Developmental changes in slow cortical potentials of young children with elevated body lead burden. Neurophysiological considerations.
The effects of age and blood lead level (PbB) on slow cortical potentials elicited during classical conditioning were studied in children aged 13 to 75 months. At normal PbB levels (less than 30 micrograms/dl), SW voltage tended to be positive in children under five years old and negative in children over five years. A reverse age-related polarity shift was observed in children with elevated PbB levels (Otto et al., 1981). Evidence indicative of excitatory (or disinhibitory) effects of Pb exposure in immature animals (associated with hyperactivity in animals and children) was reviewed. Reversals in locomotor activity (hypoactivity) and electroconvulsive activity have also been noted in adult animals exposed to Pb as pups. Neurophysiological evidence suggests that slow surface-positive potentials in very young children may reflect axodendritic inhibitory processes. As the cortex matures, the locus of inhibitory activity shifts deeper to axosomatic connections. Negative slow potentials observed in older children, therefore, are presumed to reflect the surface negative (dendritic depolarization), depth positive (somatic hyperpolarization) dipole hypothesized by McSherry (1973) as the neurophysiological substrate of the CNV. Although the developmental model of the CNV proposed here is based on preliminary data and is clearly speculative, the implications of the model and data merit consideration. Positive shifts observed in very young children during the CS-UCS interval suggest why previous investigators have had great difficulty in eliciting contingent negative variation in subjects younger than five years. The data also suggest that body Pb burden at any level produces an observable effect on CNS function. Further study is needed to replicate and clarify these findings.