Visualization of the Spatiotemporal Propagation of Interictal Spikes in Temporal Lobe Epilepsy: A MEG Pilot Study

Magnetoencephalography (MEG) is clinically used to localize interictal spikes in discrete brain areas of epilepsy patients through the equivalent current dipole (ECD) method, but does not account for the temporal dynamics of spike activity. Recent studies found that interictal spike propagation beyond the temporal lobe may be associated with worse postsurgical outcomes, but studies using whole-brain data such as in MEG remain limited. In this pilot study, we developed a tool that visualizes the spatiotemporal dynamics of interictal MEG spikes normalized to spike-free sleep activity to assess their onset and propagation patterns in patients with temporal lobe epilepsy (TLE). We extracted interictal source data containing focal epileptiform activity in awake and asleep states from seven patients whose MEG ECD clusters localized to the temporal lobe and normalized the data against spike-free sleep recordings. We calculated the normalized activity over time per cortical label, confirmed maximal activity at onset, and mapped the activity over a 10 ms interval onto each patient's brain using a custom-built Multi-Modal Visualization Tool. The onset of activity in all patients appeared near the clinically determined epileptogenic zone. By 10 ms, four of the patients had propagated source activity restricted to within the temporal lobe, and three had propagated source activity spread to extratemporal regions. Using this tool, we show that noninvasively identifying the onset and propagation of interictal spike activity in MEG can be achieved, which may help provide further insight into epileptic networks and guide surgical planning and interventions in patients with TLE.

Deficiency in Re-Orienting of Attention in Adults with Attention-Deficit Hyperactivity Disorder

Objective: To characterize potential brain indexes of attention deficit hyperactivity disorder (ADHD) in adults. Methods: In an effort to develop objective, laboratory-based tests that can help to establish ADHD diagnosis, the brain indexes of distractibility was investigated in a group of adults. We used event-related brain potentials (ERPs) and performance measures in a forced-choice visual task. Results: Behaviorally aberrant distractibility in the ADHD group was significantly higher. Across three ERP components of distraction: N1 enhancement, P300 (P3a), and Reorienting Negativity (RON) the significant difference between ADHD and matched controls was found in the amplitude of the RON. We used non-parametric randomization tests, enabling us to statistically validated this difference between-group. Conclusions: Our main results of this feasibility study suggest that among other ERP components associated with auditory distraction, the RON response is promising index for a potential biomarker of deficient re-orienting of attention in adults s with ADHD.

Child Neurology: Presurgical Evaluation of Dominant Hemisphere Function in a Child With Rasmussen Encephalitis Using Magnetoencephalography

Rasmussen's encephalitis (RE) is a devastating progressive inflammatory disorder that leads to debilitating neurological deficits and intractable epilepsy. Surgical treatment of the dominant hemisphere has been attempted with hesitation given the lack of effective diagnostic tools to determine the potential functional deficits from disconnection procedures.We present the case of a 15-year-old male with RE, right hemiparesis, profound aphasia, and recurrent status epilepticus, who underwent language assessment using magnetoencephalography (MEG) prior to urgent hemispherectomy for epilepsia partialis continua. Cortical responses in the passive auditory task were localized to the left and right hemispheres at latencies of 200 and 380 ms, respectively from the stimulation onset. The current density reconstruction analysis showed the sources at 380 and 200 ms in the right and left temporal-parietal junctions, respectively. These findings confirmed that the patient's language was represented bilaterally. Other tests conventionally used to assess cortical language function were not attempted given his poor functional status and ongoing seizures. The left functional hemispherectomy has resulted in seizure freedom and significantly improved language function.The MEG-based evaluation of the language function could provide valuable information regarding cortical language organization prior to hemispherectomy in patients with RE. Such approach of mapping the eloquent cortical functions can be used in other structural and autoinflammatory disorders of the brain, especially in patients who cannot participate in the conventional diagnostic modalities designed to assess critical brain functions like language and memory.

