IN VIVO analysis of ocular morphological changes during phakic accommodation

The principal theme of this thesis is the in vivo examination of ocular morphological changes during phakic accommodation, with particular attention paid to the ciliary muscle and crystalline lens. The investigations detailed involved the application of high-resolution imaging techniques to facilitate the acquisition of new data to assist in the clarification of aspects of the accommodative system that were poorly understood. A clinical evaluation of the newly available Grand Seiko Auto Ref/ Keratometer WAM-5500 optometer was undertaken to assess its value in the field of accommodation research. The device was found to be accurate and repeatable compared to subjective refraction, and has the added advantage of allowing dynamic data collection at a frequency of around 5 Hz. All of the subsequent investigations applied the WAM-5500 for determination of refractive error and objective accommodative responses. Anterior segment optical coherence tomography (AS-OCT) based studies examined the morphology and contractile response of youthful and ageing ciliary muscle. Nasal versus temporal asymmetry was identified, with the temporal aspect being both thicker and demonstrating a greater contractile response. The ciliary muscle was longer in terms of both its anterior (r = 0.49, P <0.001) and overall length (r = 0.45, P = 0.02) characteristics, in myopes. The myopic ciliary muscle does not appear to be merely stretched during axial elongation, as no significant relationship between thickness and refractive error was identified. The main contractile responses observed were a thickening of the anterior region and a shortening of the muscle, particularly anteriorly. Similar patterns of response were observed in subjects aged up to 70 years, supporting a lensocentric theory of presbyopia development. Following the discovery of nasal/ temporal asymmetry in ciliary muscle morphology and response, an investigation was conducted to explore whether the regional variations in muscle contractility impacted on lens stability during accommodation. A bespoke programme was developed to analyse AS-OCT images and determine whether lens tilt and decentration varied between the relaxed and accommodated states. No significant accommodative difference in these parameters was identified, implying that any changes in lens stability with accommodation are very slight, as a possible consequence of vitreous support. Novel three-dimensional magnetic resonance imaging (MRI) and analysis techniques were used to investigate changes in lens morphology and ocular conformation during accommodation. An accommodative reduction in lens equatorial diameter provides further evidence to support the Helmholtzian mechanism of accommodation, whilst the observed increase in lens volume challenges the widespread assertion that this structure is incompressible due to its high water content. Wholeeye MRI indicated that the volume of the vitreous chamber remains constant during accommodation. No significant changes in ocular conformation were detected using MRI. The investigations detailed provide further insight into the mechanisms of accommodation and presbyopia, and represent a platform for future work in this field.

Aspects of tonic accommodation and tonic vergence

Under conditions of reduced visual stimulation, the systems of accommodation and vergence tend towards physiological resting states that are intermediate within their functional range. The terms tonic accommodation (TA) and tonic vergence (TV) are used in the study to describe these stimulus-free, intermediate adjustments and to represent the systems as being in a state of innervational tonicity. The literature relating to TA and TV and the various experiments of this thesis are reviewed. Methodology has been developed enabling the determination of TA and TV under conditions of total darknessl laser optometry for TA and ~ernier-alignment for TV. The thesis describes a series of experiments designed to investigate various aspects of TA and TV, and their role in ametropia, binocular vision and their adaptation to sustained visual tasks. Measurements of TA were also utilised to investigate the effect of various autonomic effector drugs on the ciliary muscle. The effects of ethanol on binocular function are shown to be directly proportional to the .initial level of TVJ which is itself unaffected. These results support the concept of TV as the reference point for normal vergence responses. The results of the pharmacological investigations indicate the presence of a small but significant, beta-receptor mediated inhibitory sympathetic input to the ciliary muscle, and that the wide distribution in TA is a consequence of inter-observer variations in parasympathetic, rather than sympathetic tone. Following interaction with visual tasks of t5mins duration, the levels of TA and TV are found to be biased in the direction of, and proportional to, the task position: except during near-task viewing where the task-to-TA stimulus-distance exceeds 1.5D (for TA) and 3.5deg (for TV). Under these conditions the expected level of bias is attenuated, Adaptive models are discussed, proposing TA and TV as the reference points of the accommodative and vergence system.

