Optometry and Vision Sciences - Research Publications

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    A comparison of microelectrodes for a visual cortical prosthesis using finite element analysis.
    Brunton, E ; Lowery, AJ ; Rajan, R (Frontiers Media SA, 2012)
    Altering the geometry of microelectrodes for use in a cortical neural prosthesis modifies the electric field generated in tissue, thereby affecting electrode efficacy and tissue damage. Commonly, electrodes with an active region located at the tip ("conical" electrodes) are used for stimulation of cortex but there is argument to believe this geometry may not be the best. Here we use finite element analysis to compare the electric fields generated by three types of electrodes, a conical electrode with exposed active tip, an annular electrode with active area located up away from the tip, and a striped annular electrode where the active annular region has bands of insulation interrupting the full active region. The results indicate that the current density on the surface of the conical electrodes can be up to 10 times greater than the current density on the annular electrodes of the same height, which may increase the propensity for tissue damage. However choosing the most efficient electrode geometry in order to reduce power consumption is dependent on the distance of the electrode to the target neurons. If neurons are located within 10 μm of the electrode, then a small conical electrode would be more power efficient. On the other hand if the target neuron is greater than 500 μm away-as happens normally when insertion of an array of electrodes into cortex results in a "kill zone" around each electrode due to insertion damage and inflammatory responses-then a large annular electrode would be more efficient.
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    Driver self-regulation and depressive symptoms in cataract patients awaiting surgery: a cross-sectional study
    Fraser, ML ; Meuleners, LB ; Ng, JQ ; Morlet, N (BMC, 2013-09-10)
    BACKGROUND: Cataract is an extremely common visual condition of ageing. Evidence suggests that visual impairment influences driving patterns and self-regulatory behavior among older drivers. However, little is known about the psychological effects of driver self-regulation among older drivers. Therefore, this study aimed to describe driver self-regulation practices among older bilateral cataract patients and to determine the association between self-regulation and depressive symptoms. METHODS: Ninety-nine older drivers with bilateral cataract were assessed the week before first eye cataract surgery. Driver self-regulation was measured via the Driving Habits Questionnaire. Depressive symptoms were assessed using the 20-item Center for Epidemiological Studies Depression Scale. Visual, demographic and cognitive data were also collected. Differences between self-regulators and non self-regulators were described and linear regression modeling used to determine the association between driver self-regulation and depressive symptoms score. RESULTS: Among cataract patients, 48% reported self-regulating their driving to avoid at least one challenging situation. The situations most commonly avoided were driving at night (40%), on the freeway (12%), in the rain (9%) and parallel parking (8%). Self-regulators had significantly poorer contrast sensitivity in their worse eye than non self-regulators (p = 0.027). Driver self-regulation was significantly associated with increased depressive symptoms after controlling for potential confounding factors (p = 0.002). CONCLUSIONS: Driver self-regulation was associated with increased depressive symptoms among cataract patients. Further research should investigate this association among the general older population. Self-regulation programs aimed at older drivers may need to incorporate mental health elements to counteract unintended psychological effects.
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    Clinical Applications of Wavefront Refraction
    Bruce, AS ; Catania, LJ (LIPPINCOTT WILLIAMS & WILKINS, 2014-10)
    PURPOSE: To determine normative reference ranges for higher-order wavefront error (HO-WFE), compare these values with those in common ocular pathologies, and evaluate treatments. METHODS: A review of 17 major studies on HO-WFE was made, involving data for a total of 31,605 subjects. The upper limit of the 95% confidence interval (CI) for HO-WFE was calculated from the most comprehensive of these studies using normal healthy patients aged 20 to 80 years. There were no studies identified using the natural pupil size for subjects, and for this reason, the HO-WFE was tabulated for pupil diameters of 3 to 7 mm. Effects of keratoconus, pterygium, cataract, and dry eye on HO-WFE were reviewed and treatment efficacy was considered. RESULTS: The calculated upper limit of the 95% CI for HO-WFE in a healthy normal 35-year-old patient with a mesopic pupil diameter of 6 mm would be 0.471 μm (471 nm) root-mean-square or less. Although the normal HO-WFE increases with age for a given pupil size, it is not yet completely clear how the concurrent influence of age-related pupillary miosis affects these findings. Abnormal ocular conditions such as keratoconus can induce a large HO-WFE, often in excess of 3.0 μm, particularly attributed to coma. For pterygium or cortical cataract, a combination of coma and trefoil was more commonly induced. Nuclear cataract can induce a negative spherical HO-WFE, usually in excess of 1.0 μm. CONCLUSIONS: The upper limit of the 95% CI for HO-WFE root-mean-square is about 0.5 μm with normal physiological pupil sizes. With ocular pathologies, HO-WFE can be in excess of 1.0 μm, although many devices and therapeutic and surgical treatments are reported to be highly effective at minimizing HO-WFE. More accurate normative reference ranges for HO-WFE will require future studies using the subjects' natural pupil size.