Cortico-striato-thalamo-cerebellar networks of structural covariance underlying different epilepsy syndromes associated with generalized tonic-clonic seizures

Generalized tonic-clonic seizures (GTCS) are the severest and most remarkable clinical expressions of human epilepsy. Cortical, subcortical, and cerebellar structures, organized with different network patterns, underlying the pathophysiological substrates of genetic associated epilepsy with GTCS (GE-GTCS) and focal epilepsy associated with focal to bilateral tonic-clonic seizure (FE-FBTS). Structural covariance analysis can delineate the features of epilepsy network related with long-term effects from seizure. Morphometric MRI data of 111 patients with GE-GTCS, 111 patients with FE-FBTS and 111 healthy controls were studied. Cortico-striato-thalao-cerebellar networks of structural covariance within the gray matter were constructed using a Winner-take-all strategy with five cortical parcellations. Comparisons of structural covariance networks were conducted using permutation tests, and module effects of disease duration on networks were conducted using GLM model. Both patient groups showed increased connectivity of structural covariance relative to controls, mainly within the striatum and thalamus, and mostly correlated with the frontal, motor, and somatosensory cortices. Connectivity changes increased as a function of epilepsy durations. FE-FBTS showed more intensive and extensive gray matter changes with volumetric loss and connectivity increment than GE-GTCS. Our findings implicated cortico-striato-thalamo-cerebellar network changes at a large temporal scale in GTCS, with FE-FBTS showing more severe network disruption. The study contributed novel imaging evidence for understanding the different epilepsy syndromes associated with generalized seizures.

Auditory-vocal control system is object for predictive processing within seconds time range

Predictive processing across hierarchically organized time scales is one of the fundamental principles of neural computations in the cerebral cortex. We hypothesize that relatively complex aggregation of auditory and vocal brain systems that use auditory feedback for reflexive control of vocalizations can be an object for predictive processing. We used repetitive patterns of perturbations in auditory feedback during vocalizations to elicit implicit expectations that were violated by surprising direction of perturbations in one of the experimental conditions. Our results provide empirical support for the idea that formation of expectancy for integrated auditory-vocal brain systems, within the time range of seconds, resulted in two sequential neuronal processes. The first process reflects monitoring and error detection in prediction about perturbations in auditory feedback during vocalizations within the time range of seconds. The second neuronal process can be attributed to the optimization of brain predictions for sensory contingencies during vocalizations at separable and distinct timescales.

Insomnia in shift work disorder relates to occupational and neurophysiological impairment

STUDY OBJECTIVES

To determine whether occupational and neurophysiological decrements within shift work disorder (SWD) are differentially related to its two diagnostic symptoms, insomnia and excessive sleepiness.

METHODS

Thirty-four permanent night workers participated in an overnight lab protocol including a multiple sleep latency test (MSLT) and an event-related brain potential (ERP) task testing auditory target detection (P3a and P3b). At 16:00, each subject completed an Endicott Work Productivity Scale (EWPS), two Insomnia Severity Indices (ISI-Day, ISI-Night), and an Epworth Sleepiness Scale (ESS). Subjects were grouped by ISI and ESS scores into clinical phenotypes. This study compared EWPS and ERP results between alert insomniacs ("AI," reporting insomnia without sleepiness), sleepy insomniacs ("SI," reporting both insomnia and sleepiness), and controls.

RESULTS

The AI group was most impaired on the EWPS, significantly more impaired than controls (25.8 ± 14.8 vs. 12.3 ± 9.4, p < 0.05). SI were not statistically different from controls (19.5 ± 8.7 vs. 12.3 ± 9.4, p > 0.05). Compared to controls, AI showed significantly attenuated P3a response (Fcz, Czp, Cpz, mean difference [MD] 1.62-1.77, p < 0.05) and target-detection P3b response (Fcz, Czp, Cpz, MD 1.28-1.64, p < 0.05). P3b in SI was not different from controls (p > 0.10), and P3a was only different at one electrode site (Cpz, MD 1.43, p < 0.01). Neither the MSLT nor the ESS correlated with EWPS scores or ERP (P3a/P3b) amplitudes (p > 0.10). However, the mean of the ISI measurements correlated with the EWPS (r = 0.409, p < 0.01) and the attention-to-novelty P3a (r = -0.410, p < 0.01).

CONCLUSIONS

Among shift work disorder patients, insomnia is linked to functional and cognitive impairments. Insomniacs with normal sleepiness showed more severe impairments than insomniacs who also reported excessive sleepiness.

Differential sleep, sleepiness, and neurophysiology in the insomnia phenotypes of shift work disorder

STUDY OBJECTIVES

To characterize and compare insomnia symptoms within two common phenotypes of Shift Work Disorder.

DESIGN

Observational laboratory and field study.

SETTING

Hospital sleep center.

