EEG source identification: frequency analysis during sleep

This article deals with a new approach in sleep characterization that combines EEG source localisation methods with standard frequency analysis of multielectrode EEGs. First, we describe the theoretical methodology and the benefits that we get from a three-dimensional image (LORETA) of the cerebral activity related to a frequency band. Then, this new application is used as signal-processing technique on sleep EEG recordings obtained from young male adults using four frequency bands (delta 0.5-3.5 Hz, theta 4.0-7.5 Hz, alpha 8.0-12.5 Hz and beta 13.0-32.0 Hz) in different sleep stages. Finally, we show that the obtained results are highly consistent with other physiological assessments (standard EEG mapping, functional magnetic resonance imaging, etc.), but give us more realistic additional information on the generators of electromagnetic cerebral activity.     

Segmentation of EEG during sleep using time-varying autoregressive modeling

Time-varying AR modeling is applied to sleep EEG signal, in order to perform parameter estimation and detect changes in the signal characteristics (segmentation). Several types of basis functions have been analyzed to determine how closely they can approximate parameter changes characteristics of the EEG signal. The TV-AR model was applied to a large number of simulated signal segments, in order to examine the behaviour of the estimation under various conditions such as variations in the EEG parameters and in the location of segment boundaries, and different orders of the basis functions. The set of functions that is the basis for the Discrete Cosine Transform (DCT), and the Walsh functions were found to be the most efficient in the estimation of the model parameters. A segmentation algorithm based on an “Identification function” calculated from the estimated model parameters is suggested.     

Interhemispheric EEG asymmetries during unilateral bright-light exposure and subsequent sleep in humans

We tested whether evening exposure to unilateral photic stimulation has repercussions on interhemispheric EEG asymmetries during wakefulness and later sleep. Because light exerts an alerting response in humans, which correlates with a decrease in waking EEG theta/alpha-activity and a reduction in sleep EEG delta activity, we hypothesized that EEG activity in these frequency bands show interhemispheric asymmetries after unilateral bright light (1,500 lux) exposure. A 2-h hemi-field light exposure acutely suppressed occipital EEG alpha activity in the ipsilateral hemisphere activated by light. Subjects felt more alert during bright light than dim light, an effect that was significantly more pronounced during activation of the right than the left visual cortex. During subsequent sleep, occipital EEG activity in the delta and theta range was significantly reduced after activation of the right visual cortex but not after stimulation of the left visual cortex. Furthermore, hemivisual field light exposure was able to shift the left predominance in occipital spindle EEG activity toward the stimulated hemisphere. Time course analysis revealed that this spindle shift remained significant during the first two sleep cycles. Our results reflect rather a hemispheric asymmetry in the alerting action of light than a use-dependent recovery function of sleep in response to the visual stimulation during prior waking. However, the observed shift in the spindle hemispheric dominance in the occipital cortex may still represent subtle local use-dependent recovery functions during sleep in a frequency range different from the delta range.     

EEG patterns and body temperatures in man during sleep in arctic winter nights

Two Caucasian males, aged 19 and 22, slept at night in sleeping bags (9.0 clo) in an unheated tent at ambient temperatures between –25 and –35°C in the Arctic. Electroencephalographic (EEG) sleep studies were conducted for two baseline nights (19–21°C), 10 cold exposure nights and 2 recovery nights (19–21°C). Rectal and skin temperatures, and heart rates were also recorded. The subjects suffered disturbances in sleep patterns involving an insomnia composed of an increased wakefulness, a decrease in slow wave sleep and a deprivation in rapid eye movement (REM) sleep. Dissimilarities appeared between the subjects which may be related to differences in thermoregulatory responses.     

Changes in respiratory muscle activity in conscious cats during experimental dives at 101 ATA.

The rationale for the present study was to test the hypothesis that increased work of breathing during experimental deep diving may lead to respiratory muscle fatigue. For this purpose, electromyograms (EMGs) of respiratory and skeletal muscles, plus electrocardiogram and electroencephalogram (EEG) derivatives, were continuously recorded in conscious cats. In each muscle group, the ratio of power in a high (H) to that in a low (L) band of EMG frequencies was computed. Direct diaphragmatic stimulation in selected animals produced a mass action potential to obtain the muscle fiber conduction velocity (MFCV). The maximal pressure was 101 ATA (1,000 msw) with a maximal duration of 72 h. Four cats breathed an He-O2 mixture and five others a ternary mixture (10% N2 in He-O2). Inspired O2 partial pressure was 350 Torr. With the He-O2 mixture, all the animals died within 2-54 h during the study at maximal depth. EEG signs of high-pressure nervous syndrome (HPNS) were present in all cats, and low-frequency (11-14 Hz) hyperbaric tremor discontinuously contaminated all EMG tracings. The H/L ratio computed from diaphragmatic and intercostal muscle EMGs increased after 12 h at 101 ATA. With the He-N2-O2 mixture, the cats survived until the end of the sojourn at 101 ATA, during which no hyperbaric tremor was detected from EMG tracings, and EEG signs of HPNS were weak or absent. From 31 ATA, the H/L ratio decreased significantly in respiratory but not in skeletal muscles; this was associated with decreased MFCV in the diaphragm after several hours at maximal depth.(ABSTRACT TRUNCATED AT 250 WORDS)     