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    A computational study of how orientation bias in the lateral geniculate nucleus can give rise to orientation selectivity in primary visual cortex
    Kuhlmann, L ; Vidyasagar, TR (FRONTIERS MEDIA SA, 2011)
    Controversy remains about how orientation selectivity emerges in simple cells of the mammalian primary visual cortex. In this paper, we present a computational model of how the orientation-biased responses of cells in lateral geniculate nucleus (LGN) can contribute to the orientation selectivity in simple cells in cats. We propose that simple cells are excited by lateral geniculate fields with an orientation-bias and disynaptically inhibited by unoriented lateral geniculate fields (or biased fields pooled across orientations), both at approximately the same retinotopic co-ordinates. This interaction, combined with recurrent cortical excitation and inhibition, helps to create the sharp orientation tuning seen in simple cell responses. Along with describing orientation selectivity, the model also accounts for the spatial frequency and length-response functions in simple cells, in normal conditions as well as under the influence of the GABA(A) antagonist, bicuculline. In addition, the model captures the response properties of LGN and simple cells to simultaneous visual stimulation and electrical stimulation of the LGN. We show that the sharp selectivity for stimulus orientation seen in primary visual cortical cells can be achieved without the excitatory convergence of the LGN input cells with receptive fields along a line in visual space, which has been a core assumption in classical models of visual cortex. We have also simulated how the full range of orientations seen in the cortex can emerge from the activity among broadly tuned channels tuned to a limited number of optimum orientations, just as in the classical case of coding for color in trichromatic primates.
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    Direct visualization and characterization of erythrocyte flow in human retinal capillaries
    Bedggood, P ; Metha, A (OPTICAL SOC AMER, 2012-12-01)
    Imaging the retinal vasculature offers a surrogate view of systemic vascular health, allowing noninvasive and longitudinal assessment of vascular pathology. The earliest anomalies in vascular disease arise in the microvasculature, however current imaging methods lack the spatiotemporal resolution to track blood flow at the capillary level. We report here on novel imaging technology that allows direct, noninvasive optical imaging of erythrocyte flow in human retinal capillaries. This was made possible using adaptive optics for high spatial resolution (1.5 μm), sCMOS camera technology for high temporal resolution (460 fps), and tunable wavebands from a broadband laser for maximal erythrocyte contrast. Particle image velocimetry on our data sequences was used to quantify flow. We observed marked spatiotemporal variability in velocity, which ranged from 0.3 to 3.3 mm/s, and changed by up to a factor of 4 in a given capillary during the 130 ms imaging period. Both mean and standard deviation across the imaged capillary network varied markedly with time, yet their ratio remained a relatively constant parameter (0.50 ± 0.056). Our observations concur with previous work using less direct methods, validating this as an investigative tool for the study of microvascular disease in humans.
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    Limitations to adaptive optics image quality in rodent eyes
    Zhou, X ; Bedggood, P ; Metha, A (OPTICAL SOC AMER, 2012-08-01)
    Adaptive optics (AO) retinal image quality of rodent eyes is inferior to that of human eyes, despite the promise of greater numerical aperture. This paradox challenges several assumptions commonly made in AO imaging, assumptions which may be invalidated by the very high power and dioptric thickness of the rodent retina. We used optical modeling to compare the performance of rat and human eyes under conditions that tested the validity of these assumptions. Results showed that AO image quality in the human eye is robust to positioning errors of the AO corrector and to differences in imaging depth and wavelength compared to the wavefront beacon. In contrast, image quality in the rat eye declines sharply with each of these manipulations, especially when imaging off-axis. However, some latitude does exist to offset these manipulations against each other to produce good image quality.
