Lithium battery cells are commonly modeled using an equivalent circuit with large lookup tables for each circuit element, allowing flexibility for the model to closely match measured data. Pulse discharge curves and charge curves are collected experimentally to characterize the battery performance at various operating points. It can be extremely difficult to fit the simulation model to the experimental data using optimization algorithms, due to the number of values in the lookup tables. This challenge is addressed using a layered approach to break the parameter estimation problem into smaller tasks. The size of each estimation task is reduced to a small subset of data and parameter values, so that the optimizer can better focus on a specific problem. The layered approach was successful in fitting an equivalent circuit model to a lithium iron phosphate (LFP) cell data set to within a mean of 0.7mV residual error, and max of 9.2mV error at a transient.

We are developing a vestibular implant to electrically stimulate vestibular neurons in the semicircular canals in order to alleviate vertigo, which is a commonly occurring problem. However, since electrical stimulation causes synchronous (phase-locked) neural responses, such electrical stimulation might also cause inappropriate vestibuloocular eye movements, which might, in turn, cause visual blurring. We investigated the eye movements evoked in the guinea pig using electric stimulation with a constant rate of 250 pulses per second (pps), and measured 0.010(°) peak-to-peak eye movements on an average at 250 Hz, with an average peak velocity amplitude of 8.1(°)/s, which might cause visual blurring. However, after half an hour of stimulation, that component reduced to 1.6(°)/s (0.0020(°) peak-to-peak). The average time constant for this reduction was 5.0 min. After one week of constant stimulation, the 250-Hz response component was only slightly smaller, at 1.2(°)/s (0.0015(°) peak-to-peak). We conclude that although an electrical prosthesis with a resting rate of 250 pps may cause some visual blurring when first turned on, such blurring is very likely to attenuate and be imperceptible within several minutes.

Vestibular loss, which can manifest as dizziness, imbalance, or spatial disorientation, is widespread and often caused by inner ear hair cell malfunction. To address these problems, we are developing a vestibular implant analogous to cochlear implants for the deaf. This vestibular prosthesis provides pulsatile electrical stimulation to the vestibular nerve. Prosthesis effectiveness is assessed using the vestibulo-ocular reflex (VOR), since the VOR helps stabilize gaze in healthy individuals by evoking eye movements that compensate for head movements. In this thesis, the prosthesis was used to probe the high frequency VOR in squirrel monkeys and guinea pigs. In two studies, modulated stimulation was applied acutely to characterize the VOR between 1.5 and 701 Hz. A third study characterized the VOR response to chronic stimulation with a constant rate of 250 Hz. The VOR has previously been characterized up to 50 Hz in monkeys and 2 Hz in guinea pigs by physically rotating subjects. This range was extended in these studies, by using electrical stimulation from the prosthesis. Eye movement spectral peaks were used to characterize the VOR frequency response. The VOR was measurable up to 267 Hz in squirrel monkeys and 151 Hz in guinea pigs. The magnitude response was similar in both species-it increased gradually with frequency, peaked (at 140 Hz in squirrel monkeys and 50 Hz in guinea pigs), and then rolled off. The high frequency fall-off was consistent with the low-pass nature of the oculomotor plant. The phase responses had a linear lag with frequency, consistent with a fixed 4 ms delay of the VOR three-neuron-arc. Since the VOR responds at high frequencies, this raises the question whether the prosthesis causes eye movements at the prosthesis pulse rate, since electrical stimulation elicits neural responses that are phase-locked with the stimulation. Such responses might cause visual blurring for a patient using the device. This thesis shows that such eye movements are measurable, and have substantial velocity magnitude of 8.1 deg/s initially, but within 30 minutes the magnitude reduces by 80% and probably does not yield perceptible visual blurring. Vertigo and imbalance are commonly occurring problems. Vestibular vertigo affects approximately 5% of adults each year (Neuhauser et al. 2005), and can be a life-altering problem. Furthermore, people who report dizziness are 12 times more likely to experience falls (Agrawal et al. 2009), which are a leading cause of severe health problems. Chronic vestibular vertigo can be caused by malfunction of …

We are developing two types of vestibular prosthetics that electrically stimulate afferent neurons. One type replaces absent sensory function by providing stimulation that modulates above and below a baseline established with the head stationary. The other type provides constant stimulation and is turned on only when necessary, for example, to override unnatural variations like those experienced by patients suffering from Meniere's syndrome; this prosthesis does not provide motion information. Both prostheses require neural plasticity, which we investigated by providing chronic constant-rate stimulation to semicircular canal neurons in three guinea pigs. The stimulation was alternately switched on or off for eight consecutive weeks before being switched daily. A brisk horizontal nystagmus was measured when the stimulation was first turned on and then dissipated over the course of a day. The nystagmus demonstrated an after-effect in the opposite direction when the stimulation was turned off. The nystagmus that we measured after just a few (2 to 5) off-to-on transitions returned to baseline more rapidly than when first turned on. In fact, after many such off-to-on or on-to-off transitions, little nystagmus was evoked by turning the stimulation on or off. These findings show that the brain acclimates to constant-rate stimulation