Topographic distribution of sleep spindles in young healthy subjects

The application of an automatic sleep spindle detection procedure allowed the documentation of the topographic distribution of spindle characteristics, such as number, amplitude, frequency and duration, as a function of sleep depth and of recording time. Multichannel all-night EEG recordings were performed in 10 normal healthy subjects aged 20–35 years. Although the interindividual variability in the number of sleep spindles was very high (2.7±2.1 spindles per minute stage 2 sleep), all but two subjects showed maximal spindle activity in centro-parietal midline leads. Moreover, this topography was seen in all sleep stages and changed only slightly – to a more central distribution – towards the end of the night. On the other hand, slow (11.5–14 Hz) and fast (14–16 Hz) spindles showed a completely different topography, with slow spindles distributed anteriorly and fast spindles centro-parietally. The number of sleep spindles per min was significant depending on sleep stages, with the expected highest occurrence in stage 2, and on recording time, with a decrease in spindle density from the beginning towards the end of the night. However, spindle amplitude, frequency and individual duration was not influenced by sleep depth or time of the night.     

Genetic risk factors for schizophrenia associate with sleep spindle activity in healthy adolescents

Schizophrenia has been associated with disturbed sleep, even before the onset of the disorder, and also in non‐schizophrenic first‐order relatives. This may point to an underlying genetic influence. Here we examine whether weighted polygenic risk scores (PRS) for schizophrenia are associated with sleep spindle activity in healthy adolescents. Our sample comes from a community‐based cohort of 157 non‐schizophrenic adolescents (57% girls) having both genetic data and an overnight sleep EEG measurement available. Based on a recent genome‐wide association study, we calculated PRS for schizophrenia across the whole genome. We also calculated PRS for the CACNA1l gene region, which has been associated with both schizophrenia and sleep spindle formation. We performed an overnight sleep EEG at the homes of the participants. Stage two sleep spindles were detected using an automated algorithm. Sleep spindle amplitude, duration, intensity and density were measured separately for central and frontal derivations and for fast (13–16 Hz) and slow (10–13 Hz) spindles. PRS for schizophrenia was associated with higher fast spindle amplitude (p = 0.04), density (p = 0.006) and intensity (p = 0.04) at the central derivation, and PRS in the CACNA1l region associated with higher slow spindle amplitude (p = 0.01), duration (p = 0.03) and intensity (p = 0.002) at the central derivation. A positive association between genetic variants for schizophrenia and sleep spindle activity among healthy adolescents supports a view that sleep spindles and schizophrenia share similar genetic pathways. This study suggests that altered sleep spindle activity might serve as an endophenotype of schizophrenia.     

Schizotypal traits are associated with sleep spindles and rapid eye movement in adolescence

Research suggests an association between schizophrenia and a decrease in sleep spindle activity, as well as a change in sleep architecture. It is unknown how the continuum of psychotic symptoms relates to different features in the sleep electroencephalogram. We set out to examine how sleep architecture and stage 2 spindle activity are associated with schizotypy in a healthy adolescent population. The participants in our study (n = 176, 61% girls) came from a community‐based cohort. Schizotypal traits were evaluated using the Schizotypal Personality Scale (STA) in early adolescence (mean age 12.3 years, SD = 0.5) and the participants underwent ambulatory overnight polysomnography at mean age 16.9 years (SD = 0.1). Sleep was scored in 30‐s epochs into stages 1, 2, 3 and rapid eye movement (REM) sleep. Stage 2 spindles were detected using an automated algorithm. Spindle analyses from central and frontal derivations included spindle duration and density for slow (10–13 Hz) and fast (13–16 Hz) ranges. Covariates included sex and age. Those with the highest STA scores had a higher percentage of REM (B = 2.07 [95% CI, 0.17, 4.0]; p = .03) than those with the lowest scores. Those with the highest scores had shorter spindle duration, as derived from the frontal regions, and a slower oscillation range (B = ?0.04 [95% CI, ?0.07, ?0.01]; p = .023) than those with the lowest scores. We conclude that high levels of schizotypy characteristics measured in early adolescence may be associated with distinguished features of sleep architecture, namely with spindle morphology and a higher proportion of REM sleep.     

