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

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.

Impact of focus cue presentation on perceived realism of 3-D scene structure: Implications for scene perception and for display technology

Stereoscopic imagery often aims to evoke three-dimensional (3-D) percepts that are accurate and realistic-looking. The "gap" between 3-D imagery and real scenes is small, but focus cues typically remain incorrect because images are displayed on a single focal plane. Research has concentrated on the resulting vergence-accommodation conflicts. Yet, incorrect focus cues may also affect the appearance of 3-D imagery. We investigated whether incorrect focus cues reduce perceived realism of 3-D structure ("depth realism"). Experiment 1 used a multiple-focal-planes display to compare depth realism with correct focus cues vs. conventional stereo presentation. The stimuli were random-dot stereograms, which isolated the role of focus cues. Depth realism was consistently lower with incorrect focus cues, providing proof-of-principle evidence that they contribute to perceptual realism. Experiments 2 and 3 examined whether focus cues play a similar role with realistic objects, presented with an almost complete set of visual cues using a high-resolution, high-dynamic-range multiple-focal-planes display. We also examined the efficacy of approximating correct focus cues via gaze-contingent depth-of-field rendering. Improvements in depth realism with correct focus cues were less clear in more realistic scenes, indicating that the role of focus cues in depth realism depends on scene content. Rendering-based approaches, if anything, reduced depth realism, which we attribute to their inability to present higher-order aspects of blur correctly. Our findings suggest future general 3-D display solutions may need to present focus cues correctly to maximise perceptual realism.

Using a video-based eye tracker to analyse the binocular near-reflex dynamics response

PURPOSE

This study presents a novel video-based eye-tracking system for analysing the dynamics of the binocular near-reflex response. The system enables the simultaneous measurement of convergence, divergence and pupillary size during accommodation and disaccommodation to aid the comprehensive understanding of the three-component near-reflex.

METHODS

A high-speed (90 Hz) video-based eye tracker was used to capture changes in eye gaze and pupil radius in 15 participants in response to altering stimulus conditions. An offline analysis involved separating the gaze vector components and pupil radius, which were fitted to a hyperbolic tangent function to characterise the dynamics of the near-reflex process.

RESULTS

Significant differences in the temporal parameters of the pupil radius were observed between the near-to-far and far-to-near vision changes, with faster miosis compared with mydriasis. Additionally, differences in response times were found between gaze angle components, with longer convergence times compared to changes in the vertical direction (saccades). The steady-state values of the gaze components and pupil radius were in line with theoretical expectations and previous reports.

CONCLUSIONS

The proposed system provides a non-invasive, portable and cost-effective method for evaluating near-reflex dynamics under natural viewing conditions using a video-based eye tracker. The sampling rate ensures the accurate assessment of vergence eye movements and pupillary dynamics. By simultaneously measuring eye convergence, divergence and pupil size, the system offers a more comprehensive assessment of the near-reflex response. This makes it a valuable tool for clinical diagnosis, research studies and investigating the effects of near work on the visual system.

Comparison of the influence of alcohol and cannabis on the dynamics of the accommodative response

PURPOSE

To assess and compare the changes produced by the two most commonly used substances, alcohol and cannabis, on accommodation dynamics.

METHODS

A total of 38 young participants (19 females) were enrolled in the study. They were assigned to two groups: a cannabis group (N = 19) and an alcohol group. Participants in the cannabis group underwent two randomized sessions: a baseline session and a session after smoking a cigarette. Participants in the alcohol group underwent three randomized sessions: a baseline session, a session after the intake of 300 ml of red wine (Alcohol 1), and other after the ingestion of 450 ml of wine (Alcohol 2). For the accommodation assessment, the open-field autorefractor WAM-5500 was used.

RESULTS

The decrease of the mean velocity of the accommodative response produced by Alcohol 2 condition was significantly greater than that observed for Alcohol 1 and Cannabis (p = 0.046). The direction of the accommodation (near-distance and distance-near) had no effect on the deterioration of the accommodation dynamics following substance use. The target distance had a significant effect on the decrease of the mean velocity following substance use (p = 0.002). The decrease of the amplitude of the accommodative response was associated with a decrease of the peak velocity (p = 0.004) and the increase of the accommodative lag (p < 0.001).

CONCLUSIONS

A moderate-high dose of alcohol impairs accommodation dynamics to a greater extent that lower dose of alcohol or smoked cannabis. The deterioration of the accommodation mean speed was higher for a shorter target distance.

Using a smartphone on the move: do visual constraints explain why we slow walking speed?

