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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 103–105 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.

EEG activation patterns during the performance of tasks involving different components of mental calculation

In this study we demonstrate the existence of different patterns of EEG activation during the performance of 4 different tasks involving different components of mental calculation in normal subjects. The EEG was recorded in all monopolar leads of the 10/20 system using linked ear lobes as reference. Absolute and relative power were calculated in the delta (1.5-3.5 Hz), theta (3.5-7.5 Hz), alpha (7.5-12.5 Hz) and beta (12.5-19 Hz) bands. The tasks were presented randomly and the EEG segments preceding presentation of the stimulus were considered as the rest corresponding to the task requested by the stimulus. Tasks were of 4 different types, involving number comprehension, recognition of mathematical symbols, the calculation process and the spatial component. ANOVAs between the rest periods showed no differences in any band. Neither did ANOVAs between tasks. However, other variables (task minus rest), which were calculated as the differences in power between task and rest respectively, showed significant differences between tasks in the delta and beta bands in the frontal lobes. In addition, new variables were calculated as the difference between tasks, since many factors were common across several tasks. These variables correspond to the EEG change due to a specific component of mental calculation. Significant differences were obtained in delta and theta bands in right posterior areas and in the beta band in frontal areas. We concluded that the EEG differences observed during different components of mental calculation suggest the participation of different networks.

Effect of sex, psychosocial disadvantages and biological risk factors on EEG maturation

Broad-band frequency analysis was performed on the EEGs of school-age children in order to study the effects of sex, socioeconomic status (SES) and biological risk factors on EEG maturation. Absolute power and relative power in delta, theta, alpha and beta frequency bands were computed in monopolar recordings from F3, F4, C3, C4, P3, P4, O1, O2, F7, F8, T3, T4, T5 and T6. Total absolute power and power in the delta and theta bands decreased with age, following a quadratic polynomial expression. Relative power followed a linear regression with age. Delta and theta decreased while alpha and beta increased. Sex differences in relative power were observed, possibly related to a pubertal spurt: slopes of regression equations were steeper in girls than in boys. Children from low SES (very low income and/or illiterate mother) had higher values of absolute power and a higher percentage of delta and lower of alpha than children with good SES, suggesting a maturational lag. A group of children with personal antecedents of risk factors associated with brain damage were compared with children without antecedents. The former group had higher values of absolute power. Depending on the severity of the risk, children with more severe antecedents had greater differences from children with no antecedents than children in whom antecedents were considered as slight. Risk antecedents had no effect on relative power.

The functional significance of delta oscillations in cognitive processing.

Ample evidence suggests that electroencephalographic (EEG) oscillatory activity is linked to a broad variety of perceptual, sensorimotor, and cognitive operations. However, few studies have investigated the delta band (0.5–3.5 Hz) during different cognitive processes. The aim of this review is to present data and propose the hypothesis that sustained delta oscillations inhibit interferences that may affect the performance of mental tasks, possibly by modulating the activity of those networks that should be inactive to accomplish the task. It is clear that two functionally distinct and potentially competing brain networks can be broadly distinguished by their contrasting roles in attention to the external world vs. the internally directed mentation or concentration. During concentration, EEG delta (1–3.5 Hz) activity increases mainly in frontal leads in different tasks: mental calculation, semantic tasks, and the Sternberg paradigm. This last task is considered a working memory task, but in neural, as well as phenomenological, terms, working memory can be best understood as attention focused on an internal representation. In the Sternberg task, increases in power in the frequencies from 1 to 3.90 Hz in frontal regions are reported. In a Go/No-Go task, power increases at 1 Hz in both conditions were observed during 100–300 ms in central, parietal and temporal regions. However, in the No-Go condition, power increases were also observed in frontal regions, suggesting its participation in the inhibition of the motor response. Increases in delta power were also reported during semantic tasks in children. In conclusion, the results suggest that power increases of delta frequencies during mental tasks are associated with functional cortical deafferentation, or inhibition of the sensory afferences that interfere with internal concentration. These inhibitory oscillations would modulate the activity of those networks that should be inactive to accomplish the task.

