Age changes in the interactions between the accommodation and vergence systems

Convergence insufficiency: Review of clinical diagnostic signs

Convergence insufficiency (CI) is a common binocular vision (BV) disorder characterized by difficulty in maintaining motor fusion at near, which affects approximately 7.5 percent of the population. Diagnostic criteria for the disorder are inconsistent, ranging from one to many clinical signs. Methodology for clinical tests is inconsistent in measurement technique, visual targets, required repetitions, and normative values. This manuscript demonstrates the inconsistencies amongst published studies, and highlights the importance of consistent clinical diagnostic signs, measurement techniques, visual targets, and cut-off criteria. For each clinical sign, the recommended methodology for the procedure is described. Several studies do not take age into account when diagnosing CI in their cohorts. As such, the review emphasizes changes in diagnostic signs with age. This manuscript highlights the need for consistent and clear procedures and diagnostic criteria amongst clinicians and provides the basis for future studies in terms of diagnostic testing required for CI of varying age groups.

Preoperative binocular vision characteristics in the age-related cataract population

Background

This study is the first part of the “Binocular Vision Anomalies after Cataract Surgery” study that aimed to investigate the impact of cataract surgery on binocular vision status in adults with age-related cataract. This study aimed to investigate the preoperative binocular vision status of participants with age-related cataract.

Methods

Patients who elected to undergo bilateral cataract surgery (≥50 years of age) were recruited. Clinical measures of binocular vision including stereopsis, ocular alignment, fusional vergence, vergence facility, convergence amplitude and a symptom survey related to binocular vision anomalies were administered. A detailed classification protocol was established to identify the presence of binocular vision anomalies. The frequency of specific binocular vision anomalies and normative data of binocular vision measures were reported.

Results

A total of 73 subjects were evaluated. No strabismus was detected in the cohort. Non-strabismic binocular vision anomalies were detected in 24 subjects (32.9%), of whom 18 (24.7%) had convergence insufficiency, 3 (4.1%) had basic exophoria, 2 (2.7%) had convergence excess, and 1 (1.4%) had fusional vergence dysfunction. Decreased vergence facility and convergence amplitude were more common compared to the pre-presbyopes (P < 0.01).

Conclusion

Binocular vision problems, especially convergence insufficiency, are common in the adults with age-related cataract. The study results demonstrate that the lack of normative binocular vision data for the presbyopic population is a significant gap in the literature and suggest the need for a study of normative data for this population.

Trial registration

The study was registered at ClinicalTrials.gov (NCT03592615, USA).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-022-02418-7.

Visual outcomes in hyperopic myopic and emmetropic patients with customized aspheric ablation (Q factor) and micro-monovision

PURPOSE

To evaluate the visual outcomes, visual performance, and stereoacuity in presbyopic patients following treatment by a change in the corneal asphericity and micro-monovision.

METHODS

Presbyopic patients with diverse refractive errors and emmetropes (n = 30 eyes) were treated with a custom Q-ablation profile and micro-monovision in the non-dominant eye. There with a difference of Q - 0.30 in the Q profiles between dominant and non-dominant eyes. Patients were assigned in two groups based on the preoperative spherical equivalent (Group 1  + 4.00 to + 0.50, and group 2 neutral to - 3.00). Binocular uncorrected distance visual acuity (binocular UCVA), best-corrected visual acuity (BCVA), binocular uncorrected near visual acuity (binocular UNVA) preoperative and postoperative, spherical equivalent refraction, contrast sensitivity, and stereopsis were analyzed at 1, 3, and 6 months.

RESULTS

The mean age was 52.6 ± 5.1 (SD) years. At six months post-operation, the mean binocular uncorrected distance visual acuity (binocular UDVA) was 0.15 ± 0.04 logMAR (20/25-) in group 1, and 0.11 ± 0.05 logMAR (20/25) in group 2, and binocular uncorrected near vision UNVA was 0.5 ± 0.1 M (20/25 J2) in group 1 and 0.45 ± 0.2 M (20/25 J2) group 2. An increase in stereoacuity was found in both groups.

CONCLUSIONS

The correction of refractive defects using customized corneal asphericity was an effective treatment in presbyopic patients. Furthermore, the treatment was well tolerated in this group of patients. Following surgery, the quality of vision was adequate, and the stereovision improved in this cohort of patients.

