Primary task demands modulate P3a amplitude.

Auditory event-related brain potentials (ERPs) were recorded from 10 subjects in two different conditions: (1) subjects were required to reorder five visually presented letters in order to form a word and provide a verbal response (task condition); (2) subjects were presented with a control stimulus with the same physical characteristics as the experimental stimulus, but containing just one type of letter (i.e., AAAAA). Subjects had to verbally respond to such stimuli by saying "A" (control condition). Tones of 1000 Hz (standard) and 1050 Hz (deviant) were also presented to the subjects in a 85%-15% probability paradigm 2 s before, during and 8 s after the presentation of the visual stimuli. Recordings were obtained from Fpz, Fz, Cz and Pz vs. linked ears. Auditory ERPs to the auditory stimuli after the presentation of the visual letter string and during the performance of the task were averaged for the standard and deviant tones in both conditions. Only correct responses were considered for the averages. The N100 was affected by stimulus type (standard vs. deviant) but not by condition (task vs. control); however, larger P3a waves were observed during the control than during the task condition. No significant differences between conditions were observed in the mismatch negativity (MMN) latency range. These results suggest that primary task demands modulate involuntary attention processing.

Relationship of specific EEG frequencies at specific brain areas with performance.

This study shows that incorrect responses are preceded by different EEG characteristics than correct responses, and that these differences appear in specific brain regions that participate in each particular task. EEGs were recorded in children during three different tasks: color discrimination (CDT), verbal working memory (VWM), and word categorization task (WCT). EEG segments previous to the presentation of the stimulus were analysed. Incorrect responses were preceded by lower EEG power values at 7.8 Hz in posterior temporal and right parietal leads in CDT, 8.59 and 9.36 Hz in frontal areas in VWM, and 10.72 Hz in the left hemisphere in WCT. In the former task > 1.56 Hz power in frontal areas prior to an incorrect response was also observed.

Specific EEG frequencies at specific brain areas and performance.

In this study it was shown that in adults, the frequency characteristics of EEG preceding stimuli that were followed by incorrect responses were different from the characteristics of EEG preceding stimuli that were followed by correct responses. In the recording during three different tasks that explore different neuronal networks, higher values of current preceding incorrect performance in those areas directly related to the task were found in frequencies within the delta (1.56 and 3.12 Hz) and beta bands (13.26, 14.04, 14.82, 15.6, 17.16 and 17.94 Hz), suggesting that these frequencies signal inhibition. Frequencies within the alpha band (9.36 and 12.48 Hz) showed greater energy preceding correct responses in task-specific areas, supporting previous results observed in children.

Cerebral blood flow and sources of abnormal EEG activity (VARETA) in neurocysticercosis.

OBJECTIVES: To compare two different functional procedures in the assessment of brain ischemia in patients with neurocysticercosis (NCC): (1) electroencephalography (EEG) evaluated by brain maps and EEG current sources in the frequency domain using variable resolution electromagnetic tomography and (2) blood flow analyzed by computerized tomography assessed with stable Xe (Xe-CT). METHODS: Eleven patients with NCC at different evolution stages were studied. CT and Xe-CT scans, as well as quantitative electroencephalography with source calculation in the frequency domain, were obtained. All patients showed cysts and in 6 of them there were also vascular complications: two of them presented calcifications of the middle cerebral artery, two other subjects showed calcifications of the vessels in the circle of Willis and the remaining two had brain infarctions. RESULTS: In the cyst areas important hypoperfused zones were observed, as intense as those observed in infarcted areas. Damage to the blood-brain barrier was originated by parasites in colloidal phase (final cysticerci stage) producing large areas of edema and hypoperfusion. Abnormal delta EEG activity was observed in very large lesions, probably generated by partial cortical deafferentation; and abnormal theta activity was mainly related to the presence of edema. CONCLUSIONS: Sources of abnormal EEG activity were very similar in topography to the hypoperfused areas.

EEG changes during word and figure categorization.

OBJECTIVE: To analyze whether the EEG changes observed during figure and word categorization are compatible with either the dual, the common amodal, or the alternative model (modality-specific codes for words and pictures, where meaning is represented for both in a higher-order amodal system) for semantic knowledge. METHODS: EEG was recorded during word and figure categorization of animals or non-animals in a group of 28 children 8-10 years old. Computation of EEG sources in the frequency domain using variable resolution electrical tomography (VARETA) and their statistical evaluation by statistical parametric mapping were carried out. RESULTS: At all frequencies, there were significant changes between EEG segments prior to the presentation of the stimuli and EEG segments recorded after the stimuli. Post-segments showed more power from 1.56 to 7.02 Hz, and less power than pre-segments from 8 to 12.48 Hz. EEG changes were only observed in the word task at: 3.9 (left occipital), 4.68, 5.46, and 6.24 Hz (temporo-occipital regions). These changes may be associated with visual encoding of words. Frequencies 7.8 and 17.94 Hz increased in prefrontal, anterior cingulate, and anterior temporal regions only during figure categorization. The prefrontal region may be related to object working memory. Thus, these frequencies might be related to figure codification. No significant differences between tasks were observed at 3.12 and 7.02 Hz in very wide brain areas (all lobes except occipital), suggesting that the amodal semantic system storage could be the model compatible with figure and word categorization. CONCLUSIONS: Thus, our results support the modified amodal semantic hypothesis, which advocates that the meanings of both kinds of stimuli are represented in a conceptual memory that receives input from the logogen and iconogen systems.

