To provide an alternative means of evaluating this nested cortical network in younger adults and its potentially disconnected nature in older adults, we explored the phase-angle preference and information flow direction of memory-related theta-gamma PAC within left temporal electrodes across subjects. Unlike younger adults who showed intact working memory network nesting on memory trials, evidenced by significantly nonuniform theta phase sorted gamma amplitude values (z = −2.724, p = 0.003), older adults had sorted amplitude values that did not differ significantly from a random distribution (z = −0.540, p = 0.286) (Fig. 3c). Analysis of PAC directionality using phase-slope index (PSI) corroborated this pattern of results. Younger adults showed theta phase predictive of gamma amplitude over left temporal electrodes, rather than the converse (t41 = 3.029, p = 0.004, dz = 0.467), but this directionality was markedly reduced for older adults relative to younger subjects (t82 = 2.735, p = 0.009, dz = 0.422) (Fig. 3d).