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.

The Effect of Age on Binocular Vision Normative Values

PURPOSE

To establish a relationship between age and horizontal heterophoria, horizontal fusional vergence amplitudes, and vergence facility testing.

METHODS

The sample consisted of 112 patients with a mean age of 39.8 ± 14.97 years (range: 18 to 65 years) and was composed of 61 women (54.5%) and 51 men (45.5%). The non-presbyopic group included patients 18 to 39 years old (n = 49) and the presbyopic group included patients 41 to 65 years old (n = 63). Binocular vision was studied by heterophoria horizontal magnitude (prism diopters [PD]), horizontal fusional vergences amplitudes (PD), and vergence facility testing (cycles per minute [cpm]) and quantified with a combination of 3 PD base-in and 12 PD base-out prisms.

RESULTS

Significant differences were obtained in near heterophoria with compensation (exophoria increased by 3.74 PD, t = 2.12, P < .05), distance positive fusional vergence (PFV) recovery (decreased by 2.86 PD, t = 3.03, P < .01), near PFV blur (decreased by 3.13 PD, t = 1.98, P = .05), near PFV break (decreased by 4.45 PD, t = 2.75, P < .01), near PFV recovery (decreased by 4.69 PD, t = 3.30, P < .01), and vergence facility testing (decreased by 2.63 PD, t = 2.77, P < .01).

CONCLUSIONS

The results indicated an increase of exophoria, a decrease in near positive horizontal fusional vergences, and vergence facility was dependent on age; thus, the authors suggest that changes in the normal values should be considered for each age range. [J Pediatr Ophthalmol Strabismus. 2020;57(6):363-371.].

The Effect of Focal Distance, Age, and Brightness on Near-Field Augmented Reality Depth Matching

Many augmented reality (AR) applications operate within near-field reaching distances, and require matching the depth of a virtual object with a real object. The accuracy of this matching was measured in three experiments, which examined the effect of focal distance, age, and brightness, within distances of 33.3 to 50 cm, using a custom-built AR haploscope. Experiment I examined the effect of focal demand, at the levels of collimated (infinite focal distance), consistent with other depth cues, and at the midpoint of reaching distance. Observers were too young to exhibit age-related reductions in accommodative ability. The depth matches of collimated targets were increasingly overestimated with increasing distance, consistent targets were slightly underestimated, and midpoint targets were accurately estimated. Experiment II replicated Experiment I, with older observers. Results were similar to Experiment I. Experiment III replicated Experiment I with dimmer targets, using young observers. Results were again consistent with Experiment I, except that both consistent and midpoint targets were accurately estimated. In all cases, collimated results were explained by a model, where the collimation biases the eyes' vergence angle outwards by a constant amount. Focal demand and brightness affect near-field AR depth matching, while age-related reductions in accommodative ability have no effect.

Objective Evaluation of Visual Fatigue Using Binocular Fusion Maintenance

Purpose

In this study, we investigated whether an individual's visual fatigue can be evaluated objectively and quantitatively from their ability to maintain binocular fusion.

Methods

Binocular fusion maintenance (BFM) was measured using a custom-made binocular open-view Shack-Hartmann wavefront aberrometer equipped with liquid crystal shutters, wherein eye movements and wavefront aberrations were measured simultaneously. Transmittance in the liquid crystal shutter in front of the subject's nondominant eye was reduced linearly, and BFM was determined from the transmittance at the point when binocular fusion was broken and vergence eye movement was induced. In total, 40 healthy subjects underwent the BFM test and completed a questionnaire regarding subjective symptoms before and after a visual task lasting 30 minutes.

Results

BFM was significantly reduced after the visual task (P < 0.001) and was negatively correlated with the total subjective eye symptom score (adjusted R² = 0.752, P < 0.001). Furthermore, the diagnostic accuracy for visual fatigue was significantly higher in BFM than in the conventional test results (aggregated fusional vergence range, near point of convergence, and the high-frequency component of accommodative microfluctuations; P = 0.007).

Conclusions

These results suggest that BFM can be used as an indicator for evaluating visual fatigue.

