Event-related synchronization (ERS): an electrophysiological correlate of cortical areas at rest

Oscillations in the alpha and beta bands can display either an event-related blocking response or an event-related amplitude enhancement. The former is named event-related desynchronization (ERD) and the latter event-related synchronization (ERS). Examples of ERS are localized alpha enhancements in the awake state as well as sigma spindles in sleep and alpha or beta bursts in the comatose state. It was found that alpha band activity can be enhanced over the visual region during a motor task, or during a visual task over the sensorimotor region. This means ERD and ERS can be observed at nearly the same time; both form a spatiotemporal pattern, in which the localization of ERD characterizes cortical areas involved in task-relevant processing, and ERS marks cortical areas at rest or in an idling state.     

Event-related synchronization (ERS): an electrophysiological correlate of cortical areas at rest.

Oscillations in the alpha and beta bands can display either an event-related blocking response or an event-related amplitude enhancement. The former is named event-related desynchronization (ERD) and the latter event-related synchronization (ERS). Examples of ERS are localized alpha enhancements in the awake state as well as sigma spindles in sleep and alpha or beta bursts in the comatose state. It was found that alpha band activity can be enhanced over the visual region during a motor task, or during a visual task over the sensorimotor region. This means ERD and ERS can be observed at nearly the same time; both form a spatiotemporal pattern, in which the localization of ERD characterizes cortical areas involved in task-relevant processing, and ERS marks cortical areas at rest or in an idling state.     

Event-related alpha synchronization/desynchronization in a memory-search task in adolescent survivors of childhood cancer.

OBJECTIVES: Event-related desynchronization (ERD) and synchronization (ERS) of the 8-10 and 10-12 Hz frequency bands of the background EEG were studied in 19 adolescent survivors of childhood cancer (11 leukemias, 8 solid tumors) and in 10 healthy control subjects performing an auditory memory task. METHODS: The stimuli were auditory Finnish words presented as a Sternberg-type memory-scanning paradigm. Each trial started with the presentation of a 4 word set for memorization whereafter a probe word was presented to be identified by the subject as belonging or not belonging to the memorized set. RESULTS: Encoding of the memory set elicited ERS and retrieval ERD at both frequency bands. However, in the survivors of leukemia, ERS was turned to ERD during encoding at the lower alpha frequency band. ERD was lasting longer at the lower frequency band than at the higher frequency band, in each study group. At both frequency bands, the maximum of ERD was achieved later in the cancer survivors than in the control group. CONCLUSION: The previously reported type of ERD/ERS during an auditory memory task was reproducible also in the survivors of childhood cancer, at different alpha frequency bands. However, the temporal deviance in ERD/ERS magnitudes, in the cancer survivors, was interpreted to indicate that both survivor groups had prolonged information processing time and/or they used ineffective cognitive strategies. This finding was more pronounced in the group of leukemia survivors, at the lower alpha frequency band, suggesting that the main problem of this patient group might be in the field of attention.     

Test-retest consistency of the event-related desynchronization/event-related synchronization of the 4-6, 6-8, 8-10 and 10-12 Hz frequency bands during a memory task.

OBJECTIVE: The aim of this study was to determine the test-retest consistency of the event-related desynchronization/event-related synchronization (ERD/ERS) method during the performance of an auditory memory task. METHODS: The EEG was recorded while 12 subjects performed an auditory memory task on two separate occasions (mean test-retest interval 9 days). Differences in the ERD/ERS responses between the first and second registrations were examined in 4 EEG frequency bands of interest: 4-6, 6-8, 8-10 and 10-12 Hz. RESULTS: We observed significant differences in the ERD/ERS responses between the first and second registration in all studied frequency bands when examined as a function of time and experimental task. The test-retest reliability of the ERD/ERS values was highest in the theta frequency range (4-6 and 6-8 Hz), poorer in the 8-10 Hz and poorest in the 10-12 Hz alpha frequency range. CONCLUSIONS: The test-retest effect was seen in the ERD/ERS responses of all studied frequency bands during an auditory memory task.     