PARTICIPANTS

34 permanent night workers. Subjects were classified by Epworth Sleepiness Scale and Insomnia Severity Index into 3 subgroups: asymptomatic controls, alert insomniacs (AI), and sleepy insomniacs (SI).

MEASUREMENTS

Sleep parameters were assessed by sleep diary. Circadian phase was evaluated by dim-light salivary melatonin onset (DLMO). Objective sleepiness was measured using the multiple sleep latency test (MSLT). Brain activity was measured using the N1 event-related potential (ERP). A tandem repeat in PER3 was genotyped from saliva DNA.

RESULTS

(1) AI group showed normal MSLT scores but elevated N1 amplitudes indicating cortical hyperarousal. (2) SI group showed pathologically low MSLT scores but normal N1 amplitudes. (3) AI and SI groups were not significantly different from one another in circadian phase, while controls were significantly phase-delayed relative to both SWD groups. (4) AI showed significantly longer sleep latencies and lower sleep efficiency than controls during both nocturnal and diurnal sleep. SI significantly differed from controls in nocturnal sleep parameters, but differences during diurnal sleep periods were smaller and not statistically significant. (5) Genotype × phenotype χ² analysis showed significant differences in the PER3 VNTR: 9 of 10 shift workers reporting sleepiness in a post hoc genetic substudy were found to carry the long tandem repeat on PER3, while 4 of 14 shift workers without excessive sleepiness carried the long allele.

CONCLUSIONS

Our results suggest that the sleepy insomnia phenotype is comprehensively explained by circadian misalignment, while the alert insomnia phenotype resembles an insomnia disorder precipitated by shift work.

Length polymorphism in the Period 3 gene is associated with sleepiness and maladaptive circadian phase in night-shift workers

The objective of the current study was to determine if night-shift workers carrying the five-repeat variant of the Period 3 gene show elevated levels of nocturnal sleepiness and earlier circadian phase compared with homozygotes for the four-repeat allele. Twenty-four permanent night-shift workers were randomly selected from a larger study. Participants took part in an observational laboratory protocol including an overnight multiple sleep latency test and half-hourly saliva collection for calculation of dim-light melatonin onset. Period 3(-/5) shift workers had significantly lower multiple sleep latency test during overnight work hours compared with Period 3(4/4) workers (3.52 ± 23.44 min versus 10.39 ± 6.41 min, P = 0.003). We observed no significant difference in sleepiness during early morning hours following acute sleep deprivation. Long-allele carriers indicated significantly higher sleepiness on the Epworth Sleepiness Scale administered at 17:00 hours (12.08 ± 2.55 versus 8.00 ± 1.94, P < 0.001). We observed a significantly earlier melatonin onset in Period 3(-/5) individuals compared with Period 3(4/4) shift workers (20:44 ± 6:37 versus 02:46 ± 4:58, P = 0.021). Regression analysis suggests that Period 3 genotype independently predicts sleepiness even after controlling for variations in circadian phase, but we were unable to link Period 3 to circadian phase when controlling for sleepiness. Period 3(-/5) shift workers showed both subjective and objective sleepiness in the pathological range, while their Period 3(4/4) counterparts showed sleepiness within normal limits. Period 3(-/5) night workers also show a mean circadian phase 6 h earlier (i.e. less adapted) than Period 3(4/4) workers. Because Period 3(-/5) workers have maladaptive circadian phase as well as pathological levels of sleepiness, they may be at greater risk for occupational and automotive accidents. We interpret these findings as a call for future research on the role of Period 3 in sleepiness and circadian phase, especially as they relate to night work.

Effects of armodafinil on simulated driving and alertness in shift work disorder

STUDY OBJECTIVES

Forty-one percent of shift workers report dozing while driving. This study tested whether armodafinil improves driving simulator performance in subjects with shift work disorder (SWD). A primary outcome was performance late in the shift when workers are typically driving home.

DESIGN

Randomized, double-blind, crossover. During each 12-h test session (21:30-09:30), subjects were kept awake except for multiple sleep latency testing (MSLT: 01:30, 03:30, 05:30, and 07:30). Subjective sleepiness (Karolinska Sleepiness Scale, KSS), driving performance, and cognitive performance (digit symbol substitution test and creativity on the Remote Associates Test, RAT) were evaluated during the night shift and commute home times.

SETTING

Hospital-based sleep research laboratory.

