Attempt to modify rate and duration of licking in rats by operant conditioning

Lick-rate in rats is said to be constant for a given animal, despite variations of internal and external stimuli. On the other hand, small changes can be observed due to changes in the construction of the licking device. However, variations do not exceed 20%. In an attempt to gain operant control over the ILI (interlick interval - the time between two lick-onsets) the delivery of reinforcement (20 μl water) was made dependent on the emission of ILIs of a predetermined length longer than during baseline licking. It could be observed that rats could not shift the peak of their ILI distribution within the reinforced range but - to increase the number of reinforcements - they increased the scatter of the ILI distribution or developed a "harmonic" peak at double ILI length. When the animals were forced in a second experiment to prolong the lick-duration (time of tongue-spout contact) to obtain water, they failed if the restriction from the drinking spout made a closer approach impossible. It is argued that the ability to obtain reinforcement under both schedules is due to postural changes of the animal. The mechanisms controlling licking seem to be relatively constant, which allows good coordination with other behaviours which have to be performed during drinking, such as breathing and swallowing. It can be concluded that the amount of water consumed by rats is controlled by the length of time spent in licking and not by changing the lick-rate.