Effects of lick-contingent timeout on schedule-induced polydipsia

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to conditions in which each lick on the drinking tube produced a timeout period during which the food-schedule lever was retracted and the fixed-interval timer either did or did not continue to operate.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. Water-intake 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 from 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.