Frontal midline theta oscillations during working memory maintenance and episodic encoding and retrieval

Abstract

Neural oscillations in the theta band (4-8 Hz) are prominent in the human electroencephalogram (EEG), and many recent electrophysiological studies in animals and humans have implicated scalp-recorded frontal midline theta (FMT) in working memory and episodic memory encoding and retrieval processes. However, the functional significance of theta oscillations in human memory processes remains largely unknown. Here, we review studies in human and animals examining how scalp-recorded FMT relates to memory behaviors and also their possible neural generators. We also discuss models of the functional relevance of theta oscillations to memory processes and suggest promising directions for future research.

Figure 1

Power spectrum showing prominent peak at theta (4-8Hz) frequency band. Frequency decomposition was computed on scalp-recorded EEG data from Fz channel during the maintenance of temporal order information in a working memory task (i.e., the “ORDER” trial condition in Figure 2A). The power spectrum was computed on data during the delay period of ORDER working memory task on trials in which temporal order information was correctly identified. Power spectrum was computed based on scalp-recorded EEG data collected from one example participant in Hsieh et al. (2011).

Figure 2

FMT power increases during maintenance of temporal order information. (A) Schematic diagram of the working memory tasks. (B) Time-frequency spectrogram illustrates the difference in oscillatory power between correct order and correct item trials during working memory delay. The x-axis represents time relative to the onset of the 4s delay period and the y-axis represents logarithmically spaced frequencies. The shown spectrogram is the average of three time-frequency spectrograms from Fz, F1, and F2 channels. (C) Topographic map of the difference in oscillatory power between correct order and correct item trials in theta frequency band during working memory delay. Oscillatory power is computed from current source density (CSD) estimates of scalp-recorded EEG that can more sharply localize EEG activities from superficial neocortical sources. Figures are adapted from Hsieh et al. (2011).

Figure 3

Pre-stimulus FMT enhancement before successful episodic retrieval. (A) Schematic diagram of the episodic retrieval task. (B) (Upper) time course of theta power for item+source, item-only, and item-incorrect trials at Fz electrode site. (Lower) Time-frequency spectrogram illustrates the difference in oscillatory power between item+source versus item-only retrieval at Fz electrode site. The zero time point on the x-axis represents the onset of test stimulus. (C) Topographic map of the difference in oscillatory power between item+source and item-only in theta frequency band during the pre-stimulus period (i.e., −150 to 0 ms before test stimulus onset). (D) Pre-stimulus theta power differences at Fz were positively correlated with individual differences in source memory accuracy. Figures are adapted from Addante et al. (2011).