PARTICIPANTS

Twenty night workers (age: 42.7 ± 8.7 y, 17 F) with excessive sleepiness (≥ 10 on the Epworth Sleepiness Scale), meeting International Classification of Sleep Disorders, Second Edition (ICSD-2) criteria for SWD, and having no other medical conditions.

INTERVENTIONS

Armodafinil (150 mg) or placebo at (23:45 h) on counterbalanced nights separated by 7-14 days.

MEASUREMENT AND RESULTS

Primary endpoints were driving simulator performance (standard deviation of lateral position (SDLP) and off-road deviations) with four sessions starting 3.25 h after drug administration, objective sleepiness (MSLT; 1.75 to 7.75 h post-drug), and creativity (5 h post-drug). Significant effects of drug were observed for each driving measure (P < 0.05). Armodafinil significantly improved SDLP for simulator sessions at 05:30, 07:30, and 09:30, and off-road deviations at 7 h, 15 min and 9 h, 15 min post-drug (P < 0.05). Armodafinil also improved objective sleepiness from 3.7 ± 0.6 min to 9.7 ± 5.2 min (P < 0.001) and RAT score from 8.75 ± 4.9 to 11.25 ± 6.0 (P < 0.005).

CONCLUSIONS

Armodafinil 150 mg early in the night shift improves driving simulator performance in SWD. Effects on sleepiness, cognition, and driving were found up to 9.5 h post-ingestion, during the critical time when many night workers are driving home.

Sleep loss, circadian mismatch, and abnormalities in reorienting of attention in night workers with shift work disorder

STUDY OBJECTIVES

Permanent night-shift workers may develop shift-work disorder (SWD). In the current study, we evaluated neurophysiological and behavioral indices of distractibility across times prior to the night shift (T1), during night hours (T2), and after acute sleep deprivation (T3) in permanent hospital night workers with and without SWD.

METHODS

Ten asymptomatic night workers (NW) and 18 NW with SWD participated in a 25-h sleep deprivation study. Circadian phase was evaluated by dim-light salivary melatonin onset (DLMO). Objective sleepiness was evaluated using the Multiple Sleep Latency Test (MSLT). Electrophysiological distractibility was evaluated by brain event-related potentials (ERP), whereas behavioral distractibility was evaluated by performance on a visual task in an auditory-visual distraction paradigm.

STATISTICAL ANALYSES

Comparisons of ERP results were performed by repeated-measures analysis of variance, and t-tests were used where appropriate. A Mann-Whitney U test was used for comparison of variables (MLST, Stanford Sleepiness Scale, and DLMO) that deviated from normal.

RESULTS

First, in the SWD group, the reorienting negativity ERP amplitude was significantly attenuated compared to that in the NW group. Second, the SWD group had shorter MSLT during night shift hours (4.8 ± 4.9 min) compared to that in NW (7.8 ± 3.7 min; U = 47; z = -2.1; P < 0.03). Third, NW with SWD had a DLMO at 20:27 ± 5.0 h, whereas healthy NW had a DLMO at 05:00 ± 3.4 h (U = 43.5; z = -2.22, P < 0.03). Finally, acute sleep deprivation impaired behavioral performance and the P3a ERP in both groups.

CONCLUSIONS

Our results demonstrate specific deficits in neurophysiological activity in the attentional domain among the shift-work disorder group relative to night workers.

Physiological correlates of insomnia

Insomnia is a prevalent sleep disorder that is typically comorbid with medical, psychiatric, and other sleep disorders. Yet, it is a disorder with its own course and morbidity that can persist if untreated. This chapter describes the physiological correlates of insomnia expressed during sleep and during the daytime. Together, the data from nighttime and daytime electrophysiology, event-related brain potential recording, neuroimaging studies, sympathetic nervous system, and HPA axis monitoring all suggest that insomnia is a 24 h disorder of hyperarousal.