N400 and lexical decisions: automatic or controlled processing?

OBJECTIVE: To investigate whether the N400 effect is sensitive to automatic or controlled processes. METHODS: Two experiments were performed. In one experiment, directly related word pairs were used. In the other experiment, mediated-related word pairs were used. In order to reduce controlled processes, each experiment consisted of 3 tasks: Low- and high-proportion of related pairs, and single presentation lexical decision task. RESULTS: In the first experiment, the amount of priming was equivalent for the 3 tasks. The N400 effect appeared in the high and low proportion of directly related words, but not in the single presentation task. In the second experiment, behavioral priming was also found in the 3 tasks. However, the N400 effect was observed only in the task with low proportion of related pairs. CONCLUSION: These results suggest that the N400 effect may be related to controlled processes.

Do specific EEG frequencies indicate different processes during mental calculation?

EEG recordings during mental calculation and a control task (with presentation of stimuli with similar physical characteristics to the arithmetic symbols) were obtained in 10 subjects. Narrow band analyses of the EEG and distributed sources for each EEG frequency were calculated using variable resolution electromagnetic tomography. Significant differences between the sources for arithmetic and control tasks were observed at 3.9 Hz within Broca's and left parietotemporal cortices, suggesting that this frequency may be related with the production of internal speech, storage and rehearsal of verbal working memory. Differences at 5.46 Hz within the right dorsolateral prefrontal cortex were considered to be associated to sustained attention. The decrease at 12.46 Hz within the left parietal cortex was interpreted as a sign of retrieval of arithmetic facts from long term memory.

Event-related brain potentials during a continuous performance test (CPT) task in normal children.

BACKGROUND: Continuous Performance Test (CPT) is a commonly used paradigm to assess attention disorders that could involve working memory processes. METHODS: Event-related potentials (ERPs) during a CPT (X-AX) test were obtained in 16 healthy male students, with ages ranging from 9 to 11 years (X = 10.3). In grouped averaged ERPs, an endogenous slow positive potential was recorded in the first task (infrequent letter detection); maximum was at 460 msec with a slight lateralization tendency toward left parietal area. In the second task (target detection with an A as warning signal), an early (maximum at 330 msec) and more acute peak was detected without evidence of any lateralization. RESULTS: Reaction times were significantly shorter for the second task. Electrophysiologic differences between both target conditions showed an early, remarkable, and statistically significant component located at the parietal area at 340 msec. CONCLUSIONS: These ERPs findings could be interpreted as correlates with working memory processes in children.

Electroencephalographic coherences discriminate between children with different pedagogical evaluation.

The relationship of reading-writing ability and EEG coherences was studied in 84 subjects from two age groups 7.0-8.9 and 9-11.2 years old. All children were divided into three groups according to their performance on a pedagogical test: ped1, normal children; ped2, children with mild problems; ped3, children with reading-writing disability. The following results were obtained: in general, children showed higher coherences in groups with poor performance in the delta, theta and beta bands. In the alpha band, higher coherence values were related to better performance. The exceptions to this general pattern were rare. Group ped2 had higher coherences in delta, theta and alpha bands than ped1 and ped3, in left temporal leads. In older children the same tendency was observed, but group differences in the theta, alpha and beta bands were few. In this age range, the significant group differences were almost all interhemispheric coherences. The discriminant analysis that classified subjects by their coherence values gave very good results, fact that demonstrates, that EEG coherence is a highly sensitive measurement indicating not only the existence of a reading-writing problem, but also the degree of its severity.

EEG delta activity: an indicator of attention to internal processing during performance of mental tasks.

