Oscillations in the Alpha Band (9-12 Hz) Increase with Memory Load during Retention in a Short-term Memory Task

TLDR

Hypotheses concerning the role of alpha band activity in working memory are discussed. To study the role of brain oscillations in working memory, we recorded scalp EEG during the retention interval of a modified Sternberg task. We recorded scalp EEG during the retention interval of a modified Sternberg task. EEG analysis revealed that alpha‑band power (9–12 Hz) increased with memory load, was strongest over posterior and bilateral central regions, persisted throughout most of the 2.8‑s retention interval, and rapidly fell after the probe, indicating a load‑dependent, temporally precise link between alpha activity and working‑memory circuits, while theta activity was also observed in some subjects.

Abstract

To study the role of brain oscillations in working memory, we recorded the scalp electroencephalogram (EEG) during the retention interval of a modified Sternberg task. A power spectral analysis of the EEG during the retention interval revealed a clear peak at 9–12 Hz, a frequency in the alpha band (8–13 Hz). In apparent conflict with previous ideas according to which alpha band oscillations represent brain ‘idling’, we found that the alpha peak systematically increased with the number of items held in working memory. The enhancement was prominent over the posterior and bilateral central regions. The enhancement over posterior regions is most likely explained by the well known alpha rhythm produced close to the parietal-occipital fissure, whereas the lateral enhancement could be explained by sources in somato-motor cortex. A time-frequency analysis revealed that the enhancement was present throughout the last 2.5 s of the 2.8 s retention interval and that alpha power rapidly diminished following the probe. The load dependence and the tight temporal regulation of alpha provide strong evidence that the alpha generating system is directly or indirectly linked to the circuits responsible for working memory. Although a clear peak in the theta band (5–8 Hz) was only detectable in one subject, other lines of evidence indicate that theta occurs and also has a role in working memory. Hypotheses concerning the role of alpha band activity in working memory are discussed.