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Background and objectives The pathophysiologic aspects of parasympathetic nerve (PN) function during sleep in patients with obstructive sleep apnea (OSA) studied by classical power spectrum analysis on heart rate variability (HRV) are highly controversial. The controversy is attributed to methodologic concerns, such as poor time resolution involved in power spectrum analysis. We aimed to establish the appropriate method for the investigation of PN function in OSA patients with apneas and hypopneas using instantaneous time–frequency analysis with complex demodulation (CD) and sufficient time resolution. Methods A total of 30 patients with PSG-confirmed mild to severe OSA were recruited for the analysis of frequency spectra contained in R-R intervals (RRI) of overnight electrocardiograph (ECG) tracings. High-frequency (HF) domains ranging between 0.15 and 0.40 Hz were selected for analysis. Among these domains, the HF domain with the maximum instantaneous amplitude was defined as the main HF peak and was used as the surrogate marker of PN discharge. Based on density spectrum array (DSA) map for main HF peak constructed with a time scale of 1 s and a frequency resolution of 0.002 Hz (HF-DSA map), the shift in central frequency (CF) of main HF peak over time was continuously monitored. When the main HF peak with the same CF lasted for more than 20 s or 5 min on HF-DSA map, the PN function was considered to be stable or very stable. The measurements were then repeated after continuous positive airway pressure (CPAP) treatment. Results The extent of PN-evoked modulation of RRI was enhanced in nonrapid eye movement (NREM) sleep, though the stability was reduced in both NREM and rapid eye movement (REM) sleep. These peculiar behaviors of PN function were reversed by CPAP treatment. Conclusion We found that instantaneous time–frequency analysis allowed estimation of transitional changes in PN function during sleep in OSA patients.

To study brain mechanisms of memory scan, we have measured event-related potentials (ERPs) during the execution of a Sternberg task concerned with movement directions of random dot patterns (RDPs). Five healthy male subjects participated in the experiment. Event-related potentials were recorded at 20 electrode positions over the entire head. A prominent positive ERP component peaking at 380–390 ms known as P300 was observed in the parietal region after the presentation of the probe moving random dot pattern, whereas the P300 was not observed after the presentation of a control stationary RDP. On the other hand, a positive ERP component peaking at 127 ms was commonly observed in the occipital region after the presentation of both moving and control stationary RDPs. These findings suggest that P300 may reflect cortical activities related to the memory scan of movement directions of RDP.