Sensory stimulation triggers spindles during sleep stage 2

STUDY OBJECTIVES: Toward understanding the function of sleep spindle, we examined whether sensory stimulation triggers sleep spindles. PARTICIPANTS: Eleven normal subjects participated in the experiments. INTERVENTION: The subjects had a nap in the afternoon, and sensory stimulation was applied during sleep stage 2. MEASUREMENTS: 21-channel EEG was recorded during the 2-3 hour experiment carried out between 13:00 and 16:00. Somatosensory, auditory, or visual stimulation was performed over a 5-minute period during stage 2. The frequency and duration of spindles were compared in 2 different segments of 5 minutes, with and without sensory stimulation. The latency from the onset of a sensory stimulus to the succeeding spindle was also analyzed. To estimate the active brain regions during a spindle, the EEG recordings were modeled with a single equivalent moving dipole (SEMD) model. RESULTS: In the period with stimulation, spindle frequency and duration increased compared with the period without stimulation. Statistical tests revealed that with stimulation, the interval between 2 consecutive spindles was significantly shorter (p < 0.05, regardless of the modality) and that the duration of the spindles was significantly longer with stimulation (p < 0.05, regardless of the modality). The latency was approximately 2 s. During a spindle after somatosensory stimulation brain activities were observed near the somatosensory area, while with auditory stimulation active regions were observed near the auditory cortex. CONCLUSIONS: A sensory stimulus appeared to trigger a sleep spindle during sleep stage 2. SEMD trajectories suggest that active brain regions during spindle are different according to the modality of the preceding stimulus.     

Development and comparison of four sleep spindle detection methods

OBJECTIVE: The objective of the present work was to develop and compare methods for automatic detection of bilateral sleep spindles. METHODS AND MATERIALS: All-night sleep electroencephalographic (EEG) recordings of 12 healthy subjects with a median age of 40 years were studied. The data contained 6043 visually scored bilateral spindles occurring in frontopolar or central brain location. In the present work a new sigma index for spindle detection was developed, based on the fast Fourier transform (FFT) spectrum, aiming at approximating our previous fuzzy spindle detector. The sigma index was complemented with spindle amplitude analysis, based on finite impulse response (FIR) filtering, to form of a combination detector of bilateral spindles. In this combination detector, the spindle amplitude distribution of each recording was estimated and used to tune two different amplitude thresholds. This combination detector was compared to bilaterally extracted sigma indexes and fuzzy detections, which aim to be independent of absolute spindle amplitudes. As a fourth method a fixed spindle amplitude detector was included. RESULTS: The combination detector provided the best overall performance; in S2 sleep a 70% true positive rate was reached with a specificity of 98.6%, and a false-positive rate of 32%. The bilateral sigma indexes provided the second best results, followed by fuzzy detector, while the fixed amplitude detector provided the poorest results so that in S2 sleep a 70% true positive rate was reached with a specificity of 97.7% and false-positive rate of 46%. The spindle amplitude distributions automatically determined for each recording by the combination detector were compared to amplitudes of visually scored spindles and they proved to correspond well. Inter-hemispheric amplitude variation of visually scored bilateral spindles is also presented. CONCLUSION: Flexibility is beneficial in the detection of bilateral spindles. The present work advances automated spindle detection and increases the knowledge of bilateral sleep spindle characteristics.     

REM sleep behaviour disorder is associated with lower fast and higher slow sleep spindle densities

To investigate differences in sleep spindle properties and scalp topography between patients with rapid eye movement sleep behaviour disorder (RBD) and healthy controls, whole‐night polysomnograms of 35 patients diagnosed with RBD and 35 healthy control subjects matched for age and sex were compared. Recordings included a 19‐lead 10–20 electroencephalogram montage and standard electromyogram, electrooculogram, electrocardiogram and respiratory leads. Sleep spindles were automatically detected using a standard algorithm, and their characteristics (amplitude, duration, density, frequency and frequency slope) compared between groups. Topological analyses of group‐discriminative features were conducted. Sleep spindles occurred at a significantly (e.g. t₃₄ = ?4.49; P = 0.00008 for C3) lower density (spindles?min^?1) for RBD (mean ± SD: 1.61 ± 0.56 for C3) than for control (2.19 ± 0.61 for C3) participants. However, when distinguishing slow and fast spindles using thresholds individually adapted to the electroencephalogram spectrum of each participant, densities smaller (31–96%) for fast but larger (20–120%) for slow spindles were observed in RBD in all derivations. Maximal differences were in more posterior regions for slow spindles, but over the entire scalp for fast spindles. Results suggest that the density of sleep spindles is altered in patients with RBD and should therefore be investigated as a potential marker of future neurodegeneration in these patients.     