Decreased movement-related beta desynchronization and impaired post-movement beta rebound in amyotrophic lateral sclerosis

ObjectiveThis study explored event-related desynchronization (ERD) and synchronization (ERS) in amyotrophic lateral sclerosis (ALS) to quantify cortical sensorimotor processes during volitional movements. We furthermore compared ERD/ERS measures with clinical scores and movement-related cortical potential (MRCP) amplitudes.MethodsElectroencephalograms were recorded while 21 ALS patients and 19 controls performed two self-paced motor tasks: sniffing and right index finger flexion. Based on Wavelet analysis the alpha and beta frequency bands were selected for subsequent evaluation.ResultsPatients generated significantly smaller resting alpha spectral power density (SPD) and smaller beta ERD compared to controls. Additionally patients exhibited merely unilateral post-movement ERS (beta rebound) whereas this phenomenon was bilateral in controls. ERD/ERS amplitudes did not correlate with corresponding MRCPs for either patients or controls.ConclusionsThe smaller resting alpha SPD and beta ERD and asymmetrical appearance of beta ERS in patients compared to controls could be the result of pyramidal cell degeneration and/or corpus callosum involvement in ALS.SignificanceThese results support the notion of reduced movement preparation in ALS involving also areas outside the motor cortex. Furthermore post-movement cortical inhibition seems to be impaired in ALS. ERD/ERS and MRCP are found to be independent measures of cortical motor functions in ALS.     

Long-term stability and consistency of EEG event-related (de-)synchronization across different cognitive tasks.

OBJECTIVE: We examined whether task-related band power changes (event-related desynchronization/synchronization; ERD/ERS) that have been linked to individual differences in cognitive ability demonstrate satisfying temporal stability and cross-situational consistency. METHODS: Multi-channel EEG recordings from 29 adults, assessed at three different occasions over 2 years were examined. Between-session correlations and consistency coefficients (Cronbach's alpha) across the three experiments were evaluated for both, spectral power features of the resting EEG and ERD/ERS estimates while the participants performed some cognitive task (i.e. different elementary cognitive tasks that put comparable demands on the participants). RESULTS: ERD/ERS values, while subjects performed a cognitive task, demonstrated satisfactory stability and consistency (i.e. >0.7), whereby the degree of consistency varied as a function of frequency band and brain region. Highest consistency was found for the 8-10 Hz ERD in parieto-occipital recording sites (i.e. >0.9). In resting EEG, mean alpha (gravity) frequency was the most stable EEG feature. CONCLUSIONS: The present data suggest that ERD/ERS phenomena in different narrow frequency bands are rather stable over time and across different situations. The relatively high reproducibility of ERD/ERS promotes the usefulness of this measure in assessing individual differences of physiological activation patterns accompanying cognitive performance. SIGNIFICANCE: This study addresses the issue of reproducibility of EEG in general and ERD/ERS experiments in particular, which is a prerequisite for both basic research and clinical studies.     

Event-related desynchronization/synchronization during an auditory-verbal working memory task in mild Parkinson's disease.

OBJECTIVE: Event-related desynchronization (ERD) and synchronization (ERS) were studied during an auditory-verbal working memory task in patients suffering from Parkinson's disease (PD). METHODS: PD patients (n=7, mean age 59) at a mild stage of the disease volunteered in the study. A group of healthy subjects (n=10, mean age 61) served as control group. ERD and ERS of the 1-25 Hz EEG frequencies were studied using wavelet transforms during memory encoding and retrieval. RESULTS: Both groups performed equally well on the memory task. Statistically significant differences in ERD/ERS responses were observed in posterior electrodes during encoding of the memory set due to the fact that alpha ( approximately 10-15 Hz) ERS was elicited in the controls, but not in the PD group. In broad frequency bands ( approximately 6-25 Hz) ERD responses were observed in both groups during memory retrieval. A pre-stimulus alpha ERS seen in the control group was absent in the PD group. CONCLUSIONS: The results of this preliminary study indicate that Parkinson's disease might affect brain oscillatory responses in the alpha frequency range in the encoding phase of auditory-verbal working memory. SIGNIFICANCE: The ERD/ERS patterns may reflect neurophysiological alterations in the processes underlying working memory deficits in PD.     

Cortical activation related to auditory semantic matching of concrete versus abstract words.