Synkinesis of accommodation and vergence during sustained rear vision

It is well established that a synkinetic relationship exists between the accommodation and vergence components of the oculomotor near response such that increased accommodation will initiate a vergence response (i.e. accommodative convergence) and conversely increased vergence will drive accommodation (i.e. convergent accommodation) . The synkinesis associated with sustained near-vision was examined in a student population consisting of emmetropes, late-onset myopes (LOMs) i.e. myopia onset at 15 years of age or later and early-onset myopes (EOMs) i.e. myopia onset prior to 15 years of age. Oculomotor synkinesis was investigated both under closed-loop conditions and with either accommodation or vergence open-loop. Objective measures of the accommodative response were made using an infra-red optometer. Differences in near-response characteristics were observed between LOMs and EOMs under both open- and closed-loop conditions. LOMs exhibit significantly higher levels of disparity-induced accommodation (accommodation driven by vergence under closed-loop conditions) and lower response accommodative convergence/accommodation (AC/A) ratios when compared with EOMs. However no difference in convergent accommodation/convergence (CA/C) ratios were found between the three refractive groups. Accommodative adaptation was examined by comparing the pre- to post-task shift in dark focus (DF) following near-vision tasks. Accommodative adaptation was observed following tasks as brief as 15s. Following a 45s near-vision task, subjects having pre-task DF greater than +0.750 exhibited a marked negative shift in post-task DF which was shown to be induced by beta-adrenergic innervation to the ciliary muscle. However no evidence was found to support the proposal of reduced adrenergic innervation to the ciliary muscle in LOMs. Disparity-vergence produced a reduction in accommodative adaptation suggesting that oculomotor adaptation was not driven by the output of the near-response crosslinks. In order to verify this proposition, the effect of vergence adaptation on CA/C was investigated and it was observed that prism adaptation produced no significant change in the CA/C ratio. This would indicate that in a model of accommodation-vergence interaction, the near response cross-links occur after the input to the adaptive components of the oculomotor response rather than before the adaptive elements as reported in previous literature. The findings of this thesis indicate differences in the relative composition of the aggregate accommodation and vergence responses in the three refractive groups examined. They may also have implications with regard to the aetiology of late-onset myopia.

An investigation into the relationship between pupil and accommodation responses during near vision

A Hamamatsu Video Area Analyser has been coupled with a modified Canon IR automatic optometer. This has allowed simultaneous recording of pupil diameter and accommodation response to be made both statically and continuously, a feature not common in previous studies. Experimental work concerned pupil and accommodation responses during near vision tasks under a variety of conditions. The effects of sustained near vision tasks on accommodation have usually been demonstrated by taking post-task measures under darkroom conditions. The possibility of similar effects on pupil diameter was assessed using static and continuous recordings following a near vision task. Results showed that is luminance levels remained unchanged by using a pre-and post-task bright-empty field then, although accommodation regressed to pre-task levels,pupil diameter remained for several minutes at the contstricted level induced by the task. An investigation into the effect of a sinusoidally-modulated blur-only accommodative stimulus on pupil response demonstrated that response may be reduced or absent despite robust accommodation responses. This suggests that blur-driven acommodation alone may not be sufficient to produce a pupil near response and that the presence of other cues may be necessary. Pupil response was investigated using a looming stimulus which produced an inferred-proximity cue. It was found that a pupil response could be induced which was in synchrony with the stimulus while closed-loop accommodation response was kept constant by the constraints of optical blur. The pupil diameter of young and elderly subjects undertaking a 5 minute reading task was measured to assess the contribution of pupil constriction to near vision function in terms of depth-of-focus. Results showed that in the young subjects pupil diameter was too large to have a significant effect on depth-of-focus although it may be increased in the elderly subjects. Pupil and accommodation reponses to a temporally-modulated stimulus containing all cues present in a normal visual environment was assessed and results showed that as stimulus temporal frequency increased, pupil response showed increasing phase lag relative to closed-loop accommodation. The results of this study suggest that it may be necessary to change the accepted view of the function of pupil response as part of the near vision triad and that further study would be of benefit in particular to designers of vision aids such as, for example, bifocal contact lenses.

