Optometry and Vision Sciences - Research Publications

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    Pattern Motion Processing by MT Neurons
    Eskikand, PZ ; Kameneva, T ; Burkitt, AN ; Grayden, DB ; Ibbotson, MR (Frontiers Media, 2019-06-21)
    Based on stimulation with plaid patterns, neurons in the Middle Temporal (MT) area of primate visual cortex are divided into two types: pattern and component cells. The prevailing theory suggests that pattern selectivity results from the summation of the outputs of component cells as part of a hierarchical visual pathway. We present a computational model of the visual pathway from primary visual cortex (V1) to MT that suggests an alternate model where the progression from component to pattern selectivity is not required. Using standard orientation-selective V1 cells, end-stopped V1 cells, and V1 cells with extra-classical receptive fields (RFs) as inputs to MT, the model shows that the degree of pattern or component selectivity in MT could arise from the relative strengths of the three V1 input types. Dominance of end-stopped V1 neurons in the model leads to pattern selectivity in MT, while dominance of V1 cells with extra-classical RFs result in component selectivity. This model may assist in designing experiments to further understand motion processing mechanisms in primate MT.
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    Feasibility of Nitrogen Doped Ultrananocrystalline Diamond Microelectrodes for Electrophysiological Recording From Neural Tissue
    Wong, YT ; Ahnood, A ; Maturana, M ; Kentler, W ; Ganesan, K ; Grayden, DB ; Meffin, H ; Prawer, S ; Ibbotson, MR ; Burkitt, AN (FRONTIERS MEDIA SA, 2018-06-22)
    Neural prostheses that can monitor the physiological state of a subject are becoming clinically viable through improvements in the capacity to record from neural tissue. However, a significant limitation of current devices is that it is difficult to fabricate electrode arrays that have both high channel counts and the appropriate electrical properties required for neural recordings. In earlier work, we demonstrated nitrogen doped ultrananocrystalline diamond (N-UNCD) can provide efficacious electrical stimulation of neural tissue, with high charge injection capacity, surface stability and biocompatibility. In this work, we expand on this functionality to show that N-UNCD electrodes can also record from neural tissue owing to its low electrochemical impedance. We show that N-UNCD electrodes are highly flexible in their application, with successful recordings of action potentials from single neurons in an in vitro retina preparation, as well as local field potential responses from in vivo visual cortex tissue. Key properties of N-UNCD films, combined with scalability of electrode array fabrication with custom sizes for recording or stimulation along with integration through vertical interconnects to silicon based integrated circuits, may in future form the basis for the fabrication of versatile closed-loop neural prostheses that can both record and stimulate.
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    Electrical receptive fields of retinal ganglion cells: Influence of presynaptic neurons
    Maturana, MI ; Apollo, NV ; Garrett, DJ ; Kameneva, T ; Cloherty, SL ; Grayden, DB ; Burkitt, AN ; Ibbotson, MR ; Meffin, H ; Fine, I (PUBLIC LIBRARY SCIENCE, 2018-02)
    Implantable retinal stimulators activate surviving neurons to restore a sense of vision in people who have lost their photoreceptors through degenerative diseases. Complex spatial and temporal interactions occur in the retina during multi-electrode stimulation. Due to these complexities, most existing implants activate only a few electrodes at a time, limiting the repertoire of available stimulation patterns. Measuring the spatiotemporal interactions between electrodes and retinal cells, and incorporating them into a model may lead to improved stimulation algorithms that exploit the interactions. Here, we present a computational model that accurately predicts both the spatial and temporal nonlinear interactions of multi-electrode stimulation of rat retinal ganglion cells (RGCs). The model was verified using in vitro recordings of ON, OFF, and ON-OFF RGCs in response to subretinal multi-electrode stimulation with biphasic pulses at three stimulation frequencies (10, 20, 30 Hz). The model gives an estimate of each cell's spatiotemporal electrical receptive fields (ERFs); i.e., the pattern of stimulation leading to excitation or suppression in the neuron. All cells had excitatory ERFs and many also had suppressive sub-regions of their ERFs. We show that the nonlinearities in observed responses arise largely from activation of presynaptic interneurons. When synaptic transmission was blocked, the number of sub-regions of the ERF was reduced, usually to a single excitatory ERF. This suggests that direct cell activation can be modeled accurately by a one-dimensional model with linear interactions between electrodes, whereas indirect stimulation due to summated presynaptic responses is nonlinear.
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    A Possible Role for End-Stopped V1 Neurons in the Perception of Motion: A Computational Model
    Eskikand, PZ ; Kameneva, T ; Ibbotson, MR ; Burkitt, AN ; Grayden, DB ; Chacron, MJ (PUBLIC LIBRARY SCIENCE, 2016-10-14)
    We present a model of the early stages of processing in the visual cortex, in particular V1 and MT, to investigate the potential role of end-stopped V1 neurons in solving the aperture problem. A hierarchical network is used in which the incoming motion signals provided by complex V1 neurons and end-stopped V1 neurons proceed to MT neurons at the next stage. MT neurons are categorized into two types based on their function: integration and segmentation. The role of integration neurons is to propagate unambiguous motion signals arriving from those V1 neurons that emphasize object terminators (e.g. corners). Segmentation neurons detect the discontinuities in the input stimulus to control the activity of integration neurons. Although the activity of the complex V1 neurons at the terminators of the object accurately represents the direction of the motion, their level of activity is less than the activity of the neurons along the edges. Therefore, a model incorporating end-stopped neurons is essential to suppress ambiguous motion signals along the edges of the stimulus. It is shown that the unambiguous motion signals at terminators propagate over the rest of the object to achieve an accurate representation of motion.
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    Retinal ganglion cells electrophysiology: the effect of cell morphology on impulse waveform
    Maturana, MI ; Wong, R ; Cloherty, SL ; Ibbotson, MR ; Hadjinicolaou, AE ; Grayden, DB ; Burkitt, AN ; Meffin, H ; O'Brien, BJ ; Kameneva, T (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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    First-in-Human Trial of a Novel Suprachoroidal Retinal Prosthesis
    Ayton, LN ; Blamey, PJ ; Guymer, RH ; Luu, CD ; Nayagam, DAX ; Sinclair, NC ; Shivdasani, MN ; Yeoh, J ; McCombe, MF ; Briggs, RJ ; Opie, NL ; Villalobos, J ; Dimitrov, PN ; Varsamidis, M ; Petoe, MA ; McCarthy, CD ; Walker, JG ; Barnes, N ; Burkitt, AN ; Williams, CE ; Shepherd, RK ; Allen, PJ ; Mori, K (PUBLIC LIBRARY SCIENCE, 2014-12-18)
    UNLABELLED: Retinal visual prostheses ("bionic eyes") have the potential to restore vision to blind or profoundly vision-impaired patients. The medical bionic technology used to design, manufacture and implant such prostheses is still in its relative infancy, with various technologies and surgical approaches being evaluated. We hypothesised that a suprachoroidal implant location (between the sclera and choroid of the eye) would provide significant surgical and safety benefits for patients, allowing them to maintain preoperative residual vision as well as gaining prosthetic vision input from the device. This report details the first-in-human Phase 1 trial to investigate the use of retinal implants in the suprachoroidal space in three human subjects with end-stage retinitis pigmentosa. The success of the suprachoroidal surgical approach and its associated safety benefits, coupled with twelve-month post-operative efficacy data, holds promise for the field of vision restoration. TRIAL REGISTRATION: Clinicaltrials.gov NCT01603576.