loop state and a slow drift to a bias level, under the influence of a leaky integrator.

Binocular accommodation reaction and response times for normal observers

Accommodation was recorded from right and left eyes of visually normal observers in both binocular and monocular viewing. Reaction and response times were similar in monocular and binocular viewing and are not influenced by eye dominance. Far-to-near responses were significantly quicker than near-to-far responses. The origin of this difference may be a feature of the elastic properties of the accommodation mechanism. Limited data are presented that indicate that the slowing of accommodation speed with age affects the near-to-far response disproportionally. Errors in the initial direction of response were fewer in binocular viewing in comparison with monocular viewing.

Accommodative fluctuations in Adie's syndrome

Recent reports suggest that the higher frequency components (around 2 Hz) of accommodative fluctuations may arise from the accommodative 'plant' and may have no role in the control of the accommodation system. The dynamic responses of accommodation in two cases with Adie's syndrome were recorded. The results showed that the velocity of accommodative responses was reduced, and the accommodative fluctuations in the higher frequency components were completely diminished. It can be concluded that the origin of fast fluctuations is not located at a more peripheral site than the lesion of tonic accommodation. Since the possible lesion exists at the ciliary ganglion, the origin of the fast oscillation may not be the accommodative 'plant', but the neural one.

Dynamics of accommodation: measurements for clinical application

Accommodation dynamics have not been used in clinical diagnosis as have eye movement and pupillary dynamics; the difficulty of clinical observation is matched by limitations in measurement methods. An instrument suitable for clinical use is described that allows measurement of step response latencies and especially time constants. With computer analysis, phase plane trajectories, and noise spectra can be quickly obtained also. The utility of these dynamical parameters for clinical diagnosis is illustrated by a study of changes of time constants with age in prepresbyopia.

Accommodative disfacility presenting as intermittent exotropia

Accommodative disfacility and intermittent exotropia are two common clinically-observed ocular abnormalities. However, records of individual system dynamics are rare and mechanisms for possible interactive effects between the accommodation and the vergence system remain obscure in cases of system dysfunction. We present objective, quantitative, static and dynamic records of accommodation, vergence and related system responses in a symptomatic patient having accommodation disfacility and intermittent exotropia and propose that these two conditions worked in a pathogenic symbiotic manner.

Indications for prescribing spectacles

Indications for spectacle correction are reviewed in the context of symptoms, pathophysiology, and prognostic efficacy. Criteria include improving acuity, restoring comfortable vision, enhancing visual efficiency, preventing progression, and assorted miscellaneous goals. Alternatives to spectacles and side effects are analyzed. Specific guidelines for individualized management are evaluated in terms of current research.