Translational Relevance

BFM can be used to evaluate the visual fatigue caused by the new visual devices, such as head-mount display, objectively.

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.

Self-motion perception in the elderly

Self-motion through space generates a visual pattern called optic flow. It can be used to determine one's direction of self-motion (heading). Previous studies have already shown that this perceptual ability, which is of critical importance during everyday life, changes with age. In most of these studies subjects were asked to judge whether they appeared to be heading to the left or right of a target. Thresholds were found to increase continuously with age. In our current study, we were interested in absolute rather than relative heading judgments and in the question about a potential neural correlate of an age-related deterioration of heading perception. Two groups, older test subjects and younger controls, were shown optic flow stimuli in a virtual-reality setup. Visual stimuli simulated self-motion through a 3-D cloud of dots and subjects had to indicate their perceived heading direction after each trial. In different subsets of experiments we varied individually relevant stimulus parameters: presentation time, number of dots in the display, stereoscopic vs. non-stereoscopic stimulation, and motion coherence. We found decrements in heading performance with age for each stimulus parameter. In a final step we aimed to determine a putative neural basis of this behavioral decline. To this end we modified a neural network model which previously has proven to be capable of reproduce and predict certain aspects of heading perception. We show that the observed data can be modeled by implementing an age related neuronal cell loss in this neural network. We conclude that a continuous decline of certain aspects of motion perception, among them heading, might be based on an age-related progressive loss of groups of neurons being activated by visual motion.

Convergence accommodation to convergence CA/C ratio: convergence versus divergence

AIM

To determine whether the convergence accommodation to convergence (CA/C) ratio during divergence with base-in (BI) prisms is of a similar or different magnitude to that measured during convergence with base-out (BO) prisms.

METHODS

Eighteen participants with normal binocular single vision were recruited. The participants viewed a pseudo-Gaussian target, which consisted of a light emitting diode (LED) behind a diffusing screen at 40 cm. After 5 minutes of dark adaptation, the refractive status of the eye was measured without any prism using a Shin-Nippon SRW-5000 autorefractor. The participant held the selected prism (5Δ or 10Δ BO or BI, counterbalanced) in front of their right eye and obtained a single, fused image of the target while refractive measures were taken with each. A 30-second rest period was given between measurements. The mean age of the participants was 20.6±3.22 years.

RESULTS

The mean CA/C ratios for the 5Δ BO, 10Δ BO, 5Δ BI, and 10Δ BI were 0.108 (±0.074) D/Δ, 0.110 (±0.056) D/Δ, 0.100 (±0.090) D/Δ, and 0.089 (±0.055) D/Δ, respectively. A 2-factor repeated measures ANOVA found that the CA/C ratio did not significantly change with differing levels of prism-induced convergence and divergence (p=0.649).

CONCLUSIONS

Change in accommodation induced by manipulating vergence is similar whether convergence or divergence are induced. The CA/C ratio did not show any change with differing levels of prism-induced convergence and divergence.

Spatial frequency selectivity of visual suppression during convergence eye movements

Visual suppression of low-spatial frequency information during eye movements is believed to contribute to a stable perception of our visual environment. While visual perception has been studied extensively during saccades, vergence has been somewhat neglected. Here, we show that convergence eye movements reduce contrast sensitivity to low spatial frequency information around the onset of the eye movements, but do not affect sensitivity to higher spatial frequencies. This suggests that visual suppression elicited by convergence eye movements may have the same temporal and spatial characteristics as saccadic suppression.

The changes in phoria and convergence to divergence peak velocity ratio are correlated

PURPOSE

Phoria is used in the diagnosis of binocular dysfunctions, such as convergence insufficiency. A common treatment for convergence insufficiency is vision therapy, yet it is controversial whether the phoria changes as a result of rehabilitation. Phoria can vary due to the duration and/or the type of prior visual tasks. The variability potentially observed within phoria measurements leads to difficulties in confidently assessing changes within longitudinal studies. Hence, we propose to measure phoria and vergence peak velocities on separate days to evaluate whether a more robust measurement can be attained.

