Transcranial Doppler Ultrasonography and EEG Recording: A Novel Approach to Investigating Resting State

Transcranial Doppler (TCD) ultrasonography enables non-invasive, real-time assessment of cerebral hemodynamics. This study investigated the relationship between EEG, TCD, and photoplethysmography (PPG) during rest, testing the hypothesis that hemodynamic metrics possess intrinsic dynamics beyond immediate neural demand or autonomic control. Fifty-seven healthy adults underwent simultaneous EEG, bilateral middle cerebral artery TCD, and PPG recordings. We employed time-lagged correlation and quantified the duration of stable "states" for each modality. Specific neurovascular couplings were confirmed, including an association between posterior alpha-rhythm suppression and reduced flow velocity, and between decreased theta power and increased vascular resistance. Crucially, we found a temporal divergence: hemodynamic states identified via TCD were significantly more prolonged (mean ~ 22.6 seconds) than neural states captured by EEG (mean ~ 7.2 seconds). PPG-derived indices showed no significant correlation with steady-state TCD metrics, suggesting the observed hemodynamics are not primarily driven by autonomic control. Furthermore, the duration of stable TCD states correlated with baseline motor performance. We conclude that the resting-state brain is characterized by dissociable neural, vascular, and autonomic activity on distinct timescales. While rapid neurovascular coupling links momentary neural activity to vascular tone, the cerebrovascular system exhibits its own slower dynamics, independent of direct autonomic modulation, likely reflecting the integration of local metabolic and neurogenic processes.