Viewing one’s smartphone whilst walking commonly leads to a slowing of walking. Slowing walking speed may occur because of the visual constraints related to reading the hand-held phone whilst in motion. We determine how walking-induced phone motion affects the ability to read on-screen information. Phone-reading performance (PRP) was assessed whilst participants walked on a treadmill at various speeds (Slow, Customary, Fast). The fastest speed was repeated, wearing an elbow brace (Braced) or with the phone mounted stationary (Fixed). An audible cue (‘text-alert’) indicated participants had 2 s to lift/view the phone and read aloud a series of digits. PRP was the number of digits read correctly. Each condition was repeated 5 times. 3D-motion analyses determined phone motion relative to the head, from which the variability in acceleration in viewing distance, and in the point of gaze in space in the up-down and right-left directions were assessed. A main effect of condition indicated PRP decreased with walking speed; particularly so for the Braced and Fixed conditions (p = 0.022). Walking condition also affected the phone’s relative motion (p < 0.001); post-hoc analysis indicated that acceleration variability for the Fast, Fixed and Braced conditions were increased compared to that for Slow and Customary speed walking (p ≤ 0.05). There was an inverse association between phone acceleration variability and PRP (p = 0.02). These findings may explain why walking speed slows when viewing a hand-held phone: at slower speeds, head motion is smoother/more regular, enabling the motion of the phone to be coupled with head motion, thus making fewer demands on the oculomotor system. Good coupling ensures that the retinal image is stable enough to allow legibility of the information presented on the screen.

Dynamics of the accommodative response after smoking cannabis

PURPOSE

Cannabis is the most widely consumed illicit drug worldwide. It has been suggested that cannabis could generate blurred vision during reading tasks. The goal of this study was to objectively assess the acute effects of smoking cannabis on the dynamics of ocular accommodation. The influence of other factors, including target distance and the direction of accommodation, as well as personal characteristics, were also analysed.

METHODS

Nineteen young people who were occasional cannabis users participated in the study (mean age 22.53 [3.12] years). Their usage profiles were evaluated by means of the Cannabis Use Disorders Identification Test-revised (CUDIT-r). The dynamics of the accommodative response were evaluated using an open-field auto refractor (Grand Seiko WAM-5500). The participants completed two different experimental sessions, one week apart, and in random order (baseline session and after smoking cannabis). During these sessions, the amplitude of the response (D), mean velocity (D/s), peak velocity (D/s), response time (s), accommodative lag (D) and accommodation variability (D) were measured.

RESULTS

The results indicated that cannabis use had a significant main effect on the mean accommodation/disaccommodation velocity (F_1,13  = 7.21; p = 0.02; η p 2  = 0.396). Cannabis consumption also interacted significantly with other factors. Response time showed a significant two-way interaction between condition × target distance (F_1,13  = 11.71; p = 0.005; η p 2  = 0.474) and condition × accommodation direction (F_1,13  = 8.71; p = 0.01; η p 2  = 0.401). For mean velocity, two-way interactions were found between condition × age (F_1,13  = 6.03; p = 0.03; η p 2  = 0.354), condition × CUDIT-r score (F_1,13  = 6.03; p = 0.03; η p 2  = 0.356) and condition × target distance (F_1,13  = 7.20; p = 0.02; η p 2  = 0.396).

CONCLUSIONS

These findings suggest that cannabis use can alter the accommodation process, although further studies should be carried out to explore the role of attention deficits. According to these results, certain daily activities that depend on an accurate accommodative function may be affected by cannabis use.

Difference in Pupillary Diameter as an Important Factor for Evaluating Amplitude of Accommodation: A Prospective Observational Study

Presbyopia is increasing globally due to aging and the widespread use of visual display terminals. Presbyopia is a decrease in the eye's amplitude of accommodation (AA) due to loss of crystalline lens elasticity. AA differs widely among individuals. We aimed to determine the factors that cause presbyopia, other than advanced age, for early medical intervention. We examined 95 eyes of 95 healthy volunteers (33 men, 62 women) aged 22-62 years (mean: 37.22 ± 9.77 years) with a corrected visual acuity of ≥1.0 and without other eye afflictions except ametropia. Subjective refraction, AA, maximum and minimum pupillary diameters during accommodation, axial length of the eye, and crystalline lens thickness were measured. AA was measured using an auto refractometer/keratometer/tonometer/pachymeter. The difference between maximum and minimum pupillary diameters was calculated. On multiple regression analysis, age and difference in pupillary diameter were both significantly and independently associated with AA in participants aged <44 years, but not in those aged ≥45 years. Our results suggest that the difference in pupillary diameter could be an important age-independent factor for evaluating AA in healthy individuals without cataract. Thus, improving the difference in pupillary diameter values could be an early treatment target for presbyopia.

