Period-amplitude Analysis Research Articles

Differential modulation of NREM sleep regulation and EEG topography by chronic sleep restriction in mice

Compensatory elevation in NREM sleep EEG delta power has been typically observed following prolonged wakefulness and widely used as a sleep homeostasis indicator. However, recent evidence in human and rodent chronic sleep restriction (CSR) studies suggests that NREM delta power is not progressively increased despite of accumulated sleep loss over days. In addition, there has been little progress in understanding how sleep EEG in different brain regions responds to CSR. Using novel high-density EEG electrode arrays in the mouse model of CSR where mice underwent 18-h sleep deprivation per day for 5 consecutive days, we performed an extensive analysis of topographical NREM sleep EEG responses to the CSR condition, including period-amplitude analysis of individual slow waves. As previously reported in our analysis of REM sleep responses, we found different patterns of changes: (i) progressive decrease in NREM sleep duration and consolidation, (ii) persistent enhancement in NREM delta power especially in the frontal and parietal regions, and (iii) progressive increases in individual slow wave slope and frontal fast oscillation power. These results suggest that multiple sleep-wake regulatory systems exist in a brain region-specific manner, which can be modulated independently, especially in the CSR condition.

Effects of partial sleep deprivation on slow waves during non-rapid eye movement sleep: A high density EEG investigation

ObjectiveChanges in slow waves during non-rapid eye movement (NREM) sleep in response to acute total sleep deprivation are well-established measures of sleep homeostasis. This investigation utilized high-density electroencephalography (hdEEG) to examine topographic changes in slow waves during repeated partial sleep deprivation. MethodsTwenty-four participants underwent a 6-day sleep restriction protocol. Spectral and period-amplitude analyses of sleep hdEEG data were used to examine changes in slow wave energy, count, amplitude, and slope relative to baseline. ResultsChanges in slow wave energy were dependent on the quantity of NREM sleep utilized for analysis, with widespread increases during sleep restriction and recovery when comparing data from the first portion of the sleep period, but restricted to recovery sleep if the entire sleep episode was considered. Period-amplitude analysis was less dependent on the quantity of NREM sleep utilized, and demonstrated topographic changes in the count, amplitude, and distribution of slow waves, with frontal increases in slow wave amplitude, numbers of high-amplitude waves, and amplitude/slopes of low amplitude waves resulting from partial sleep deprivation. ConclusionsTopographic changes in slow waves occur across the course of partial sleep restriction and recovery. SignificanceThese results demonstrate a homeostatic response to partial sleep loss in humans.

Period-Amplitude Analysis Reveals Wake-Dependent Changes in the Electroencephalogram during Sleep Deprivation

Electroencephalographic slow wave activity (SWA) during non-rapid eye movement (NREM) sleep results from the synchronous oscillation of cortical neurons and is the standard measurement of sleep homeostasis. SWA is not a direct measure of sleep pressure accumulation, but rather a measure of the NREM-sleep response to accumulated sleep pressure. Currently, no practical standard for the direct measurement of sleep pressure accumulation exists. Recently, it was demonstrated that rat cortical neurons undergo oscillations during wake that are similar to the cortical oscillations responsible for SWA. Furthermore, these oscillations increase in number as time awake increases. Here we hypothesize that period-amplitude analysis of the electroencephalogram (EEG), which treats the EEG as a series of discrete waves, can measure these cortical oscillations, and thus, is a measure of sleep-pressure accumulation during extended wake. Mice were sleep deprived for 24 h by confinement to a slowly rotating wheel in order to assess wake-dependent changes in EEG wave incidence. Continuous period-amplitude analysis of the waking EEG across 24 h of sleep deprivation revealed that the incidence of 2 to 6 Hz waves increased exponentially over the deprivation period. This increase in wave incidence appeared to occur in two phases with exponential time constants of approximately 0.12 h and 3 h. Further analysis revealed that the changes in wave incidence were significantly correlated with two established markers of sleep pressure, SWA and NREM sleep latency. The data suggest that wave incidence is an effective method of measuring sleep homeostasis in the waking EEG that provides better temporal resolution than spectral power analysis.

Longitudinal sleep EEG trajectories indicate complex patterns of adolescent brain maturation.

