Autonomic correlates of ocular accommodation and cardiovascular function

PURPOSE: To evaluate the hypothesis that objective measures of open- and closed-loop ocular accommodation are related to systemic cardiovascular function, and ipso facto autonomic nervous system activity. METHODS: Sixty subjects (29 male; 31 female) varying in age from 18 to 33 years (average: 20.3 +/- 2.9 years) with a range of refractive errors [mean spherical equivalent (MSE): -7.12 to +1.82 D] participated in the study. Five 20-s continuous objective recordings of the accommodative response, measured with an open-view IR autorefractor (Shin-Nippon SRW-5000), were obtained for a variety of open- and closed-loop accommodative demands while simultaneous continuous measurement of heart rate was recorded with a finger-mounted piezo-electric pulse transducer for 5 min. Fast Fourier Transformation of cardiovascular function allowed the absolute and relative power of the autonomic components to be assessed in the frequency-domain, whereas heart period gave an indication of the time-domain response. RESULTS: Increasing closed-loop accommodative demand led to a concurrent increase in heart rate of approximately 2 beats/min for a 4.0 D increase in accommodative demand. The increase was attributable to a reduction in the absolute (p < 0.05) and normalised (p < 0.001) input of the systemic parasympathetic nervous system, and was unaffected by refractive group. The interaction with refractive group failed to reach significance. CONCLUSIONS: For sustained accommodation effort, the data demonstrate covariation between the oculomotor and cardiovascular systems which implies that a near visual task can significantly influence cardiovascular behaviour. Accommodative effort alone, however, is not a sufficient stimulus to induce autonomic differences between refractive groups. The data suggest that both the oculomotor and cardiovascular systems are predominantly attributable to changes in the systemic parasympathetic nervous system.

Influence of accommodation and refractive status on the peripheral refractive profile

AIM: The aim of the study was to determine, objectively and non-invasively, whether changes in accommodative demand modify differentially the peripheral refraction in emmetropic and myopic human eyes. METHODS: Forty subjects (19 male, 21 female) aged 20-30 years (mean 22.7 (SD 2.8) years), 21 emmetropes (mean spherical equivalent refractive error (MSE) -0.13 (SD 0.29) D) and 19 myopes (MSE -2.95 (SD 1.76) D) participated in the study. Ametropia was corrected with soft contact lenses (etafilcon A, 58% water content). Subjects viewed monocularly a stationary, high contrast (85%) Maltese cross at 0.0, 1.0, 2.0 and 3.0 D of accommodative demand and at 0, 10, 20 and 30 degrees field angle (nasal and temporal) through a +3.0 D Badal optical system. Static recordings of the accommodation response were obtained for each accommodative level, at each field angle, with an objective, open-view, infrared optometer. RESULTS: Peripheral mean spherical equivalent (M) data showed that the emmetropic cohort exhibited relative myopic shifts into the periphery, while the myopic group showed hypermetropic shifts. Increasing accommodative demand did not alter the peripheral refractive profile in either the temporal (p = 0.25) or nasal (p = 0.07) periphery with no differential accommodative effect between refractive groups in either the temporal (p = 0.77) or nasal (p = 0.73) field. Significant shifts in the J(0) astigmatic component were seen in the temporal (p<0.0005) and nasal (p<0.0005) fields with increasing eccentricity. Interaction effects between eccentricity and accommodative demand illustrated that increasing accommodative demand significantly altered the peripheral refractive profile in the temporal J(0) astigmatic component (p<0.0005). The nasal periphery, however, failed to show such an effect (p = 0.65). CONCLUSIONS: Alterations in peripheral refraction augmented by changes in ocular accommodation are relatively unaffected by refractive error for young, healthy human eyes.

Three-dimensional magnetic resonance imaging of the phakic crystalline lens during accommodation

To quantify changes in crystalline lens curvature, thickness, equatorial diameter, surface area, and volume during accommodation using a novel two-dimensional magnetic resonance imaging (MRI) paradigm to generate a complete three-dimensional crystalline lens surface model.

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass transport in bubbling fluidised beds

The fluid–particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the heat, momentum and mass transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user-defined function (UDF). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors, Part A: Eulerian computation of momentum transport in bubbling fluidised beds

The fluid–particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase, according to the literature. FLUENT 6.2 has been used as the modelling framework of the simulations with a completely revised drag model, in the form of user defined function (UDF), to calculate the forces exerted on the particle as well as its velocity components. 2-D and 3-D simulations are tested and compared. The study is the first part of a complete pyrolysis model in fluidised bed reactors.

Computational modelling of the impact of particle size to the heat transfer coefficient between biomass particles and a fluidised bed

The fluid–particle interaction and the impact of different heat transfer conditions on pyrolysis of biomass inside a 150 g/h fluidised bed reactor are modelled. Two different size biomass particles (350 µm and 550 µm in diameter) are injected into the fluidised bed. The different biomass particle sizes result in different heat transfer conditions. This is due to the fact that the 350 µm diameter particle is smaller than the sand particles of the reactor (440 µm), while the 550 µm one is larger. The bed-to-particle heat transfer for both cases is calculated according to the literature. Conductive heat transfer is assumed for the larger biomass particle (550 µm) inside the bed, while biomass–sand contacts for the smaller biomass particle (350 µm) were considered unimportant. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Biomass reaction kinetics is modelled according to the literature using a two-stage, semi-global model which takes into account secondary reactions. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of User Defined Function (UDF).