Impact of monovision on dynamic accommodation of early presbyopes

PURPOSE

To examine the impact of monovision on dynamic changes in accommodation, pupil responses, spherical aberration and resultant image quality in early presbyopes.

METHODS

Refractive state, pupil size and spherical aberration levels were monitored in nine early presbyopes who exhibited some accommodation (40-50 years, mean = 42 ± 2.37 years) using a Shack-Hartmann aberrometer as a binocularly viewed stimulus stepped closer (from 2 m to 40 cm), or farther (from 40 cm to 2 m). Comparison data from two fully presbyopic (i.e. non-accommodating) subjects (ages 46 and 61 years) and two young adults (ages 26 and 29 years) were also collected. Each subject was fit with four different refractive strategies: (1) both eyes corrected for 2 m, (2) both eyes corrected for 40 cm, (3) monovision with the measured right eye corrected for 2 m and, (4) monovision with the right eye corrected for 40 cm. Monochromatic image quality was quantified using the AreaMTF metric.

RESULTS

When fit with monovision, the largest number of early presbyopes produce an accommodative response dominated by the right eye correction (distance or near) as the stimulus is abruptly changed from the retinal conjugate plane of one eye to that of the other eye. However, the accommodative responses in some early presbyopes were always dominated by the distance corrected eye, the near corrected eye, or by convergence. When the stimulus approached, the near corrected eye experienced high image quality only if there was no accommodative response. However, reduced image quality was observed if an accommodative response was initiated. Neither accommodation nor pupil response latencies were longer with monovision corrections compared with bilateral distance corrections (p > 0.05). In the early presbyopes, spherical aberration was reduced during near viewing, but primarily due to pupil miosis and not lens shape changes.

CONCLUSION

As the stimulus was abruptly changed from the retinal conjugate plane of the distance corrected eye to that of the near corrected eye, most early presbyopes fit with monovision accommodated, which resulted in a decline, not an increase in image quality in the near corrected eye. These results reveal a non-optimal accommodative strategy in early presbyopes fit with monovision.

Stimulus Unpredictability in Time, Magnitude, and Direction on Accommodation

SIGNIFICANCE

The effect of predictability in changes of time, magnitude, and direction of the accommodation demand on the accommodation response latency and its magnitude are insignificant, which suggests that repetitive accommodative tasks such as the clinical accommodative facility test may not be influenced by potential anticipation effects.

PURPOSE

The purpose of this study was to investigate the effect of stimulus' time, magnitude, and direction predictability, as well as their interactions, on accommodation latency and response magnitude.

METHODS

Monocular accommodative response and latency were measured in 12 young subjects for nine different conditions where the stimulus accommodative demand changed several times in a steplike fashion for a period of 120 seconds. Each change in accommodative demand could have different time duration (i.e., 1, 2, or 3 seconds), magnitude (1, 2, or 3 diopters), and/or direction (i.e., accommodation or disaccommodation). All conditions were created permuting the factors of time, magnitude, and direction with two levels each: random and not random. The baseline condition was a step signal from 0 to 2 diopters persisting for 2 seconds in both accommodative demands. After each condition, subjects were asked to provide a score from 1 to 5 in their perceived predictability.

RESULTS

Friedman test conducted on the perceived predictability of each condition resulted in statistically significant differences between the nine conditions (χ = 56.57, P < .01). However, repeated-measures analysis of variance applied to latency and accommodative response magnitude did not show significant differences (P > .05). In addition, no correlation was found between the perceived predictability scores and both latency and accommodative response magnitudes between the most predictable and the most unpredictable conditions.

CONCLUSIONS

Subjects were able to perceptually notice whether the stimulus was predictable or not, although our results indicate no significant effect of stimuli predictability on either the accommodation latency or its magnitude.

Validation of Novel Metrics from the Accommodative Dynamic Profile

Objective and subjective methods of assessing time taken for accommodative change (ToAC) include accommodative dynamics (AD) and accommodative facility (AF). This study investigates the validity of novel metrics derived from the AD-profile and explores their relationship with AF. AD were assessed using a modified open-field autorefractor in 43 healthy adults. Non-linear regression curves were fitted to the data to derive: latency-of-accommodation (nLoA) and -disaccomodation (nLoD), Time-for-accommodation (ToA) and -disaccommodation (ToD), and objective-ToAC (oToAC). Latencies were also calculated through visual inspection of the AD data as in previous studies (pLoA and pLoD). AF was used to assess subjective-ToAC. Statistical analysis explored the relationships between the AD-metrics and AF. Subjects were assessed on three visits to examine intra- and inter-observer repeatability. nLoA and nLoD were greater than pLoA (p = 0.001) and pLoD (p = 0.004) respectively. nLoA and nLoD also demonstrated greater intra- and inter-observer repeatability than pLoA and pLoD. AF demonstrated a moderate, inverse correlation with ToA (p = 0.02), ToD (p = 0.007), and oToAC (p = 0.007). ToD was the single best accommodative predictor of AF (p = 0.011). The novel method for deriving latency was more repeatable, but not interchangeable with the techniques used in previous studies. ToD was the most repeatable metric with the greatest association with AF.