Reliability of sleep spindle measurements in adolescents: How many nights are necessary?

Evidence of night‐to‐night variation in adolescent sleep spindle characteristics is lacking. Twelve adolescents (M = 15.8 ± 0.8 years, eight males) participated in a laboratory study involving 9 nights with 10 hr sleep opportunity. Sleep electroencephalograph was analysed and intra‐class coefficients calculated to determine the reliability of sleep spindles across multiple nights of recording. Slow spindle amplitude and fast spindle density, duration and amplitude characteristics all had acceptable reliability within a single night of sleep recording. Slow spindle density and duration measurements needed a minimum of 4 and 2 nights, respectively, for reliable estimation. Theoretical and methodological implications are discussed.     

Validating an automated sleep spindle detection algorithm using an individualized approach

The goal of the current investigation was to develop a systematic method to validate the accuracy of an automated method of sleep spindle detection that takes into consideration individual differences in spindle amplitude. The benchmarking approach used here could be employed more generally to validate automated spindle scoring from other detection algorithms. In a sample of Stage 2 sleep from 10 healthy young subjects, spindles were identified both manually and automatically. The minimum amplitude threshold used by the prana ^® (PhiTools, Strasbourg, France) software spindle detection algorithm to identify a spindle was subject‐specific and determined based upon each subject’s mean peak spindle amplitude. Overall sensitivity and specificity values were 98.96 and 88.49%, respectively, when compared to manual scoring. Selecting individual amplitude thresholds for spindle detection based on systematic benchmarking data may validate automated spindle detection methods and improve reproducibility of experimental results. Given that interindividual differences are accounted for, we feel that automatic spindle detection provides an accurate and efficient alternative approach for detecting sleep spindles.     

Phase‐amplitude coupling of sleep slow oscillatory and spindle activity correlates with overnight memory consolidation

Initially independent lines of research suggest that sleep‐specific brain activity patterns, observed as electroencephalographic slow oscillatory and sleep spindle activity, promote memory consolidation and underlying synaptic refinements. Here, we further tested the emerging concept that specifically the coordinated interplay of slow oscillations and spindle activity (phase‐amplitude coupling) support memory consolidation. Particularly, we associated indices of the interplay between slow oscillatory (0.16–1.25 Hz) and spindle activity (12–16 Hz) during non‐rapid eye movement sleep (strength [modulation index] and phase degree of coupling) in 20 healthy adults with parameters of overnight declarative (word‐list task) and procedural (mirror‐tracing task) memory consolidation. The pattern of results supports the notion that the interplay between oscillations facilitates memory consolidation. The coincidence of the spindle amplitude maximum with the up‐state of the slow oscillation (phase degree) was significantly associated with declarative memory consolidation (r = .65, p = .013), whereas the overall strength of coupling (modulation index) correlated with procedural memory consolidation (r = .45, p = .04). Future studies are needed to test for potential causal effects of the observed association between neural oscillations during sleep and memory consolidation, and to elucidate ways of modulating these processes, for instance through non‐invasive brain‐stimulation techniques.     

The effect of daytime napping and full‐night sleep on the consolidation of declarative and procedural information

Many studies investigating sleep and memory consolidation have evaluated full‐night sleep rather than alternative sleep periods such as daytime naps. This multi‐centre study followed up on, and was compared with, an earlier full‐night study (Schabus et al., 2004) investigating the relevance of daytime naps for the consolidation of declarative and procedural memory. Seventy‐six participants were randomly assigned to a nap or wake group, and performed a declarative word‐pair association or procedural mirror‐tracing task. Performance changes from before to after a 90‐min retention interval filled with sleep or quiet wakefulness were evaluated between groups. Associations between performance changes, sleep architecture, spindles, and slow oscillations were investigated. For the declarative task we observed a trend towards stronger forgetting across a wake period compared with a nap period, and a trend towards memory increase over the full‐night. For the procedural task, accuracy was significantly decreased following daytime wakefulness, showed a trend to increase with a daytime nap, and significantly increased across full‐night sleep. For the nap protocol, neither sleep stages, spindles, nor slow oscillations predicted performance changes. A direct comparison of day and nighttime sleep revealed that daytime naps are characterized by significantly lower spindle density, but higher spindle activity and amplitude compared with full‐night sleep. In summary, data indicate that daytime naps protect procedural memories from deterioration, whereas full‐night sleep improves performance. Given behavioural and physiological differences between day and nighttime sleep, future studies should try to characterize potential differential effects of full‐night and daytime sleep with regard to sleep‐dependent memory consolidation.     