New longitudinal sleep data spanning ages 6-10 yr are presented and combined with previous data to analyze maturational trajectories of delta and theta EEG across ages 6-18 yr in non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM delta power (DP) increased from age 6 to age 8 yr and then declined. Its highest rate of decline occurred between ages 12 and 16.5 yr. We attribute the delta EEG trajectories to changes in synaptic density. Whatever their neuronal underpinnings, these age curves can guide research into the molecular-genetic mechanisms that underlie adolescent brain development. The DP trajectories in NREM and REM sleep differed strikingly. DP in REM did not initially increase but declined steadily from age 6 to age 16 yr. We hypothesize that the DP decline in REM reflects maturation of the same brain arousal systems that eliminate delta waves in waking EEG. Whereas the DP age curves differed in NREM and REM sleep, theta age curves were similar in both, roughly paralleling the age trajectory of REM DP. The different maturational curves for NREM delta and theta indicate that they serve different brain functions despite having similar within-sleep dynamics and responses to sleep loss. Period-amplitude analysis of NREM and REM delta waveforms revealed that the age trends in DP were driven more by changes in wave amplitude rather than incidence. These data further document the powerful and complex link between sleep and brain maturation. Understanding this relationship would shed light on both brain development and the function of sleep.

Differences in EEG Delta Frequency Characteristics and Patterns in Slow-Wave Sleep Between Dementia Patients and Controls

To evaluate the modifications of EEG activity during slow-wave sleep in patients with dementia compared with healthy elderly subjects, using spectral analysis and period-amplitude analysis. Five patients with dementia and 5 elderly control subjects underwent night polysomnographic recordings. For each of the first three nonrapid eye movement-rapid eye movement sleep cycles, a well-defined slow-wave sleep portion was chosen. The delta frequency band (0.4-3.6 Hz) in these portions was analyzed with both spectral analysis and period-amplitude analysis. Spectral analysis showed an increase in the delta band power in the dementia group, with a decrease across the night observed only in the control group. For the dementia group, period-amplitude analysis showed a decrease in well-defined delta waves of frequency lower than 1.6 Hz and an increase in such waves of frequency higher than 2 Hz, in incidence and amplitude. Our study showed (1) a loss of the dynamics of delta band power across the night sleep, in dementia, and (2) a different distribution of delta waves during slow-wave sleep in dementia compared with control subjects. This kind of computer-based analysis can highlight the presence of a pathologic delta activity during slow-wave sleep in dementia and may support the hypothesis of a dynamic interaction between sleep alteration and cognitive decline.

Automatic breath-to-breath analysis of nocturnal polysomnographic recordings

Diagnosis of sleep-disordered breathing is based on the presence of an abnormal breathing pattern during sleep. In this study, an algorithm was developed for the offline breath-to-breath analysis of the nocturnal respiratory recordings. For that purpose, respiratory signals (nasal airway pressure, thoracic and abdominal movements) were divided into half waves using period amplitude analysis. Individual breaths were characterized by the parameters of the half waves (duration, amplitude, and slope). These values can be used to discriminate between normal and abnormal breaths. This algorithm was applied to six polysomnographic recordings to distinguish abnormal breathing events (apneas and hypopneas). The algorithm was robust for the identification of breaths (sensitivity=96.8%, positive prediction value (PPV)=99.5%). The detection of apneas and hypopneas was compared to the manual scoring of two experienced sleep technicians: sensitivity was, respectively, 89.2 and 88.9%, PPV was 54.1 and 59.3%. The classification of apneas into central, obstructive, or mixed was in concordance with the observers in 68% of the apneas. Although the algorithm tended to detect more hypopneas than the clinical standard, this study shows that the extraction of breath-to-breath parameters is useful for detection of abnormal respiratory events and provides a basis for further characterization of these events.

EEG gamma frequency and sleep–wake scoring in mice: Comparing two types of supervised classifiers

There is growing interest in sleep research and increasing demand for screening of circadian rhythms in genetically modified animals. This requires reliable sleep stage scoring programs. Present solutions suffer, however, from the lack of flexible adaptation to experimental conditions and unreliable selection of stage-discriminating variables. EEG was recorded in freely moving C57BL/6 mice and different sets of frequency variables were used for analysis. Parameters included conventional power spectral density functions as well as period-amplitude analysis. Manual staging was compared with the performance of two different supervised classifiers, linear discriminant analysis (LDA) and Classification Tree. Gamma activity was particularly high during REM (rapid eye movements) sleep and waking. Four out of 73 variables were most effective for sleep–wake stage separation: amplitudes of upper gamma-, delta- and upper theta-frequency bands and neck muscle EMG. Using small sets of training data, LDA produced better results than Classification Tree or a conventional threshold formula. Changing epoch duration (4 to 10 s) had only minor effects on performance with 8 to 10 s yielding the best results. Gamma and upper theta activity during REM sleep is particularly useful for sleep–wake stage separation. Linear discriminant analysis performs best in supervised automatic staging procedures. Reliable semi-automatic sleep scoring with LDA substantially reduces analysis time.

