Optometry and Vision Sciences - Research Publications
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ItemA Three-Dimensional Atlas of the Honeybee Neck(PUBLIC LIBRARY SCIENCE, 2010-05-24)Three-dimensional digital atlases are rapidly becoming indispensible in modern biology. We used serial sectioning combined with manual registration and segmentation of images to develop a comprehensive and detailed three-dimensional atlas of the honeybee head-neck system. This interactive atlas includes skeletal structures of the head and prothorax, the neck musculature, and the nervous system. The scope and resolution of the model exceeds atlases previously developed on similar sized animals, and the interactive nature of the model provides a far more accessible means of interpreting and comprehending insect anatomy and neuroanatomy.
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ItemFocal Activation of Primary Visual Cortex Following Supra-choroidal Electrical Stimulation of the Retina: Intrinsic Signal Imaging and Linear Model Analysis(IEEE, 2010)UNLABELLED: We performed optical intrinsic signal imaging of cat primary visual cortex (Area 17 and 18) while delivering bipolar electrical stimulation to the retina by way of a supra-choroidal electrode array. Using a general linear model (GLM) analysis we identified statistically significant (p < 0.01) activation in a localized region of cortex following supra-threshold electrical stimulation at a single retinal locus. OUR RESULTS: (1) demonstrate that intrinsic signal imaging combined with linear model analysis provides a powerful tool for assessing cortical responses to prosthetic stimulation, and (2) confirm that supra-choroidal electrical stimulation can achieve localized activation of the cortex consistent with focal activation of the retina.
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ItemEpiretinal Electrical Stimulation and the Inner Limiting Membrane in Rat Retina(IEEE, 2012)In this paper we aim to quantify the effect of the inner limiting membrane (ILM) of the retina on the thresholds for epiretinal electrical stimulation of retinal ganglion cells by a microelectronic retinal prosthesis. A pair of bipolar stimulating electrodes was placed either above (on the epiretinal surface) or below the ILM while we made whole-cell patch-clamp recordings from retinal ganglion cells in an isolated rat retinal whole-mount preparation. Across our cell population we found no significant difference in the median threshold stimulus amplitudes when the stimulating electrodes were placed below as opposed to above the ILM (p = 0.08). However, threshold stimulus amplitudes did tend to be lower when the stimulating electrodes were placed below the ILM (30 µA vs 56 µA).
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ItemRetinal ganglion cells electrophysiology: the effect of cell morphology on impulse waveform(IEEE, 2013)There are 16 morphologically defined classes of rats retinal ganglion cells (RGCs). Using computer simulation of a realistic anatomically correct A1 mouse RGC, we investigate the effect of the cell's morphology on its impulse waveform, using the first-, and second-order time derivatives as well as the phase plot features. Using whole cell patch clamp recordings, we recorded the impulse waveform for each of the rat RGCs types. While we found some clear differences in many features of the impulse waveforms for A2 and B2 cells compared to other cell classes, many cell types did not show clear differences.
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ItemEffects of saccades on visual processing in primate MSTd(PERGAMON-ELSEVIER SCIENCE LTD, 2010-12)In surveying their visual environment, primates, including humans make frequent rapid eye movements known as saccades. Saccades result in rapid motion of the retinal image and yet this motion is not perceived. We recorded saccade-related changes in neural activity in the dorsal medial superior temporal area (MSTd) of alert macaque monkeys. We show that the spontaneous activity of neurons in MSTd is modulated around the time of saccades. Some cells show considerable suppression of spontaneous activity, while most show early and significant enhancement. While this modulation of spontaneous activity is variable, the concomitant modulation of neural responses evoked by flashed visual stimuli is uniform and stereotypical - visual responses are suppressed for stimuli presented around the time of saccades and enhanced for stimuli presented afterwards. The combined modulation of spontaneous activity and evoked visual responses likely serves to reduce the detectability of peri-saccadic stimuli and promote the perceptual awareness of visual stimuli between saccades.