Spindles are highly heritable as identified by different spindle detectors

Study ObjectivesSleep spindles, a defining feature of stage N2 sleep, are maximal at central electrodes and are found in the frequency range of the electroencephalogram (EEG) (sigma 11–16 Hz) that is known to be heritable. However, relatively little is known about the heritability of spindles. Two recent studies investigating the heritability of spindles reported moderate heritability, but with conflicting results depending on scalp location and spindle type. The present study aimed to definitively assess the heritability of sleep spindle characteristics.MethodsWe utilized the polysomnography data of 58 monozygotic and 40 dizygotic same-sex twin pairs to identify heritable characteristics of spindles at C3/C4 in stage N2 sleep including density, duration, peak-to-peak amplitude, and oscillation frequency. We implemented and tested a variety of spindle detection algorithms and used two complementary methods of estimating trait heritability.ResultsWe found robust evidence to support strong heritability of spindles regardless of detector method (h² > 0.8). However not all spindle characteristics were equally heritable, and each spindle detection method produced a different pattern of results.ConclusionsThe sleep spindle in stage N2 sleep is highly heritable, but the heritability differs for individual spindle characteristics and depends on the spindle detector used for analysis.     

Influence and implications of the renin–angiotensin–aldosterone system in obstructive sleep apnea: An updated systematic review and meta-analysis

Obstructive sleep apnea is a chronic, sleep-related breathing disorder, which is an independent risk factor for cardiovascular disease. The renin–angiotensin–aldosterone system regulates salt and water homeostasis, blood pressure, and cardiovascular remodelling. Elevated aldosterone levels are associated with excess morbidity and mortality. We aimed to analyse the influence and implications of renin–angiotensin–aldosterone system derangement in individuals with and without obstructive sleep apnea. We pooled data from 20 relevant studies involving 2828 participants (1554 with obstructive sleep apnea, 1274 without obstructive sleep apnea). The study outcomes were the levels of renin–angiotensin–aldosterone system hormones, blood pressure and heart rate. Patients with obstructive sleep apnea had higher levels of plasma renin activity (pooled wmd+ 0.25 [95% confidence interval 0.04–0.46], p = 0.0219), plasma aldosterone (pooled wmd+ 30.79 [95% confidence interval 1.05–60.53], p = 0.0424), angiotensin II (pooled wmd+ 5.19 [95% confidence interval 3.11–7.27], p < 0.001), systolic (pooled wmd+ 5.87 [95% confidence interval 1.42–10.32], p = 0.0098) and diastolic (pooled wmd+ 3.40 [95% confidence interval 0.86–5.94], p = 0.0086) blood pressure, and heart rate (pooled wmd+ 3.83 [95% confidence interval 1.57–6.01], p = 0.0009) compared with those without obstructive sleep apnea. The elevation remained significant (except for renin levels) when studies involving patients with resistant hypertension were removed. Sub-group analysis demonstrated that levels of angiotensin II were significantly higher only among the Asian population with obstructive sleep apnea compared with those without obstructive sleep apnea. Body mass index accounted for less than 10% of the between-study variance in elevation of the renin–angiotensin–aldosterone system parameters. Patients with obstructive sleep apnea have higher levels of renin–angiotensin–aldosterone system hormones, blood pressure and heart rate compared with those without obstructive sleep apnea, which remains significant even among patients without resistant hypertension.     

