The seed region and frequencies chosen for the PLV analysis were based on the location and frequencies involved in significant changes in PAC. Because no localized sources of PAC were observed for older adults in the sham condition of Experiment 1, we used the seed region identified from younger adults in the sham condition. However, we confirmed the results from this analysis by recalculating source PLV for older adults in the sham condition using seed regions defined by significant PAC identified from the active stimulation condition in the same subjects. For the seeded PLV analysis, we averaged the cross-spectral density matrices between 500 and 3000 ms in each condition and projected the results of each frequency bin within the theta or gamma band in source space using the dynamic imaging of coherent sources beamformer (see PAC section for description of forward model) 89. For each frequency bin separately, PLV was computed from every seed voxel to all other voxels in the 3294 grid, averaged across all considered frequency bins, and Fisher z transformed. This resulted in two synchronization measures per voxel, one for the memory and one for the control block, for each age group and stimulation condition.