Circadian phase, sleepiness, and light exposure assessment in night workers with and without shift work disorder

Most night workers are unable to adjust their circadian rhythms to the atypical hours of sleep and wake. Between 10% and 30% of shiftworkers report symptoms of excessive sleepiness and/or insomnia consistent with a diagnosis of shift work disorder (SWD). Difficulties in attaining appropriate shifts in circadian phase, in response to night work, may explain why some individuals develop SWD. In the present study, it was hypothesized that disturbances of sleep and wakefulness in shiftworkers are related to the degree of mismatch between their endogenous circadian rhythms and the night-work schedule of sleep during the day and wake activities at night. Five asymptomatic night workers (ANWs) (3 females; [mean ± SD] age: 39.2 ± 12.5 yrs; mean yrs on shift = 9.3) and five night workers meeting diagnostic criteria (International Classification of Sleep Disorders [ICSD]-2) for SWD (3 females; age: 35.6 ± 8.6 yrs; mean years on shift = 8.4) participated. All participants were admitted to the sleep center at 16:00 h, where they stayed in a dim light (<10 lux) private room for the study period of 25 consecutive hours. Saliva samples for melatonin assessment were collected at 30-min intervals. Circadian phase was determined from circadian rhythms of salivary melatonin onset (dim light melatonin onset, DLMO) calculated for each individual melatonin profile. Objective sleepiness was assessed using the multiple sleep latency test (MSLT; 13 trials, 2-h intervals starting at 17:00 h). A Mann-Whitney U test was used for evaluation of differences between groups. The DLMO in ANW group was 04:42 ± 3.25 h, whereas in the SWD group it was 20:42 ± 2.21 h (z = 2.4; p < .05). Sleep did not differ between groups, except the SWD group showed an earlier bedtime on off days from work relative to that in ANW group. The MSLT corresponding to night work time (01:00-09:00 h) was significantly shorter (3.6 ± .90 min: [M ± SEM]) in the SWD group compared with that in ANW group (6.8 ± .93 min). DLMO was significantly correlated with insomnia severity (r = -.68; p < .03), indicating that the workers with more severe insomnia symptoms had an earlier timing of DLMO. Finally, SWD subjects were exposed to more morning light (between 05:00 and 11:00 h) as than ANW ones (798 vs. 180 lux [M ± SD], respectively z = -1.7; p < .05). These data provide evidence of an internal physiological delay of the circadian pacemaker in asymptomatic night-shift workers. In contrast, individuals with SWD maintain a circadian phase position similar to day workers, leading to a mismatch/conflict between their endogenous rhythms and their sleep-wake schedule.

Habitual short sleep impacts frontal switch mechanism in attention to novelty

STUDY OBJECTIVES

Reduced time in bed relative to biological sleep need is common. The impact of habitual short sleep on auditory attention has not been studied to date. In the current study, we utilized novelty oddball tasks to evaluate the effect of habitual short sleep on brain function underlying attention control processes measured by the mismatch negativity (MMN, index of pre-attentive stage), P3a (attention-dependent), and P3b (memory-dependent) event related brain potentials (ERPs). An extended time in bed in a separate study was used to evaluate the possible reversal of the impairments of these processes in habitual short sleepers.

METHODS

Ten self-defined short sleepers (total sleep time [TST] ≤ 6 h) and 9 normal-sleeping subjects with TST 7-8 h, participated. ERPs were recorded via a 64-channel EEG system. Two test conditions: "ignore" and "attend" were implemented. The ERPs were analyzed and compared between groups on the 2 task conditions and frontal/central/parietal electrodes by 3-factor ANOVA. Sleep diary data were compared between groups by t-test. Sleep was recorded by the Zeo sleep monitoring system for a week in both habitual and extended sleep conditions at home.

RESULTS

The main findings of the present study show that short sleeping individuals had deficiency in activity of the MMN and P3a brain responses over frontal areas compared to normal-sleeping subjects. The P3b amplitude was increased over frontal areas and decreased over parietal with respect to the control group. Extension of time in bed for one week increased TST (from 5.7 h to 7.4 h), and concomitantly MMN amplitude increased from -0.1 μV up to -1.25 μV over frontal areas.

CONCLUSIONS

Reduced time in bed is associated with deficiency of the neuronal process associated with change detection, which may recover after one week of sleep extension, whereas attention-dependent neural processes do not normalize after this period of time in habitually short sleeping individuals and may require longer recovery periods.

An assessment of MEG coherence imaging in the study of temporal lobe epilepsy

PURPOSE

This study examines whether magnetoencephalographic (MEG) coherence imaging is more sensitive than the standard single equivalent dipole (ECD) model in lateralizing the site of epileptogenicity in patients with drug-resistant temporal lobe epilepsy (TLE).

