Effect of lead during different developmental periods in the monkey on concurrent discrimination performance

A total of 52 monkeys (Macaca fascicularis) were dosed orally with vehicle or 1.5 mg/kg/day of lead according to one of four dosing regimens (13 monkeys/group): Group 1, vehicle only; Group 2, dosed with lead continuously from birth; Group 3, dosed with lead from birth to 400 days of age and vehicle thereafter; Group 4, dosed with vehicle from birth to 300 days of age and lead thereafter. Blood lead concentrations averaged 3-6 micrograms/dl when monkeys were not dosed with lead, 32-36 micrograms/dl when being dosed with lead and having access to infant formula, and 19-26 micrograms/dl when being dosed with lead after weaning from infant formula. When monkeys were 8-9 years old, they were tested on two sets of concurrent discrimination tasks, each consisting of 6 stimulus pairs. Group 2 required more sessions on Task 1 for 5 of the 6 pairs to reach a criterion of 90% correct, while Groups 3 and 4 required more or marginally more sessions on 4 of the 6 pairs. On Task 2, Group 2 required more sessions to reach criterion for most pairs, Group 4 was marginally impaired, and Group 3 was unimpaired. All three treated groups made more perseverative errors than controls on Task 1, but not on Task 2. There were no differences between groups for response latency or position bias. This extends previous research with these monkeys, in which impairment was observed on spatial and nonspatial discrimination reversal, and spatial delayed alternation.