The EEG of mildly retarded children: Developmental, classificatory, and topographic aspects

Our investigation was concerned with 25 children, 10-13 years old, with an IQ 50-70 ('mild mental retardation,' following the ICD). Among these, 14 attended a school for the mentally retarded and 11 one for the learning disabled. A control group was recruited, matched in age, sex and social class. The unipolar 8-channel record of the EEG at rest was subjected to blind clinical rating, and a computerized analysis (broad band spectral parameters delta, theta, alpha₁, alpha₂, beta₁, beta₂). A significantly higher frequency of paroxysms was found by the clinical rating. It also allowed the diagnosis of a maturational lag with respect to the items 'maturity' and 'prominence of alpha rhythm'). Spectral parameters differentiated the two matched groups particularly in bands and leads of developmental relevance (theta, delta and fronto-central beta in absolute power and theta, delta occipitally and alpha₂, with the exception of frontal leads for relative power). As is well known, the mentally retarded constitute a heterogeneous group: this could also be verified with respect to EEG activity for the segment of mild mental retardation. A multivariate classification by nonmetric multidimensional scaling yielded a subgroup of 10 children deviant with respect to its overall EEG activity and a group of 15 children within the normal range. This assignment did not overlap with the assignment to the two schools. By computing ratios of broad band power in antero-posterior and symmetric-interhemispheric leads a reduced topographic differentiation was found for the experimental group in their antero-posterior distribution.