Functional roles of theta- and alpha-band neural oscillations in memory and attention

Synchronized neural activity involving widespread networks is common in the central nervous system. This activity often manifests itself as oscillations, which at one point were considered to be background noise or an indication of an idling state of the brain. It is now generally accepted that these oscillations play a role in higher-order cognitive processes, and these roles are currently under active investigation. In this dissertation, we study the roles of theta (4--8 Hz) and alpha (8--12 Hz) band oscillations in two higher-order cognitive processes: memory and attention.;First, we studied the role of theta (4--8 Hz) oscillations in the communication between two distant brain regions that are both involved in memory processes. The medial temporal lobe (MTL) and the prefrontal cortex (PFC) are known to be critical structures for human memory processes. Furthermore, it has been suggested that they are part of a memory network. While memory-modulated interaction between PFC and MTL has been observed at the hemodynamic level, it remains unclear what the neuronal process is that mediates the communication between these two areas. Experiments in rodents suggest that field oscillations in the theta band (4--8 Hz) facilitate PFC-MTL interaction. No such evidence has been reported in humans. To address this problem, cortical electrical activity from MTL, PFC and lateral temporal lobe was recorded from implanted electrode grids in three epilepsy patients performing a verbal free-recall memory task. The data were analyzed using a parametric spectral method to obtain estimates of power, coherence, and Granger causality. A task-modulated increase in coherence values between PFC and MTL was seen during free recall as opposed to a baseline condition. Concurrently, the number of coherent PFC-MTL site pairs was significantly increased during recall. Granger causality analysis further revealed that the increased coherence is a consequence of higher bidirectional information flow between the two regions, with a generally greater driving from MTL to PFC, namely, (MTL→PFC) > (PFC→MTL).;We then investigated the role of mu and alpha (8--12 Hz) oscillations in somatosensory spatial attention. Neural oscillations with a frequency of around 10 Hz are thought to be a ubiquitous phenomenon in sensory cortices, and it has been hypothesized that the level of 10 Hz activity is related to local cortical excitability. During spatial attention, the visual alpha rhythm has been found to be modulated according to the direction of attention. Specifically, a desynchronization (decrease in amplitude) of the alpha rhythm over visual cortex contralateral to the direction of attention as well as a synchronization (increase in amplitude) over visual cortex ipsilateral to the direction of attention have been reported. These modulations have been associated with both a facilitation and an inhibition of sensory processing, respectively. It has been proposed that the somatosensory mu rhythm serves a similar function to the visual alpha rhythm, and the two rhythms have been found to have similar behaviors in cognitive tasks such as working memory. In this chapter, we investigate whether the somatosensory mu rhythm is somatotopically modulated by spatial attention in a way similar to the visual alpha rhythm. 128 channel EEG was recorded while subjects performed a somatosensory spatial attention task. In addition to analyses on scalp recorded data, a spatial filtering method was utilized to investigate spatial attention effects in the source space. The direction of spatial attention was found to have an effect on the ongoing mu rhythm occurring in primary somatosensory cortex as well as stimulus evoked activity. Lastly, an analysis was performed to investigate the correlation between the level of prestimulus mu activity and subsequent evoked activity in primary somatosensory cortex.;Finally, we further investigated the previous findings regarding the mu rhythm and its relationship with evoked activity by utilizing microelectrode recordings through the cortical laminae of Area 3b in the primary somatosensory cortex of a rhesus monkey during somatosensory stimulation as well as during a baseline period. We were able to confirm that oscillatory activity in the mu band indeed occurs in primary somatosensory cortex. By examining the stimulus evoked P20 component, a homologue of the human P50 (also known as the P1) somatosensory evoked component, we found evidence supporting the previous interpretations that the human P50 is associated with local inhibition.