The studies described here show how the spatiotemporal dynamics of neuronal oscillations can be used to investigate the large scale brain networks involved in language comprehension and attention. In addition, these studies show that the electromagnetic fields generated by active brain networks exhibit a higher-order statistical dependence---termed cross-frequency coupling or nested oscillations---which may help coordinate activity across a wide range of spatial and temporal scales. The purpose of the first chapter of this dissertation is to provide background information about oscillations needed to evaluate the empirical studies described in following chapters. In particular, the first chapter focuses on the cellular and network origins of neuronal oscillations as well as the functional roles such oscillations are suggested to play.;The second chapter describes how we examined the spatiotemporal dynamics of word processing by recording the electrocorticogram (ECoG) from the lateral frontotemporal cortex of neurosurgical patients chronically implanted with subdural electrode grids. Subjects engaged in a target detection task where proper names served as infrequent targets embedded in a stream of task-irrelevant verbs and nonwords. Verbs described actions related to the hand (e.g., throw) or mouth (e.g., blow), while unintelligible nonwords were sounds which matched the verbs in duration, intensity, temporal modulation, and power spectrum. Complex oscillatory dynamics were observed in the delta, theta, alpha, beta, low and high gamma bands in response to presentation of all stimulus types. High gamma activity (HG, 80--200 Hz) in the ECoG tracked the spatiotemporal dynamics of word processing and identified a network of cortical structures involved in early word processing. HG was used to determine the relative onset, peak, and offset times of local cortical activation during word processing. Listening to verbs compared to nonwords sequentially activates first the posterior superior temporal gyrus (STG), then the middle superior temporal gyrus (mid-STG), followed by the superior temporal sulcus (STS). We also observed strong phase-locking between pairs of electrodes in the theta band, with weaker phase-locking occurring in the delta, alpha, and beta frequency ranges. These results provide details on the first few hundred milliseconds of the spatiotemporal evolution of cortical activity during word processing and provide evidence consistent with the hypothesis that an oscillatory hierarchy coordinates the flow of information between distinct cortical regions during goal-directed behavior. The second chapter was published as (Canolty and Soltani, et al., 2007).;Chapter three shifts from language to attention. Several prior studies have shown that distinct cortical areas within the frontal and parietal lobes are important nodes in the large-scale brain network regulating attention, target detection, and vigilance. However, studies of attention often reach divergent results depending upon the methodology employed. I addressed this apparent electrophysiological-fMRI dissociation by recording the electrocorticogram (ECoG) during an auditory target detection task. ECoG high gamma (HG, 80--160 Hz) activation was observed in superior temporal areas for all auditory stimuli, while lateral frontal and parietal were selectively activated by targets as opposed to distractors, with electrodes over frontal cortex exhibiting significant HG activity before electrodes over parietal cortex. In addition, while low frequency (theta ∼ 6 Hz) phase coherence was strongest between electrode pairs exhibiting HG activity, the phase difference between ECoG electrodes depended upon both stimulus type and the inferred functional role of the cortical areas involved. In addition to suggesting that neuronal oscillations transiently modulate activity in brain networks, these results support the general method of (1) identifying functional nodes via localized HG activity and then (2) investigating the effective connectivity between nodes via low frequency phase coherence. The third chapter was presented in poster form at the 10 th International Conference on Cognitive Neuroscience (ICON X), in Bodrum, Turkey.;Chapter four, the last empirical data chapter, focuses on nested oscillations. We observed robust coupling between the high and low frequency bands of ongoing electrical activity in the human brain. In particular, the phase of the low frequency theta (4--8 Hz) rhythm modulates power in the high gamma (80--150 Hz) band of the electrocorticogram, with stronger modulation occurring at higher theta amplitudes. Furthermore, different behavioral tasks evoke distinct patterns of theta/high gamma coupling across the cortex. The results indicate that transient coupling between low and high frequency brain rhythms coordinates activity in distributed cortical areas providing a mechanism for effective communication during cognitive processing in humans. The fourth chapter as published as (Canolty, et al., 2006).;The final chapter notes several trends and pursues their implications, before concluding that the theme of oscillations provide an ideal platform from which to explore almost any aspect of nervous system function. (Abstract shortened by UMI.)
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