Vergence analysis reveals the influence of axial distances on accommodation with age and axial ametropia

Despite numerous investigations, the aetiology and mechanism of accommodation and presbyopia remains equivocal. Using Gaussian first-order ray tracing calculations, we examine the contribution that ocular axial distances make to the accommodation response. Further, the influence of age and ametropia are also considered. The data show that all changes in axial distances during accommodation reduce the accommodation response, with the reduction in anterior chamber depth contributing most to this overall attenuation. Although the total power loss due to the changes in axial distances remained constant with increasing age, hyperopes exhibited less accommodation than myopes. The study, therefore, enhances our understanding of biometric accommodative changes and demonstrates the utility of vergence analysis in the assessment of accommodation.

Flow patterns, performance and scale-up of distillation trays

This work is concerned with the nature of liquid flow across industrial sieve trays operating in the spray, mixed, and the emulsified flow regimes. In order to overcome the practical difficulties of removing many samples from a commercial tray, the mass transfer process was investigated in an air water simulator column by heat transfer analogy. The temperature of the warm water was measured by many thermocouples as the water flowed across the single pass 1.2 m diameter sieve tray. The thermocouples were linked to a mini computer for the storage of the data. The temperature data were then transferred to a main frame computer to generate temperature profiles - analogous to concentration profiles. A comprehensive study of the existing tray efficiency models was carried out using computerised numerical solutions. The calculated results were compared with experimental results published by the Fractionation Research Incorporation (FRl) and the existing models did not show any agreement with the experimental results. Only the Porter and Lockett model showed a reasonable agreement with the experimental results for cenain tray efficiency values. A rectangular active section tray was constructed and tested to establish the channelling effect and the result of its effect on circular tray designs. The developed flow patterns showed predominantly flat profiles and some indication of significant liquid flow through the central region of the tray. This comfirms that the rectangular tray configuration might not be a satisfactory solution for liquid maldistribution on sieve trays. For a typical industrial tray the flow of liquid as it crosses the tray from the inlet to the outlet weir could be affected by the mixing of liquid by the eddy, momentum and the weir shape in the axial or the transverse direction or both. Conventional U-shape profiles were developed when the operating conditions were such that the froth dispersion was in the mixed regime, with good liquid temperature distribution while in the spray regime. For the 12.5 mm hole diameter tray the constant temperature profiles were found to be in the axial direction while in the spray regime and in the transverse direction for the 4.5 mm hole tray. It was observed that the extent of the liquid stagnant zones at the sides of the tray depended on the tray hole diameter and was larger for the 4.5 mm hole tray. The liquid hold-up results show a high liquid hold-up at the areas of the tray with low liquid temperatures, this supports the doubts about the assumptions of constant point efficiency across an operating tray. Liquid flow over the outlet weir showed more liquid flow at the centre of the tray at high liquid loading with low liquid flow at both ends of the weir. The calculated results of the point and tray efficiency model showed a general increase in the calculated point and tray efficiencies with an increase in the weir loading, as the flow regime changed from the spray to the mixed regime the point and the tray efficiencies increased from approximately 30 to 80%.Through the mixed flow regime the efficiencies were found to remain fairly constant, and as the operating conditions were changed to maintain an emulsified flow regime there was a decrease in the resulting efficiencies. The results of the estimated coefficient of mixing for the small and large hole diameter trays show that the extent of liquid mixing on an operating tray generally increased with increasing capacity factor, but decreased with increasing weir loads. This demonstrates that above certain weir loads, the effect of eddy diffusion mechanism on the process of liquid mixing on an operating tray to be negligible.

Continuous measurement of accommodation in human factor applications

It has long been sought to measure ocular accommodation continuously in human factor applications such as driving or flying. Open-field autorefractors such as the Canon R-1 could be converted to allow continuous, objective recording, but steady eye fixation and head immobilisation were essential for the measurements to be valid. Image analysis techniques utilised by newer open-view autorefractors such as the Shin-Nippon SRW-5000 are more tolerant to head and eye movements, but perhaps the technique with the greatest potential for the measurement of accommodation in human factor applications is photoretinoscopy. This paper examines the development of techniques for high temporal measurements of accommodation and reports on the tolerance of one such recent commercial instrument, the PowerRefractor (PlusOptiX). The instrument was found to be tolerant to eye movements from the optical axis of the instrument (∼0.50 DS change in apparent accommodation with gaze 25° eccentric to the optical axis), longitudinal head movement (<0.25 DS from 8 cm towards and 20 cm away from the correct photorefractor to eye distance) and changes in background illuminance (<0.25 DS from 0.5 to 20 cd m-2 target luminance). The PowerRefractor also quantifies the direction of gaze and pupil size, but is unable to take measurements with small pupils <3.7 ±1.0 mm. © 2002 The College of Optometrists.

Mass transfer coefficient for drying of moist particulate in a bubbling fluidized bed

Experiments on drying of moist particles by ambient air were carried out to measure the mass transfer coefficient in a bubbling fluidized bed. Fine glass beads of mean diameter 125?µm were used as the bed material. Throughout the drying process, the dynamic material distribution was recorded by electrical capacitance tomography (ECT) and the exit air condition was recorded by a temperature/humidity probe. The ECT data were used to obtain qualitative and quantitative information on the bubble characteristics. The exit air moisture content was used to determine the water content in the bed. The measured overall mass transfer coefficient was in the range of 0.0145–0.021?m/s. A simple model based on the available correlations for bubble-cloud and cloud-dense interchange (two-region model) was used to predict the overall mass transfer coefficient. Comparison between the measured and predicted mass transfer coefficient have shown reasonable agreement. The results were also used to determine the relative importance of the two transfer regions.

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.