Microstructural analysis of successive negative contrast in free-feeding and deprived rats

Rats shifted from 1.0 M to 0.1 M sucrose lick at lower rates for the weaker solution than rats that have continual access to the 0.1 M sucrose solution only. This effect, referred to as successive negative contrast, has been investigated primarily in food-deprived rats and, in all cases, using total licks or total volume consumed as the dependent measure. The present experiment used a microstructural analysis of licking patterns to examine the changes in behavior that constitute the contrast effect in total licks in both deprived and free-feeding rats. Although the magnitude of the effect was similar, deprived rats recovered from contrast more rapidly than free-feeding rats. Furthermore, the patterns of licking behavior associated with contrast differed under the two deprivation conditions. Specifically, when compared with the unshifted controls, the contrast effect in deprived rats was accomplished through a decrease in the number of licks per burst, an increase in the number of bursts initiated, a brief increase in the length of the interburst intervals, and no change in length of the interlick intervals. In free-feeding animals, contrast was associated with a decrease in the number of licks per burst, a brief increase in the length of the interburst interval, and no changes in either the number of bursts initiated or in the length of the interlick intervals. Together, these data demonstrate that patterns of licking behavior are differentially affected by solution concentration, deprivation state, and relative aspects of reward value.