METHODS

An archival review of ECD MEG analyses of 30 presurgical patients with TLE was undertaken with data extracted subsequently for coherence analysis by a blinded reviewer for comparison of accuracy of lateralization. Postoperative outcome was assessed by Engel classification. MEG coherence images were generated from 10 min of spontaneous brain activity and compared to surgically resected brain areas outlined on each subject's magnetic resonance image (MRI). Coherence values were averaged independently for each hemisphere to ascertain the laterality of the epileptic network. Reliability between runs was established by calculating the correlation between epochs. Match rates compared the results of each of the two MEG analyses with optimal postoperative outcome.

KEY FINDINGS

The ECD method provided an overall match rate of 50% (13/16 cases) for Engel class I outcomes, with 37% (11/30 cases) found to be indeterminate (i.e., no spikes identified on MEG). Coherence analysis provided an overall match rate of 77% (20/26 cases). Of 19 cases without evidence of mesial temporal sclerosis, coherence analysis correctly lateralized the side of TLE in 11 cases (58%). Sensitivity of the ECD method was 41% (indeterminate cases included) and that of the coherence method 73%, with a positive predictive value of 70% for an Engel class Ia outcome. Intrasubject coherence imaging reliability was consistent from run-to-run (correlation > 0.90) using three 10-min epochs.

SIGNIFICANCE

MEG coherence analysis has greater sensitivity than the ECD method for lateralizing TLE and demonstrates reliable stability from run-to-run. It, therefore, improves upon the capability of MEG in providing further information of use in clinical decision-making where the laterality of TLE is questioned.

Shift work sleep disorder is associated with an attenuated brain response of sensory memory and an increased brain response to novelty: an ERP study

STUDY OBJECTIVE

To study the neurophysiological changes in attention and memory functions in shift work sleep disorder (SWSD), using event-related brain potentials (ERPs).

PARTICIPANTS

9 healthy night workers (NW) (mean age = 40 y; SD +/- 8.9 y); 8 night workers meeting diagnostic criteria for SWSD (mean age = 37 y +/- 9.4 y) and 9 healthy day workers (DW) (mean age = 35 y +/- 7.3 y).

METHODS AND PROCEDURE

Using standard PSG the sleep related measures (TIB, TST, SOL, SE, and sleep stage distribution) were obtained prior to EEG/ERP study. Measures of habitual sleep were obtained from 2 week sleep logs and sleepiness was assessed with standardized measures. Using 32-EEG leads the ERPs to 3 types of sounds (novel, duration deviant, and simple tone) were obtained. The mismatch negativity (MMN) reflecting memory processing and P3a-reflecting the shift of involuntary attention were obtained.

STATISTICAL ANALYSIS

The statistical comparisons of ERPs and sleep related parameters were performed using repeated measured ANOVAs and t-tests where appropriate.

RESULTS

Patients with SWSD had reduced TST and increased WASO relative to healthy workers. ERP results demonstrated significant attenuation of MMN amplitude over frontal regions in SWSD patients relative to NW and DW. In the SWSD patients, the P3a was increased to novelty across frontocentral brain regions with respect to the same locations in healthy controls.

CONCLUSION

The ERP evidence of sensory memory reduction and attentional hyper-reaction to novel sound in conjunction with disturbed sleep suggests the need for more neurophysiological studies in SWSD workers.

Cortical locations of maximal spindle activity: magnetoencephalography (MEG) study

The aim of this study was to determine the main cortical regions related to maximal spindle activity of sleep stage 2 in healthy individual subjects during a brief morning nap using magnetoencephalography (MEG). Eight volunteers (mean age: 26.1 +/- 8.7, six women) all right handed, free of any medical psychiatric or sleep disorders were studied. Whole-head 148-channel MEG and a conventional polysomnography montage (EEG; C3, C4, O1 and O2 scalp electrodes and EOG, EMG and ECG electrodes) were used for data collection. Sleep MEG/EEG spindles were visually identified during 15 min of stage 2 sleep for each participant. The distribution of brain activity corresponding to each spindle was calculated using a combination of independent component analysis and a current source density technique superimposed upon individual MRIs. The absolute maximum of spindle activation was localized to frontal, temporal and parietal lobes. However, the most common cortical regions for maximal source spindle activity were precentral and/or postcentral areas across all individuals. The present study suggests that maximal spindle activity localized to these two regions may represent a single event for two types of spindle frequency: slow (at 12 Hz) and fast (at 14 Hz) within global thalamocortical coherence.