Effect of infant sleeping position on sleep spindles

Sleep spindles play an active role in inducing and maintaining sleep and may affect arousal by blocking the transmission of external stimuli through the thalamus to the cortex. Previously we have demonstrated that sleeping in the prone position impairs arousal in infants at 2-3 months of age, but not at 5-6 months. We aimed to examine if sleeping position and postnatal age affected duration and/or density of sleep spindles. Twenty-one healthy term infants were studied using daytime polysomnography at 2-3 months and 16 were again studied at 5-6 months. Infants slept both prone and supine at each study. The mean duration of non-rapid eye movement (NREM) sleep was not different between the two studies in either position. At 2-3 months both spindle density (P < 0.001) and proportion of NREM sleep (P < 0.025) with spindles were significantly greater in the supine than in the prone position. At 5-6 months spindle duration was longer in the supine than in the prone position (P < 0.03). Spindle density in the supine position was not different between the two studies, however, when infants slept prone, it was significantly increased at 5-6 months compared with 2-3 months (P < 0.001). Arousal threshold was not correlated with either spindle density or percentage of NREM sleep with spindles in either position at either study. In this study spindle density and the percentage time spent with spindles were not well correlated with infant arousability, and hence may not be able to be used as markers of depressed arousal responses in infants.     

The adolescent pattern of sleep spindle development revealed by HD-EEG

Sleep spindles are developmentally relevant cortical oscillatory patterns; however, they have mostly been studied by considering the entire spindle frequency range (11–15 Hz) without a distinction between the functionally and topographically different slow and fast spindles, using relatively few electrodes and analysing wide age-ranges. Here, we employ high-density night sleep electroencephalography in three age-groups between 12 and 20 years of age (30 females and 30 males) and analyse the adolescent developmental pattern of the four major parameters of slow and fast sleep spindles. Most of our findings corroborate those very few previous studies that also make a distinction between slow and fast spindles in their developmental analysis. We find spindle frequency increasing with age. A spindle density change is not obvious in our study. We confirm the declining tendencies for amplitude and duration, although within narrower, more specific age-windows than previously determined. Spindle frequency seems to be higher in females in the oldest age-group. Based on the pattern of our findings, we suggest that high-density electroencephalography, specifically targeting slow and fast spindle ranges and relatively narrow age-ranges would advance the understanding of both adolescent cortical maturation and development and the functional relevance of sleep spindles in general.     

The space-time profiles of sleep spindles and their coordination with slow oscillations on the electrode manifold

Sleep spindles are important for sleep quality and cognitive functions, with their coordination with slow oscillations (SOs) potentially organizing cross-region reactivation of memory traces. Here, we describe the organization of spindles on the electrode manifold and their relation to SOs. We analyzed the sleep night EEG of 34 subjects and detected spindles and SOs separately at each electrode. We compared spindle properties (frequency, duration, and amplitude) in slow wave sleep (SWS) and Stage 2 sleep (S2); and in spindles that coordinate with SOs or are uncoupled. We identified different topographical spindle types using clustering analysis that grouped together spindles co-detected across electrodes within a short delay (±300 ms). We then analyzed the properties of spindles of each type, and coordination to SOs. We found that SWS spindles are shorter than S2 spindles, and spindles at frontal electrodes have higher frequencies in S2 compared to SWS. Furthermore, S2 spindles closely following an SO (about 10% of all spindles) show faster frequency, shorter duration, and larger amplitude than uncoupled ones. Clustering identified Global, Local, Posterior, Frontal-Right and Left spindle types. At centro-parietal locations, Posterior spindles show faster frequencies compared to other types. Furthermore, the infrequent SO-spindle complexes are preferentially recruiting Global SO waves coupled with fast Posterior spindles. Our results suggest a non-uniform participation of spindles to complexes, especially evident in S2. This suggests the possibility that different mechanisms could initiate an SO-spindle complex compared to SOs and spindles separately. This has implications for understanding the role of SOs-spindle complexes in memory reactivation.     

