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Abstract

EEG signals are the most global brain signals which reflect a brain’s functional state, primarily by the frequency composition of oscillatory signal components. Numerous studies have shown that oscillations accompany many neuronal processes underlying cognitive function. Although the role of particular frequency bands is starting to emerge, their combined occurrence and dynamical interplay is scarcely understood with respect to their topological impact on neuronal processes. We set out to determine temporal and spatial properties of various EEG rhythms in the best established animal model for studying the neuronal mechanisms of cognition. Two monkeys were trained to perform a visuomotor task, moving a lever as instructed by a moving visual stimulus while fixation was maintained. At the end of each successful trial, a liquid reward was given and the monkey was waiting for the next trial to start. EEG was recorded from 64 electrodes chronically implanted in the bone bilaterally above numerous cortical areas: visual, auditory, parietal, sensorimotor, premotor and prefrontal areas, digitized at 5 kHz and analyzed for changes in signal power by sliding window FFT. These EEG signals are characterized by a broad distribution of oscillation frequencies, ranging from delta (1-3 Hz) to high gamma frequencies (>150 Hz). Different epochs of the task exhibited continual coming and going of prominent power clusters in the time-frequency domain. Reliable effects (z-scores > 2) could be observed in both monkeys: when attending the visual stimulus and precisely controlling the lever position, a prominent beta rhythm (12-30 Hz) occurred with a latency of 240 ms to the visual stimulus. As soon as the monkey initiated further lever movements, this beta rhythm was replaced by prominent power in the delta and in the high gamma band (50-140 Hz). The topography of the frequency bands differed: while beta oscillations could be seen mostly over visual, parietal and premotor areas, the delta band dominated for prefrontal and premotor electrodes and gamma rhythms were observed over prefrontal areas. In contrast, the period just after reward was dominated by power in the alpha band (8-13 Hz) distributed over the entire brain. In sum, we identified task-dependent EEG oscillations in diverse frequency bands which alternated through the different stages of the task following their typical topographical distributions. The observation that different EEG rhythms like in the delta and gamma frequency band co-occurred repeatedly suggests that interactions across frequencies might play a crucial role in processing task relevant information.