Intracranial recording and source localization of auditory brain responses elicited at the 50 ms latency in three children aged from 3 to 16 years

Maturational studies of the auditory-evoked brain response at the 50 ms latency provide an insight into why this response is aberrant in a number of psychiatric disorders that have developmental origin. Here, using intracranial recordings we found that neuronal activity of the primary contributors to this response can be localised at the lateral part of Heschl's gyrus already at the age of 3.5 years. This study provides results to support the notion that deviations in cognitive function(s) attributed to the auditory P50 in adults might involve abnormalities in neuronal activity of the frontal lobe or in the interaction between the frontal and temporal lobes. Validation and localisation of progenitors of the adults' P50 in young children is a much-needed step in the understanding of the biological significance of different subcomponents that comprise the auditory P50 in the adult brain. In combination with other approaches investigating neuronal mechanisms of auditory P50, the present results contribute to the greater understanding of what and why neuronal activity underlying this response is aberrant in a number of brain dysfunctions. Moreover, the present source localisation results of auditory response at the 50 ms latency might be useful in paediatric neurosurgery practice.

The maturation of human evoked brain potentials to sounds presented at different stimulus rates

The current study assessed the normal development of cortical auditory evoked potentials (CAEPs) in humans presented with pure tone stimuli at relatively fast stimulus rates. Traditionally, maturation of sound processing indexed by CAEPs has been studied in paradigms using inter-stimulus intervals (ISIs) generally slower than 1 Hz. While long ISIs may enhance the amplitude of CAEP components, speech information generally occurs at more rapid rates. These slower rates of sound presentation may not accurately assess auditory cortical functions in more realistic sound environments. We examined the effect of temporal rate on the elicitation of the P1-N1-P2-N2 components to unattended sounds at four levels of stimulus onset asynchrony (SOA, onset to onset, 200, 400, 600, and 800 ms) in children grouped separately by year (ages 8, 9, 10, 11 years), in adolescents (age 16 years) and in one group of young adults (ages 22-40 years). We found that both age and stimulus rate produced profound changes in CAEP morphology. Between the ages of 8-11 years, the P1 and N2 components dominated the ERP waveform at all stimulus rates. N1, the dominant CAEP component in adults, appeared as a bifurcation in a broad positive peak at earlier ages, and did not emerge as a separate component until adolescence. While the P1-N1-P2 components are more "adult-like" than "child-like" in the adolescent subjects, the N2 component, a hallmark of the child obligatory response, was still present. Faster rates resulted in the suppression of discrete components such that by 200 ms, only P1 in the adults and adolescents, and both P1 and N2 in the youngest children were discernable. We conclude that both age and ISI are important variables in the assessment of auditory cortex function and maturation. The presence of N2 in adolescents indicates that auditory cortical maturation persists into teen years.

Organization of sequential sounds in auditory memory

A repeating five-tone pattern was presented at several stimulus rates (200, 400, 600, and 800 ms onset-to-onset) to determine at what temporal proximity the five-tone repeating unit would be represented in memory. The mismatch negativity component of event-related brain potentials was used to index how the sounds were organized in memory when participants had no task with the sounds. Only at the 200-ms onset-to-onset pace was the five-tone sequence unitized in memory. At presentation rates of 400 ms and above, the regularity (a different frequency tone occurred every fifth tone) was not detected and mismatch negativity was elicited by these tones in the sequence. The results show that temporal proximity plays a role in unitizing successive sounds in auditory memory. These results also suggest that global relationships between successive sounds are represented at the level of auditory cortices.

Electrophysiological evidence of enhanced distractibility in ADHD children

Abnormal involuntary attention leading to enhanced distractibility may account for different behavioral and cognitive problems in children with attention deficit hyperactivity disorder (ADHD). This was investigated in the present experiment by recording event-related brain potentials (ERPs) to distracting novel sounds during performance of a visual discrimination task. The overall performance in the visual task was less accurate in the ADHD children than in the control children, and the ADHD children had a higher number of omitted responses following novel sounds. In both groups, the distracting novel sounds elicited a biphasic P3a ERP component and a subsequent frontal Late Negativity (LN). The early phase of P3a (180-240 ms) had significantly smaller amplitudes over the fronto-central left-hemisphere recording sites in the ADHD children than in the control group presumably due to an overlapping enhanced left-hemisphere dominant negative ERP component elicited in the ADHD group. Moreover, the late phase of P3a (300-350 ms) was significantly larger over the left parietal scalp areas in the ADHD children than in the controls. The LN had a smaller amplitude and shorter latency over the frontal scalp in the ADHD group than in the controls. In conclusion, the ERP and behavioral effects caused by the novel sounds reveal deficient control of involuntary attention in ADHD children that may underlie their abnormal distractibility.

