Behavioral deficit (delayed matching to sample) in monkeys exposed from birth to low levels of lead

Monkeys (Macaca fascicularis) were dosed po from birth with 500 micrograms/kg/day of lead as lead acetate. Blood levels peaked at an average of 55 micrograms/dl by 100 days of age, and dropped after 200 days of age to a steady-state level averaging 33 micrograms/dl. No overt signs of lead toxicity were observed. Between 3 and 3.5 years of age, monkeys were tested on both a nonspatial (color) and a spatial delayed matching to sample paradigm. For the nonspatial paradigm, the monkey was required to press a button that was lit with one of three colors a specified number of times, which turned the light off. After a specified delay period (varying from 0 seconds to several minutes), three test buttons were lit, one with each of the three colors. The monkey pressed the button corresponding to the color that had appeared on the sample button in order to receive a fruit juice reward. For the spatial matching to sample, one of the three test buttons was lit. The monkey responded on this button a specified number of times, which turned the light off. After a predetermined delay of variable duration, all three test buttons were lit and the monkey responded on the previously lit one in order to be reinforced. The treated monkeys were impaired relative to controls on both the spatial and nonspatial matching tasks, even though they learned the matching tasks as readily as controls and performed as well as 0 sec delay.