Accommodation and intraocular pressure

The relationship between accommodation and intraocular pressure (lOP) has not been addressed as a research question for over 20 years, when measurement of both of these parameters was less advanced than today. Hence the central aim of this thesis was to evaluate the effects of accommodation on lOP. The instrument of choice throughout this thesis was the Pulsair EasyEye non-contact tonometer (NCT) due principally to its slim-line design which allowed the measurement of lOP in one eye and simultaneous stimulation of accommodation in the other eye. A second reason for using the Pulsair EasyEye NCT was that through collaboration with the manufacturers (Keeler, UK) the instrument's operational technology was made accessible. Hence, the principle components underpinning non-contact lOP measures of 0.1mmHg resolution (an order of magnitude greater than other methods) were made available. The relationship between the pressure-output and corneal response has been termed the pressure-response relationship, aspects of which have been shown to be related to ocular biometric parameters. Further, analysis of the components of the pressure-response relationship together with high-speed photography of the cornea during tonometry has enhanced our understanding of the derivation of an lOP measure with the Pulsair EasyEye NCT. The NCT samples the corneal response to the pressure pulse over a 19 ms cycle photoelectronically, but computes the subject's lOP using the data collected in the first 2.34 ms. The relatively instantaneous nature of the lOP measurement renders the measures susceptible to variations in the steady-state lOP caused by the respiratory and cardiac cycles. As such, the variance associated with these cycles was minimised by synchronising the lOP measures with the cardiac trace and maintaining a constant pace respiratory cycle at 15 breathes/minute. It is apparent that synchronising the lOP measures with the peak, middle or trough of the cardiac trace significantly reduced the spread of consecutive measures. Of the 3 locations investigated, synchronisation with the middle location demonstrated the least variance (coeflicient of variation = 9.1%) and a strong correlation (r = 0.90, p = <0.001) with lOP values obtained with Goldmann contact tonometry (n = 50). Accordingly lOP measures synchronised with the middle location of the cardiac cycle were taken in the RE while the LE fixated low (L; zero D), intermediate (I; 1.50 D) and high (H; 4 D) accommodation targets, Quasi-continuous measures of accommodation responses were obtained during the lOP measurement period using the portable infrared Grand Seiko FR-5000 autorefractor. The lOP reduced between L and I accommodative levels by approximately 0.61 mmHg (p <0.00 I). No significant reduction in IOP between L and H accommodation levels was elicited (p = 0.65) (n = 40). The relationship between accommodation and lOP was characterised by substantial inter-subject variations. Myopes demonstrated a tendency to show a reduction in IOP with accommodation which was significant only with I accommodation levels when measured with the NCT (r = 0.50, p = 0.01). However, the relationship between myopia and lOP change with accommodation reached significance for both I (r = 0.61, p= 0.003) and H (r = 0.531, p= 0.0 1) accommodation levels when measured with the Ocular blood Flow Analyser (OBFA). Investigation of the effects of accommodation on the parameters measured by the OBFA demonstrated that with H accommodation levels the pulse amplitude (PA) and pulse rate (PR) responses differed between myopes and emmetropes (PA: p = 0.03; PR: p = 0.004). As thc axial length increased there was a tendency for the pulsatile ocular blood flow (POBF) to reduce with accommodation, which was significant only with H accommodation levels (r = 0.38, p = 0.02). It is proposed that emmetropes arc able to regulate the POBF responses to changes in ocular perfusion pressure caused by changes in lOP with I (r = 0.77, p <0.001) and H (r = 0.73, p = 0.001) accommodation levels. However, thc relationship between lOP and POBF changes in the myopes was not correlated for both I (r = 0.33, p = 0.20) and H (r = 0.05, p = 0.85) accommodation levels. The thesis presents new data on the relationships between accommodation, lOP and parameters of the OBFA,: and provides evidence for possible lOP and choroidal blood flow regulatory mechanisms. Further the data highlight possible deficits in the vascular regulation of the myopic eye during accommodation, which may play a putative role in the aetiology of myopia development.

Modelling the human accommodation system using finite element analysis

The human accommodation system has been extensively examined for over a century, with a particular focus on trying to understand the mechanisms that lead to the loss of accommodative ability with age (Presbyopia). The accommodative process, along with the potential causes of presbyopia, are disputed; hindering efforts to develop methods of restoring accommodation in the presbyopic eye. One method that can be used to provide insight into this complex area is Finite Element Analysis (FEA). The effectiveness of FEA in modelling the accommodative process has been illustrated by a number of accommodative FEA models developed to date. However, there have been limitations to these previous models; principally due to the variation in data on the geometry of the accommodative components, combined with sparse measurements of their material properties. Despite advances in available data, continued oversimplification has occurred in the modelling of the crystalline lens structure and the zonular fibres that surround the lens. A new accommodation model was proposed by the author that aims to eliminate these limitations. A novel representation of the zonular structure was developed, combined with updated lens and capsule modelling methods. The model has been designed to be adaptable so that a range of different age accommodation systems can be modelled, allowing the age related changes that occur to be simulated. The new modelling methods were validated by comparing the changes induced within the model to available in vivo data, leading to the definition of three different age models. These were used in an extended sensitivity study on age related changes, where individual parameters were altered to investigate their effect on the accommodative process. The material properties were found to have the largest impact on the decline in accommodative ability, in particular compared to changes in ciliary body movement or zonular structure. Novel data on the importance of the capsule stiffness and thickness was also established. The new model detailed within this thesis provides further insight into the accommodation mechanism, as well as a foundation for future, more detailed investigations into accommodation, presbyopia and accommodative restoration techniques.

