Abstract
Previous research has indicated that phase-amplitude coupling between the phase of theta oscillations ( ~6 Hz) and the amplitude of gamma oscillations (~30-90 Hz) may be a primary mechanism by which information is maintained in memory (Sauseng et al., 2009). However, the degree to which theta-gamma coupling is specific to memory-related processes remains unclear. To investigate this, we collected EEG data from 60 subjects while they performed three variants on a visual change detection task sequence: one in which subjects were asked to maintain the colors of the squares in memory (memory condition); one in which subjects were asked to attend to the colors of the squares in search of a target color (attention condition); and one in which subjects simply passively viewed the colored squares (passive condition). We found evidence for cross-frequency coupling in all three conditions, suggesting that theta-gamma coupling is not unique to memory tasks. We did find small but consistent differences in the 35-50 Hz range, with the memory condition showing somewhat higher coupling than the attention or passive conditions. These differences did not correlate with behavioral performance in the memory task (p > 0.5 in all conditions), so the functional significance of this effect is unknown. Overall, it seems that theta-gamma phase-amplitude coupling is unlikely to be unique to visual working memory processes. Future studies of theta-gamma coupling may consider the use of appropriate controls to isolate generalized coupling from coupling that is related to the cognitive construct of interest.