Reliability of Sleep EEG Measures: A Comment on Van Dongen et al

WE OFFER A SOMEWHAT BELATED COMMENT ON VAN DONGEN ET AL'S1 SURVEY ON THE RELIABILITY OF SLEEP ELECTROENCEPHALOGRAM (EEG) MEASURES. In their paper, Van Dongen and coworkers reviewed the research implications of high within-subject stability (i.e., inter-night reliability) of sleep measures, including EEG measured by computer. They pointed out important research issues on which this evidence bears. However, their extensive review omitted 2 substantial studies specifically focused on the reliability of computer-measured sleep EEG frequencies. In the first, Tan et al2 reported remarkably high inter-night reliability of delta, sigma, and beta frequencies during non-rapid eye movement sleep, measured with both fast Fourier transform and period-amplitude analysis. Subjects were 10 males and 6 females (mean age = 20 years) who underwent 5 consecutive nights of baseline recording. Internight reliability coefficients in these frequency bands were approximately 0.9 for fast Fourier transform power and period-amplitude measures of wave amplitude and incidence. For all measures, the mean of nights 1 and 2 correlated at approximately 0.95 with the 5-night mean. Tan et al noted that differences in skull impedance and other extra-cerebral factors could produce consistent individual differences in EEG amplitude that might spuriously enhance internight reliability of amplitude-weighted EEG measures (fast Fourier transform power and related period-amplitude measures). However, they found equally high reliability for an amplitude-free measure (period-amplitude time in band), indicating that it is the brain electrical activity that is reliable. In agreement with Linkowski et al's3 earlier suggestion based on visually scored sleep EEG, Tan et al proposed that non-rapid eye movement delta and other sleep EEG frequencies are individual traits that are heavily genetically determined. In their second paper, Tan et al4 presented internight reliability data for sleep EEG in 19 young adults (mean age 22 years, 10 men) and 19 elderly normal adults (mean age 71 years, 8 men). This remains the most extensive study of this question. Subjects were studied for 4 nonconsecutive nights, a design that challenges internight reliability more strongly than does one using consecutive nights. Digitized EEG in both non-rapid eye movement and rapid eye movement sleep was analyzed in 26 frequency bands using the fast Fourier transform and period-amplitude modules of PASSPLUS (Delta Software). Separate non-rapid eye movement and rapid eye movement spectra for each computer measure were constructed. In both age groups, non-rapid eye movement and rapid eye movement spectral curves differed significantly. Both spectra were complex curves with highly significant linear, quadratic, and cubic components. Again, the EEG measures were highly consistent within subjects, so much so that the spectra for each measure were virtually superimposed when plotted for individual subjects. Tan et al again demonstrated high within-subject Pearson reliability coefficients (∼.8 – ∼.95) for each frequency band. In both age groups, mean values for each measure were also stable across these nonconsecutive nights. To our surprise, and contradicting a speculation by Van Dongen et al, EEG spectra in the elderly were at least as reliable as those in young adults. Tan et al proposed that “The high reliability of computer-measured sleep EEG traits found here obviously invites their genetic investigation.”4p1551 It is encouraging that Van Dongen and colleagues have recently undertaken such studies.5

