Vol. 6, No. 4, 2000 Page 1&3 |
Monkeys exposed to lead, PCBs show
behavior resembling ADHD
Monkeys exposed to lead or PCBs show patterns of behavioral impairment
resembling the deficits of children with attention deficit hyperactivity disorder (ADHD), according to research by Deborah Rice and colleagues.
In particular, the researchers say, lead- or PCB-exposed monkeys show an
inability to learn from the consequences of previous behavior, and an inability to organize their behavior according to a time sequence. Among Rice et al.'s studies:
- In one task, monkeys were required to choose one of two images (e.g., a cross instead of a square) regardless of its position. Once they mastered the task, they were then required to select the other object instead. The researchers tested control mon
nkeys and monkeys exposed to lead from birth on, during infancy only, or after infancy. "When tested as juveniles at about three years of age," the researchers say, "all lead-treated groups were impaired… relative to controls."
- Administering the same test to monkeys dosed from birth to 20 weeks of
age with types of PCBs commonly found in human breast milk, the researchers
found that "when tested as juveniles, some individuals made many more errors over the first several reversals than did control monkeys." This failure to adapt to changing patterns and respond correctly, the researchers note, "is a hallmark of ADHD."
- Exposure to either lead or PCBs during early development impaired
monkeys' ability to perform a task requiring them to alternate responses
between two positions from trial to trial, in the absence of external cues.
- To test the monkeys' ability to organize their behavior according to time sequence, the researchers employed a task that required the monkeys to make a single response after a fixed interval of time, in order to receive a reward. Because no external
cues were provided, the monkeys needed to rely on internal cues to judge the passage of time. Again, monkeys exposed to lead or PCBs during development performed poorly compared to controls.
The researchers conclude that while genetic and other factors play roles in
ADHD, "it seems reasonable to postulate that environmental neurotoxicants
contribute to the prevalence of ADHD currently being identified in children."
"Parallels between attention deficit hyperactivity disorder and behavioral
deficits produced by neurotoxic exposure in monkeys," Deborah C. Rice,
Environmental Health Perspectives, Vol. 108, Supplement 3,
June 2000, pp. 405-408. Address: Deborah C. Rice, U.S. Environmental
Protection Agency, Office of Research and Development, National Center for
Environmental Assessment, MC8623 D, Washington, D.C. 20460.