Florey Department of Neuroscience and Mental Health - Theses
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ItemMinimally invasive deep brain stimulation via an endovascular stent-electrode array( 2016)Modern neural interfaces are practical demonstrations of what multi-disciplinary teams can create in this age of medicine. From decoding brain signals in order to control prosthetic limbs, to stimulating peripheral nerves in amputees to restore sensation, the blend of engineering and medicine continues to generate novel, life-changing, therapeutics. In an attempt to create a paradigm shift towards minimally invasive implantation of such neural interfaces, the Vascular Bionics Laboratory at The Royal Melbourne Hospital has designed a stent-electrode array that can be deployed in the cerebrovasculature via a series of percutaneously positioned catheters. While the device was designed to record brain signals for the purpose of decoding them, its stimulation capabilities had not been studied. This thesis takes that next step by conducting stimulation studies with the device. Were such a stimulation process established with these proof of concept experiments, then the device would hold promise for future neural interfaces. The animal model that the stent-electrode arrays had been designed for was used for these stimulation studies. Seven Corriedale sheep had devices implanted, and then a series of stimulation experiments were conducted across a range of time points using a variety of stimulation parameters and configurations while the animals were awake; the objective of stimulation was to excite the motor cortex and thereby induce a categorical response, such as the contraction of a limb. Stimulation parameters included monophasic, either anodal or cathodal, pulses at phase widths of 200 µs or 500 µs and at frequencies of either 20 Hz or 50 Hz with currents ranging from 1 to 6 mA. These were applied across a variety of configurations including monopolar, with either a head-mounted or flank-mounted return electrode, and bipolar. Pulse trains were also studied. Induced motor activity was documented principally by visual observation and accelerometer tracking. A range of motor activity was documented across the animals including about the flank, the neck, the ear, and some facial muscles. However, these contractions did not align to the neuroanatomical position of the electrodes relative to the motor cortex, nor were the contractions consistent across time points. In addition, the majority of contractions were not elicited from bipolar stimulation and thus prone to induction by current spread. Four devices were found on autopsy to have severed cables connecting to the stent-electrode arrays, a failure not identified by the regular tacking of impedances in the device. The dataset generated from these stimulation studies suggested that the motor activity documented was a product of current spread through the devices to muscles of the sheep, rather than motor activity caused by stimulation of the motor cortex. These studies serve to guide future experiments with this device.