OBJECTIVE: The aim of this study was to examine cortical correlates of semantic memory processes in the auditory stimulus modality. METHODS: Event-related desynchronization (ERD) and synchronization (ERS) of the lower (8-10 Hz) and upper (10-12 Hz) alpha frequency bands of background EEG were studied in 10 subjects performing an auditory semantic matching task. The stimuli were abstract and concrete nouns which were presented sequentially in pairs. The task was to decide whether the two nouns belonged to the same semantic category or not. RESULTS: The presentation of the first stimulus (encoding) elicited ERS whereas the presentation of the second stimulus (semantic matching) elicited ERD. Abstract nouns presented as the second stimulus elicited ERD which was most prominent in the lower alpha frequency band, whereas the presentation of a concrete noun as the first stimulus elicited ERS, most prominently in the upper alpha frequency band. CONCLUSIONS: The auditorily elicited ERD/ERS reflects cortical activity associated with cognitive functions. The present findings demonstrate that the auditorily elicited ERD/ERS can reveal subtle differences in auditory information processing. Semantic memory processes (encoding and comparison) are reflected as varying responses in the two alpha frequency bands.     

Event-related desynchronization and synchronization during a memory task in children

OBJECTIVE: To examine the event-related desynchronization (ERD) and synchronization (ERS) responses of several narrow electroencephalographic (EEG) frequency bands in children during an auditory memory task. METHODS: ERD/ERS responses of the 4-6, 6-8, 8-10 and 10-12 Hz EEG frequency bands were studied in 12 children (mean age 12 years) while they performed an auditory memory task. Twelve adult subjects served as a control group. RESULTS: The children's ERD/ERS responses differed from those of the adult's in the 4-6, 6-8 and 8-10 Hz EEG frequency bands, especially during retrieval from memory. The children's 4-6 Hz initial ERS responses were of lesser amplitude and of delayed latency as compared to those of the adults. In the 6-8 and 8-10 Hz frequency bands, especially during retrieval from memory, the children's ERD responses were of lesser magnitude than those of the adults. In the 10-12 Hz frequency band, no differences were observed between the ERD/ERS responses between the children and adults. CONCLUSIONS: Our results suggest that theta and alpha response systems might participate in auditory information processing already at this age, although not being fully developed. Memory systems involving retrieval may be the last to mature.     

Event-related. EEG desynchronization and synchronization during an auditory memory task

Event-related desynchronization (ERD) and synchronization (ERS) of the lower (8–10 Hz) and upper (10–12 Hz) alpha bands of background EEG were studied in 10 subjects during an auditory memory scanning paradigm. Each experimental trial started with the presentation of a visual warning signal, after which an auditory 4-vowel memory set was presented for memorization. Thereafter the probe, a fifth vowel, was presented and identified by the subject as belonging or not belonging to the memorized set. In 50% of the cases, the probe was among the previously presented memory set. The presentation of the memory set elicited a significant ERS in the both alpha frequency bands. In contrast, the presentation of the probe elicited a significant bilateral ERD in both alpha frequency bands studied. The results suggest that the ERD phenomenon is closely associated with higher cortical processes such as memory functions rather than with auditory stimulus processing per se. Event-related desynchronization provides a potentially valuable tool for studying cortical activity during cognitive processing in the auditory stimulus modality.     

Event-related desynchronization and synchronization during an auditory lexical matching task

Objectives: Event-related desynchronization (ERD) and synchronization (ERS) of the 8–10 and 10–12 Hz frequency bands of the background EEG were studied in 10 subjects performing an auditory lexical matching task.Methods: The stimuli were words and pseudowords presented sequentially in pairs. The subject was prompted to answer whether the two stimuli shared the same lexical status (words or pseudowords).Results: Regardless of lexicality, the presentation of the first stimulus elicited a significant late frontal ERD in both alpha frequency bands. When preceded by a pseudoword, the presentation of the second stimulus elicited a significant ERS at 200–400 ms and a significant, long-lasting and topographically-widespread ERD at 600–2200 ms in both frequency bands. When preceded by a word, the second stimulus did not elicit ERS in the initial time window, but a late ERD which was similar to the one observed in the previous condition. The complexity of ERD/ERS changes in the present task was revealed by significant interactions that time had with frequency band, stimulus type, stimulus order and lexicality of the preceding stimulus.Conclusions: The results suggest that ERD/ERS does not reflect primary auditory stimulus processing. Rather, the ERD/ERS observed in this experiment most probably reflected task difficulty and differences between lexical–semantic and phonological memory functions.     

