Effects of Working Memory Load on Oscillatory Power in Human Intracranial EEG

Abstract

Studies of working memory load effects on human EEG power have indicated divergent effects in different frequency bands. Although gamma power typically increases with load, the load dependency of the lower frequency theta and alpha bands is uncertain. We obtained intracranial electroencephalography measurements from 1453 electrode sites in 14 epilepsy patients performing a Sternberg task, in order to characterize the anatomical distribution of load-related changes across the frequency spectrum. Gamma power increases occurred throughout the brain, but were most common in the occipital lobe. In the theta and alpha bands, both increases and decreases were observed, but with different anatomical distributions. Increases in theta and alpha power were most prevalent in frontal midline cortex. Decreases were most commonly observed in occipital cortex, colocalized with increases in the gamma range, but were also detected in lateral frontal and parietal regions. Spatial overlap with group functional magnetic resonance imaging results was minimal except in the precentral gyrus. These findings suggest that power in any given frequency band is not a unitary phenomenon; rather, reactivity in the same frequency band varies in different brain regions, and may relate to the engagement or inhibition of a given area in a cognitive task.

Potentiation of Gamma Oscillatory Activity through Repetitive Transcranial Magnetic Stimulation of the Dorsolateral Prefrontal Cortex

“…The finding that g-oscillatory activity was most pronounced at N-back conditions of greatest difficulty is consistent with earlier studies examining the role of g-oscillatory activity and WM (Basar-Eroglu et al, 2007;Howard et al, 2003;Meltzer et al, 2008). For example, Howard et al (2003) first showed a linear increase in g-oscillatory power with WM load in epileptic patients performing the Sternberg WM task (Howard et al, 2003).…”

“…For example, Howard et al (2003) first showed a linear increase in g-oscillatory power with WM load in epileptic patients performing the Sternberg WM task (Howard et al, 2003). Additional studies confirmed enhanced g-oscillatory activity with WM load using both the Sternberg (Meltzer et al, 2008) and N-back (Basar-Eroglu et al, 2007) tasks. In these studies, however, WM loads comparable with the 3-back condition were not examined.…”

“…These authors contend that measures of evoked g-activity may better mitigate such effects from miniature saccadic activity on this neurophysiological phenomenon (Yuval-Greenberg et al, 2008). Considering that induced g-activity may reflect involuntary eye movements with emerging evidence supporting the association of evoked g-activity with WM (Howard et al, 2003;Meltzer et al, 2008;Basar-Eroglu et al, 2007), we selected to measure the effect of rTMS on evoked g-activity.…”

“…Both evoked and induced g-activities have been associated with sensory and cognitive processing (Lutzenberger et al, 1995;Muller et al, 1996;Cho et al, 2006). Moreover, induced g-oscillatory activity has been shown to increase with increased cognitive control (Cho et al, 2006), whereas other studies have shown that evoked g-oscillatory activity increases with cognitive demand in WM paradigms (Howard et al, 2003;Basar-Eroglu et al, 2007;Meltzer et al, 2008). As such, we measured the mean evoked g-power from frontal electrodes while the participants completed the N-back task before (pre) and after (post) rTMS was administered over the right and left DLPFC.…”

“…For example, the N-back task requires participants to determine whether the current stimulus is the same stimulus that was presented 'N' trials back, thus allowing the evaluation of increasing cognitive demand (ie, WM load) on g-oscillatory activity. Further, a growing body of evidence has shown increased g-oscillatory activity with cognitive demand (Howard et al, 2003;Meltzer et al, 2008;Basar-Eroglu et al, 2007). For example, Cho et al (2006) reported that g-oscillatory activity increased with the increased cognitive control in the frontal regions that was related to performance (Cho et al, 2006), suggesting that g is modulated by cognitive demand, which may underlie WM performance.…”