METHODS

Eleven subjects with normal binocular vision participated in two identical experimental sessions. Four-degree convergence and divergence steps stimulated from targets at far, middle, and near initial vergence positions were recorded using a limbus tracking system. Near dissociated phoria was measured after a set of step stimuli. The vergence ratio was defined as the convergence peak velocity divided by the divergence peak velocity. Linear regression analyses calculated the correlation between the phoria and the vergence ratio, and the difference between phoria measurements and vergence ratio measurements, recorded on different days.

RESULTS

Near dissociated phoria measurements and vergence ratios were highly correlated for all three initial vergence positions (r > 0.85, p < 0.005). The change in phoria was significantly correlated to the change in vergence ratio (r > 0.94, p = 0.0001). The slope of the linear regression analysis between the phoria and the vergence ratio using individual subject data was repeatable between the sessions (r = 0.99, p < 0.00001) despite large changes in phoria observed in a few individuals.

CONCLUSIONS

The change in phoria is significantly correlated to the change in vergence ratio. For longitudinal studies including the efficacy of vision therapies for patients with binocular dysfunctions, we suggest assessing both phoria and vergence velocities to reduce the variability potentially observed from different days.

Short-term adaptation of accommodation, accommodative vergence and disparity vergence facility

Previous studies have found that subjects can increase the velocity of accommodation using visual exercises such as pencil push ups, flippers, Brock strings and the like and myriad papers have shown improvement in accommodation facility (speed) and sufficiency (amplitude) using subjective tests following vision training but few have objectively measured accommodation before and after training in either normal subjects or in patients diagnosed with accommodative infacility (abnormally slow dynamics). Accommodation is driven either directly by blur or indirectly by way of neural crosslinks from the vergence system. Until now, no study has objectively measured both accommodation and accommodative-vergence before and after vision training and the role vergence might play in modifying the speed of accommodation. In the present study, accommodation and accommodative-vergence were measured with a Purkinje Eye Tracker/optometer before and after normal subjects trained in a flipper-like task in which the stimulus stepped between 0 and 2.5 diopters and back for over 200 cycles. Most subjects increased their speed of accommodation as well as their speed of accommodative vergence. Accommodative vergence led the accommodation response by approximately 77 ms before training and 100 ms after training and the vergence lead was most prominent in subjects with high accommodation and vergence velocities and the vergence leads tended to increase in conjunction with increases in accommodation velocity. We surmise that volitional vergence may help increase accommodation velocity by way of vergence-accommodation cross links.

Retinal image quality and postnatal visual experience during infancy

Studies of animal models have demonstrated that abnormal visual experience can lead to abnormal visual development. The provision of normal optical experience for human infants and children requires an understanding of their typical retinal image quality in the natural dynamic environment. The literature related to this topic is reviewed.

Accommodative and vergence responses to conflicting blur and disparity stimuli during development

Accommodative and vergence responses of the typically developing visual system are generated using a combination of cues, including retinal blur and disparity. The developmental importance of blur and disparity cues in generating these motor responses was assessed by placing the two cues in conflict with each other. Cue-conflicts were induced by placing either -2 D lenses or 2 MA base-out prisms before both eyes of 140 subjects (2.0 months to 40.8 years) while they watched a cartoon movie binocularly at 80 cm. The frequency and amplitude of accommodation to lenses and vergence to prisms increased with age (both p < 0.001), with the vergence response (mean +/- 1 SEM = 1.38 +/- 0.05 MA) being slightly larger than the accommodative response (1.18 +/- 0.04 D) at all ages (p = 0.007). The amplitude of these responses decreased with an increase in conflict stimuli (1 to 3 D or MA) (both p < 0.01). The coupled vergence response to -2 D lenses (0.31 +/- 0.06 MA) and coupled accommodative response to 2 MA base-out prisms (0.21 +/- 0.02 D) were significantly smaller than (both p < 0.001) and poorly correlated with the open-loop vergence (r = 0.12; p = 0.44) and open-loop accommodation (r = -0.08; p = 0.69), respectively. The typically developing visual system compensates for transiently induced conflicts between blur and disparity stimuli, without exhibiting a strong preference for either cue. The accuracy of this compensation decreases with an increase in amplitude of cue-conflict.