Neuroendocrine Regulation of Sleep Disturbances in PTSD

Studies that have conducted quantitative analysis of the sleep electroencephalogram (EEG) have demonstrated decreased delta sleep in PTSD. Elevations in both hypothalamic (neurohormonal) and extrahypothalamic (neurotransmitter) corticotropin releasing factor (CRF) release is associated with decreased delta sleep activity. We present data from several studies examining the effect of metyrapone administration on the sleep EEG in PTSD and control subjects. Plasma ACTH, cortisol, and 11-deoxycorticol were obtained the morning following polysomnographic sleep recordings before and after metyrapone administration. Delta sleep was measured by period amplitude analysis. The results demonstrate: a) decreased delta sleep in male subjects with PTSD; b) metyrapone administration resulted in an activation of the sleep EEG and a robust decrease in quantitative delta sleep; c) the sleep and endocrine (increase in ACTH) responses to metyrapone were significantly decreased in PTSD in two different study samples; and d) the metyrapone-related disruption to sleep in both samples was predicted by the increase in ACTH measured the following morning. These findings strongly suggest that the delta sleep response to metyrapone is a measure of the brain response to a hypothalamic CRF challenge. The attenuated delta sleep and endocrine response to metyrapone challenge in PTSD is consistent with a model of enhanced negative feedback regulation or downregulation of CRF receptors in an environment of chronically increased CRF activity.

Asymmetric interhemispheric sigma waves during all-night sleep in humans

There are some studies that have examined the asymmetry of brain function, such as that reflected in electroencephalogram (EEG) activity, during sleep, however, the results are not consistent. To clarify the asymmetry of brain function during sleep as well as cerebral dominance, period-amplitude analysis of sigma activity was performed on polysomnograms (PSGs) in normal humans. Fifteen healthy, male volunteers aged 22–34 years (mean age 27.4 years) participated in this study. Electroencephalograms were recorded from disc electrodes placed at bilateral frontal, central, parietal and occipital (10–20 electrode system) sites using A1 + A2 for reference. Period-amplitude analysis was performed by the zero-crossing method using the Medilog Sleep Analyzing Computer (Oxford Instruments, Oxford, UK). Sigma wave counts in the right frontal regions during all-night sleep were significantly greater than those in the left. To the contrary, sigma counts in the left parietal and occipital regions were significantly greater than those in the right. There were no significant regional differences and no significant difference between the left and right hemispheres in the total alpha counts during all-night sleep. There were significant regional differences in the total beta wave counts during all-night sleep. However, there was no significant difference in beta counts between the left and right hemispheres. These results suggest distinct laterality in the number of sigma waves in the frontal and parietal regions, reflecting functional asymmetry of the brain during sleep.

Heart rate variability, sympathetic and vagal balance and EEG arousals in upper airway resistance and mild obstructive sleep apnea syndromes

Background and purpose We questioned the role of respiratory events in obstructive sleep apnea syndrome (OSAS) and of upper airway resistance syndrome (UARS) on heart rate (HR) during sleep, paying specific attention to the termination of the abnormal breathing events and examining the presence of arousals or termination with only central nervous system (CNS) activation. Patients and methods Twenty patients, 10 with UARS and 10 with mild OSAS, were studied. A nocturnal polysomnogram was performed including measurement of respiratory variables and pulse transit time (PTT). According to the presence or absence of a PTT event indicative of autonomic nervous system (ANS) activation, 148 events were extracted after having been randomly chosen in each represented sleep stage, with or without an electroencephalogram (EEG) arousal >1.5 s. RR interval (RRI) in electrocardiogram (ECG) recordings, as well as heart rate variability, was calculated during 60 and 120 s, respectively. Period amplitude analysis (PAA) was applied for RR-interval analysis, and fast Fourier transformation (FFT) was applied to perform HR variability analysis. Results Visually scored EEG arousal was significantly associated with an increase in sympathetic index of heart rate, while PTT was associated with a drop in parasympathetic index, after the respiratory events. Patients with mild OSAS presented persistently shorter RRI when compared to patients with UARS. The latter also exhibited a significant decrease in parasympathetic index (High Frequency (HF)) at the termination of a respiratory event. Conclusion The HF component was only significantly decreased in patients with UARS, which indicates a predominant involvement of the parasympathetic tone in patients with UARS in comparison to those with OSAS.

Sleep EEG evidence of sex differences in adolescent brain maturation.

The steep decline in slow-wave (delta) electroencephalogram (EEG) intensity across adolescence is a prominent feature of late brain maturation. As a first step in determining whether the adolescent delta decline is similar in both sexes, we compared cross-sectional sleep EEG data from 9- and 12-year-old boys and girls. All-night EEG recordings, 6 months apart, were conducted on each subject. EEG was recorded in the subjects' homes. Thirty-two 9-year-olds and 38 12-year-olds are enrolled in a 4-year longitudinal study of adolescent sleep. There are equal numbers of each sex in both age cohorts. N/A. Using ambulatory recorders, EEG was recorded in the subjects' homes on their normal sleep schedule. For each of the 2 semi-annual recording periods, data from the 10 subjects from each age-sex group with the cleanest (fewest artifacts) signals were selected for crosssectional comparisons of visual scoring and EEG variables. All artifact-free 20-second non-rapid eye movement epochs were analyzed with power spectral and period-amplitude analysis. In the 12-year-old cohort, delta power per minute was 37% higher in boys than girls. The 9-year-old cohort showed no sex difference. A second recording 6 months later produced similar results. These cross-sectional data indicate that girls begin the steep adolescent decline in slow-wave EEG earlier than boys. We hypothesize that this reflects an earlier onset of adolescent synaptic pruning in females.

