To be presented at the Society for Psychophysiological Research, Lisbon, 2005

Frontal midline EEG dynamics during working memory

Julie Onton and Scott Makeig

During a working memory task, an electroencephalographic (EEG) process originating in or near dorsal anterior cingulate cortex exhibited several characteristic modes of oscillatory activity in at least three frequency bands whose amplitudes varied widely across trials. During presentations of letter series to be memorized or ignored, this process, identified by independent component analysis (ICA) of the unaveraged data, expressed distinct (5-7 Hz) frontal midline theta (fmq) and low-beta (12-15 Hz) activities that increased weakly, on average, with memory load. Low-beta activity, but not fmq, was larger during memorize letters than during ignore letter presentations. Following onset of a probe letter after each letter sequence, the same fmq processes emitted a brief burst of 3-Hz activity whose amplitude was unaffected by memory load or by the latency of the succeeding subject button press. The weak mean fmq increase with increasing memory load was produced by progressively larger theta power in a relatively small fraction of fmq component trials, this increase accounting for the entire mean theta increase with memory load on the frontal midline scalp. Trial-to-trial variations in theta power co-varied moderately with theta power in other frontal and left temporal processes. Some of this variability may be linked to trial-to-trial differences in task demands and behavioral context.