Unilateral thalamic stroke does not decrease ipsilateral sleep spindles

STUDY OBJECTIVES: To measure the sleep spindle characteristics in patients with unilateral thalamic stroke. DESIGN: A prospective study of patients with thalamic stroke and age-matched healthy controls. SETTING: Department of Neurology of a University Hospital. PARTICIPANTS: Thirteen patients (mean age: 67 years, SD: 13,44) with an isolated, unilateral acute thalamic stroke and 18 healthy age-matched volunteers. INTERVENTIONS: A polysomnogram recording from 14 scalp EEG electrodes performed during 2 consecutive nights, the second or third week after the stroke. Only the sleep of the second night was analyzed. MEASUREMENTS AND RESULTS: Sleep spindles were counted during two separate 10-minute epochs of stage II. Spindles appearing synchronously in both sides with similar amplitude were called "bilateral." Spindles with twice the amplitude in one side than the other were "right" or "left-side predominant". There were 8 patients with posterolateral, 3 with global and 2 with anterior lesions. Eight were right and 5 left-sided. The number of spindles was similar in patients (39.8 +/- 23.4 in 20 minutes) than controls (26.07 +/- 29.07; p=0.173). Spindles with a centroparietal (34%) and centroparieto-occipital localization (22%) were the most frequent. In controls approximately 66% of the spindles had a bilateral and symmetric distribution over the scalp, 23% of the spindles were predominantly left-sided and 5% were predominantly right-sided. In patients, bilateral spindles decreased (p<0.0001) but asymmetric spindles did not change. CONCLUSION: Unilateral acute thalamic stroke does not decrease sleep spindles ipsilaterally; rather, it seems to produce a bilateral diminution in their number.     

Respiratory safety of lemborexant in healthy adult and elderly subjects with mild obstructive sleep apnea: A randomized,double‐blind,placebo‐controlled,crossover study

Lemborexant is a dual orexin receptor antagonist indicated for the treatment of adult and elderly individuals with insomnia. Some current pharmacologic treatments for insomnia cause respiratory depression, a serious safety concern, particularly for individuals with obstructive sleep apnea (OSA). This phase 1, randomized, double‐blind, placebo‐controlled, two‐period crossover study examined respiratory safety parameters in individuals with mild OSA following treatment with lemborexant. Participants (n = 39) were randomized to one of two treatment sequences, including placebo and lemborexant 10 mg. Each treatment period lasted 8 days and was separated by a washout of at least 14 days. Following single or multiple doses, there were no significant differences in mean apnea–hypopnea index for lemborexant 10 mg versus placebo (least squares mean [LSM] difference [95% confidence interval {CI}]: day 1, ?0.03 [?2.22, 2.17]; day 8, ?0.06 [?1.95, 1.83]) or peripheral capillary oxygen saturation during sleep (LSM difference [95% CI]: day 1, 0.07 [?0.31, 0.46]; day 8, 0.25 [?0.11, 0.61]). There were no significant differences versus placebo for the percentage of total sleep time during which peripheral capillary oxygen saturation was <80% (LSM difference [95% CI]: day 1, 0.002 [?0.019, 0.023]; day 8, 0.006 [?0.015, 0.026]), <85% (LSM difference [95% CI]: day 1, 0.067 [?0.124, 0.258]; day 8, 0.056 [?0.117, 0.228]) or <90% (LSM difference [95% CI]: day 1, 0.312 [?0.558, 1.181]; day 8, 0.088 [?0.431, 0.607]). The incidence of treatment‐emergent adverse events was low and similar for lemborexant and placebo. Lemborexant demonstrated respiratory safety in this study population and was well tolerated.     

Fast sleep spindle (13-15 hz) activity correlates with sleep-dependent improvement in visuomotor performance

STUDY OBJECTIVES: The relationship between memory enhancement and fast (13-16 Hz) versus slow (10-13 Hz) spindle activity during sleep was investigated. DESIGN: Standard polysomnographic recordings were conducted during an adaptation, control nonlearning, and learning night. Automatic spindle detection and measurement was utilized with visual confirmation. SETTING: Participants slept in individual, temperature-controlled bedrooms in a sleep laboratory. PARTICIPANTS: Twelve healthy student volunteers (9 women and 3 men, mean age: 22.3 years) participated. INTERVENTIONS: On the learning night, participants completed a presleep learning session on a modified version of mirror-tracing task followed by a postsleep test session. No learning or test sessions were performed on the adaptation and nonlearning nights. MEASUREMENTS AND RESULTS: Tracing time was reduced by 6.4 seconds (20.6% +/- 2.07%) from the presleep to the postsleep session. Mean amplitude and duration of fast spindles was greater on the learning night than on the nonlearning night (both P values < 0.05). Skill improvement and fast-spindle activity were positively correlated (density [r = 0.76, P < 0.01], amplitude [r = 0.69, P < 0.05], and duration [r = 0.67, P <0.05]). Significant correlations between fast-spindle activity and mirror-tracing performance were also evident for the nonlearning night. There was no significant relationship between mirror-tracing performance and slow-spindle activity on any night. CONCLUSIONS: The thalamocortical network underlying fast-spindle generation may contribute to or reflect plasticity during sleep.