Homeostatic sleep response to naps is similar in normal elderly and young adults

Delta homeostatic regulation can be challenged by reducing delta need with daytime naps and measuring delta in post-nap sleep. We previously demonstrated that, after a late afternoon nap, young adults reduce the amount of delta in post-nap sleep by the amount in the nap. We compared homeostatic responses of 19 young adults (mean age 22.4 years) and 19 normal elderly subjects (mean age 71.4 years). Each participated in four separate 2-day sessions that consisted of a baseline night, a nap, and post-nap sleep. Nap times were 0900, 1200, 1500 and 1800 h. The 1800 h nap contained the largest amount of delta and produced the largest reduction in post-nap delta. The young and elderly groups respectively produced 28 and 24% of baseline delta in the 1800 h nap. Both groups showed equivalent delta regulation, reducing post-nap delta by 28 and 25%, respectively. In both age groups, the decrease in post-nap delta resulted from a reduced rate of delta production (power/min) and reduced non-rapid eye movement (NREM) sleep duration. Period-amplitude analysis showed that the reduction in power/min resulted from decreases in delta wave amplitude and incidence. None of the responses to nap challenges differed significantly across age groups nor were there gender differences or age by gender interactions. These results show that delta homeostatic responses to naps in the elderly parallel those of young subjects. REM sleep showed no homeostatic reductions following naps in either group. We believe that the striking differences in the delta and REM responses point to different biological roles of the two kinds of sleep.

Chronic benzodiazepine usage and withdrawal in insomnia patients

We studied the sleep of patients with insomnia during continuous and very long-term use of benzodiazepines (BZDs), and after withdrawal. A group of 25 patients (mean age 44.3±11.8 years) with persistent insomnia, who had been taking BZDs nightly for 6.8±5.4 years was selected. The control group was comprised of 18 age-matched healthy individuals. Sleep stage parameters were analyzed during Night 1 (while taking BZDs), Night 2 (first night after completing BZD withdrawal), and Night 3 (15 days after gradual BZD withdrawal). Sleep data for control subjects was monitored in parallel. Sleep EEGs of the patients were analyzed using Period Amplitude Analysis (PAA), during Nights 1 and 3 only. During BZD use, a significant reduction of Total Sleep Time (TST) and increased sleep latency were found in the insomniac group when compared to controls. We found an increase in stage 2 non-REM (NREM) sleep, and a reduction in Slow Wave Sleep (SWS) when comparing to night 3 (after withdrawal). Sleep EEGs analysis showed an increase in sigma band and decrease in delta count in stages 2, 3, 4 NREM and REM sleep in the BZD group when comparing to night 3 (after withdrawal). During the BZD withdrawal period, six out of nine subjects taking lorazepam failed withdrawal. In the remaining 19 subjects, gradual withdrawal of BZDs was associated with immediate worsening of nocturnal sleep, as indicated by sleep parameters. However, 15 days after withdrawal (Night 3), some of the sleep structure parameters of patients were not significantly different from baseline (while taking BZDs), except for a significant increase in SWS and in delta count throughout most sleep stages, and a decrease in stage 2 NREM sleep. These values were not different from those shown by control subjects. REM sleep parameters showed no significant variation across the experimental conditions. Subjective sleep quality was significantly improved on Night 3 compared with Night 1. Conclusions: Chronic intake of BZDs may be associated with poor sleep in this population. A progressive 15-day withdrawal did not avoid an immediate worsening of sleep parameters. But at the end of the protocol, SWS, delta count, and sleep quality were improved compared to those recorded during the chronic BZD intake, despite the lack of change in sleep efficiency.

Kinetics of non-rapid eye movement delta production across sleep and waking in young and elderly normal subjects: theoretical implications.

