Rats bar pressing on a 1-min fixed-interval schedule for 45-mg food pellets became polydipsic when water was concurrently available. They were then exposed to conditions in which each lick on the drinking tube produced a timeout period during which the foodschedule lever was retracted and the fixed-interval timer either did or did not continue to operate. Licks occurring within a timeout period extended its duration. As the duration of the lick-initiated timeout period was increased logarithmically through four values from 10 sec to 80 sec, lick rates as well as water intake rates generally decreased for all three subjects. As timeout duration was progressively increased, the rate of licks occurring in the absense of, but producing, timeouts decreased for all three rats, whereas the rate of licks occurring in the presence of timeout periods remained essentially constant. Waterintake rates and, with one exception, lick rates were suppressed more by timeout periods during which the fixed-interval timer did not continue to operate. These results indicate that lick-contingent timeout froni positive reinforcement reduces schedule-induced drinking, and this suppressive effect is greater when the timeout period necessarily increases the interreinforcement interval beyond its minimum duration than when it does not. Schedule-induced polydipsia was first reported by Falk (1961). He found that fooddeprived rats, bar pressing for 45-mg food pellets on a variable-interval 1-min schedule, drank after each pellet to the point of consuming in a 3.2-hr session three to four times their normal 24-hr intake. Schedule-induced polydipsia, which has been observed in mice (Palfai, Kutscher, and Symons, 1971), in monkeys (Schuster and Woods, 1966), in the pigeon (Shanab and Peterson, 1969) as well as in rats, is characterized by a period of drinking that occurs shortly after the ingestion of a food pellet. This typical drinking pattern is observed with fixed or variable schedules whether or not food deliveries are contingent upon the emission of a specific operant response (Falk, 1969(Falk, , 1971 Although it appears to be physiologically maladaptive (Falk, 1971), schedule-induced polydipsia is a relatively strong behavioral phenomenon. Falk (1966a) observed that polydipsia occurred adjunctive to a 60-sec variable-interval food schedule even when water was concurrently available in 0. l-ml portions contingent on responding under a fixed-ratio schedule requiring as many as 50 responses. These results indicate that schedule-induced drinking is not simply a mediating or collateral behavior, but rather is an activity that is sufficiently reinforcing to sustain scheduled behavior. Further indication of the strength of adjunctive drinking behavior was provided by Flory and O'Boyle (1972), who reported that rats, bar pressing for food pellets on a 60-sec fixed-interval schedule, became polydipsic when 0.l-ml portions of water were concurrently available via a second lever on a fixed-ratio 1 basis. The session water intakes of each rat remain...
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