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.

Wide Field Of View Varifocal Near-Eye Display Using See-Through Deformable Membrane Mirrors

Accommodative depth cues, a wide field of view, and ever-higher resolutions all present major hardware design challenges for near-eye displays. Optimizing a design to overcome one of these challenges typically leads to a trade-off in the others. We tackle this problem by introducing an all-in-one solution - a new wide field of view, gaze-tracked near-eye display for augmented reality applications. The key component of our solution is the use of a single see-through, varifocal deformable membrane mirror for each eye reflecting a display. They are controlled by airtight cavities and change the effective focal power to present a virtual image at a target depth plane which is determined by the gaze tracker. The benefits of using the membranes include wide field of view (100° diagonal) and fast depth switching (from 20 cm to infinity within 300 ms). Our subjective experiment verifies the prototype and demonstrates its potential benefits for near-eye see-through displays.

Topiramate and accommodation: Does topiramate cause accommodative dysfunction?

OBJECTIVE

To investigate the accommodation function in topiramate users.

DESIGN

Case-control clinical study.

PARTICIPANTS

The participants included 16 controls and 22 patients using 100 mg/kg topiramate who were diagnosed with migraine according to the International Classification of Headache Disorders, second edition criteria.

METHODS

One-minute dynamic measurements of refraction with accommodation stimuli of 0 D, 2 D, 2.5 D, 3 D, 4 D, and 5 D were obtained using the open field refractometer WAM-5500 in.

RESULTS

In most of the accommodation stimuli ranges (0 D, 2.5 D, 3 D, and 5 D), topiramate users had a significantly higher accommodative lag compared with controls (p = 0.028, p = 0.014, p = 0.011, and p = 0.011, respectively). The most important causes of accommodative lag were found to be accommodation stimulus and inclusion in the topiramate group (p < 0.001, R² = 0.32, 95% CI 0.22-0.37 and 0.42-0.91, respectively). Multivariate linear regression analysis revealed that the 2 most important predictors of accommodative lag were accommodation stimulus and age (p < 0.001, r = 0.51, 95% CI 0.31-0.32 and 0.67-0.69, respectively) CONCLUSIONS: Even after adjustment for age, accommodative lag is greater across several accommodative stimulus levels in patients using topiramate, which may be related to visual symptoms in topiramate users.

The influence of age on adaptation of disparity vergence and phoria

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20-70years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5min of sustained fixation on a binocular target (40cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r=-0.33; p=0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p>0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

Effect of defocus on response time in different age groups: A pilot study

PURPOSE

To assess the response time associated with visual performance (VP) tasks in the presence of defocus in different presbyopic populations.

METHODS

58 eyes between the ages of 35 and 50 years were studied. Subjects were categorized as pre-presbyopic (35-39 years), early-presbyopic (40-45 years), and mid-presbyopic (46-50 years). VP measurements obtained monocularly included distance and near high contrast (HC) and low contrast (LC) optotype recognition, and contrast threshold at 12cpd for different defocus magnitudes between 0D and 3D in 1D steps. Response time defined as the time taken to recognize and verbalize an optotype, was compared among different presbyopic age groups.

RESULTS

From 58 eyes, mean (SD) response time for high contrast distance visual acuity for 0D through 3D ranged between 1.48 (0.23) and 1.87 (0.31)s, whereas low contrast distance visual acuity ranged between 1.5 (0.22) and 2.09 (0.49)s. Mean response time for high contrast near visual acuity for 0D through 3D ranged between 1.56 (0.19) and 2.23 (0.45)s. However, for low contrast near visual acuity it ranged between 1.75 (0.32) and 2.71 (0.94)s. Mean (SD) response time for 12cpd ranged between 2.11 (0.50) and 5.72 (1.09)s. ANOVA revealed a significant difference in response time for distance, near visual acuity and contrast sensitivity as a function of defocus for different age groups.

CONCLUSIONS

Response time is increased in the presence of increasing defocus for both distance and near visual acuity and could impact on performance for critical tasks. Full correction of visual acuity at distance and near in presbyopes is warranted always.

