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- All Subjects: Vision
- Creators: Chung, Sonia
Description
Growing understanding of the neural code and how to speak it has allowed for notable advancements in neural prosthetics. With commercially-available implantable systems with bi- directional neural communication on the horizon, there is an increasing imperative to develop high resolution interfaces that can survive the environment and be well tolerated by the nervous system under chronic use. The sensory encoding aspect optimally interfaces at a scale sufficient to evoke perception but focal in nature to maximize resolution and evoke more complex and nuanced sensations. Microelectrode arrays can maintain high spatial density, operating on the scale of cortical columns, and can be either penetrating or non-penetrating. The non-penetrating subset sits on the tissue surface without puncturing the parenchyma and is known to engender minimal tissue response and less damage than the penetrating counterpart, improving long term viability in vivo. Provided non-penetrating microelectrodes can consistently evoke perception and maintain a localized region of activation, non-penetrating micro-electrodes may provide an ideal platform for a high performing neural prosthesis; this dissertation explores their functional capacity.
The scale at which non-penetrating electrode arrays can interface with cortex is evaluated in the context of extracting useful information. Articulate movements were decoded from surface microelectrode electrodes, and additional spatial analysis revealed unique signal content despite dense electrode spacing. With a basis for data extraction established, the focus shifts towards the information encoding half of neural interfaces. Finite element modeling was used to compare tissue recruitment under surface stimulation across electrode scales. Results indicated charge density-based metrics provide a reasonable approximation for current levels required to evoke a visual sensation and showed tissue recruitment increases exponentially with electrode diameter. Micro-scale electrodes (0.1 – 0.3 mm diameter) could sufficiently activate layers II/III in a model tuned to striate cortex while maintaining focal radii of activated tissue.
In vivo testing proceeded in a nonhuman primate model. Stimulation consistently evoked visual percepts at safe current thresholds. Tracking perception thresholds across one year reflected stable values within minimal fluctuation. Modulating waveform parameters was found useful in reducing charge requirements to evoke perception. Pulse frequency and phase asymmetry were each used to reduce thresholds, improve charge efficiency, lower charge per phase – charge density metrics associated with tissue damage. No impairments to photic perception were observed during the course of the study, suggesting limited tissue damage from array implantation or electrically induced neurotoxicity. The subject consistently identified stimulation on closely spaced electrodes (2 mm center-to-center) as separate percepts, indicating sub-visual degree discrete resolution may be feasible with this platform. Although continued testing is necessary, preliminary results supports epicortical microelectrode arrays as a stable platform for interfacing with neural tissue and a viable option for bi-directional BCI applications.
The scale at which non-penetrating electrode arrays can interface with cortex is evaluated in the context of extracting useful information. Articulate movements were decoded from surface microelectrode electrodes, and additional spatial analysis revealed unique signal content despite dense electrode spacing. With a basis for data extraction established, the focus shifts towards the information encoding half of neural interfaces. Finite element modeling was used to compare tissue recruitment under surface stimulation across electrode scales. Results indicated charge density-based metrics provide a reasonable approximation for current levels required to evoke a visual sensation and showed tissue recruitment increases exponentially with electrode diameter. Micro-scale electrodes (0.1 – 0.3 mm diameter) could sufficiently activate layers II/III in a model tuned to striate cortex while maintaining focal radii of activated tissue.
In vivo testing proceeded in a nonhuman primate model. Stimulation consistently evoked visual percepts at safe current thresholds. Tracking perception thresholds across one year reflected stable values within minimal fluctuation. Modulating waveform parameters was found useful in reducing charge requirements to evoke perception. Pulse frequency and phase asymmetry were each used to reduce thresholds, improve charge efficiency, lower charge per phase – charge density metrics associated with tissue damage. No impairments to photic perception were observed during the course of the study, suggesting limited tissue damage from array implantation or electrically induced neurotoxicity. The subject consistently identified stimulation on closely spaced electrodes (2 mm center-to-center) as separate percepts, indicating sub-visual degree discrete resolution may be feasible with this platform. Although continued testing is necessary, preliminary results supports epicortical microelectrode arrays as a stable platform for interfacing with neural tissue and a viable option for bi-directional BCI applications.
ContributorsOswalt, Denise (Author) / Greger, Bradley (Thesis advisor) / Buneo, Christopher (Committee member) / Helms-Tillery, Stephen (Committee member) / Mirzadeh, Zaman (Committee member) / Papandreou-Suppappola, Antonia (Committee member) / Arizona State University (Publisher)
Created2018
Description
Optometry is a field in the United States dedicated to analyzing the health of eyes and offering corrective lenses and/or treatments to improve a patient’s ocular health and vision. Since its origin in the U.S. in the late 19th century, the field of optometry has been met with strong opposition from the medical community, ophthalmologists in particular. This ongoing feud between optometrists and ophthalmologists, medical doctors who also specialize in eye health and perform eye surgeries, continues today as ophthalmologists push back against optometrists’ attempts to expand their scope of practice. With this expansion to include certain eye surgeries, it would save patients both time and money. This is just one factor impacting patients, with another being the widely varied state laws surrounding eye health. Procedures optometrists are able to perform is decided by state laws, which leads to vast discrepancies. Optometrists in one state can perform laser eye surgeries, while optometrists in a nearby state cannot even provide simple treatments for ocular diseases they diagnosis. In this study, three states were analyzed to showcase these variations in possible treatment and demonstrate both the positive and negative impacts they are having on patients. First was Massachusetts which has one of the best medical care systems in the U.S., but one of the worst vision care. As the only state to not allow optometrists to treat glaucoma and one of two states to not allow optometrists to prescribe medications for patients, these limitations have caused patients the inconvenience of having to then visit an ophthalmologist for treatment which adds additional costs and delay in treatment which can cause the conditions to possibly worsen. Second was Oklahoma which was the first U.S. state to allow optometrists to perform laser eye surgeries in 1998. This legislation expanded Oklahoma residents access to treatment as before patients would have to travel to other cities or counties to visit one of the few ophthalmologists in the state. Lastly was Maine which in 2015 passed legislation to allow optometrists to regain control of their field from vision insurance companies who can no longer dictate fees patients are charged if the insurance companies will not cover it. This study concluded that there needs to be a universal vision care system across the U.S. that includes expansion of practice for optometrists and allow them to be in control of their own field, not the state government or vision insurance companies.
ContributorsFoskit, Nevada Anaid (Author) / Gaughan, Monica (Thesis director) / Chung, Sonia (Committee member) / School of Life Sciences (Contributor) / Barrett, The Honors College (Contributor)
Created2019-05