Slow potential shifts at sleep--wake transitions and shifts between NREM and REM sleep

Infra-slow fluctuations in cortical potentials and respiration drive fast cortical EEG rhythms in sleeping and waking states

OBJECTIVE

Infra-slow fluctuations (ISF, 0.008-0.1 Hz) characterize hemodynamic and electric potential signals of human brain. ISFs correlate with the amplitude dynamics of fast (>1 Hz) neuronal oscillations, and may arise from permeability fluctuations of the blood-brain barrier (BBB). It is unclear if physiological rhythms like respiration drive or track fast cortical oscillations, and the role of sleep in this coupling is unknown.

METHODS

We used high-density full-band electroencephalography (EEG) in healthy human volunteers (N = 21) to measure concurrently the ISFs, respiratory pulsations, and fast neuronal oscillations during periods of wakefulness and sleep, and to assess the strength and direction of their phase-amplitude coupling.

RESULTS

The phases of ISFs and respiration were both coupled with the amplitude of fast neuronal oscillations, with stronger ISF coupling being evident during sleep. Phases of ISF and respiration drove the amplitude dynamics of fast oscillations in sleeping and waking states, with different contributions.

CONCLUSIONS

ISFs in slow cortical potentials and respiration together significantly determine the dynamics of fast cortical oscillations.

SIGNIFICANCE

We propose that these slow physiological phases play a significant role in coordinating cortical excitability, which is a fundamental aspect of brain function.

Neurofeedback in ADHD and insomnia: vigilance stabilization through sleep spindles and circadian networks

In this review article an overview of the history and current status of neurofeedback for the treatment of ADHD and insomnia is provided. Recent insights suggest a central role of circadian phase delay, resulting in sleep onset insomnia (SOI) in a sub-group of ADHD patients. Chronobiological treatments, such as melatonin and early morning bright light, affect the suprachiasmatic nucleus. This nucleus has been shown to project to the noradrenergic locus coeruleus (LC) thereby explaining the vigilance stabilizing effects of such treatments in ADHD. It is hypothesized that both Sensori-Motor Rhythm (SMR) and Slow-Cortical Potential (SCP) neurofeedback impact on the sleep spindle circuitry resulting in increased sleep spindle density, normalization of SOI and thereby affect the noradrenergic LC, resulting in vigilance stabilization. After SOI is normalized, improvements on ADHD symptoms will occur with a delayed onset of effect. Therefore, clinical trials investigating new treatments in ADHD should include assessments at follow-up as their primary endpoint rather than assessments at outtake. Furthermore, an implication requiring further study is that neurofeedback could be stopped when SOI is normalized, which might result in fewer sessions.

Declarative memory consolidation: mechanisms acting during human sleep

Of late, an increasing number of studies have shown a strong relationship between sleep and memory. Here we summarize a series of our own studies in humans supporting a beneficial influence of slow-wave sleep (SWS) on declarative memory formation, and try to identify some mechanisms that might underlie this influence. Specifically, these experiments show that declarative memory benefits mainly from sleep periods dominated by SWS, whereas there is no consistent benefit of this memory from periods rich in rapid eye movement (REM) sleep. A main mechanism of declarative memory formation is believed to be the reactivation of newly acquired memory representations in hippocampal networks that stimulates a transfer and integration of these representations into neocortical neuronal networks. Consistent with this model, spindle activity and slow oscillation-related EEG coherence increase during early sleep after intense declarative learning in humans, signs that together point toward a neocortical reprocessing of the learned material. In addition, sleep seems to provide an optimal milieu for declarative memory reprocessing and consolidation by reducing cholinergic activation and the cortisol feedback to the hippocampus during SWS.

Spindle and slow wave rhythms at slow wave sleep transitions are linked to strong shifts in the cortical direct current potential

Electroencephalographic activity at the transition from wakefulness to sleep is characterized by the appearance of spindles (12-15 Hz) and slow wave rhythms including delta activity (1-4 Hz) and slow oscillations (0.2-1 Hz). While these rhythms originate within neocortico-thalamic circuitry, their emergence during the passage into slow wave sleep (SWS) critically depends on the activity of neuromodulatory systems. Here, we examined the temporal relationships between these electroencephalogram rhythms and the direct current (DC) potential recorded from the scalp in healthy men (n=10) using cross-correlation analyses. Analyses focused on transitions from wakefulness to SWS in the beginning of the sleep period, and from SWS to lighter sleep and rapid eye movement (REM) sleep at the end of the first sleep cycle. For spindle, delta and slow oscillatory activity strong negative correlations with the DC potential were found at the transition into SWS with peak correlation coefficients (at zero time lag) averaging r=-0.81, -0.88 and -0.88, respectively (P<0.001). Though slightly lower, distinct negative correlations between these measures were also found at the transition from SWS to REM sleep (-0.78, -0.77 and -0.77, respectively, P<0.001). Fast oscillatory activity in the beta frequency band (15-25 Hz) was correlated positively with the DC potential (r=+0.75, P<0.05, at the passage to SWS). Data indicate close links between increasing spindle, delta and slow oscillatory activity and the occurrence of a steep surface negative cortical DC potential shift during the transition from wake to SWS. Likewise, a DC potential shift toward surface positivity accompanies the disappearance of these oscillatory phenomena at the end of the non-REM sleep period. The DC potential shifts may reflect gradual changes in extracellular ionic (Ca2+) concentration resulting from the generation of spindle and slow wave rhythms, or influences of neuromodulating systems on cortical excitability thereby controlling the emergence of cortical spindle and slow wave rhythms at SWS transitions.

Scalp recorded direct current brain potentials during human sleep

The direct current (DC) potential recorded from the scalp of awake humans has been considered a reflection of general changes in cortical excitability. This study examined DC potential shifts in humans during a night of continuous sleep. Standard polysomnographic recordings and skin temperature were measured simultaneously. Contrary to expectations, average DC potential level indicated higher negativity during nonrapid eye movement (NREM) sleep than REM sleep and wakefulness. Moreover, a dynamic regulation of the DC potential level was revealed in association with the NREM-REM sleep cycle comprising four successive phases: (i) a steep 'NREM-transition-negative shift' during the initial 10-15 min of the NREM sleep period; (ii) a more subtle 'NREM-positive slope' during the subsequent NREM sleep period; (iii) a steep 'REM-transition-positive shift' starting shortly prior to the REM sleep period, and (iv) a 'REM-negative slope', characterizing the remaining greater part of the REM sleep period. DC potential changes were only weakly related to changes in slow-wave activity (r2 < 0.18). The NREM-negative slope and REM-positive slope could reflect, respectively, gradually increasing and decreasing cortical excitability resulting from widespread changes in the depolarization of apical dendrites. In contrast, the NREM-transition-negative shift and the REM-transition-positive shift may reflect the progression and retrogression, respectively, of a long-lasting hyperpolarization in deeply lying neurons.