Comparison of FC measures
FC measures can be broadly classified into methods that are sensitive to volume conduction (such as coherence, PLV and PPC) versus those that are not (for example, imaginary part of coherence (Nolte et al. , 2004), phase lag index (PLI; (Stam et al. , 2007))). We used PPC in this report since the measures that are insensitive to volume conduction also tend to be less reproducible (Duan et al., 2021). Since our main aim was to test whether stimulus-induced gamma FC could be modulated by aging or cognitive impairment, we used the measure that is widely used and has been shown to yield reproducible results (Duan and colleagues found that PLV had highest reproducibility, and PPC is an unbiased estimator of the square of PLV). Further, the potential influence of volume conduction was reduced by power-matching, which discards the common power factor (van Vliet et al. , 2018). In addition, to exclude the local connections that are corrupted by volume conduction, we considered average FC over the electrode pairs within [-0.5 to 0.5] in\(\cos\left(\Delta\theta\right)\) space. Future studies that incorporate other FC measures that are insensitive to volume conduction or incorporate this analysis in the source space will be useful to reveal the underlying neural mechanisms behind the differences in power and FC with aging and cognitive impairment.