Abstracts

Rationale: From the early days of electroencephalography (EEG), rhythmic activity in the alpha range has been known to exist in the occipital and perirolndic somatosensory and motor areas, i.e., the alpha rhythm and the mu rhythm, respectively. A third variant of alpha like rhythm, the so-called “third rhythm”, has also been reported in the temporal lobe of patients with skull bone defect (Niedermeyer, 1990 & 1991). To date, the behavioral correlates of the third rhythm and its independence from the other alpha band rhythms are unknown. Intracranial EEG (iEEG) recordings in a patient with depth electrodes implanted in the postcentral gyrus and ipsilateral Heschl gyrus gave us a rare opportunity to study these focal alpha band activities in the human brain. Methods: Simultaneous video and iEEG data were recorded from 67 intracranial and five scalp EEG contacts, including right postcentral gyrus and ipsilateral Heschl gyrus in a patient undergoing epilepsy surgery evaluation. Our methods included the analysis of power, coherence, synchronization likelihood, phase locking index, and network statistics such as PageRank and Granger Causality algorithms. Results: We found oscillatory activity in the 8-12Hz (alpha band) range recorded simultaneously in the right postcentral somatosensory and superior temporal auditory cortices. The somatosensory alpha was desynchronized with tactile stimulation or when the patient performed, or observed others perform, goal directed actions but not during active listening. In contrast, alpha band activity in the auditory belt region was desynchronized during active listening task but not during somatomotor tasks. Neither somatosensory nor auditory alpha were modulated when the subject opened or closed his eyes. We examined the temporal dynamics and inter-electrode synchronization of these focal responses using novel measures of synchronization likelihood and phase locking and found that the activity in the temporal channel was not due to volume conduction effect, but represented an independent alpha band activity. Conclusions: Our results support the earlier findings of temporal alpha and suggest that the default mode of activity of the occipital visual cortices and of the perirolandic somatomotor regions can be generalizable to the superior temporal auditory region of the brain. It needs to be determined if the occipital alpha, the mu, and the third rhythm hallmark an electrophysiological means through which the activity of these sensory motor cortices are coordinated in time when there is no external stimulus towards which a focused attention or action is required.