, 2012). In a typical working Androgen Receptor antagonist memory experiment, subjects are presented with a list of several items. This is followed by a delay period (usually <10 s) during which no information is presented. Subjects are the shown a test item and must make a response based on the properties of the information stored in their working
memory. In humans, MEG studies and intracranial recordings have reported an increase in gamma power during the delay period of working memory tasks. Importantly, this increase varies with the number of items being held in memory (Howard et al., 2003; Roux et al., 2012; van Vugt et al., 2010). An increase in spike-field coherence during the delay period has also been observed (Pesaran et al., 2002). Several studies in humans have reported sustained theta band cortical activity during the delay
period (Gevins et al., 1997; Jensen and Tesche, 2002; Raghavachari et al., 2001; Scheeringa et al., 2009), pointing to a role for theta in working memory. One objection to this conclusion is that single-unit recording of persistent firing during a working memory task did not reveal any obvious theta rhythmicity (Funahashi et al., 1991). However, rhythmicity can be difficult to detect by analysis of spikes alone and is more easily detected by determining whether spikes are phase locked to the oscillations in the local field potential (Wang, 2010). Consistent with this, experiments in monkeys have shown that persistent firing during a working memory task
next is phase locked to low-frequency (theta/delta) Crizotinib ic50 oscillations in the local field potential, both in extrastriate cortex (Lee et al., 2005; Liebe et al., 2012) and in prefrontal cortex (Siegel et al., 2009). Theta-gamma coupling during working memory has been demonstrated in humans both in cortex (Canolty et al., 2006; Lee et al., 2005) and in the hippocampus (Maris et al., 2011). Gamma power in the hippocampus is modulated by the phase of theta oscillations during working memory retention, and the strength of this cross-frequency coupling predicts individual working memory performance (Axmacher et al., 2010). Importantly, the particular cortical regions demonstrating cross-frequency coupling depend on the nature of the information being held in working memory. The spatial distribution of gamma band power in cortex can be used to predict whether working memory is maintaining an indoor or outdoor scene (Fuentemilla et al., 2010), and the gamma activity with this predictive capability is phase locked to the theta activity. In another study, gamma power at certain sites (primarily in occipital cortex) was shown to depend on the particular letter being viewed, and gamma was found to be phase locked to theta (Jacobs and Kahana, 2009).