The effects of postweaning low-level Pb exposure on sustained attention: a study of target densities, stimulus presentation rate, and stimulus predictability.

It has been suggested that behavioral effects associated with low-level lead (Pb) exposure are related to alterations in attentional processes. These "attention deficits" remain undefined, and few studies have directly addressed the effects of Pb on specific measures of attention. The current study assessed the impact of Pb on sustained attention, as one of the 3 primary clinical symptom domains of attention deficit disorder. Groups of rats were exposed from weaning (21 days of age) to either 0, 50, or 150 ppm Pb acetate in drinking water producing blood Pb levels of <5, 16.0, and 28.0 microg/dl, respectively. Rats were trained to discriminate a pulsing (target) from a non-pulsing (distracter) light. Parametric manipulations of target probability (10%-80%) and inter-stimulus interval (1 s-6 s, and variable) were imposed to evaluate whether effects of Pb were dependent upon attention demand. Sensitivity of the paradigm to changes in CNS function was validated by examining the effects of d-amphetamine (0.0-2.0 mg/kg) in a separate group of rats. Exposure to 50 ppm increased errors of omission only during sessions with long and variable inter-stimulus intervals. The 150 ppm group showed a small increase in errors of commission during sessions with a long inter-stimulus interval. No other Pb-related effects were observed. The absence of any pronounced Pb effects in this study on this sustained attention task could indicate that: 1) any Pb-induced attention deficits are modified by reinforcement and time-out contingencies; 2) Pb disrupts other aspects of attention such as inability to tolerate delays or impulsivity; and 3) Pb-related deficits are due to other behavioral dysfunctions.