In previous papers we proposed that an increase in delta EEG activity during mental tasks might be related to an increase in subjects' attention to internal processing. In this paper we have made a narrow band analysis to detect those EEG frequencies that change selectively during the performance of a mental task that requires attention to internal processing. Two different experiments were performed: (1) a difficult mental calculation task and a control stimulus with the same physical characteristics as the arithmetical symbols were presented in random order; (2) the Sternberg paradigm for the analysis of short term memory using a memory set of 5 or 3 digits was also presented in random order. Referential recordings to linked ears were obtained in all leads of the 10/20 system. In the first experiment, the increase of power from 1.56 to 5.46 Hz was observed only during the performance of the task and not during the control condition. In the Sternberg paradigm, the increase of power from 1.56 to 3.90 Hz was greater during the difficult than during the easy condition. These results support our hypothesis that an increase in delta activity may be related to attention to internal processing during the performance of a mental task.

A follow-up study of EEG coherences in children with different pedagogical evaluations.

A follow-up study on EEG coherence values was carried out with 46 school-age children divided into three pedagogical groups considering their reading and writing abilities (normal subject and children with mild and with serious reading-writing problems). EEGs were recorded at rest, with eyes closed in 15 referential derivations that gave 105 possible coherence values. Coherence was calculated after the elimination of the average reference. Repeated Measure ANOVAs were calculated to evaluate the effects of time, sex, reading-writing problems and their interactions. The second EEG recordings were taken 2-3 years after the first. The results revealed significant increase of coherence values during the time lapse studied, consisting of an increase of frontal coherences in all bands, with the exception of the theta band. The most numerous changes occurred in the alpha band. Very few differences in coherence values were observed between the two EEG recordings by pedagogical groups and by sex suggesting a similar but not igual maturation of coherences of boys and girls and of the three pedagogical groups, in this age range studied. These findings suggests an abnormal coherence development for the reading disabled group, as well as different coherence maturation for boys and girls in this time lapse studied.

Auditory event-related potentials in poor readers.

Although poor readers (PR) are considered the major group among reading-disabled children, there are not event-related potentials (ERP) studies reported of PR on the subject. In this study, attentional and memory processes were studied in an auditory oddball task in PR and normal controls. ERP to auditory stimuli were recorded in 19 leads of the 10/20 system, using linked earlobes as references, in 20 normal children (10 female) and 20 PR (10 female) of the same age (10-12 years old). Two pure tones (1000 and 3000 Hz) were used in an oddball paradigm. No significant differences were observed in the amplitudes and latencies of N100 between the groups. However, N200 to frequent stimuli and P200 to both frequent and infrequent stimuli were of higher amplitude in poor readers than in normal children. There were no differences between groups in the latency and amplitude of P300. The results suggest that PR use more attentional resources in the components occurring before P300 to both frequent and infrequent stimuli than the normal children, and this finding is particularly marked for PR girls.

Delayed P300 during Sternberg and color discrimination tasks in poor readers.

The P300 ERP component was studied in poor and normal readers, using Sternberg and color discrimination (Spaceships) tasks. During the first one, subjects must decide if a probe item belongs or not to a set of digits previously presented. In the second one, the participants must shoot violet spaceships with one key and other than violet spaceships with another key. There were no significant differences between groups with respect to reaction times, but a larger proportion of errors was observed in poor readers. Longer P300 latencies were recorded for poor readers than controls in both tasks. P300 amplitudes showed topographical differences between the two groups: Poor readers' P300 is larger frontally during the Sternberg task, but smaller at posterior sites during the Spaceships task. These results suggest that poor readers may have deficiencies during the early processing stage, such as visual stimulus evaluation. Poor readers also appear to have deficits for classifying and memorizing visual stimuli.

Electroencephalographic characteristics of children with learning disabilities.

Normal, borderline and abnormal EEG findings were investigated in 122 children with no evidence of active neurological or psychiatric disorder, or perceptual or behavioral alterations. All children had a normal IQ. Sixty-one children had a good academic achievement (Controls) and 61 were considered as learning disabled (LD). The great majority of the children had pathological antecedents considered as risk factors for brain damage. The main findings observed were: Abnormal signs were significantly more frequent in LD than in control children. Atypical high amplitude alpha, and abnormal focal paroxysmal and nonparoxysmal activities were found only in LD children with risk factors. No significant differences in the resting EEG were found between presence or absence of pathological antecedents, or between males and females, when the children were compared as a group against the 4 EEG categories. Paroxysmal reactivity to eye-closure was only observed in children with a pathological history. Excessive response to hyperventilation (Grades II and III) was found significantly more frequently in girls than in boys.

3D statistical parametric mapping of EEG source spectra by means of variable resolution electromagnetic tomography (VARETA).

