To test whether signals flow from the sender to the receiver hemisphere, we measured spectral Granger causality between LFPs in the two prefrontal hemispheres. Granger causality is a measure of how much variance in one signal can be explained by the recent history of another (here, LFPs at distinct sites), beyond what can be explained by the signal’s own dynamics. Spectral Granger causality expresses these causal interactions in the frequency domain (Dhamala et al., 2008). As predicted, causality was greater in the sender-to-receiver direction (Figure 8A, green) than in the opposite, receiver-to-sender direction (orange). Although this difference was significant for all frequencies from approximately 10–40 Hz (Figure 8B, gray stars; p < 0.01, paired t test), it was greatest at the same high-beta frequencies that synchrony peaked at (~20–40 Hz; Figure 8B, gray curve). This asymmetric directionality was not observed in no-swap trials between sites contralateral and ipsilateral to the sample location (Figures 8C and 8D). Between-region differences in signal-to-noise ratio (SNR) can result in false-positive Granger causality. As our results were pooled across left-to-right and right-to-left hemispheres, any between-hemisphere
