EEG alpha synchronization and functional coupling during top‐down processing in a working memory task

Abstract

Electroencephalogram (EEG) alpha (around 10 Hz) is the dominant rhythm in the human brain during conditions of mental inactivity. High amplitudes as observed during rest usually diminish during cognitive effort. During retention of information in working memory, however, power increase of alpha oscillations can be observed. This alpha synchronization has been interpreted as cortical idling or active inhibition. The present study provides evidence that during top-down processing in a working memory task, alpha power increases at prefrontal but decreases at occipital electrode sites, thereby reaching a state in which alpha power and frequency become very similar over large distances. Two experimental conditions were compared. In the first, visuospatial information only had to be retained in memory whereas the second condition additionally demanded manipulation of the information. During the second condition, stronger alpha synchronization at prefrontal sites and larger occipital alpha suppression was observed as compared to that for pure retention. This effect was accompanied by assimilation of prefrontal and occipital alpha frequency, stronger functional coupling between prefrontal and occipital brain areas, and alpha latency shifts from prefrontal cortex to primary visual areas, possibly indicating the control of posterior cortical activation by anterior brain areas. An increase of prefrontal EEG alpha amplitudes, which is accompanied by a decrease at posterior sites, thus may not be interpreted in terms of idling or "global" inhibition but may enable a tight functional coupling between prefrontal cortical areas, and thereby allows the control of the execution of processes in primary visual brain regions.

Synchronization during an internally directed cognitive state in healthy aging and mild cognitive impairment: a MEG study

“…These results are consistent with recent studies which consider that the alpha band is associated with internal tasks (Jensen et al 2002;Cooper et al 2003;Sauseng et al 2005;Palva and Palva 2007;Benedek et al 2011), such as mental calculation, and that may reflect an internal information processing which involves topdown control and may be related to an inhibition of external interfering input (Jensen and Mazaheri 2010). Controls also displayed a desynchronization over frontal brain areas, which could be related to the processing of semantic information such as mathematical knowledge (Klimesch et al 2007).…”

Abnormally high EEG alpha synchrony during working memory maintenance in twins discordant for schizophrenia

“…Consistent with this inhibitory function, alpha ERS is evident during the delay period of WM tasks, correlates in magnitude with memory load, and peaks over cortical areas responsible for processing task-modality specific sensory information (Sauseng et al, 2005;Jensen et al, 2002). Collectively, these findings suggest that alpha ERS reflects 'top-down' gating of sensory areas to prevent encoding of goal-irrelevant stimuli while task-relevant information is held actively in mind (reviewed, Klimesch et al, 2007).…”

Gamma-Band Activity in Human Prefrontal Cortex Codes for the Number of Relevant Items Maintained in Working Memory

“…Support for this hypothesis was first provided by WM studies in humans (Tallon-Baudry et al, 1998) and nonhuman primates (Pesaran et al, 2002), which reported a sustained increase of oscillatory activity in the gamma frequency range during the memory period of delayed response tasks. Subsequently, a large body of evidence has documented a parametric relationship between the amplitude of oscillatory activity at theta, alpha, beta, and gamma frequencies and the number of WM items memorized during memory-scanning tasks (Jensen and Tesche, 2002;Howard et al, 2003;Onton et al, 2005;Meltzer et al, 2008), visuospatial WM (Busch and Herrmann, 2003;Sauseng et al, 2005Sauseng et al, , 2009Palva et al, 2011), and auditory WM (Leiberg et al, 2006).…”

“…In contrast, modulations of spectral power at alpha frequencies have been assigned different roles. While some studies have related delay activity in the alpha band with the functional inhibition of task-irrelevant brain areas (Jensen et band activity directly to processes underlying WM maintenance (Busch and Herrmann, 2003;Herrmann et al, 2004;Sauseng et al, 2005;Leiberg et al, 2006;Palva et al, 2011). In addition to the contribution of individual frequency bands, theoretical and empirical studies have implicated the coupling between oscillation frequencies as a mechanism for WM maintenance.…”