Multichannel EEG analysis of respiratory evoked-potential components during wakefulness and NREM sleep

Airway occlusion in awake humans producesa somatosensory evoked response called the respiratory-related evokedpotential (RREP). In the present study, 29 channel evoked-potentialrecordings were obtained from seven men who were exposed to 250-msinspiratory airway occlusions during wakefulness, stage 1, stage 2, andslow-wave sleep. The RREP recorded during wakefulness was similar toprevious reports, with the unique observation of an additionalshort-latency positive peak with a mean latency of 25 ms. Short-latencyRREP components were maintained in non-rapid-eye-movement (NREM) sleep. The clearly seen N₁ vertex andlate positive complex components during wakefulness were markedlyattenuated during NREM sleep, and two large negative components(N₃₀₀ andN₅₅₀) dominated the sleep RREP.These findings indicate the maintenance of central nervous systemmonitoring of respiratory afferent information during NREM sleep,presumably to facilitate protective arousal responses topathophysiological respiratory phenomena.

     

Effects of age and exercise on physiological dead space during simulated dives at 2.8 ATA.

Physiological dead space (Vds), end-tidal CO(2) (Pet(CO(2))), and arterial CO(2) (Pa(CO(2))) were measured at 1 and 2.8 ATA in a dry hyperbaric chamber in 10 older (58-74 yr) and 10 younger (19-39 yr) air-breathing subjects during rest and two levels of upright exercise on a cycle ergometer. At pressure, Vd (liters btps) increased from 0.34 +/- 0.09 (mean +/- SD of all subjects for normally distributed data, median +/- interquartile range otherwise) to 0.40 +/- 0.09 (P = 0.0060) at rest, 0.35 +/- 0.13 to 0.45 +/- 0.11 (P = 0.0003) during light exercise, and 0.38 +/- 0.17 to 0.45 +/- 0.13 (P = 0.0497) during heavier exercise. During these conditions, Pa(CO(2)) (Torr) increased from 33.8 +/- 4.2 to 35.7 +/- 4.4 (P = 0.0059), 35.3 +/- 3.2 to 39.4 +/- 3.1 (P < 0.0001), and 29.6 +/- 5.6 to 37.4 +/- 6.5 (P < 0.0001), respectively. During exercise, Pet(CO(2)) overestimated Pa(CO(2)), although the absolute difference was less at pressure. Capnography poorly estimated Pa(CO(2)) during exercise at 1 and 2.8 ATA because of wide variability. Older subjects had higher Vd at 1 ATA but similar changes in Vd, Pa(CO(2)), and Pet(CO(2)) at pressure. These results are consistent with an effect of increased gas density.     

Correction of functional disturbances during pregnancy by the method of adaptive EEG biofeedback training

The applicability and efficiency of the electroencephalogram (EEG)-based method of biofeedback (BFB) training in correcting functional disorders in pregnancy have been evaluated in an obstetric/gynecological clinic in 65 pregnant women. Each of them took part in 2–12 examinations, which differed in voluntarily regulated EEG rhythm (a decrease in the θ, an increase in the α, or a decrease in the β rhythm), left-or right-hemispheric location of the EEG derivations, and the duration of the delay (3 or 30 s) of auditory feedback stimuli. As compared to the baseline, the most distinct and significant changes were observed in the α and θ rhythms during medical procedures, testifying to a general relaxing effect of the EEG BFB. The greatest number of successful trials (83.3 ± 4.4%) was observed for the BFB sessions aimed at decreasing the θ activity, and the lowest number (32.0 ± 6.7%), for the attempts at decreasing the EEG β rhythm. Voluntary control of the α rhythm of the EEG yielded a moderate number of successful trials (63.7 ± 5.3%), but it was characterized by the lowest efficiency (demonstrating minimal differences in positive changes in the functional state between successful and unsuccessful trials). The comparatively low effectiveness of voluntary BFB control of the α rhythm may have been determined by its high heterogeneity and suggests the necessity of using more narrowband EEG components in BFB sessions.     

T wave changes in humans and dogs during experimental dives.

Electrocardiogram (ECG) analysis was performed in three human divers studied at 21 and 23.5 ATA while they breathed various gas mixtures containing H2 and/or He (COMEX HYDRA IX experiment) and in five dogs exposed to 91 ATA of He-O2 or He-N2-O2. In all cases, the O2 partial pressure was slightly higher than its physiological value. These human and animal studies reveal that elevated pressure of different inert gases did not change the resting heart rate or its respiratory fluctuation. However, the T wave amplitude increased in proportion to the gas density in the three divers; this was also found in four of the five dogs studied. Changes in peak T wave configurations were also observed in the dog experiments. Positional changes in QRS or T vectors cannot explain these T wave changes.     