Dynamic measurement of accommodation and pupil size using the portable Grand Seiko FR-5000 autorefractor

PURPOSE. The purpose of this study was to evaluate the potential of the portable Grand Seiko FR-5000 autorefractor to allow objective, continuous, open-field measurement of accommodation and pupil size for the investigation of the visual response to real-world environments and changes in the optical components of the eye. METHODS. The FR-5000 projects a pair of infrared horizontal and vertical lines on either side of fixation, analyzing the separation of the bars in the reflected image. The measurement bars were turned on permanently and the video output of the FR-5000 fed into a PC for real-time analysis. The calibration between infrared bar separation and the refractive error was assessed over a range of 10.0 D with a model eye. Tolerance to longitudinal instrument head shift was investigated over a ±15 mm range and to eye alignment away from the visual axis over eccentricities up to 25.0°. The minimum pupil size for measurement was determined with a model eye. RESULTS. The separation of the measurement bars changed linearly (r = 0.99), allowing continuous online analysis of the refractive state at 60 Hz temporal and approximately 0.01 D system resolution with pupils >2 mm. The pupil edge could be analyzed on the diagonal axes at the same rate with a system resolution of approximately 0.05 mm. The measurement of accommodation and pupil size were affected by eccentricity of viewing and instrument focusing inaccuracies. CONCLUSIONS. The small size of the instrument together with its resolution and temporal properties and ability to measure through a 2 mm pupil make it useful for the measurement of dynamic accommodation and pupil responses in confined environments, although good eye alignment is important. Copyright © 2006 American Academy of Optometry.

Simultaneous continuous recording of accommodation and pupil size using the modified Shin-Nippon SRW-5000 autorefractor

The internal optics of the recent models of the Shin-Nippon SRW-5000 autorefractor (also marketed as the Grand Seiko WV-500) have been modified by the manufacturer so that the infrared measurement ring has been replaced by pairs of horizontal and vertical infrared bars, on either side of fixation. The binocular, open field-of-view, allowing the accommodative state to be objectively monitored while a natural environment is viewed, has made the SRW-5000 a valuable tool in further understanding the nature of the oculomotor response. It is shown that the root-mean-square of model eye measures was least (0.017 ± 0.002D) when the separation of the horizontal measurement bars were averaged twice. The separation of the horizontal bars changes by 3.59 pixels/dioptre (r2 = 0.99), allowing continuous on-line analysis of the refractive state at up to 60 Hz temporal resolution to an accuracy of <0.001D, with pupils >3 mm. The pupil edge is not obscured in the diagonal axis by the measurement bars, unlike the ring of the original optics, so in the newer model pupil size can be measured simultaneously at the same rate with a resolution of <0.001 mm. The measurements of accommodation and pupil size are relatively unaffected by eccentricity of viewing up to ±10° from the visual axis and instrument focusing inaccuracies over a range of 10 mm towards the eye and 5 mm away from the eye. The resolution and temporal properties of the analysis are therefore ideal for the simultaneous measurement of dynamic accommodation and pupil responses. © 2004 The College of Optometrists.

Sympathetic control of accommodation:evidence for inter-subject variation

Autonomic innervation of ciliary smooth muscle is mediated principally by the parasympathetic nervous system and is supplemented by the sympathetic nervous system. Previous drug and nerve stimulation experiments on humans and animals have demonstrated that sympathetic innervation is inhibitory (via β-2 adrenoceptors), relatively small, slow and augmented by concurrent levels of background parasympathetic activity. These characteristics are pertinent to the sympathetic system having a specific role in our ability to adapt successfully to sustained near vision tasks and, given the clear association between near vision and the onset and development of myopia, to a putative aetiological role in myopia development in pre-disposed individuals. A fifth characteristic, namely the variation between individuals in access to an inhibitory sympathetic facility is therefore of particular interest. A novel method for continuous recording of accommodation, currently employed in a large sample longitudinal study of myopia in young adults, was used following topical instillation of non-selective (timolol) and selective (betaxolol) sympathetic β-adrenoceptor antagonists. Measures of post-task accommodative hysteresis were taken with reference to the time-course of regression of accommodation when open-loop (Difference of Gaussian) conditions were immediately imposed following short (10 s) and long (3 min) duration far (0D) and near (3D above tonic level) tasks viewed through a Badal system. Data confirm earlier informal experimental observations that only one in three individuals are likely to have access to a sympathetic inhibitory facility during sustained near vision. © 2002 The College of Optometrists.

