Lead-induced changes in learning: evidence for behavioral mechanisms from experimental animal studies

Lead is probably the most studied of the neurotoxic agents, both in humans and in animal models. Research has focused on learning impairment and other behavioral consequences produced by developmental exposure. In children, lead exposure results in deficits in such global measures as IQ, as well as more specific deficits that are suggestive of attentional deficit disorder. Research in animals has also clearly demonstrated learning and memory deficits as a consequence of developmental lead exposure. Experiments performed in monkeys implicate the same behavioral problems as those observed in children: increased distractibility, inability to inhibit inappropriate responding, and perseveration in behaviors that are no longer appropriate. For example, lead-treated monkeys were impaired in their ability to perform discrimination reversal task, but not on the initial visual discrimination task. Deficits were more severe in the presence of distracting irrelevant stimuli. Lead-treated monkeys displayed severe perseveration on one button on a task which required them to alternate responding between two buttons. Lead-treated monkeys displayed memory impairment on a task requiring them to remember a previously observed stimulus or position, which was at least in part the result of interference from responses from previous trials. Lead-treated monkeys exhibited a higher rate of response on an intermittent schedule of reinforcement, and had difficulty inhibiting responding when required. Thus, for many of the tasks on which monkeys have been found to display learning and/or memory impairment, the deficit can be attributed at least in part to an attentional deficit and/or perseverative behavior.