To perform a within-subject analysis of the changes in absolute and normalized rates of non-rapid eye movement (NREM) delta power production across daytime naps and baseline sleep, specifically to determine whether these rates change linearly or exponentially. Subjects were studied in a 2-day protocol after screening for sleep disorders. The first night was a baseline sleep recording that was followed by a nap the next day at one of the following times: 900, 1200, 1500, 1800 with the nap order varied between subjects. Sleep was then recorded on the postnap night with the baseline bedtime schedule. Subjects completed all 4 nap conditions. A 4-bed sleep research laboratory. Each subject slept in a separate bedroom. Not applicable. Subjects were young normal (YNs, mean age 22.4 yrs) and elderly normal (ENs, mean age 71.4 yrs) adults. Electroencephalography was recorded continuously during baseline, nap, and postnap sleep. Fast Fourier transform and period-amplitude analysis and visual scoring were performed. Delta power/min and period-amplitude integrated amplitude/min were significantly lower in EN than YN subjects. Both measures declined linearly across NREM periods and increased linearly across naps in the two age groups. The linear slopes for absolute power and integrated amplitude were significantly flatter in the elderly. However, when normalized to each individual subject's baseline rate of delta production, the slopes of the ENs closely approximated those of the YNs. The observation of linear changes in delta across NREM periods and daytime naps contradicts the exponential assumptions of the two-process model. The similarity of the normalized slopes shows that waking and sleep exert the same proportionate change in NREM delta in YNs and ENs. This raises the possibility that homeostatic drive is similar in the two groups despite different rates of absolute delta production. The data here, taken in association with other findings, lead us to hypothesize that NREM delta is specifically homeostatic for the neuronal systems that underlie consciousness.

Can valerian improve the sleep of insomniacs after benzodiazepine withdrawal?

Purpose: The authors studied the sleep of patients with insomnia who complained of poor sleep despite chronic use of benzodiazepines (BZDs). The sample consisted of 19 patients (mean age 43.3±10.6 years) with primary insomnia (DSM-IV), who had taken BZDs nightly, for 7.1±5.4 years. The control group was composed of 18 healthy individuals (mean age 37±8 years). Sleep electroencephalogram (EEG) of the patients was analyzed with period amplitude analysis (PAA) and associated algorithms, during chronic BZD use (Night 1), and after 15 days of a valerian placebo trial (initiated after washout of BZD, Night 2). Sleep of control subjects was monitored in parallel. Results: Valerian subjects reported significantly better subjective sleep quality than placebo ones, after BZD withdrawal, despite the presence of a few side effects. However, some of the differences found in sleep structure between Night 1 and Night 2 in both the valerian and placebo groups may be due to the sleep recovery process after BZD washout. Example of this are: the decrease in Sleep Stage 2 and in sigma count; the increase in slow-wave sleep (SWS), and delta count, which were found to be altered by BZD ingestion. There was a significant decrease in wake time after sleep onset (WASO) in valerian subjects when compared to placebo subjects; results were similar to normal controls. Nonetheless, valerian-treated patients also presented longer sleep latency and increased alpha count in SWS than control subjects. Conclusions: The decrease in WASO associated with the mild anxiolytic effect of valerian appeared to be the major contributor to subjective sleep quality improvement found after 2-week of treatment in insomniacs who had withdrawn from BDZs. Despite subjective improvement, sleep data showed that valerian did not produce faster sleep onset; the increase in alpha count compared with normal controls may point to residual hyperarousabilty, which is known to play a role in insomnia. Nonetheless, we lack data on the extent to which a sedative drug can improve alpha sleep EEG. Thus, the authors suggest that valerian had a positive effect on withdrawal from BDZ use.

Unilateral Eye Closure and Interhemispheric EEG Asymmetry during Sleep in the Pigeon (Columba livia)