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ItemDifferential changes in perceived contrast following contrast adaptation in humans(PERGAMON-ELSEVIER SCIENCE LTD, 2010-01-11)Perceived contrast is reduced after prolonged exposure to a textured pattern (contrast adaptation). The size of this effect is dependent on the relationship between the adapting contrast and the test contrast. It is generally accepted that the greatest reductions occur when the adapting contrast is much higher than the test contrast. Here this relationship was examined for a wide range of spatial frequencies. The results show that the effect of the adapt/test ratio on perceived contrast following contrast adaptation is highly spatial frequency dependent. At high spatial frequencies >1cpd perceived contrast was reduced for all adapting contrasts, which is consistent with other studies. However, at low spatial frequencies (<1cpd) the perceived contrast was actually above veridical perception when the adapting contrast was lower than the test contrast. This finding has not been previously reported and has important implications for models of contrast perception.
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ItemPHASE SENSITIVITY OF COMPLEX CELLS IN PRIMARY VISUAL CORTEX(PERGAMON-ELSEVIER SCIENCE LTD, 2013-05-01)Neurons in the primary visual cortex are often classified as either simple or complex based on the linearity (or otherwise) of their response to spatial luminance contrast. In practice, classification is typically based on Fourier analysis of a cell's response to an optimal drifting sine-wave grating. Simple cells are generally considered to be linear and produce responses modulated at the fundamental frequency of the stimulus grating. In contrast, complex cells exhibit significant nonlinearities that reduce the response at the fundamental frequency. Cells can therefore be easily and objectively classified based on the relative modulation of their responses - the ratio of the phase-sensitive response at the fundamental frequency of the stimulus (F₁) to the phase-invariant sustained response (F₀). Cells are classified as simple if F₁/F₀>1 and complex if F₁/F₀<1. This classification is broadly consistent with criteria based on the spatial organisation of cells' receptive fields and is accordingly presumed to reflect disparate functional roles of simple and complex cells in coding visual information. However, Fourier analysis of spiking responses is sensitive to the number of spikes available - F₁/F₀ increases as the number of spikes is reduced, even for phase-invariant complex cells. Moreover, many complex cells encountered in the laboratory exhibit some phase sensitivity, evident as modulation of their responses at the fundamental frequency. There currently exists no objective quantitative means of assessing the significance or otherwise of these modulations. Here we derive a statistical basis for objectively assessing whether the modulation of neuronal responses is reliable, thereby adding a level of statistical certainty to measures of phase sensitivity. We apply our statistical analysis to neuronal responses to moving sine-wave gratings recorded from 367 cells in cat primary visual cortex. We find that approximately 60% of complex cells exhibit statistically significant (α<0.01) modulation of their responses to optimal moving gratings. These complex cells are phase sensitive and reliably encode spatial phase.
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ItemIntrinsic physiological properties of rat retinal ganglion cells with a comparative analysis(AMER PHYSIOLOGICAL SOC, 2012-10)Mammalian retina contains 15-20 different retinal ganglion cell (RGC) types, each of which is responsible for encoding different aspects of the visual scene. The encoding is defined by a combination of RGC synaptic inputs, the neurotransmitter systems used, and their intrinsic physiological properties. Each cell's intrinsic properties are defined by its morphology and membrane characteristics, including the complement and localization of the ion channels expressed. In this study, we examined the hypothesis that the intrinsic properties of individual RGC types are conserved among mammalian species. To do so, we measured the intrinsic properties of 16 morphologically defined rat RGC types and compared these data with cat RGC types. Our data demonstrate that in the rat different morphologically defined RGC types have distinct patterns of intrinsic properties. Variation in these properties across cell types was comparable to that found for cat RGC types. When presumed morphological homologs in rat and cat retina were compared directly, some RGC types had very similar properties. The rat A2 cell exhibited patterns of intrinsic properties nearly identical to the cat alpha cell. In contrast, rat D2 cells (ON-OFF directionally selective) had a very different pattern of intrinsic properties than the cat iota cell. Our data suggest that the intrinsic properties of RGCs with similar morphology and suspected visual function may be subject to variation due to the behavioral needs of the species.