We previously showed that fluctuations of the EEG alpha power at a very low frequency range reflects ability to flexibly adjust behaviors corresponding to cognitive load (SAS conference, 2021). Though activities of the autonomic nervous system (ANS) have also been reported to affect cognitive performance, details of interplays of the brain and ANS in behavioral adjustment accompanying cognitive load are not fully elucidated. Thus, we explored this issue by examining effects of the slow EEG fluctuation of the alpha power, heart rate variability (HRV) as a parasympathetic index, and skin conductance responses (SCRs) as a sympathetic index on performance of a 3-back task. Participants (n=12) conducted the task for 6 minutes. We analyzed fluctuation of EEG alpha power to estimate brain states, an index of HRV: root mean square of successive differences of R-R intervals (RMSSD), frequencies of spontaneous SCRs, as well as accuracy of the 3-back task. Greater RMSSD significantly predicted better task accuracy (r=0.59, p<0.05). A mediation analysis showed a tendency that fluctuation of the 2nd-order time series of the alpha power of EEG at around 0.01 Hz mediated the association between RMSSD and task accuracy. Frequencies of SCRs showed no consistent patterns of associations with EEG alpha activity and task accuracy. These results suggest that parasympathetic activity, compared to sympathetic activity, can modulate brain functions for more flexible behavioral adjustment during high cognitive load situations. Neural mechanisms of this phenomenon should be clarified in future.
