Early exposure to lead appears to cause deleterious effects on auditory processes. Not only are basic psychoacoustic and electrophysiological measures affected, but also higher-order processes such as speech perception and language comprehension. It is likely that such auditory system pathology contributes to more global behavioral consequences for reading skills, school problems, and IQ deficits characteristic of childhood lead exposure. The objectives of this research are to define the nature of auditory pathologies related to childhood lead exposure for an experimental, nonhuman primate model (rhesus monkey).
We aim to evaluate contributions of the peripheral and central auditory system to these abnormalities, and to characterize the contribution of exogenous factors, such as attentional processes to the auditory pathology using behavioral and neurophysiologic techniques. Results of these studies would affect screening methods, alter the composition of intervention programs, and provide the basis for determining whether a more extensive investigation of the underlying pathophysiology are warranted. Monkeys were exposed to lead daily either throughout gestation or for a comparable time postnatally beginning at birth. Lead exposure was at levels relevant to human childhood lead exposure. Control monkeys were given no added lead. The monkeys are now 9 years old and have extensive experimental test histories. Initial studies with these and other monkeys using electrophysiologic indices indicate effects of lead on peripheral and higher order auditory processes. Latencies and amplitudes of several components of the brainstem auditory evoked response were increased and electrophysiological indices of discrimination of complex sounds (phonemes) were altered as a function of lead exposure with different periods of lead exposure producing different patterns of results. Behavioral measures of attention also were disrupted following early lead exposure in the discrimination studies. Further tests of auditory system integrity are proposed including behavioral measurements of absolute and difference threshold, behavioral measures of attentional processes, behavioral measures of discrimination of complex auditory stimuli including human speech sounds, and further analysis of electrophysiologic responses to auditory stimuli including the brainstem auditory response.