This article describes a new method for 3D QEEG tomography in the frequency domain. A variant of Statistical Parametric Mapping is presented for source log spectra. Sources are estimated by means of a Discrete Spline EEG inverse solution known as Variable Resolution Electromagnetic Tomography (VARETA). Anatomical constraints are incorporated by the use of the Montreal Neurological Institute (MNI) probabilistic brain atlas. Efficient methods are developed for frequency domain VARETA in order to estimate the source spectra for the set of 10(3)-10(5) voxels that comprise an EEG/MEG inverse solution. High resolution source Z spectra are then defined with respect to the age dependent mean and standard deviations of each voxel, which are summarized as regression equations calculated from the Cuban EEG normative database. The statistical issues involved are addressed by the use of extreme value statistics. Examples are shown that illustrate the potential clinical utility of the methods herein developed.

Sources of abnormal EEG activity in the presence of brain lesions.

In routine clinical EEG, a common origin is assumed for delta and theta rhythms produced by brain lesions. In previous papers, we have provided some experimental support, based on High Resolution qEEG and dipole fitting in the frequency domain, for the hypothesis that delta and theta spectral power have independent origins related to lesion and edema respectively. This paper describes the results obtained with Frequency Domain VARETA (FD-VARETA) in a group of 13 patients with cortical space-occupying lesions, in order to: 1) Test the accuracy of FD-VARETA for the localization of brain lesions, and 2) To provide further support for the independent origin of delta and theta components. FD VARETA is a distributed inverse solution, constrained by the Montreal Neurological Institute probabilistic atlas that estimates the spectra of EEG sources. In all patients, logarithmic transformed source spectra were compared with age-matched normative values, defining the Z source spectrum. Maximum Z values were found in 10 patients within the delta band (1.56 to 3.12 Hz); the spatial extent of these sources in the atlas corresponded with the location of the tumors in the CT. In 2 patients with small metastases and large volumes of edema and in a patient showing only edema, maximum Z values were found between 4.29 and 5.12 Hz. The spatial extent of the sources at these frequencies was within the volume of the edema in the CT. These results provided strong support to the hypothesis that both delta and theta abnormal EEG activities are the counterparts of two different pathophysiological processes.

EEG and behavioral changes following neurofeedback treatment in learning disabled children.

Neurofeedback (NFB) is an operant conditioning procedure, by which the subject learns to control his/her EEG activity. On one hand, Learning Disabled (LD) children have higher values of theta EEG absolute and relative power than normal children, and on the other hand, it has been shown that minimum alpha absolute power is necessary for adequate performance. Ten LD children were selected with higher than normal ratios of theta to alpha absolute power (theta/alpha). The Test Of Variables of Attention (TOVA) was applied. Children were divided into two groups in order to maintain similar IQ values, TOVA values, socioeconomical status, and gender for each group. In the experimental group, NFB was applied in the region with highest ratio, triggering a sound each time the ratio fell below a threshold value. Noncontingent reinforcement was given to the other group. Twenty half-hour sessions were applied, at a rate of 2 per week. At the end of the 20 sessions, TOVA, WISC and EEG were obtained. There was significant improvement in WISC performance in the experimental group that was not observed in the control group. EEG absolute power decreased in delta, theta, alpha and beta bands in the experimental group. Control children only showed a decrease in relative power in the delta band. All changes observed in the experimental group and not observed in the control group indicate better cognitive performance and the presence of greater EEG maturation in the experimental group, which suggests that changes were due not only to development but also to NFB treatment.

Sources of abnormal EEG activity in brain infarctions.

EEGs from 16 patients with stroke in three different stages of evolution were recorded. EEG sources were calculated every 0.39 Hz by frequency domain VARETA. The main source was within the delta band in 2 out of 4 chronic patients, and in 67% of the patients in the acute or subacute stages when edema (cytotoxic or vasogenic) was present. Moreover, all patients showed abnormal activity in the theta band. Sources of abnormal activity in cortical or corticosubcortical infarcts were located in the cortex, surrounding the lesion. At the site of the infarct, a decrease of EEG power was observed. Sources of abnormal theta power coincided with edema and/or ischemic penumbra.

Transitory cognitive impairment in epileptic children during a CPT task.

EEGs and behavioral responses were studied in two sex matched groups of 58 epileptic and 20 healthy children between 8 and 12 years of age, during the execution of a go-no go CPT (X; A-X) task to determine transitory cognitive impairment (TCI) incidence. Paroxysmal discharges were found on 87.9% and 5% of the EEGs in the epileptic and control groups respectively, with no differences related to sex. The predominant EEG findings with respect to paroxysmal discharges were the association of two or more types of paroxysms with frequency higher than 5/minute, an average duration less than 0.5 second and topographical distribution over temporal-parietal-occipital areas without significant interhemispheric differences. TCI was detected in 36.2% of epileptic children. The epileptic group showed significantly higher numbers of behavioral errors and longer reaction times (RTs) in relation to the control group. Analyzing RTs on the two blocks of the task, linear discriminant analysis showed an acceptable classification of TCI incidence between groups.