Event-related desynchronization and synchronization. Reactivity of electrocortical rhythms in relation to the planning and execution of voluntary movement]

Cortical electroencephalographic rhythms reactivity may be quantified using event-related desynchronization (ERD) and synchronization (ERS) methods. We therefore studied cortical activation occurring during programming and performance of voluntary movement in healthy subjects. EEG power evolution within the reactive frequency bands (mu and beta central rhythms) was averaged before, during and after a minimum of 50 self-paced flexions of the thumb. Recordings in 18 normal adults showed that ERD (decrease in power) of mu rhythm started 2,000 ms before movement onset, while ERD of beta rhythm started 1,500 ms before movement onset. Early ERD of mu and beta rhythms were located over the contralateral central region covering primary motor cortex. They were followed by bilateral ERD occurring over ipsilateral and contralateral central regions during performance of the movement. At the end of the movement, an ERS (increase in power) of beta rhythm occurred. These results suggest that programming of voluntary movement induces early activation in contralateral sensorimotor areas, while performance of the movement induces bilateral activation in sensorimotor areas. ERS of beta rhythm occurring at the end of the movement could correspond to inactivation of motor areas activated by movement. Based on EEG activity, ERD and ERS prove to be useful methods to analyze cortical activation during programming and performance of voluntary movements with good spatial and temporal resolution.     

Fixational saccade‐related activity of pedunculopontine tegmental nucleus neurons in behaving monkeys

Fixational saccades are small, involuntary eye movements that occur during attempted visual fixation. Recent studies suggested that several cognitive processes affect the occurrence probability of fixational saccades. Thus, there might be an interaction between fixational saccade‐related motor signals and cognitive signals. The pedunculopontine tegmental nucleus (PPTN) in the brainstem has anatomical connections with numerous saccade‐related and limbic areas. Previously, we reported that a group of PPTN neurons showed transient phasic bursts or a pause in activity during large visually guided and spontaneous saccades, and also showed sustained tonic changes in activity with task context. We hypothesised that single PPTN neurons would relay both fixational saccade‐related and task context‐related signals, and might function as an interface between the motor and limbic systems. We recorded the activity of PPTN neurons in behaving monkeys during a reward‐biased task, and analysed neuronal activity for small fixational saccades during visual fixation, and compared it with the activity for large visually guided targeting saccades and large spontaneous saccades during intertrial intervals. A population of PPTN neurons exhibited a fixational saccade‐related phasic increase in activity, and the majority of them also showed activity modulation with large targeting saccades. In addition, a group of these neurons showed a task‐related tonic increase in activity during the fixation period, and half of them relayed the saccade signal only when the neuron exhibited higher tonic activity during the task execution period. Thus, fixational saccade‐related signals of PPTN neurons overlap with tonic task‐related signals, and might contribute to the cognitive modulation of fixational saccades.     

Preparation of saccade sequences and eye programming affect endogenous covert attention

Endogenous attention can be allocated in parallel to at least two saccade target locations of a planned sequence, but attentional resources are larger in the location of the first than the second saccade. The meridian effect that is observed in endogenous attention can be explained by eye programming, but it is not known how eye‐movement preparation and eye programming can together affect endogenous attention during sequential saccades. We used a double‐task paradigm to investigate this issue. In two experiments, we confirmed the relation between the preparation of sequential saccades and attentional selection and also showed that eye programming could eliminate deterioration of attentional resources in the second saccade location. The finding of the meridian effect in both the saccadic task and the target discrimination task additionally indicates the important role of eye programming in endogenous attention. The results were discussed in terms of the Premotor Theory of Attention.     

Event-related desynchronization in reaction time paradigms: a comparison with event-related potentials and corticospinal excitability.