The influence of first near-spectacle reading correction on accommodation and its interaction with convergence

PURPOSE

Accommodation and convergence can adapt to blur and disparity stimuli and to age-related changes in accommodative amplitude. Does this ability decline with age? The authors investigated short-term adaptation to first near-spectacle reading correction on the accommodative-stimulus response (ASR) function, accommodative amplitude (AA), AC/A, and CA/C ratios in a pre-presbyopic and an incipient presbyopic population and determined whether changes in these functions recovered after discontinuation of the use of near spectacles.

METHODS

Thirty subjects with normal vision participated; their ages ranged from 21 to 30 years (n = 15) and 38 to 44 years (n = 15). Oculomotor functions were measured before and after single-vision reading spectacles were worn for near tasks over a 2-month period and then 2 months after the use of near spectacles was discontinued.

RESULTS

The slope of the ASR function and the AC/A and CA/C ratios did not change significantly after near spectacles were worn. There was a hyperopic shift of the ASR function that significantly reduced the near point of accommodation (NPA) and lowered the far-point refraction. These changes were age invariant and did not recover after 2 months of discontinuation of near spectacle wear.

CONCLUSIONS

These results imply that the NPA may be enhanced normally by tonic bias of accommodation that elevates the entire ASR function and produces myopic refraction bias. When this bias relaxes after reading spectacles are worn, there is a hyperopic shift of the refractive state and a reduction of the NPA, specified from optical infinity.

Accommodation and vergence latencies in human infants

PURPOSE

Achieving simultaneous single and clear visual experience during postnatal development depends on the temporal relationship between accommodation and vergence, in addition to their accuracies. This study was designed to examine one component of the dynamic relationship, the latencies of the responses.

METHODS

Infants and adults were tested in three conditions (i) binocular viewing of a target moving in depth at 5 cm/s (closed loop) (ii) monocular viewing of the same target (vergence open loop) (iii) binocular viewing of a low spatial frequency Difference of Gaussian target during a prism induced step change in retinal disparity (accommodation open loop).

RESULTS

There was a significant correlation between accommodation and vergence latencies in binocular conditions for infants from 7 to 23 weeks of age. Some of the infants, as young as 7 or 8 weeks, generated adult-like latencies of less than 0.5 s. Latencies in the vergence open loop and accommodation open loop conditions tended to be shorter for the stimulated system than the open loop system in both cases, and all latencies were typically less than 2 s across the infant age range.

CONCLUSIONS

Many infants between 7 and 23 weeks of age were able to generate accommodation and vergence responses with latencies of less than a second in full binocular closed loop conditions. The correlation between the latencies in the two systems suggests that they are limited by related factors from the earliest ages tested.

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.

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.

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.

Age-related changes of vergence under natural viewing conditions

Vergence eye movements were recorded with the scleral search-coil system in 32 healthy subjects (ages 19-73 years) to characterize the age-related effects on the dynamic parameters of vergence responses to step (transient components) and ramp or sinusoidal targets (sustained components) under natural viewing conditions. Transient vergence showed an age-related increase in latency and decreases in peak velocity and acceleration in the binocular stimulus condition but not in accommodative vergence. Sustained vergence showed no age-related effect in the binocular condition, but there was an age-related decrease in accommodative vergence steady-state velocity and an increase in latency. Age-related changes of the transient and sustained components were very similar to those reported for saccades and smooth pursuit; they thus might support a distinction between a sustained and transient vergence system. Furthermore, such age-related effects have to be taken into account when assessing eye movement disorders in neurodegenerative and cerebrovascular diseases.

Gain and movement time of convergence-accommodation in preschool children

BACKGROUND

Convergence-accommodation is the synkinetic change in accommodation driven by vergence. A few studies have investigated the static and dynamic properties of this cross-link in adults but little is known about convergence-accommodation in children. The purpose of this study was to develop a technique for measuring convergence-accommodation and to study its dynamics (gain and movement time) in a sample of pre-school children.

METHOD

Convergence-accommodation measures were examined on thiry-seven normal pre-school children (mean age = 4.0 +/- 1.31 yrs). Stimulus CA/C (sCA/C) ratios and movement time measures of convergence-accommodation were assessed using a photorefractor while subjects viewed a DOG target. Repeated measures were obtained on eight normal adults (mean age = 23 +/- 0.2 yrs).

