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.     

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.     

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.     

Effects of electromagnetic field emitted by cellular phones on the EEG during a memory task

The effects of electromagnetic fields (EMF) emitted by cellular phones on the ERD/ERS of the 4-6 Hz, 6-8 Hz, 8-10 Hz and 10-12 Hz EEG frequency bands were studied in 16 normal subjects performing an auditory memory task. All subjects performed the memory task both with and without exposure to a digital 902 MHz EMF in counterbalanced order. The exposure to EMF significantly increased EEG power in the 8-10 Hz frequency band only. Nonetheless, the presence of EMF altered the ERD/ERS responses in all studied frequency bands as a function of time and memory task (encoding vs retrieval). Our results suggest that the exposure to EMF does not alter the resting EEG per se but modifies the brain responses significantly during a memory task.     

Brain oscillatory EEG event-related desynchronization (ERD) and -sychronization (ERS) responses during an auditory memory task are altered in children with epilepsy.

PURPOSE: Children with epilepsy are in risk for cognitive impairment, but reliable methods, other than neuropsychological testing, to verify such a decline are few. The purpose of this study was to assess the effect of infrequent seizures on cognitive skills in children with non-symptomatic focal epilepsy taking antiepileptic medication but still having infrequent seizures. METHODS: EEG (electroencephalogram) brain electric oscillatory responses of the 4-6Hz, 6-8Hz, 8-10Hz and 10-12Hz EEG frequency bands were studied. These responses, assessed by means of the event-related desynchronization (ERD) and synchronization (ERS) method, were recorded in 6 children with epilepsy (mean age 11.3 years) and in 11 control children (mean age 12 years) while they performed an auditory memory task. All subjects also underwent a comprehensive neuropsychological test battery. RESULTS: The differences in the 4-6Hz ERD/ERS responses between encoding and recognition were smaller in the children with epilepsy as compared to those of the control children. In the 6-8Hz frequency band, the responses of the two groups dissociated most notably in the frontal electrodes. No statistically significant differences in the alpha frequency range (8-12Hz) were observed between the groups. CONCLUSIONS: Significant alterations in the lower EEG frequency (4-8Hz) ERD/ERS responses in children with epilepsy during auditory memory processing, as compared to age-matched, healthy children may suggest that seizures affect memory and underlying brain processes, indexed also by poorer performance particularly in neuropsychological subtests related to language functions.     

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.     

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.     

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. 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.     

Frontal theta event-related synchronization: comparison of directed attention and working memory load effects

Summary. Early studies showed that long-term encoding and retrieval of new information is associated with modulation of the theta rhythm. More recently, changes in theta power amplitude over frontal electrode sites were reported during working memory, yet their relative significance in regard to attentional and memory processes remains unclear. Event-related synchronisation responses in the 4–7.5 Hz theta EEG frequency band was studied in 12 normal subjects performing four different tasks: two working memory tasks in which load varied from one (1-back task) to two (2-back task) items, an oddball detection (attention) task and a passive fixation task. A phasic theta increase was observed following stimulus apparition on all electrode sites within each task, with longer culmination peak and maximal amplitude over frontal electrodes. Frontal theta event-related synchronization (ERS) was of higher amplitude in the 1-back, 2-back and detection tasks as compared to the passive fixation task. Additionally, the detection task elicited a larger frontal and central theta ERS than the 2-back task. By analyzing theta ERS characteristics in various experimental conditions, the present study reveals that early phasic theta response over frontal regions primarily reflects the activation of neural networks involved in allocation of attention related to target stimuli rather than working memory processes.     

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 EEG theta and alpha band oscillatory responses during language translation

Recent investigations on oscillatory EEG dynamics by means of event-related synchronisation and desynchronisation (ERS/ERD) suggest that first language semantic information processing is primarily reflected in the theta (4-7 Hz) and alpha (7-13 Hz) frequency bands. In this pilot study we explore whether similar ERS/ERD patterns emerge during language translation and which frequency bands sensitively respond to the difficulty of translation and the translation success. Thirteen female students of translation and interpreting were visually presented high and low frequency English words that had to be translated into German. Time-frequency representations of ERS/ERD between 2 and 50 Hz displayed a theta ERS response about 200-600 ms after word presentation, a beta ERD from about 400 ms, and alpha ERS and ERD patterns about 200-400 ms after word presentation. Statistical analyses of the ERS/ERD data in the theta (4-7 Hz), two alpha frequency bands (7-10 Hz and 10-13 Hz), and a beta band (20-30 Hz) predominantly revealed: (a) higher parietal theta ERS and frontal upper alpha ERD during the translation of low as compared to high frequency words, and (b) generally stronger ERD in the lower alpha band and larger left-hemispheric upper alpha ERD for successfully translated in contrast to not translated low frequency words. These findings provide first evidence of the sensitivity of the theta and alpha ERS/ERD measure to lexical-semantic processes involved in language translation.     