OBJECTIVES: To study cortical activity in different motor tasks, we compared event-related desynchronization (ERD) and event-related potentials (ERPs) in different reaction time (RT) paradigms with the time course of corticospinal excitability. METHODS: Nine right-handed, normal subjects performed right or left thumb extensions in simple, choice and go/no go auditory RT paradigms. Eight subjects had participated in a previous study evaluating changes in corticospinal excitability during the same paradigms. Twenty-nine EEG channels with electrooculogram and bilateral EMG monitoring were collected. ERPs and ERD of 10 and 18-22 Hz bands were obtained with respect to tone administration and EMG onset. RESULTS: Trials with movement showed lateralized ERP components, corresponding to the motor potential (MP), both in the averages on the tone and on EMG. The MP corresponded well in time and location to the rise in corticospinal excitability on the moving side observed in the previous study. Sensorimotor ERD, followed by event-related synchronization (ERS), was present for trials with movements and for the no go. ERD was present contralaterally during movement preparation and in no go trials, while it was bilateral during motor execution. No go ERD was followed more rapidly by ERS than in trials with movement. This finding suggests that in no go trials, there is a brief active process in the sensorimotor areas. ERD and ERS do not correspond, respectively, in time and location to increases and decreases in corticospinal excitability. In fact, ERD is bilateral during movement execution, when corticospinal inhibition of the side at rest is observed. Contralateral no go ERS occurs later than corticospinal inhibition, which is bilateral. CONCLUSIONS: These findings may suggest that ERD is compatible with both corticospinal activation and inhibition, ERS indicating the removal of either, resulting in cortical idling.     

Event-related beta EEG-changes during passive and attempted foot movements in paraplegic patients

A number of electroencephalographic (EEG) studies report on motor event-related desynchronization and synchronization (ERD/ERS) in the beta band, i.e. a decrease and increase of spectral amplitudes of central beta rhythms in the range from 13 to 35 Hz. Following an ERD that occurs shortly before and during the movement, bursts of beta oscillations (beta ERS) appear within a 1-s interval after movement offset. Such a post-movement beta ERS has been reported after voluntary hand movements, passive movements, movement imagination, and also after movements induced by functional electrical stimulation. The present study compares ERD/ERS patterns in paraplegic patients (suffering from a complete spinal cord injury) and healthy subjects during attempted (active) and passive foot movements. The aim of this work is to address the question, whether patients do have the same focal beta ERD/ERS pattern during attempted foot movement as healthy subjects do. The results showed midcentral-focused beta ERD/ERS patterns during passive, active, and imagined foot movements in healthy subjects. This is in contrast to a diffuse and broad distributed ERD/ERS pattern during attempted foot movements in patients. Only one patient showed a similar ERD/ERS pattern. Furthermore, no significant ERD/ERS patterns during passive foot movement in the group of the paraplegics could be found.     

The effects of memory load on event-related EEG desynchronization and synchronization.

OBJECTIVES: To examine the effects of working memory load on the event-related desynchronization (ERD) and synchronization (ERS) of several narrow EEG frequency bands. METHODS: ERD/ERS responses of the 4-6, 6-8, 8-10 and 10-12 Hz EEG frequency bands were studied in 24 normal subjects performing a visual sequential letter task (so-called n-back task) in which memory load was varied from 0 to 2. RESULTS: In the 4-6 Hz theta frequency band, a long-lasting synchronization was observed in the anterior electrodes, especially after the presentation of targets. In the 6-8 and 8-10 Hz frequency bands, anterior ERS was elicited especially in the 2-back condition (highest memory load). In contrast to the responses of the 8-10 Hz frequency band, in the 10-12 Hz frequency band the 2-back experimental condition elicited the greatest ERD. CONCLUSIONS: In the highest memory load (2-back) experimental condition the attentional capacities were most probably exceeded, resulting in 6-8 and 8-10 Hz ERS. This might reflect an inhibition of such brain areas (frontal cortices) no longer involved in task completion when alternative strategies are needed and utilized. These more 'cognitive' strategies were then reflected as an increase in 10-12 Hz ERD. Additionally, our results support the assumption that the simultaneously recorded ERD/ERS responses of different narrow EEG frequency bands differ and reflect distinct aspects of information processing.     

