Sleep spindles comprise a subset of a broader class of electroencephalogram events.

STUDY OBJECTIVES: Sleep spindles are defined based on expert observations of waveform features in the electroencephalogram (EEG) traces. This is a potentially limiting characterization, as transient oscillatory bursts like spindles are easily obscured in the time domain by higher amplitude activity at other frequencies or by noise. It is therefore highly plausible that many relevant events are missed by current approaches based on traditionally defined spindles. Given their oscillatory structure, we reexamine spindle activity from first principles, using time-frequency activity in comparison to scored spindles. METHODS: Using multitaper spectral analysis, we observe clear time-frequency peaks in the sigma (10-16 Hz) range (TFσ peaks). While nearly every scored spindle coincides with a TFσ peak, numerous similar TFσ peaks remain undetected. We therefore perform statistical analyses of spindles and TFσ peaks using manual and automated detection methods, comparing event cooccurrence, morphological similarities, and night-to-night consistency across multiple datasets. RESULTS: On average, TFσ peaks have more than three times the rate of spindles (mean rate: 9.8 vs. 3.1 events/minute). Moreover, spindles subsample the most prominent TFσ peaks with otherwise identical spectral morphology. We further demonstrate that detected TFσ peaks have stronger night-to-night rate stability (ρ = 0.98) than spindles (ρ = 0.67), while covarying with spindle rates across subjects (ρ = 0.72). CONCLUSIONS: These results provide compelling evidence that traditionally defined spindles constitute a subset of a more generalized class of EEG events. TFσ peaks are therefore a more complete representation of the underlying phenomenon, providing a more consistent and robust basis for future experiments and analyses.

Increased Conductivity and Reduced Settling Time of Carbon-Based Electrodes By Addition of Sea Salt for Wearable Application.

A carbon-based dry electrode is designed to measure bio-potential from skin surface without hydrogel. Consequently, unlike Ag/AgCl electrodes, the carbon-based electrodes require some settling time before a high-fidelity signal is obtained due to the process for impedance matching among skin surface, electrode and amplifiers in biometric system. Besides, especially, when electrocardiogram (ECG) is measured at some distance away from the chest using carbon-based electrodes for wearable application, the settling time could be a critical concern for immediate data collection due to the smaller bio-potential and bigger motion artifact noises. The settling time was defined as the time it takes for the carbon-based electrodes to have the same impedance as that of Ag/AgCl electrodes at a particular frequency (< 1 kHz) for bio-signals. In this study, we investigated the characteristics of the skin contact impedance as a function of time using carbon-based electrodes with and without sea salt and different thickness. Specifically, sea salt was added to the carbon black (SCB)/polydimethlysiloxane (PDMS) electrode to examine the level of enhanced conductivity and reduction of settling time. We used SCB/PDMS and CB/PDMS electrodes with thickness of 1.0 mm and 1.5 mm, examined their electrode and skin contact impedance values and compared them to Ag/AgCl electrodes. We collected impedance data from seven subjects using both SCB and CB/PDMS electrodes every 10 minutes for 50 minutes. A SCB/PDMS electrode showed lower impedance than a CB/PDMS electrode, and for both types of electrodes, higher thickness resulted in lower impedance. The same results were found for skin contact impedance. The settling times of the SCB/PDMS electrodes were found to be $20 \pm 10$ minutes and $40 \pm 10$ minutes for widths of 1.0 mm and 1.5 mm, respectively. The settling time for CB/PDMS without sea salt resulted in significantly higher settling time (> 50 minutes) when compared to SCB/PDMS electrodes. In summary, when carbon-based electrodes are used to measure bio-signals from skin surface for wearable application, its settling time can be partially offset by adding sea salt to CB/PDMS electrode and by making it thinner.