Continuous recording of accommodation and pupil size using the Shin-Nippon SRW-5000 autorefractor

A newly released commercial autorefractor, the Shin-Nippon SRW-5000 (Japan), has been found to be valid compared to subjective refraction and repeatable over a wide prescription range. Its binocular open field-of-view allows the accommodative state to be monitored while a natural environment is viewed. In conventional static mode, the device can take up to 45 readings in 1min using digital image analysis of the reflected retinal image of a measurement ring. Continuous on-line analysis of the ring provides high (up to 60Hz) temporal resolution of the refractive state to an accuracy of <0.001D. Pupil size can also be analysed to a resolution of <0.001mm. The measurement of accommodation and pupil size was relatively unaffected by eccentricity of viewing up to ±10° and instrument focusing inaccuracies of ±5mm. The resolution properties of the analysis are shown to be ideal for measurement of dynamic accommodation and pupil responses. Copyright © 2001 The College of Optometrists.

Green@CU:an environmental game for residential accommodation

The importance of using games for supporting behavioural and attitudinal change has been explored in the literature, most recently the games for change movement has promulgated the use of games for supporting altruistic changes that have a positive impact upon the environment. This paper presents a Serious Game designed for University students and its main aim is to educate them about environmental issues. In particular, the focus lies in the importance of saving energy. A user study with 42 participants assessed the feeling of presence of the whole virtual learning experience.

Accommodation in young adults wearing multifocal soft contact lenses under long- and short- wavelength lighting

Purpose: Several studies have suggested accommodative lags may serve as a stimulus for myopic growth, and while a blurred foveal image is believed to the main stimulus for accommodation, spectral composition of the retinal image is also believed to influence accommodative accuracy. Of particular interest is how altering spectral lighting conditions influences accommodation in the presence of soft multifocal contact lenses, which are currently being used off-label for myopia control. Methods: Accommodative responses were assessed using a Grand Seiko WAM-5500 autorefractor for four target distances: 25, 33, 50, and 100cm for 30 young adult subjects (14 myopic, 16 emmetropic; mean refractive errors (±SD, D) -4.22±2.04 and -0.15±0.67 respectively). Measurements were obtained with four different soft contact lenses, Single vision distance (SVD), Single vision near (SVN), Centre-Near (CN) and Centre-Distance (CD) (+1.50 add), and three different lighting conditions: red (peak λ 632nm), blue (peak λ 460nm), and white (peak λ 560nm). Corrections for chromatic differences in refraction were made prior to calculating accommodative errors. Results: The size of accommodative errors was significantly affected by lens design (p<0.001), lighting (p=0.027), and target distance (p=0.009). Mean accommodative errors were significantly larger with the SV lenses compared to the CD and CN designs (p<0.001). Errors were also significantly larger under blue light compared to white (p=0.004) and a significant interaction noted between lens design and lighting (p<0.001). Blue light generally decreased accommodative lags and increased accommodative leads relative to white and red light, the opposite was true of red light (p≤0.001). Lens design also significantly influenced direction of accommodative error (i.e. lag or lead) (p<0.001). Interactions with or between refractive groups were not found to be statistically significant for either the magnitude or direction of accommodative error (p>0.05 for all). Conclusions: Accuracy of accommodation is affected by both lens design and by wavelength of lighting. These accommodative lag data lend some support to recent speculation about the potential therapeutic value of lighting with a spectral bias towards blue during near work for myopia, although such treatment effects are likely to be more subtle under broad compared to the narrow spectrum lighting conditions used here.

Mass-transport and slope accommodation : implications for turbidite sandstone reservoirs

Peer reviewed

Postmodern Paralysis: The Critical Impasse in Feminist Perspectives on Consumers

Critical perspectives on theory play an important and valued role in disciplines across the academy. Feminist perspectives might be expected to be at or near the forefront of critical engagement with consumer behaviour theory, especially given the importance of gender in consumer research. Following a brief upsurge during the 1990s, critical feminist voices have been muted of late. This paper explores some reasons for this. It begins with a brief overview of research on gender and consumer behaviour and how insights from feminist theories and feminist activism began to alter our understanding of gendered consumption. It then discusses how postmodern and postfeminist perspectives have diluted feminism as a critique of gendered consumption. Finally, it argues that a return to materialist feminism would open up possibilities for new and more critical analyses of gendered consumption.