Aquatic mammals (i.e., Cetaceans, eared seals and manatees) and birds show interhemispheric asymmetries (IA) in slow-wave sleep-related electroencephalographic (EEG) activity, suggesting that the depth of sleep differs between hemispheres. In birds, an association between unilateral eye closure and IA has been reported in five species from three orders (i.e., Galliformes, Charadriiformes, and Anseriformes). Moreover, unilateral eye closure has been observed during behaviorally defined sleep in 29 species from 13 avian orders, suggesting that birds in general display IA during sleep. Despite the apparent prevalence of unilateral eye closure and IA in birds, previous work did not detect IA in the pigeon, thereby challenging the conclusion that this is a general feature of birds. Using digital period amplitude analysis, an objective method for quantifying EEG power (a measure of wave amplitude) across different frequencies, we demonstrate that pigeons do, in fact, display an association between unilateral eye closure and IA. For a given hemisphere, standardized 2–4 Hz EEG power was greater when the contralateral eye was closed when compared to open. We also found that pigeons used the open eye during IA to monitor their environment. In addition, individual pigeons showed a bias for keeping one eye open more than the other. The direction (left vs. right) of this bias differed across birds, and appeared to be influenced by the structure of the surrounding environment. Finally, with the addition of pigeons (order Columbiformes), IA associated with unilateral eye closure has been recorded in four avian orders, suggesting that this form of sleep is widespread in birds.

Effects of hypnotics on the sleep EEG of healthy young adults: new data and psychopharmacologic implications

Benzodiazepine hypnotics increase NREM sleep and alter its EEG by reducing delta (0.3–3 Hz) and increasing sigma (12–15 Hz) and beta (15–23 Hz) activity. We tested whether the nonbenzodiazepine hypnotic, zolpidem (10 mg), produced the same pattern of sleep and EEG changes as two “classical” benzodiazepines, triazolam (0.25 mg) and temazepam (30 mg). Sleep EEG of 16 subjects was analyzed with period amplitude analysis for 3 nights during drug administration or placebo. The effects of zolpidem were in the same direction but generally of smaller magnitude than those of the classical benzodiazepines. These differences are more likely the result of non-equivalent dosages than different pharmacologic actions. Period amplitude analysis showed that the decreased delta activity resulted mainly from a decrease in wave amplitude. In contrast, the increased sigma and beta activity were produced by increased wave incidence. Delta suppression increased with repeated drug administration but sigma and beta stimulation did not. While these findings have little relevance for the clinical choice of hypnotics they may hold important implications for the brain mechanisms involved in hypnotic tolerance and withdrawal delirium.

High internight reliability of computer-measured NREM delta, sigma, and beta: biological implications

Background: Computer analysis of the sleep electroencephalogram (EEG) waveforms is widely employed, but there have been no systematic studies of its reliability. Methods: The most commonly used computer methods are power spectral analysis with the fast-Fourier transform (FFT) and period amplitude analysis (PAA) with zero cross or zero first derivative half-wave measurement. We applied all three computer methods to the digitized EEG of 16 normal subjects who underwent 5 consecutive nights of baseline (placebo) recording. We evaluated the internight reliability of three non–rapid eye movement (NREM) frequency bands of special importance to sleep research: delta (0.3–3 Hz), sigma (12–15 Hz), and beta (15–23 Hz). Results: Both FFT and the two methods of PAA gave excellent internight reliability for delta and sigma. Even a single night of recording correlated highly ( r > .9) with the 5-night mean. Beta reliability was lower but still highly significant for both the PAA and the FFT measures. Conclusions: Computer-analyzed sleep EEG data are highly reliable. Period amplitude methods demonstrate that wave incidence and period as well as amplitude are reliable, indicating that the reliability of composite measures (FFT power, PAA integrated amplitude) is not solely based on individual differences in EEG amplitude. The high internight stability of NREM delta indicates that it possesses traitlike characteristics and is relatively independent of day-to-day variations in state.

Asymmetric interhemispheric delta waves during all-night sleep in humans

Objective: In order to better understand the asymmetry of brain function during sleep, period-amplitude analysis of delta EEG activity was performed on polysomnograms (PSGs) in normal humans. Twenty healthy, right-handed male volunteers aged 22–35 years (mean age 27.2 years) served as subjects in this study.Methods: EEGs were recorded from disc electrodes placed at bilateral frontal, central, parietal, occipital, anterotemporal and posterotemporal (10–20 electrode system) sites using A1+A2 for reference. Period-amplitude analysis was performed by the zero-crossing method using the Medilog Sleep Analyzing Computer.Results: Delta counts in the right frontal and central regions during all-night sleep were significantly greater than in those of the left; total delta counts of the right frontal region were greater than those of the left in 18 of the 20 subjects. There were no significant differences in delta counts between the left and right hemispheres in parietal, occipital, anterotemporal, and postero-temporal regions.Conclusions: These results suggest distinct laterality in the number of delta waves in the frontal and central regions, reflecting functional asymmetry of the brain during sleep.