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    Interventions to improve cultural competency in healthcare: a systematic review of reviews
    Truong, M ; Paradies, Y ; Priest, N (BMC, 2014-03-03)
    BACKGROUND: Cultural competency is a recognized and popular approach to improving the provision of health care to racial/ethnic minority groups in the community with the aim of reducing racial/ethnic health disparities. The aim of this systematic review of reviews is to gather and synthesize existing reviews of studies in the field to form a comprehensive understanding of the current evidence base that can guide future interventions and research in the area. METHODS: A systematic review of review articles published between January 2000 and June 2012 was conducted. Electronic databases (including Medline, Cinahl and PsycINFO), reference lists of articles, and key websites were searched. Reviews of cultural competency in health settings only were included. Each review was critically appraised by two authors using a study appraisal tool and were given a quality assessment rating of weak, moderate or strong. RESULTS: Nineteen published reviews were identified. Reviews consisted of between 5 and 38 studies, included a variety of health care settings/contexts and a range of study types. There were three main categories of study outcomes: patient-related outcomes, provider-related outcomes, and health service access and utilization outcomes. The majority of reviews found moderate evidence of improvement in provider outcomes and health care access and utilization outcomes but weaker evidence for improvements in patient/client outcomes. CONCLUSION: This review of reviews indicates that there is some evidence that interventions to improve cultural competency can improve patient/client health outcomes. However, a lack of methodological rigor is common amongst the studies included in reviews and many of the studies rely on self-report, which is subject to a range of biases, while objective evidence of intervention effectiveness was rare. Future research should measure both healthcare provider and patient/client health outcomes, consider organizational factors, and utilize more rigorous study designs.
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    Optical Imaging of Human Cone Photoreceptors Directly Following the Capture of Light
    Bedggood, P ; Metha, A ; Barnes, S (PUBLIC LIBRARY SCIENCE, 2013-11-15)
    Capture of light in the photoreceptor outer segment initiates a cascade of chemical events that inhibit neurotransmitter release, ultimately resulting in vision. The massed response of the photoreceptor population can be measured non-invasively by electrical recordings, but responses from individual cells cannot be measured without dissecting the retina. Here we used optical imaging to observe individual human cones in the living eye as they underwent bleaching of photopigment and associated phototransduction. The retina was simultaneously stimulated and observed with high intensity visible light at 1 kHz, using adaptive optics. There was marked variability between individual cones in both photosensitivity and pigment optical density, challenging the conventional assumption that photoreceptors act as identical subunits (coefficient of variation in rate of photoisomerization = 23%). There was also a pronounced inverse correlation between these two parameters (p<10(-7)); the temporal evolution of image statistics revealed this to be a dynamic relationship, with cone waveguiding efficiency beginning a dramatic increase within 3 ms of light onset. Beginning as early as 2 ms after light onset and including half of cells by ∼7 ms, cone intensity showed reversals characteristic of interference phenomena, with greater delays in reversal corresponding to cones with more photopigment (p<10(-3)). The timing of these changes is argued to best correspond with either the cessation of dark current, or to related events such as changes in intracellular cGMP. Cone intensity also showed fluctuations of high frequency (332±25 Hz) and low amplitude (3.0±0.85%). Other groups have shown similar fluctuations that were directly evoked by light; if this corresponds to the same phenomenon, we propose that the amplitude of fluctuation may be increased by the use of a bright flash followed by a brief pause, to allow recovery of cone circulating current.
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    Ocellar structure and neural innervation in the honeybee
    Hung, Y-S ; Ibbotson, MR (FRONTIERS MEDIA SA, 2014-02-19)
    Honeybees have a visual system composed of three ocelli (simple eyes) located on the top of the head, in addition to two large compound eyes. Although experiments have been conducted to investigate the role of the ocelli within the visual system, their optical characteristics, and function remain controversial. In this study, we created three-dimensional (3-D) reconstructions of the honeybee ocelli, conducted optical measurements and filled ocellar descending neurons to assist in determining the role of ocelli in honeybees. In both the median and lateral ocelli, the ocellar retinas can be divided into dorsal and ventral parts. Using the 3-D model we were able to assess the viewing angles of the retinas. The dorsal retinas view the horizon while the ventral retinas view the sky, suggesting quite different roles in attitude control. We used the hanging drop technique to assess the spatial resolution of the retinas. The lateral ocelli have significantly higher spatial resolution compared to the median ocellus. In addition, we established which ocellar retinas provide the input to five pairs of large ocellar descending neurons. We found that four of the neuron pairs have their dendritic fields in the dorsal retinas of the lateral ocelli, while the fifth has fine dendrites in the ventral retina. One of the neuron pairs also sends very fine dendrites into the border region between the dorsal and ventral retinas of the median ocellus.
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    Does the Swedish Interactive Threshold Algorithm (SITA) accurately map visual field loss attributed to vigabatrin?
    Conway, ML ; Hosking, SL ; Zhu, H ; Cubbidge, RP (Springer Science and Business Media LLC, 2014-12-23)
    BACKGROUND: Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss. METHODS: The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years ± 14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm. RESULTS: SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots , defect area and defect severity. CONCLUSIONS: Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.