Transcranial electrical currents to probe EEG brain rhythms and memory consolidation during sleep in humans

Previously the application of a weak electric anodal current oscillating with a frequency of the sleep slow oscillation (～0.75 Hz) during non-rapid eye movement sleep (NonREM) sleep boosted endogenous slow oscillation activity and enhanced sleep-associated memory consolidation. The slow oscillations occurring during NonREM sleep and theta oscillations present during REM sleep have been considered of critical relevance for memory formation. Here transcranial direct current stimulation (tDCS) oscillating at 5 Hz, i.e., within the theta frequency range (theta-tDCS) is applied during NonREM and REM sleep. Theta-tDCS during NonREM sleep produced a global decrease in slow oscillatory activity conjoint with a local reduction of frontal slow EEG spindle power (8-12 Hz) and a decrement in consolidation of declarative memory, underlining the relevance of these cortical oscillations for sleep-dependent memory consolidation. In contrast, during REM sleep theta-tDCS appears to increase global gamma (25-45 Hz) activity, indicating a clear brain state-dependency of theta-tDCS. More generally, results demonstrate the suitability of oscillating-tDCS as a tool to analyze functions of endogenous EEG rhythms and underlying endogenous electric fields as well as the interactions between EEG rhythms of different frequencies.     

Comparative analysis of EEG correlation synchronism and EEG amplitude relationships in all-night sleep

We used a new methodological approach to the evaluation of EEG synchronization based on correlation between amplitude modulation processes (EEG envelopes). We revealed: left-hemispheric dominance and dominance of frontal over occipital regions characteristic of all sleep stages; differences in synchronization in frequency bands and their patterns characteristic of a specific sleep stage; stage-dependent differences in inter-hemispheric synchrony and patterns of their changes from the frontal to occipital regions; and stage-dependent topographical distributions of high synchronization foci with respect to frequency domains. Analysis of amplitude topography also revealed left-hemispheric dominance and many significant differences in activity distribution patterns over parasagittal chains of electrodes (meridians) depending on sleep stages and frequency domains. The combination of EEG synchrony estimates with the amplitude spectral estimates made it possible to perform a reliable discriminant recognition of five sleep stages with errors in the range of 3-20%.     

Cardiovascular changes during deep breath-hold dives in a pressure chamber

Ferrigno, Massimo, Guido Ferretti, Avery Ellis, DanWarkander, Mario Costa, Paolo Cerretelli, and Claes E. G. Lundgren. Cardiovascular changes during deep breath-hold dives ina pressure chamber. J. Appl. Physiol.83(4): 1282-1290, 1997.Electrocardiogram, cardiac output, andblood lactate accumulation were recorded in three elite breath-holddivers diving to 40-55 m in a pressure chamber in thermoneutral(35°C) or cool (25°C) water. In two of the divers, invasiverecordings of arterial blood pressure were also obtained during divesto 50 m in cool water. Bradycardia during the dives was more pronouncedand developed more rapidly in the cool water, with heart rates droppingto 20-30 beats/min. Arrhythmias occurred, particularly during thedives in cool water, when they were often more frequent than sinusbeats. Because of bradycardia, cardiac output decreased during thedives, especially in cool water (to <3 l/min in 2 of the divers).Arterial blood pressure increased dramatically, reaching values as highas 280/200 and 290/150 mmHg in the two divers, respectively. Thishypertension was secondary to peripheral vasoconstriction, which alsoled to anaerobic metabolism, reflected in increased blood lactateconcentration. The diving response of these divers resembles the onedescribed for diving animals, although the presence of arrhythmias andlarge increases in blood pressure indicate a less perfect adaptation inhumans.

     

Cardiovascular changes during underwater static and dynamic breath-hold dives in trained divers

Limited information exists concerning arterial blood pressure (BP) changes in underwater breath-hold diving. Simulated chamber dives to 50 m of freshwater (mfw) reported very high levels of invasive BP in two divers during static apnea (SA), whereas a recent study using a noninvasive subaquatic sphygmomanometer reported unchanged or mildly increased values at 10 m SA dive. In this study we investigated underwater BP changes during not only SA but, for the first time, dynamic apnea (DA) and shortened (SHT) DA in 16 trained breath-hold divers. Measurements included BP (subaquatic sphygmomanometer), ECG, and pulse oxymetry (arterial oxygen saturation, SpO?, and heart rate). BP was measured during dry conditions, at surface fully immersed (SA), and at 2 mfw (DA and SHT DA), whereas ECG and pulse oxymetry were measured continuously. We have found significantly higher mean arterial pressure (MAP) values in SA (～40%) vs. SHT DA (～30%). Postapneic recovery of BP was slightly slower after SHT DA. Significantly higher BP gain (mmHg/duration of apnea in s) was found in SHT DA vs. SA. Furthermore, DA attempts resulted in faster desaturation vs. SA. In conclusion, we have found moderate increases in BP during SA, DA, and SHT DA. These cardiovascular changes during immersed SA and DA are in agreement with those reported for dry SA and DA.