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.

Defocus blur discrimination in natural images with natural optics

The lens system in the human eye is able to best focus light from only one distance at a time.Therefore, many objects in the natural environment are not imaged sharply on the retina. Furthermore, light from objects in the environment is subject to the particular aberrations of the observer's lens system (e.g., astigmatism and chromatic aberration). We refer to blur created by the observer's optics as "natural" or "defocus" blur as opposed to "on-screen" blur created by software on a display screen. Although blur discrimination has been studied extensively, human ability to discriminate defocus blur in images of natural scenes has not been systematically investigated. Here, we measured discrimination of defocus blur for a collection of natural image patches, sampled from well-focused photographs. We constructed a rig capable of presenting stimuli at three physical distances simultaneously. In Experiment 1, subjects viewed monocularly two simultaneously presented natural image patches through a 4-mm artificial pupil at ±1° eccentricity. The task was to identify the sharper patch. Discrimination thresholds varied substantially between stimuli but were correlated between subjects. The lowest thresholds were at or below the lowest thresholds ever reported. In a second experiment, we paralyzed accommodation and retested a subset of conditions from Experiment 1. A third experiment showed that removing contrast as a cue to defocus blur had only a modest effect on thresholds. Finally, we describe a simple masking model and evaluate how well it can explain our experimental results and the results from previous blur discrimination experiments.

Effect of the accommodation-vergence conflict on vergence eye movements

With the broader use of stereoscopic displays, a flurry of research activity about the accommodation-vergence conflict has emerged to highlight the implications for the human visual system. In stereoscopic displays, the introduction of binocular disparities requires the eyes to make vergence movements. In this study, we examined vergence dynamics with regard to the conflict between the stimulus-to-accommodation and the stimulus-to-vergence. In a first experiment, we evaluated the immediate effect of the conflict on vergence responses by presenting stimuli with conflicting disparity and focus on a stereoscopic display (i.e. increasing the stereoscopic demand) or by presenting stimuli with matched disparity and focus using an arrangement of displays and a beam splitter (i.e. focus and disparity specifying the same locations). We found that the dynamics of vergence responses were slower overall in the first case due to the conflict between accommodation and vergence. In a second experiment, we examined the effect of a prolonged exposure to the accommodation-vergence conflict on vergence responses, in which participants judged whether an oscillating depth pattern was in front or behind the fixation plane. An increase in peak velocity was observed, thereby suggesting that the vergence system has adapted to the stereoscopic demand. A slight increase in vergence latency was also observed, thus indicating a small decline of vergence performance. These findings offer a better understanding and document how the vergence system behaves in stereoscopic displays. We describe what stimuli in stereo-movies might produce these oculomotor effects, and discuss potential applications perspectives.

Accommodative changes after photorefractive keratectomy in myopic eyes

PURPOSE

To determine the changes of amplitude of accommodation (AA) and facility of accommodation (FA) in myopic patients after photorefractive keratectomy (PRK).

METHODS

Using Technolas 217Z excimer laser, 160 myopic eyes of 80 patients underwent PRK. The patients were categorized into two age groups: <30 and ≥30 years. Changes in AA and FA were observed before PRK and at 2 weeks, 1 month, and 3 months after PRK. The role of preoperative AA, FA, refractive spherical equivalent, age, and sex on postoperative AA and FA was evaluated.

RESULTS

In younger patients, the preoperative AA and FA values 7.77 ± 1.75 D and 7.75 ± 3.97 cpm changed to 8.36 ± 1.26 D and 11.57 ± 4.20 cpm (p < 0.001), respectively, at 3 months after PRK. In older patients, the preoperative AA and FA values 6.66 ± 1.41 D and 5.05 ± 3.26 cpm changed to 6.72 ± 1.26 D (p = 1.000) and 9.58 ± 4.29 cpm (p < 0.001), respectively. Two weeks after surgery, preoperative AA and spherical equivalent had a significant effect on postoperative AA, whereas preoperative AA and age had a significant effect on postoperative AA after 3 months (p < 0.001). Postoperative FA was positively related to preoperative FA and female sex (p < 0.05).

CONCLUSIONS

This study suggests that some of the near-vision problems in younger myopes in early postoperative days after PRK might be due to decrease in AA and FA, which will eventually increase. However, in older patients, despite increase in FA, AA did not change.

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.

Short-term adaptive modification of dynamic ocular accommodation

PURPOSE

Indirect observations suggest that the neural control of accommodation may undergo adaptive recalibration in response to age-related biomechanical changes in the accommodative system. However, there has been no direct demonstration of such an adaptive capability. This investigation was conducted to demonstrate short-term adaptation of accommodative step response dynamics to optically induced changes in neuromuscular demands.