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 Brocas 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.

Maturation of the coherence of EEG activity in normal and learning-disabled children.

The age effect on coherence has been studied in control (98) and learning-disabled (LD, 54) school-aged children (from 6.0 to 16.8 years old). The EEG recordings were made at rest in 15 leads, and 105 pairwise combinations for coherence were calculated (each lead was compared with all the rest) for delta, theta, alpha, beta and total frequency bands. A significant increase of coherence with age was found in both groups, with a different pattern of maturation. In the control group, a significant increase with age was found in the coherences between posterior regions and vertex (Cz). A significant decrease with age in the coherence between frontal areas was observed, especially in the theta band. The LD group showed a different pattern: no significant relation with age was found in the coherence between any lead and vertex. A high effect of age on coherence between temporal regions was observed with a predominance of the left side in comparison with the contralateral and the ipsilateral. No decrease in frontal coherence was found: in the same region where the control group showed negative values with age, the LD groups had no age effect. The results obtained are discussed as differences in brain organization, in myelogenesis and synaptogenesis and an explanation of the etiology of LD is proposed.

Correlation Between Eeg Spectral Parameters and an Educational Evaluation

EEG spectral parameters were computed in a group of children with different degrees of difficulty in learning to read and write. For statistical analyses, Z-transformed values according to normative age-regression equations were used to control the age effects. Canonical Correlation Analysis between absolute power in different bands and the categories of the educational evaluation (good, regular, poor and very poor) showed that more delta was probably related to a poor evaluation and more alpha in occipital areas to a good one. MONOVA also showed highly significant differences in the absolute power in many leads between children with different evaluations. As children with a poor evaluation very frequently had antecedents of risk factors related to brain damage and were from a low socioeconomic status, and both factors have been shown to affect absolute power, it may be that the differences observed were due to these causes. However, relative power correlated more with the learning problems. Children with minor difficulties, with no antecedents and with good socioeconomic status had more theta in almost all leads than children with a good evaluation and with the same characteristics. Children with a poor, or very poor, evaluation had more delta in left frontal and temporal areas (F3, F7 and T3) which may reflect underlying cerebral dysfunction of these regions directly involved in reading and writing processes.

Repetitive transcranial magnetic stimulation decreases the number of seizures in patients with focal neocortical epilepsy

PURPOSE To evaluate the number of seizures and interictal epileptiform discharges (IEDs) in patients with focal neocortical epilepsy before, during and after rTMS. METHODS Twelve patients (seven men and five women, mean age 29.3+/-15.8 years) were studied. An open-label study with baseline (4 weeks), intervention (2 weeks) and follow-up (8 weeks) periods was carried out. Repetitive transcranial magnetic stimulation (rTMS) with 900 pulses, intensity of 120% motor resting threshold and 0.5Hz frequency was used. A 120 channel EEG was recorded; an electrical source analysis of IEDs with Variable Resolution Electromagnetic Tomography (VARETA) was performed. The number of seizures per week and IEDs per minute were measured and compared in the three periods. RESULTS During the basal period the mean seizure frequency was 2.25 per week; in the intervention period it decreased to 0.66 per week (F=2.825; p=0.0036) which corresponds to a 71% reduction. In the follow-up period the mean frequency was 1.14 seizures per week, that is, a 50% reduction in the number of seizures. In the visual EEG analysis, the baseline IED frequency was 11.9+/-8.3events/min; it decreased to 9.3+/-7.9 during 2 weeks of rTMS with a further reduction to 8.2+/-6.6 in the follow-up period. These differences however were not significant (p=0.190). CONCLUSION We conclude that 2 weeks of rTMS at 0.5Hz with a figure-of-eight coil placed over the epileptic focus, determined with VARETA, decreases the number of seizures in patients with focal epilepsy, without reduction in IEDs.

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.

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 9.36Hz 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.