Operant control of the pattern of licking in rats

In an attempt to slow down the highly regular rate of licking by instrumental conditioning, 10 rats were trained to obtain their daily ration of water in an apparatus equipped with a retractable drinking spout. Termination of a photoelectrically monitored lick started a computer controlled delay during which the spout was made inaccesible. The subsequent return of the spout was either permanent or limited to a time window (D or W conditions). The cycle was reset by each lick. With stationary spout, the interlick intervals (ILIs) were around 210 +/- 16 ms (median and interquartile range). The spout return was gradually delayed during 22 sessions from 140 to 260 ms and limited to a 260-340 ms window during 11 sessions. A session consisted of 512 licks with stationary spout followed by 7 X 512 licks under D or W conditions. Two rats were able to smoothly adjust to the limited spout availability by increasing median ILI to 290 ms. Four rats generated bimodal ILI distributions indicating gradual reduction of tongue protrusion to an undetected lick followed by abrupt increase of lick amplitude. Three rats increased the median ILI to 260 ms and the interquartile range to 80-150 ms. The above changes of lick pattern allowed the rats to attain tongue-spout contact in 41 to 89% licks. It is concluded that the rats can substantially slow down their lick rate provided that they receive feedback information about the failure or success of each lick.