Changes in direct current (DC) potentials and infra-slow EEG oscillations at the onset of the luteinizing hormone (LH) pulse

Brain Infraslow Activity Correlates With Arousal Levels

The functional role of the brain’s infraslow activity (ISA, 0.01–0.1 Hz) in human behavior has yet to be elucidated. To date, it has been shown that the brain’s ISA correlates with behavioral performance; task performance is more likely to increase when executed at a specific ISA phase. However, it is unclear how the ISA correlates behavioral performance. We hypothesized that the ISA phase correlation of behavioral performance is mediated by arousal. Our data analysis results showed that the electroencephalogram (EEG) ISA phase was correlated with the galvanic skin response (GSR) amplitude, a measure of the arousal level. Furthermore, subjects whose EEG ISA phase correlated with the GSR amplitude more strongly also showed greater EEG ISA modulation during meditation, which implies an intimate relationship between brain ISA and arousal. These results may help improve understanding of the functional role of the brain’s ISA.

Differential modulation of behavior by infraslow activities of different brain regions

The oscillation phase of electroencephalograms (EEGs) is associated with behavioral performance. Several studies have demonstrated this association for relatively fast oscillations (>1 Hz); a similar finding has also been reported for slower oscillations, showing that behavioral performance is correlated with the phase of infraslow activity (ISA, 0.01-0.1 Hz) of electroencephalography (EEG). However, the previous study only investigated ISA in a local brain region using a relatively simple task (somatosensory discrimination task), leaving it difficult to determine how the EEG ISA for various brain regions is associated with behavioral performance. In addition, it is not known whether the EEG ISA phase modulates more complex behavioral task performance. In the present study, we analyzed the ISA of whole-brain EEG of participants performing various behaviors while playing video games. We found that behavior was associated with the specific oscillation phase of EEG ISA when that behavior was independent of other behaviors. In addition, we found that the EEG ISA oscillation phases modulating the different behaviors varied across brain regions. Our results suggest that the EEG ISA for different brain regions modulates behavioral performance in different ways and such modulation of EEG ISA can be generalized to diverse behaviors. This study may deepen the understanding of how EEG ISA modulates behavior and increases the applicability of EEG ISA.

Infra-slow fluctuations in electrophysiological recordings, blood-oxygenation-level-dependent signals, and psychophysical time series

Converging electrophysiological and neuroimaging data show that mammalian brain dynamics are governed by spontaneous modulations of neuronal activity levels in cortical and subcortical structures. The time scales of these fluctuations form a continuum from seconds to tens and hundreds of seconds corresponding to slow (0.1-1Hz), infra-slow (0.01-0.1Hz), and "ultradian" (<0.01Hz) frequency bands, respectively. We focus here on the spontaneous neuronal dynamics in the infra-slow frequency band, infra-slow fluctuations (ISFs), and explore their electrophysiological substrates and behavioral correlates. Although electrophysiological ISFs and the associated infra-slow modulations of fast (here, >1Hz) neuronal activities have been recognized on numerous occasions since late 50's, a resurgence in interest towards this frequency band has been driven by a discovery that ISFs in blood-oxygenation-level dependent (BOLD) signals are correlated among specific constellations of brain regions, which constitute intrinsic connectivity networks and define the dynamic architecture of spontaneous brain activity at large. Importantly, electrophysiological and BOLD signal ISFs are directly correlated both with ISFs in amplitudes of fast neuronal activities and with ISFs in behavioral performance. Moreover, both electrophysiological and neuroimaging data suggest that the apparently scale-free ISFs may arise from more local quasi-periodic infra-slow oscillations with a contribution of time-scale-specific cellular-level mechanisms. We conclude that ISFs in electrophysiological recordings, BOLD signals, neuronal activity levels, and behavioral time series are likely to reflect the same underlying phenomenon; a superstructure of interacting and transiently oscillatory ISFs that regulate both the integration within and decoupling between concurrently active neuronal communities.

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.

Learning increases human electroencephalographic coherence during subsequent slow sleep oscillations

Learning is assumed to induce specific changes in neuronal activity during sleep that serve the consolidation of newly acquired memories. To specify such changes, we measured electroencephalographic (EEG) coherence during performance on a declarative learning task (word pair associations) and subsequent sleep. Compared with a nonlearning control condition, learning performance was accompanied with a strong increase in coherence in several EEG frequency bands. During subsequent non-rapid eye movement sleep, coherence only marginally increased in a global analysis of EEG recordings. However, a striking and robust increase in learning-dependent coherence was found when analyses were performed time-locked to the occurrence of slow oscillations (<1 Hz). Specifically, the surface-positive half-waves of the slow oscillation resulting from widespread cortical depolarization were associated with distinctly enhanced coherence after learning in the slow-oscillatory, delta, slow-spindle, and gamma bands. The findings identify the depolarizing phase of the slow oscillations in humans as a time period particularly relevant for a reprocessing of memories in sleep.

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.

Grouping of spindle activity during slow oscillations in human non-rapid eye movement sleep

Based on findings primarily in cats, the grouping of spindle activity and fast brain oscillations by slow oscillations during slow-wave sleep (SWS) has been proposed to represent an essential feature in the processing of memories during sleep. We examined whether a comparable grouping of spindle and fast activity coinciding with slow oscillations can be found in human SWS. For negative and positive half-waves of slow oscillations (dominant frequency, 0.7-0.8 Hz) identified during SWS in humans (n = 13), wave-triggered averages of root mean square (rms) activity in the theta (4-8 Hz), alpha (8-12 Hz), spindle (12-15 Hz), and beta (15-25 Hz) range were formed. Slow positive half-waves were linked to a pronounced and microV (23.4%; p < 0.001, with reference to baseline) at the midline central electrode (Cz). In contrast, spindle activity was suppressed during slow negative half-waves, on average by -0.65 +/- 0.06 microV at Cz (-22%; p < 0.001). An increase in spindle activity 400-500 msec after negative half-waves was more than twofold the increase during slow positive half-waves (p < 0.001). A similar although less pronounced dynamic was observed for beta activity, but not for alpha and theta frequencies. Discrete spindles identified during stages 2 and 3 of non-rapid eye movement (REM) sleep coincided with a discrete slow positive half-wave-like potential preceded by a pronounced negative half-wave (p < 0.01). These results provide the first evidence in humans of grouping of spindle and beta activity during slow oscillations. They support the concept that phases of cortical depolarization during slow oscillations, reflected by surface-positive (depth-negative) field potentials, drive the thalamocortical spindle activity. The drive is particularly strong during cortical depolarization, expressed as surface-positive field potentials.

Repetitive concentric wave-ring spread of oligemia/hyperemia in the sensorimotor cortex accompanying K(+)-induced spreading depression in rats and cats

Vascular changes accompanying spreading depression (SD) remain controversial. We examined dynamic alterations of local cerebral blood volume (CBV) during SD by observing light transmission at an isosbestic point of hemoglobin (550 nm) in seven rats and five cats under alpha-chloralose/urethane anesthesia. The two species were used for comparison between the lissencephalic and gyrencephalic brains. We found that a concentrated K(+) solution microinjected into the sensorimotor cortex provoked CBV changes that appeared as a repetitive propagation of concentric wave-rings of ischemia followed by hyperemia expanding peripherally from the injection site at speeds of 1.9-3.2 mm/min. The dynamic CBV changes continued repeatedly every 1-5 min for more than 30 min in three rats, ceased within 30 min in three rats and remained at the site of K(+) injection in one rat. Similar repeated CBV changes occurred in two out of five cats.