RESULTS

The mean sCA/C ratios and movement times were not significantly different between adults and children (0.10 D/Delta [0.61 D/M.A.], 743 +/- 70 ms and 0.11 D/Delta [0.50 D/M.A.], 787 +/- 216 ms). Repeated measures on adults showed a non-significant mean difference of 0.001 D/Delta.

CONCLUSION

The results suggest that the possible differences in crystalline lens (plant) characteristics between children and adults do not appear to influence convergence-accommodation gain or duration.

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.

Subjective and objective measurement of human accommodative amplitude

PURPOSE

To assess objective and subjective methods to measure accommodation in a young human population.

SETTING

Research laboratory, University of Houston, College of Optometry, Houston, Texas, USA.

METHODS

Accommodation was measured in the right eye of 15 young subjects (9 women and 6 men) whose ages ranged from 23 to 28 years and 1 36-year-old subject. The mean age of all subjects was 26 years. Accommodation was stimulated and measured with 4 techniques. Two subjective measures (focometer and minus-lens procedures) were used. Accommodation was also stimulated with minus-lens-induced blur and with pilocarpine 6% and measured objectively with a Hartinger coincidence refractometer.

RESULTS

Accommodative amplitudes measured with the 2 subjective methods agreed with each other but differed from the objectively measured amplitudes. Objectively measured accommodative amplitudes were similar in all subjects, with a mean of about 7.0 diopters. Accommodation stimulated with pilocarpine reached a maximum 33 minutes after administration. Individuals with light irides showed a stronger accommodative response to pilocarpine than subjects with dark irides.

CONCLUSIONS

Subjective measures of accommodation tend to overestimate true accommodative amplitude. Methods exist to measure accommodation objectively. These include stimulating accommodation with trial lenses or pilocarpine 6% and measuring the accommodative response with an objective optometer such as a Hartinger coincidence refractometer. Objective measures of accommodation should be used to determine whether accommodation can be restored in presbyopes.

A longitudinal study of vergence adaptation in incipient presbyopia

BACKGROUND

To investigate vergence adaptation during the incipient phase of presbyopia, when the amplitude of accommodation approaches the level where the first reading addition is required. The study aimed to assess the ability of the vergence system to counteract changes in the component contributions to the overall vergence response with the decline in the amplitude of accommodation in presbyopia, although previous reports on the nature of changes in accommodative, tonic and proximal vergence are equivocal.

METHODS

Using a 'flashed' Maddox rod technique, an assessment of vergence adaptation to 6delta base-out and 6delta base-in prism was made for 28 subjects (aged 35-45 years at the commencement of the study). The measurements were taken four times over a 2-year period.

RESULTS

Using a repeated measures analysis of variance, the results show that with the decline in amplitude of accommodation, there is a statistically significant reduction in the magnitude of vergence adaptation to both base-out (p < 0.05) and base-in prism (p < 0.01).

CONCLUSIONS

This study shows that with ageing, there is a decrease in the ability of the slow vergence mechanism to overcome a change in fusional vergence demand and would suggest that either the fast component of fusional vergence must cope with any change in fusional vergence demand or that the sum of the accommodative, tonic and proximal vergence responses are virtually stable with age.

The use of video refraction to measure the dynamic properties of the near triad in observers of a 3-D display

OBJECTIVE

Recording of the dynamic behaviour of the near triad during viewing of a three-dimensional (3-D) display is attempted. This may then be used to identify the possible origin of asthenopia and other ocular problems which occur whilst viewing such displays.

METHODS

Subjects viewed a 3-D display through an image splitter. Video recordings of ocular responses (pupil diameter, vergence and accommodation) were made with a commercially available video refraction unit at a distance of 1.2 m from the subject. Continuous recordings were made whilst a step-wise disparity of up to 2.6 degrees was introduced between the targets presented to the two eyes, but the accommodative stimulus remained unchanged. Image analysis from the recordings was carried out on a personal computer.

RESULTS

Results showed various complex patterns of response, with the accommodation and vergence recorded reflecting the effect of stimulus conflict. Difficulty in fusional convergence was recorded when disparity was large, and the recordings reflected the subjects' comments.

CONCLUSION

The video refraction method is a useful tool for studying the dynamic behaviour of the near triad.