EEG synchronization likelihood in mild cognitive impairment and Alzheimer's disease during a working memory task.

OBJECTIVE: Synchronization likelihood analysis of resting state EEG has shown that cognitive dysfunction in Alzheimer's disease (AD) and its precursor mild cognitive impairment (MCI) are associated with a loss of functional connectivity in high (upper alpha and beta) frequency bands. Working memory tasks are known to change functional connectivity, but it is unknown whether this increases the differences between AD, MCI and healthy controls. Our objective was to investigate the behavior of synchronization likelihood of multichannel EEG in AD, MCI and cognitively healthy controls, both at rest and during a working memory task. METHODS: EEGs (200 Hz sample frequency, 21 channels, average reference) were recorded at rest as well as during a visual working memory task in 14 patients with AD according to the NINCDS-ADRDA criteria (mean age 76.4; SD 13.6), 11 patients with MCI according to the criteria of Petersen (mean age 78.4; SD 6.4) and 14 with subjective memory complaints but no demonstrable memory disturbance (mean age 61.6; SD 26.6). The synchronization likelihood was computed over 19 channels, comparing each channel with all the other channels for the 0.5-4, 4-8, 8-10, 10-12, 12-30, 30-50 Hz frequency bands. RESULTS: The synchronization likelihood was significantly decreased in the upper alpha (10-12) and beta (12-30) bands in AD compared to persons with subjective memory complaints. The working memory task scores strongly correlated with Mini-Mental State Examination scores. During the working memory task the synchronization likelihood was significantly higher in MCI compared to the control subjects in the lower alpha band (8-10 Hz). CONCLUSIONS: Decrease of beta band synchronization occurs in mild AD, both in a resting condition and during a working memory task. SIGNIFICANCE: Decrease of beta band synchronization in mild AD is a robust finding. The present study confirms our findings in a different cohort of patients, using alternative frequency bands. The diagnostic value of the synchronization likelihood in AD and MCI needs to be further established.     

EEG alpha band dissociation with increasing task demands

In assuming functional differences between different EEG alpha frequency bands, recent studies emphasize the importance of using narrow (8-10 Hz or 10-12 Hz) instead of broad alpha frequency ranges (8-12 Hz). Due to individual differences in alpha activity, it has also been suggested to adjust alpha frequency bands individually for each participant. The present paper highlights the dissociating role of different task demands on the extent of event-related desynchronization (ERD) in different alpha frequency bands. In analyzing the data of four large-scale EEG studies (with sample sizes of 51, 58, 55, and 66, respectively) employing a wide range of cognitive tasks, we found evidence that the correlations between lower and upper alpha band ERD systematically decline as task demands increase.     

Modality matters: the effects of stimulus modality on the 4- to 30-Hz brain electric oscillations during a lexical decision task

The aim of the current study was to assess modality-specific brain oscillatory responses during cognitive processing. Brain oscillatory ERD/ERS responses of the 4- to 30-Hz EEG frequency bands were examined during lexical decision where the task is to identify whether the presented stimulus is a word or a pseudoword. Seven subjects performed the task with visual stimuli and twelve subjects with auditory stimuli. Visual stimuli elicited greater theta ERS responses as compared to the auditory stimuli. Both stimulus modalities elicited alpha and beta frequency ERD, these being greater for the auditory stimuli. Auditory stimuli elicited also later emerging beta ERS responses, absent for the visual stimuli. The lexicality effects (words vs. pseudowords) were greater for the auditory than for the visual stimuli. When studying brain oscillatory correlates of cognitive processing, the stimulus modality matters. Some effects may arise and some vanish depending on in which modality a cognitive experiment is being conducted.     

Probability interrelations between pre-/post-stimulus intervals and ERD/ERS during a memory task.

OBJECTIVES: To investigate the functional relationship between oscillatory electroencephalographic (EEG) components (pre-/post-stimulus intervals) with audio-event-related desynchronization/synchronization (ERD/ERS). METHODS: In an experimental study (9 subjects), the probability-classification analysis of single-trial spectral EEG changes was utilized. Results were compared with auditory ERD/ERS. RESULTS: It was shown that (1) variability of EEG spectral patterns was considerable, (2) EEG activity was different at various task stages, (3) probability measures were different from the results of conventional frequency analysis, (4) probability of trials with alpha- and theta-patterns was characteristically different in various task stages, (5) the occurrence of alpha- and theta-trials were most probable, but not frequent enough to characterize all the trials. The results suggest that the ERD/ERS responses are influenced by EEG characteristics in the pre-stimulus interval, which also have a strong influence on the EEG in the post-stimulus interval. CONCLUSIONS: Alpha- and theta-ERD/ERS responses during memory task performance are not typical for all trials. They reflect EEG changes only in approximately 39% for alpha-activity and approximately 43% for theta-oscillations of all trials, what reflects piecewise stationary EEG structure.     