METHODS

Repetitive changes in accommodative effort were induced in 15 subjects (18-34 years) with a double-step adaptation paradigm wherein an initial 2-D step change in blur was followed 350 ms later by either a 2-D step increase in blur (increasing-step paradigm) or a 1.75-D step decrease in blur (decreasing-step paradigm). Peak velocity, peak acceleration, and latency of 2-D single-step test responses were assessed before and after 1.5 hours of training with these paradigms.

RESULTS

Peak velocity and peak acceleration of 2-D step responses increased after adaptation to the increasing-step paradigm (9/12 subjects), and they decreased after adaptation to the decreasing-step paradigm (4/9 subjects). Adaptive changes in peak velocity and peak acceleration generalized to responses that were smaller (1 D) and larger (3 D) than the 2-D adaptation stimulus. The magnitude of adaptation correlated poorly with the subject's age, but it was significantly negatively correlated with the preadaptation dynamics. Response latency decreased after adaptation, irrespective of the direction of adaptation.

CONCLUSIONS

Short-term adaptive changes in accommodative step response dynamics could be induced, at least in some of our subjects between 18 and 34 years, with a directional bias toward increasing rather than decreasing the dynamics.

Edinger--Westphal stimulated accommodative dynamics in anesthetized, middle-aged rhesus monkeys

The relationships between peak velocity and amplitude of Edinger-Westphal (EW) stimulated accommodation and disaccommodation were investigated in anesthetized, middle-aged rhesus monkeys. Accommodative responses were recorded at 30Hz with infrared photorefraction. Peak velocity of accommodation and disaccommodation increased linearly with stimulus amplitude. Peak velocities of accommodation continued to increase with stimulus amplitudes greater than required to produce the maximum response. The peak velocity of disaccommodation did not further increase with supramaximal stimulus amplitudes beyond that achieved with maximal stimulus amplitudes. Although maximum accommodative response amplitude is reduced in older rhesus monkeys, within the methodological constraints of this study, older monkeys appear to achieve accommodative and disaccommodative peak velocities similar to adolescent monkeys for the same response amplitudes.

Accommodation during Static Retinal Imaging with a Confocal Scanning Laser Ophthalmoscope

PURPOSE

Accommodation of the eye can adversely affect optical tomography of the retina using a confocal scanning laser ophthalmoscope (cSLO). This study measured the dynamic accommodative response of young adult eyes while acquiring static retinal images with a cSLO without cycloplegia and also examined whether a scanned fixation target improves stability of accommodation.

DESIGN

Experimental study.

PARTICIPANTS

Four eyes of 4 controls whose ages ranged from 24 to 35 years.

METHODS

Dynamic accommodation was measured by an infrared autorefractor during static cSLO imaging with a fixation target, without a target, while in darkness, and while viewing a Snellen chart at 20-feet distance.

MAIN OUTCOME MEASURES

Mean accommodation across viewing conditions and rate of change of accommodation (velocity).

RESULTS

Accommodation significantly increased in amplitude and in variability during imaging with the cSLO. Average peak velocity of accommodation was 1.37 diopters (D)/second, with a maximum of 2.27 D/second. Presenting a fixation target in the cSLO did not decrease accommodation but increased accommodation velocity in 3 of 4 subjects.

CONCLUSION

Accommodation dynamically changes during acquisition of retinal images with a cSLO. Concurrent scanning of a visible target did not improve stability of accommodation reliably. Potential factors affecting accommodation include depth of focus (DOF), use of monochromatic stimuli, and nearby cues. Using the model eye, the error induced by accommodation is about 491 mum/second, with a maximum of 813 mum/second. In terms of the time to acquire individual optical sections, these estimates are consistent with previous reports of increased variability of topographic measurements in nondilated eyes.

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.

A pulse-step model of accommodation dynamics

Abrupt step changes in human ocular accommodation have been traditionally modeled using a continuous feedback control system supplied by a step-position control signal. However, recent behavioral data show that, while the velocity of the step response increases proportionally with response magnitude, the peak acceleration remains constant. This argues against a step input control signal and suggests the existence of a dual-mode control of accommodation: an initial fixed innervation component related to the constant acceleration followed by an innervation component that increases with response amplitude. Specifically, we proposed a pulse-step that provides a velocity-coded input to the system that is integrated to form two position-input signals, that when combined produce high velocity responses. The pulse height controls the acceleration; the pulse width controls the velocity and the step height controls the position of the accommodation response. The pulse-step model simulations were similar to empirical observations and illustrated an enhancement of the peak velocity of accommodation when compared to when the pulse component was removed from the model. The main functional advantage of the pulse is to overcome the high viscosity of the crystalline lens and achieve rapid step responses.

