Intermittent schedules of reinforcement have been used in behavioral pharmacology and toxicology for over 50 years. Descriptions of these schedules are available from numerous sources (e.g., [11, 12]). Simple schedules such as fixed interval (FI), variable interval (VI), fixed ratio (FR), and differential reinforcement of low (response) rate (DRL) schedules are acquired reasonably rapidly by animals, and performance is similar across species, including humans. The schedule used most often in lead research was the FI, presumably because it offers a number of advantages. Although this schedule requires the subject to make only one response at the end of a specified (uncued) interval, FI performance is typically characterized by an initial pause followed by a gradually accelerating rate of response, terminating in reinforcement. The schedule does not differentially reinforce any particular response rate (other than no or very low rate of responding) and may therefore be sensitive to toxicant-induced differences in the rate of response. In addition, temporal discrimination can be examined by measuring the shape of the response pattern across the interval (e.g., quarter life or index of curvature). Lower doses of lead produced increased response rates on the FI schedule in rats and monkeys [13-21], whereas high doses resulted in lower response rates [22, 23]. In general, temporal discrimination per se, as measured by the pattern of responses across the interval, was not affected by lead (but see Rice  and Mele et al. ). When a time-out (TO) period (during which responses had no scheduled consequences) was included in the assessment of performance on the FI, lead exposure resulted in increased TO rates of response [18, 19].
Attention deficit hyperactivity disorder (ADHD) was associated with increased response rates on FI performance in 7- to 12-year-old boys, as well as a "bursting" pattern of response produced by a run of closely spaced responses separated by a short pause . This pattern was also observed in 3-year-old monkeys exposed to lead from birth (Figure 6.1) . Children with ADHD also responded more in the extinction (TO) portion of the schedule, as did lead-exposed monkeys. FI performance predicted poorer performance on a test of impulsivity in normal children [26, 27]: children with high response rates and shorter post-reinforcement pause times
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