Altered pattern of motor cortical activation–inhibition during voluntary movements in Tourette syndrome

In patients with Gilles de la Tourette syndrome (GTS) alterations of motor cortex (M1) excitability at rest have been evidenced. In contrast, there has so far been little research into changes of motor cortical reactivity during the time course of voluntary movements in GTS patients. The present study investigates neuromagnetic event‐related desynchronization (ERD) and event‐related synchronization (ERS) of bilateral M1 in 11 GTS patients and 11 healthy control subjects. ERD represents motor cortical activation, whereas ERS most likely indicates its inhibition. Subjects performed a self‐paced finger movement task while magnetoencephalography was used to record neuromagnetic activity. In GTS patients, ERD at beta frequency was significantly increased in the contralateral hemisphere before and during movements, whereas ERS following movement termination was increased in M1 ipsilateral. Ipsilateral ERS was inversely correlated with tic severity as determined by the Yale Global Tic Severity Rating Scale. The data of the present study support the hypothesis that during voluntary movements, motor cortical reactivity is pathologically altered in GTS patients. The observed pattern of increased activation (ERD) prior to and during movement execution followed by increased inhibition (ERS) after movement termination at beta frequency suggests abnormally increased motor cortical activation, possibly driving stronger inhibition. The stronger this inhibition is, the better symptoms appear to be controlled. © 2010 Movement Disorder Society     

Brief and sustained movements: differences in event-related (de)synchronization (ERD/ERS) patterns.

OBJECTIVE: (1) To determine if there are changes in event-related desynchronization/event-related synchronization (ERD/ERS) patterns when the movement is sustained? (2) To determine, from a technical point of view for ERD calculation, if it is possible to take the reference period during muscular activation? METHODS: Eight healthy subjects performed two series of brief and sustained self-paced extensions with their dominant wrist. The end of the sustained movement was externally triggered by the examinator. ERD/ERS was calculated in mu and beta bands from 13 source derivations covering motor areas, computed from 29 scalp electrodes. Movement onset and offset were determined by electromyographic activity (EMG) of wrist extensors. RESULTS: When the movement was sustained, power in the mu and beta bands returned to baseline values within 4-5 s. Movement duration had little effect, if at all, on both pre and post-movement periods. Compared to brief movement, after the onset of the prolonged movement, mu ERD just returned to baseline, without synchronization. In contrast, beta ERS was still present though earlier and much lower. CONCLUSIONS: The reference period for ERD calculation may be taken during muscular activation if its duration is long enough. Beta synchronization may occur despite a non-deactivated motor cortex, suggesting a contribution from afferent somesthetic inputs.     

Synchronization of gamma oscillations increases functional connectivity of human hippocampus and inferior-middle temporal cortex during repetitive visuomotor events

Do recency processes associated with repetitive sensorimotor events modulate the magnitude and functional coupling of brain rhythmicity in human temporal cortex? Intracranial stereo electroencephalographic activity (SEEG; 256 Hz sampling rate) was recorded from hippocampus, and inferior (BA20) and middle (BA21) temporal cortex in four epilepsy patients. The repetitive events were represented by predicted imperative somatosensory stimuli (CNV paradigm) triggering hand movements (‘repetitive visuomotor’) or counting (‘repetitive counting’). The non‐repetitive events were ‘rare’ (P3 paradigm) somatosensory stimuli triggering hand movements (‘non‐repetitive visuomotor’) or counting (‘non‐repetitive counting’). Brain rhythmicity was indexed by event‐related desynchronization/synchronization (ERD/ERS) of SEEG data, whereas the functional coupling was evaluated by spectral SEEG coherence between pairs of the mentioned areas. The frequency bands of interest were theta (4–8 Hz), alpha (8–12 Hz), beta (14–30 Hz), and gamma (32–46 Hz). Compared to the non‐repetitive events, the ‘repetitive visuomotor’ events showed a significant beta and gamma ERS in the hippocampus and a significant theta ERD in the inferior temporal cortex. Furthermore, the ‘repetitive visuomotor’ events induced a task‐specific significant gamma coherence among the examined areas. These results suggest that recency processes do modulate the magnitude and functional coupling of brain rhythmicity (especially gamma) in the human temporal cortex.