Sensitivity study of human crystalline lens accommodation

A nonlinear axisymmetric finite element method (FEM) analysis was employed to determine the critical geometric and material properties that affect human accommodation. In this model, commencing at zero, zonular traction on all lens profiles resulted in central lenticular surface steepening and peripheral surface flattening, with a simultaneous increase in central lens thickness and central optical power. An age-related decline in maximum zonular tension appears to be the most likely etiology for the decrease in accommodative amplitude with age.

Dynamic performance of accommodating intraocular lenses in a negative feedback control system: a simulation-based study

A dynamic model of ocular accommodation is used to simulate the stability and dynamic performance of accommodating intraocular lenses (A-IOLs) that replace the hardened natural ocular lens that is unable to change focus. Accommodation simulations of an older eye with A-IOL materials having biomechanical properties of a younger eye illustrate overshoots and oscillations resulting from decreased visco-elasticity of the A-IOL. Stable dynamics of an A-IOL are restored by adaptation of phasic and tonic neural-control properties of accommodation. Simulations indicate that neural control must be recalibrated to avoid unstable dynamic accommodation with A-IOLs. An interactive web-model of A-IOL illustrating these properties is available at http://schorlab.berkeley.edu.

Refractive group differences in accommodation microfluctuations with changing accommodation stimulus

PURPOSE

Microfluctuations of accommodation are known to increase in magnitude with increasing accommodation stimulus. Reduced sensitivity to blur in myopic subjects could also lead to increases in the magnitude of the microfluctuations. The aim of this study is to examine the effect of variations in accommodation stimulus upon the microfluctuations in different refractive groups.

METHODS

Thirty-six subjects were divided into three groups depending upon their refractive error and age of onset of their myopia; 12 emmetropes (EMMs), 12 early onset myopes (EOMs) and 12 late-onset myopes (LOMs). Steady-state accommodation responses were recorded continuously for 2 min using the Shin-Nippon SRW-5000 autorefractor at a sampling rate of 52 Hz while viewing targets at accommodation stimuli levels of 0, 1, 2, 3 and 4 D in a Badal (+5 D) optical system.

RESULTS

The EMMs and EOMs showed systematic increases in the root mean square (r.m.s.) value of the microfluctuations with increasing accommodation stimulus. In contrast, no systematic variation with accommodation stimulus was found for the LOMs. Power spectrum analysis demonstrated that increases in the size of the microfluctuations were mediated by increases in the power of the low frequency components of the accommodation response.

CONCLUSIONS

The magnitude of the microfluctuations in the EMMs and EOMs may be influenced primarily by accommodation response-induced zonular relaxation effects or to changes in the physical properties of the accommodation plant with increasing accommodation response. The LOMs may have an increased baseline neural blur threshold, which appears to modulate the magnitude of the accommodative microfluctuations for low accommodation levels. At higher accommodation demands, the changes in the physical properties of the accommodation plant or the zonular relaxation effects appear to exceed the blur threshold, and the known association between microfluctuations and accommodation stimulus level is restored.

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.

Age related changes in accommodative dynamics in humans

Age related changes in the dynamics of accommodation (far to near focus) and disaccommodation (near to far focus) are reported in this study. Dynamic responses to step stimulus demands from 1D to 6D, in 1D steps, were recorded with a PowerRefractor in 66 subjects in the age range 14-45 years. The accommodative and disaccommodative responses were fit with exponential functions to calculate response amplitude, time constant and peak velocity. The latency of accommodation did not change and the latency of disaccommodation increased with age. For accommodation, time constant increased and peak velocity decreased with age. For disaccommodation, no change in time constant or peak velocity was found with age. The form of the peak velocity vs response amplitude relationship (main sequence) of accommodation changed with age. The differences in the dynamics of accommodation and disaccommodation with age are discussed with reference to the age related changes in the eye leading to presbyopia.

The effect of monocular and binocular viewing on the accommodation response to real targets in emmetropia and myopia

PURPOSE

Decreased blur-sensitivity found in myopia has been linked with reduced accommodation responses and myopigenesis. Although the mechanism for myopia progression remains unclear, it is commonly known that myopic patients rarely report near visual symptoms and are generally very sensitive to small changes in their distance prescription. This experiment investigated the effect of monocular and binocular viewing on static and dynamic accommodation in emmetropes and myopes for real targets to monitor whether inaccuracies in the myopic accommodation response are maintained when a full set of visual cues, including size and disparity, is available.

