Prefrontal Cortex Modulation during Anticipation of Working Memory Demands as Revealed by Magnetoencephalography

Abstract

During the anticipation of task demands frontal control is involved in the assembly of stimulus-response mappings based on current goals. It is not clear whether prefrontal modulations occur in higher-order cortical regions, likely reflecting cognitive anticipation processes. The goal of this paper was to investigate prefrontal modulation during anticipation of upcoming working memory demands as revealed by magnetoencephalography (MEG). Twenty healthy volunteers underwent MEG while they performed a variation of the Sternberg Working Memory (WM) task. Beta band (14-30 Hz) SAM (Synthetic Aperture Magnetometry) analysis was performed. During the preparatory periods there was an increase in beta power (event-related synchronization) in dorsolateral prefrontal cortex (DLPFC) bilaterally, left inferior prefrontal gyrus, left parietal, and temporal areas. Our results provide support for the hypothesis that, during preparatory states, the prefrontal cortex is important for biasing higher order brain regions that are going to be engaged in the upcoming task.

Figure 1

Trial sequence in the working memory (letters) and control (arrows) conditions. Three consecutive working memory trials alternated with three consecutive control trials. ITI = intertrial interval.

Figure 2

ROI templates drawn on a representative participant's MRI scan along with the average time courses from each of the left hemisphere ROIs. a: anticipatory period; e: encoding period; d: delay; r: response period.

Figure 3

Time courses from each of the right hemisphere ROIs. a: anticipatory period; e: encoding period; d: delay; r: response period.

Figure 4

Group maps corresponding to 200 ms interval (from −1700 ms to −1500 ms) preceding the trials onset at beta frequency (14–30 Hz); P < .01 corrected; DLPFC: dorsolateral prefrontal cortex; IFG: Inferior frontal gyrus; IPL: inferior parietal lobe; PC: postcentral gyrus; SFG: superior frontal gyrus. Red color coding indicates task related power increase; blue color coding indicates task related power decrease. The figure shows data rendered onto a Talairach-space surface template.

Figure 5

Group maps during the encoding, delay and retrieval epochs of the working memory task at beta frequency (14–30 Hz); P < .005 corrected. (a) The encoding epoch was associated with fairly symmetrical ERD in primary visual and visual association cortex. The map shows an epoch of 500 ms duration (from 200 ms to 700 ms after the memory set presentation). (b) The delay epoch was associated with beta ERS in bilateral visual areas and beta ERDs in left IFG and left premotor regions. The map shows an epoch of 400 ms duration (from 300 ms to 700 ms after the delay began). (c) The response period beta was associated with beta ERDs over a distributed network including left DLPFC, left IFG and premotor areas, right IFG, temporal and parietal regions. The map shows an epoch of 300 ms duration (from 0 ms to 300 ms after the presentation of the probe. DLPFC: dorsolateral prefrontal cortex; IFG: inferior frontal gyrus; PM: premotor area; IPL: inferior parietal lobe; STG: superior temporal gyrus; MTG: middle temporal gyrus; MOG: middle occipital gyrus. Red color coding indicates task related power increase; blue color coding indicates task related power decrease. The figure shows data rendered onto a Talairach-space surface template.