Individual consistency and modifiability of lapping rates in rats: a new look at the variance--invariance question

Lapping rates were recorded from water-deprived rats over 3-week periods when they were young adults and again, 9.4 months later, when they were middle-aged. Test-retest correlation coefficients of 0.95, that were obtained for lapping rates recorded as long as a week apart, reflect extremely consistent frequency control, which is in line with the proposal by Wiesenfeld et al. (1977) that licking/lapping is under control of a hypoglossal oscillator. These high test-retest correlations were obtained both in young adulthood and again, later in middle-aged rats. However, correlations across the 9.4 months were close to zero, although mean water lapping rate did not change. Our data indicate that the effects of aging on lap rate were of two kinds. (1) Producing randomized shifts in the lapping rate of individual rats over long periods, with conservation of the stable mean rate, and (2) accentuating the tendency for reduction in the lapping rate for the last 50 laps compared with that of the first 50. It also appeared that the relevance of the fluid being lapped, in relation to water deprivation, was another important variable in determining the lapping rate. These and other data are considered in relation to the variance-invariance issue.