METHODS

Monocular and binocular steady-state accommodation responses were measured with a Canon R1 autorefractor for target vergences ranging from 0-5 D in emmetropes (EMM), late-onset myopes (LOM), and early-onset myopes (EOM). Dynamic closed-loop accommodation responses for a stationary target at 0.25 m and step stimuli of two different magnitudes were recorded for both monocular and binocular viewing.

RESULTS

All refractive groups showed similar accommodation stimulus response curves consistent with previously published data. Viewing a stationary near target monocularly, LOMs demonstrated slightly larger accommodation microfluctuations compared with EMMs and EOMs; however, this difference was absent under binocular viewing conditions. Dynamic accommodation step responses revealed significantly (p < 0.05) longer response times for the myopic subject groups for a number of step stimuli. No significant difference in either reaction time or the number of correct responses for a given number of step-vergence changes was found between the myopic groups and EMMs.

CONCLUSION

When viewing real targets with size and disparity cues available, no significant differences in the accuracy of static and dynamic accommodation responses were found among EMM, EOM, and LOM. The results suggest that corrected myopes do not experience dioptric blur levels that are substantially different from emmetropes when they view free space targets.

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.

A pulse-step model of accommodation dynamics in the aging eye

We have developed a dynamic model of accommodation that combines independent phasic-velocity and tonic-position neural signals to control position, velocity and acceleration properties of accommodative step responses. Phasic and tonic signals were obtained from neural integration of a fixed-height acceleration-pulse and variable-height velocity-step respectively to control independent acceleration and velocity properties of the step response. Duration and amplitude of the acceleration-pulse are increased with age to compensate for age-related increases of visco-elastic properties of the lens to maintain youthful velocity. The model illustrates a neural control strategy that is similar to the classical neural control model of step changes by the saccadic and vergence systems.

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, G0, and the cut-off frequency, fc, at which the gain fell to zero. G0 reduced with age but fc 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 G0. The linear changes in phase lag with temporal frequency corresponded to simple time delays td. 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 G0 also tended to have low values of td, 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 fc with age is the result of the ciliary body maintaining its efficiency, whereas the fall in G0 and increase in td result from increases in lens rigidity.

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.

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.

Accommodation dynamics as a function of age

Experiments are described in which the dynamic accommodation response to a stimulus whose vergence was varying sinusoidally with time between 1.33 and 2.38 D was measured as a function of frequency for 19 subjects, with ages distributed between 18 and 49 years. Response to abrupt stimulus change between the same levels was also measured. The results showed that at any age, for the sinusoidal stimuli, gain fell with frequency and phase lag increased: at fixed frequency, gain fell with age and phase lag increased. Neither reaction nor response times for step stimuli changed with age. Analysis of the sinusoidal data suggests the possible existence of a cut-off frequency of about 2 Hz, which varies little with age and above which the system cannot respond. The phase data is compatible with the existence of a frequency-independent time delay, which increases with age from about 0.17 s at 20 years to 0.48 s at 40 years. The results are discussed in terms of current knowledge of the factors contributing to the development of presbyopia.

Dynamic accommodation in rhesus monkeys

The dynamics of Edinger-Westphal (EW) stimulated accommodation were studied in two young rhesus monkeys to understand the relationships between accommodative amplitude and rates of accommodation and disaccommodation. Accommodative responses were recorded with infrared photorefraction at five different amplitudes spanning the full EW stimulated accommodative range available to each eye. Combined exponential and polynomial functions were fit to the accommodation and disaccommodation responses. Derivatives of these functions provided the maximum rates of accommodation and disaccommodation as well as time constants for each amplitude. Maximum rates of EW stimulated accommodation and disaccommodation were found to increase linearly with amplitudes from 0.58 to 17.41 D in the two monkeys. The results suggests that the rate of EW stimulated accommodation is dictated by the amplitude. We conclude that if dynamic accommodative responses are to be compared in monkeys of different ages it is necessary to compare responses for the same accommodative amplitudes in order to draw conclusions about age related changes.

Age changes in the interactions between the accommodation and vergence systems

Experiments are described in which static and dynamic accommodation (Ar), accommodative convergence (AC), vergence (C) and convergence accommodation (CA) responses to small stimuli were measured dynamically in 13 subjects with ages in the range 16 to 48 years. Analysis showed that the amplitudes of both blur and disparity-driven accommodation declined significantly with age, whereas the two types of vergence response did not. As a result, the AC/Ar ratio rose significantly with age, whereas the CA/C ratio fell. No significant change with age was found in response latencies and durations.