Patterns of the EEG theta rhythm modulations when attending to speech by orphans in early childhood

The first years of a child’s life are a critical period for the development of cognitive functions, in particular, a capacity for speech. Rearing conditions when applied to orphans may have an effect on neurophysiological mechanisms underlying their speech understanding, which is reflected in the EEG theta rhythm power changes. Individual theta rhythm frequency ranges are to be determined first, as they may vary for each child depending on age and individual characteristics of the central nervous system. The aim of our work was to analyze the EEG theta rhythm power reactivity in its individually determined frequency ranges when institutionally reared early-aged orphans attended to normal speech and speech-like signals, compared to their peers reared in their biological families. The study involved 107 children aged 24 to 42 months. The main group included 54 children (19 girls and 35 boys) brought up in the orphanage. The control group comprised 53 children (21 girls and 32 boys) reared by their biological parents. The EEG was recorded at frontal (F3, F4, F7, F8), central (C3, C4), temporal (T3, T4, T5, T6) and parietal (P3, P4) sites in three experimental conditions: in a wakeful rest state with open eyes (baseline EEG, duration 20-30 s); when listening to a normal speech (a verse with animal-characters, duration 20 s, female voice); when listening to the same speech reversed with the help of the Wave Editor software application (reversed speech). The order of presentation for the second and third conditions was randomized. To determine individual theta rhythm frequency bands, we calculated means of spectral density power, first log-transformed to help normalize distributions. To compare the EEG theta rhythm power reactivity when children in both groups attended to the normal and reversed speech stimuli relative to the baseline condition, power reactivity indices were calculated for each of the EEG sites. Significance of changes in the EEG theta rhythm power when attending to the normal or reversed speech relative to the baseline, as well as inter-group differences in reactivity indices, were assessed with the repeated measures ANOVA. The study revealed the patterns of the EEG theta rhythm power changes depending on experimental situations and EEG sites characteristic for children in both groups (See Table 1). We found ssignificant theta rhythm power changes yet of opposite directions for the control group under the normal speech condition. Theta rhythm power increased in the right frontal locus, the left anterior temporal locus and the parietal loci in both hemispheres, and decreased in the left frontal and posterior temporal loci and the right central and posterior loci (See Fig. 1, a). It seems plausible that the process of speech perception in typically developing children is associated both with searching one’s memory for phonological, syntactic and semantic features in heard words and their integration into a holistic message, and with attentional processes, associated with increases and decreases in theta rhythm power in different areas of the cerebral cortex. Compared to normal speech, presentation of a reversed signal to the control group children caused a more pronounced and widespread theta activity growth in different cortex areas. This phenomenon is likely to be associated with an increase in load on memory when children tried to recognize familiar words in a reversed audio signal. In the group of institution-reared children, when they attended both to the normal and reversed speech stimuli, significant changes in theta rhythm power were found at fewer sites compared to the control group children. When they attended to the normal speech, a significant power increase occurred only at the right posterior temporal site. When attending to the reversed signal, theta rhythm power increased in the frontal (F3, F8) and posterior temporal (T5, T6) loci (See Fig. 1, b). We found differences in theta rhythm reactivity indices under conditions of attending to the normal and reversed speech in children as depending on group and site factors (See Table 2). Theta rhythm reactivity when attending to the normal and reversed speech in the main group compared to the control revealed a large number of significant differences at different sites (See Fig. 2). It is suggested that when recognizing or trying to recognize individual words, meaning and emotional load in sentences, institutionally reared children tend to contribute considerably less cognitive resources than family-reared children. We consider the reduced orphans’capacity to engage cognitive resources in mentioned situations to be the result of insufficient experience of close social interactions with adults. Given the established association between theta oscillations and memory processes, we can assume that speech memory traces in children belonging to the studied groups are represented differently in various neocortex areas due to a different experience of their formation when raised in the family and the orphanage. The paper contains 2 Tables, 2 Figures and 40 References.

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