Sub-second "temporal attention" modulates alpha rhythms. A high-resolution EEG study

In the present high-resolution electroencephalographic (EEG) study, event-related desynchronization/synchronization (ERD/ERS) of alpha rhythms was computed during an S1-S2 paradigm, in which a visual cue (S1) predicted a SHORT (600 ms) or LONG (1400 ms) foreperiod, preceding a visual go stimulus (S2) triggering right or left finger movement. Could orienting attention to a selective point in time influence the alpha rhythms as a function of the SHORT vs. LONG foreperiod? Stronger selective attentional modulations were predicted for the SHORT than LONG condition. EEG data from 54 channels were "depurated" from phase-locked visual evoked potentials and spatially enhanced by surface Laplacian estimation (i.e., final data analysis was conducted on 16 subjects having a sufficient number of artifact-free EEG single trials). Low-band alpha rhythms (about 6-10 Hz) were supposed to be related to anticipatory attentional processes, whereas high-band alpha rhythms (10-12 Hz) would indicate task-specific visuo-motor processes. Compared to the LONG condition (foreperiod), the SHORT condition induced a quicker and stronger ERS at low-band alpha rhythm (about 6-8 Hz) over midline and bilateral prefrontal, sensorimotor, and posterior parietal areas. In contrast, the concomitant high-band alpha (about 10-12 Hz) ERD/ERS showed no significant difference between the two conditions. In conclusion, temporal attention for a sub-second delay (800 ms) did modulate low-band alpha rhythm over large regions of both cortical hemispheres.     

The effects of memory load and stimulus relevance on the EEG during a visual selective memory search task: an ERP and ERD/ERS study.

OBJECTIVE: Psychophysiological correlates of selective attention and working memory were investigated in a group of 18 healthy children using a visually presented selective memory search task. METHODS: Subjects had to memorize one (load1) or 3 (load3) letters (memory set) and search for these among a recognition set consisting of 4 letters only if the letters appeared in the correct (relevant) color. Event-related potentials (ERPs) as well as alpha and theta event-related synchronization and desynchronization (ERD/ERS) were derived from the EEG that was recorded during the task. RESULTS: In the ERP to the memory set, a prolonged load-related positivity was found. In response to the recognition set, effects of relevance were manifested in an early frontal positivity and a later frontal negativity. Effects of load were found in a search-related negativity within the attended category and a suppression of the P3-amplitude. Theta ERS was most pronounced for the most difficult task condition during the recognition set, whereas alpha ERD showed a load-effect only during memorization. CONCLUSIONS: The manipulation of stimulus relevance and memory load affected both ERP components and ERD/ERS. SIGNIFICANCE: The present paradigm may supply a useful method for studying processes of selective attention and working memory and can be used to examine group differences between healthy controls and children showing psychopathology.     

Discrimination between phase-locked and non-phase-locked event-related EEG activity

Differentiation between phase-locked and non-phase-locked event-related EEG activity is an important task in the evaluation of event-related EEG activity. Event-related changes of EEG activity such as event-related desynchronization (ERD) or event-related synchronization (ERS) can be quantified by either instantaneous band power or intertrial variance calculations. In the former case the ERD or ERS can be masked by an event-related potential while in the latter it is not. Examples from sensory stimulation and movement experiments, where the ERD (ERS) is calculated by both methods, are shown and discussed.     

Do children with ADHD and/or PDD-NOS differ in reactivity of alpha/theta ERD/ERS to manipulations of cognitive load and stimulus relevance?

ObjectiveWe examined whether the method of event-related (de-)synchronization (ERD/ERS) revealed differential effects of selective attention and working memory load in children (8–11 years) with pervasive developmental disorder – not otherwise specified (PDD-NOS) or attention-deficit/hyperactivity disorder (ADHD).MethodsFifteen healthy controls and three equally large groups of children with symptoms of PDD-NOS, ADHD or both (PDD/HD) performed a visual selective memory search task. The EEG was recorded from which occipital alpha and frontal theta were derived.ResultsThe effects of the overall task manipulations of task load, relevance and target/nontarget were clearly present in the overall analyses of alpha and theta ERD/ERS. However, no significant differences with respect to these manipulations existed between any of the subject groups.ConclusionsThe results supply no evidence for a distinction in information processing abilities of selective attention and working memory as reflected by alpha and theta ERD/ERS between children diagnosed with either ADHD, PDD-NOS or healthy controls.SignificanceAlpha and theta ERD/ERS are sensitive to manipulations of task load, relevance and target/nontarget, but supply no additional information on possible group differences in comparison to the more frequently used method of event-related potentials.