Endogenous Theta as a Function of Task Load and Working Memory Capacity: Revisiting the Role of Slower Frequencies in Theta-Gamma Coupling

Recent findings have shown the influential role of theta-gamma cross-frequency coupling in the maintenance of items in Working Memory (WM). Specifically, it has been proposed that individual items are represented within gamma oscillations that are nested within slower theta waves. Thus, longer theta cycles would theoretically allow the retention of more items. On the basis of this theory, the present preregistered study aimed to test whether slowing theta rhythms via 4 Hz transcranial alternating current stimulation (tACS) over the frontoparietal network improves WM capacity. Given the potential role of individual differences in stimulation effects, participants were divided into high and low WM capacity groups based on initial screening. Additionally, task difficult was also manipulated to maximize the likelihood of obtaining an improvement in performance. Contrary to our hypothesis, tACS did not improve behavioral outcomes regardless of task difficulty or baseline WM capacity. Post-stimulation EEG effects were also analyzed, but no modulation of theta power, peaks, or phase synchronization was found. Importantly, differences in mean theta frequency were observed before any stimulation between high- and low-capacity WM participants, with higher frequencies present in participants with better WM performance. Furthermore, higher mean frequency was also observed in high-load trials versus low-load trials. These findings contradict the predictions of the theta-gamma coupling theory. Taken together, these results raise questions about the validity of the theta-gamma coupling theory to explain cortical dynamics related to WM.