Spectral-power associations reflect amplitude modulation and within-frequency interactions on the sub-second timescale and cross-frequency interactions on the seconds timescale

We investigated the global structure of intrinsic cross-frequency dynamics by systematically examining power-based temporal associations among a broad range of oscillation frequencies both within and across EEG-based current sources (sites). We focused on power-based associations that could reveal unique timescale dependence (independently of interacting frequencies). Large spectral-power fluctuations across all sites occurred at two characteristic timescales, sub-second and seconds, yielding distinct patterns of cross-frequency associations. On the (fast) sub-second timescale, within-site (local) associations were consistently between pairs of β-γ frequencies differing by a constant Δf (particularly Δf~10Hz at posterior sites and Δf~16Hz at lateral sites) suggesting that higher-frequency oscillations are organized into Δf amplitude-modulated packets, whereas cross-site (long-distance) associations were all within-frequency (particularly in the 6-12Hz and >30Hz ranges, suggestive of feedback and feedforward interactions). On the (slower) seconds timescale, within-site (local) associations were characterized by a broad range of frequencies selectively associated with ~10Hz at posterior sites and associations among higher (>20Hz) frequencies at lateral sites, whereas cross-site (long-distance) associations were characterized by a broad range of frequencies at posterior sites selectively associated with ~10Hz at other sites, associations among higher (>20Hz) frequencies among lateral and anterior sites, and prevalent associations at ~10Hz. Regardless of timescale, within-site (local) cross-frequency associations were weak at anterior sites indicative of frequency-specific operations. Overall, these results suggest that the (fast) sub-second-timescale coordination of spectral power is limited to local amplitude modulation and insulated within-frequency long-distance interactions (likely feedforward and feedback interactions), while characteristic patterns of cross-frequency interactions emerge on the (slower) seconds timescale. The results also suggest that the occipital α oscillations play a role in organizing higher-frequency oscillations into ~10Hz amplitude-modulated packets to communicate with other regions. Functional implications of these timescale-dependent cross-frequency associations await future investigations.