Growth spurts in brain maturation during middle childhood as indexed by EEG power spectra

Topography of visual EEG reactivity in school-age children

In order to quantify the visual reactivity of EEG to opening the eyes, the topography of EEG power spectra for a sample of 72 healthy subjects of age ranging from 7 to 15 years was investigated. The EEGs were recorded from 16 scalp sites with eyes open (EOP) and eyes closed (ECL). Ln-transformed absolute EEG powers, acquired under these two states, were tested with Wilcoxon's paired test for differences between the powers. The absolute powers in alpha band and in total, were significantly higher in all derivations, under the ECL, as compared with the EOP, condition. Absolute powers in theta band under the ECL condition were also significantly higher than those under the EOP condition, except for frontal derivations. Changes in delta power were insignificant. Beta 1 band activity, when eyes were closed, was maximal in posterior and minimal in anterior derivations. When the eyes were open, the greatest beta 1 power was found in the frontal derivations. Beta 2 was the only band in which a frequent increase in power took place with eyes opening. Eyes opening appeared to decrease significantly, the beta 1 and beta 2 powers only in posterior derivations. The results showed that the visual blocking of EEG was mostly due to a higher degree of EEG desynchronization for the subjects aged 7-5 years.

EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis

Evidence is presented that EEG oscillations in the alpha and theta band reflect cognitive and memory performance in particular. Good performance is related to two types of EEG phenomena (i) a tonic increase in alpha but a decrease in theta power, and (ii) a large phasic (event-related) decrease in alpha but increase in theta, depending on the type of memory demands. Because alpha frequency shows large interindividual differences which are related to age and memory performance, this double dissociation between alpha vs. theta and tonic vs. phasic changes can be observed only if fixed frequency bands are abandoned. It is suggested to adjust the frequency windows of alpha and theta for each subject by using individual alpha frequency as an anchor point. Based on this procedure, a consistent interpretation of a variety of findings is made possible. As an example, in a similar way as brain volume does, upper alpha power increases (but theta power decreases) from early childhood to adulthood, whereas the opposite holds true for the late part of the lifespan. Alpha power is lowered and theta power enhanced in subjects with a variety of different neurological disorders. Furthermore, after sustained wakefulness and during the transition from waking to sleeping when the ability to respond to external stimuli ceases, upper alpha power decreases, whereas theta increases. Event-related changes indicate that the extent of upper alpha desynchronization is positively correlated with (semantic) long-term memory performance, whereas theta synchronization is positively correlated with the ability to encode new information. The reviewed findings are interpreted on the basis of brain oscillations. It is suggested that the encoding of new information is reflected by theta oscillations in hippocampo-cortical feedback loops, whereas search and retrieval processes in (semantic) long-term memory are reflected by upper alpha oscillations in thalamo-cortical feedback loops.

Ocular artifacts in children's EEG: selection is better than correction

The electroencephalogram (EEG) during middle childhood may be highly distorted by the occurrence of eye and head movement artifacts. Between 5 and 12 years children display a great number of such artifacts. In the present study we studied different methods to assess EEG artifacts in children. Three artifact treatments were compared with the uncorrected EEG: one widely used method which corrected the EEG for electrooculogram (EOG) EEG transfer and two methods which selected artifact-free EEG segments. The most effective method should selectively reduce the spectral power in the lower frequency bands and at the frontal regions which are most susceptible to eye artifacts. The results demonstrated that the selection procedure, which combined two criteria for the selection of artifact-free EEG segments, was superior. The procedure that corrected the EEG for EOG-EEG transfer unselectively removed spectral power across the whole scalp and across all frequency bands. Furthermore, part of the maturational change in frontal Alpha power was filtered out by the correction procedure. It was concluded that for the background EEG in children, it is better to carefully select artifact-free EEG segments than to correct for EOG-EEG transfer.

EEG power spectra of normal preadolescent twins. Gender differences of quantitative EEG maturation

To determine developmental changes of quantitative EEG maturation, we investigated the electroencephalogram (EEG) power spectra in a sample of 72 pairs of healthy twins (144 subjects, 75 girls and 69 boys) ranging in age from 7-15 years. Main spectral parameters were measured in 16 EEG channels. The statistical significance of differences in the EEG power spectra was compared in relation to age and gender. Gender differences were tested by comparing the EEG parameters of boys and girls in the whole sample and separately in the subgroup of dizygotic twins of the opposite sex (DZO twin pairs). A statistically significant decrease of absolute power occurred with increasing age. A significant redistribution of relative power also appeared in higher age, mainly consisting of a progressive increase of alpha 2 power inversely related to relative alpha 1 and theta power. Polynomial regression models of the relation between alpha 2 and theta relative power with age were best described by a 5-order function. The mean frequency of activity across all spectra also increased with age and was maximal in the eldest subjects (13-15 years), with a significantly higher mean frequency of alpha 2 power in females. Similar age and gender differences were less marked in the subgroup of DZO twin pairs. The topographic distribution of spectral bands in normal subjects suggested an earlier maturation of midparietal or occipital than frontocentral regions, and indicated that the growth spurt does not occur simultaneously over homologous right and left hemisphere regions at the same age.