To test whether age differences in working memory resulted from a lack of temporal integration within large-scale cortical networks, we computed PAC at all electrodes for different combinations of low (2–16 Hz) and high (18–120 Hz) frequencies during the memory maintenance period of the task (500 to 3000 ms post-target onset), and used cluster permutation statistics to identify clusters of electrodes and frequencies showing significant differences in PAC between memory and control blocks for each age group. Figure 3a shows the results. For older adults, no significant clusters in electrode or source space emerged as significant. In contrast, for younger adults, one cluster of left temporal electrodes showed significantly increased memory-specific PAC (pcorrected < 0.01) between 7–9 Hz phase frequencies and 26–34 Hz amplitude frequencies. In source space, cluster analysis estimated this PAC effect to the left temporal cortex. Further, a group x block interaction was significant for PAC at the 8-Hz to 30-Hz center frequencies collapsed across left temporal electrodes (pcorrected < 0.01; Fig. 3b), demonstrating frequency specificity of the PAC deficiency older adults experienced on memory trials.
