Investigation of neural frequencies within the human cortex and their clinical manifestations

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Many neurological conditions present in today’s society require expert analysis from imaging and electrophysiological studies with a significant portion of effort dedicated to understanding the processes that underpin the physiological and pathological activity identified. This rapidly expanding field of neuroscience has provided insight into potentially the most complex and enigmatic system, the brain, which is under constant dynamical change in both time and frequency. However, this field has only begun to scratch the surface of neurological oscillations and the clinical manifestations that it produces. The purpose of this research directly relates to these oscillations within the brain that produce the physiological and psychological requirements of man, and also pathological conditions. The thesis is broken into five sections, the first of which explores the limitations, modalities, methodological approaches and standard frequency ranges that are used in modern neuroscience. The wide spectrum of frequencies from all regions of the brain, are then explored, in pursuit of understanding voluntary movement. Historically, this task was performed visually or semi-automated with visual confirmation. These cortical oscillations are addressed with a multi-frequency framework for identification and quantification of defining characteristics, such as onset, duration, frequency and amplitude. This provides an objective and automatic methodology to better understand the distribution of these oscillations for several movement tasks. Extending the previous work, a sample of cortical oscillations is isolated from key motoric regions to identify the plausibility of different architecture for a brain computer interface. Although brain computer interfaces have existed for a few decades, a hierarchical structure is proposed that is informed by the previously identified distribution of movement related cortical potentials. This work also expands on a significant impediment for future utility by incorporating ‘non- movement’ into the classification procedure and hence paves the way for out-of-the-lab brain computer interfaces. The remaining two chapters then investigate pathological activities within the cerebral cortex to understand their relevance within a significant neurological population suffering from refractory epilepsy. A significant debate in the literature stems from the whether movements during nocturnal hours are pathological or physiological in nature. Previous reports have strong arguments for both possibilities, however the results are primarily from visual reports. To provide new insight into this field, a high frequency band is explored in relation to nocturnal activity, seizure semiology and ictal activity. The novel exploration and signal processing methods used substantially indicate in favour of a physiological response. However this also harmonises with pathological hypotheses as the network that is responsible for the movements is identified revealing how its instability can be provoked by epileptic discharges. Lastly, a highly accurate and clinically relevant tool for the localisation of epilepsy for patients undergoing stereo-electroencephalographic monitoring is produced. The method used builds upon a previous method however, significant modification is applied to the extraction and quantification of two features used in their work. These features, a multi-band, high frequency descending chirp and significant loss of lower frequencies are extracted using robust image processing techniques and quantified for classification. Exponential weights are added to the features after a leave-one-out analyses to identify the ratio of the features which best predict channels that are within the epileptogenic zone. This zone represents the minimum required volume to be resected for patients to achieve seizure freedom. This method provides a second, objective opinion about the presentation of refractory epilepsy, assisting clinicians with the difficult task of surgical evaluation. The method is also fully autonomous, non-parametric and provides a rank value, all of which are novel with respect to most previous localisation programs.