Effects of auditory distraction on electrophysiological brain activity and performance in children aged 8-13 years

Distractibility was investigated in three age groups of children (8-9, 10-11, and 12-13 years) with event-related brain potentials (ERPs) and performance measures in a forced-choice visual task. Distraction was reflected by increased reaction times (RTs) and decreased performance accuracy in the visual discrimination task following presentation of unexpected novel sounds. The amplitude of the late portion of the P3a elicited by novel sounds was largest for the youngest group and showed a centrally dominant scalp distribution and smallest for the oldest group with a frontal scalp distribution. A frontally dominant late negativity (LN) that was largest in the youngest group followed the P3a. Correlation between the RT increase caused by the distracting novel sounds and the amplitude of the LN elicited by these sounds suggested that the LN is associated with the degree of attention engaged by the distracting sounds.

Electric brain responses indicate preattentive processing of abstract acoustic regularities in children

This study investigated the preattentive processing of abstract acoustic regularities in children aged 8-14 years. Event-related brain potentials (ERPs) were elicited by frequent (standard) pairs ascending in pitch (the second tone having a higher frequency than the first tone) and by infrequent (deviant) pairs descending in pitch. In the easy condition, the second tone of the pair was always one step higher (standard) or lower (deviant) than the first tone, while in the hard condition, the second tone was randomly 1-10 steps higher or lower than the first tone. In the easy condition we found the mismatch negativity (MMN) and a subsequent positive P3a-like deflection. In the hard condition, the amplitude of MMN was lower over frontal sites than in the easy condition, while the temporal component of MMN was not impaired by complexity of abstract regularities. These results suggest that the complexity of the auditory stimulation affects preattentive auditory change detection in children.

Temporary and longer term retention of acoustic information

Though many studies suggest that fine acoustic details fade from memory after 15 s or even less, everyday experience tells us that the voice of a person or a musical instrument can be recognized long after it was last heard. We wished to determine whether tones leave a lasting memory trace using an experimental model of implicit recognition and testing whether exact pitch information can be retrieved even after 30 s. Event-related brain potentials demonstrated the survival of an accurate representation of tone pitch in the auditory cortex. This result provides a link between short-duration buffering and permanent storage of acoustic information.

Brain activity index of distractibility in normal school-age children

Children's attention is easily diverted from a current activity to a new event in the environment. This was indexed in school-age children by diminished performance speed and accuracy in a visual discrimination task caused by task-irrelevant novel sounds. Event-related brain potentials (ERPs) elicited by these distracting sounds showed a prominent positive deflection that was generated by brain processes associated with involuntary switching of attention to novel sounds. Recordings of the magnetoencephalographic (MEG) counterpart of this brain activity revealed a major bilateral generator source in the superior temporal cortex. However, ERP scalp distributions indicated also overlapping brain activity generated in other brain areas involved in involuntary attention switching. Moreover, differences in ERP amplitudes and in their correlations with the reaction times between younger (7-10 years) and older (11-13 years) children indicated developmental changes in attentional brain functions.

Electromagnetic responses of the human auditory cortex generated by sensory-memory based processing of tone-frequency changes

Event-related brain potentials (ERPs) and magnetoencephalographic (MEG) responses to infrequent ('deviant') tones occurring among frequent ('standard') tones of different pitch were compared with responses to rare tones presented alone. The subjects were to ignore the tones. Deviant tones elicited the mismatch negativity (MMN) and its MEG counterpart (MMNm), while the rare tones delivered alone elicited a larger N1 and its MEG counterpart (N1m) than did standard tones. Source modeling of MEG responses indicated a difference in auditory-cortex source locations between the MMNm to deviant tones and the enhanced N1m to the rare tones presented alone. Thus, the MMN/MMNm is elicited by infrequent sounds only when they occur among frequent sounds. This supports the idea that a sensory-memory trace formed in the auditory cortex by preceding repetitive sounds is a necessary precondition for MMN/MMNm elicitation.