Ontogenetic dissociation of two classes of habituation.

The rates of decrement of two classes of response (an elicited startle reflex and emitted exploratory behavior) were determined in rats of two different ages (15 and 36 days). The rate of decrement in the startle reflex was not clearly differentiated as a function of age. In contrast, there was no evidence of habituation of exploration in the younger animals, whereas the older rats uniformly showed profound response decrements. This ontogenetic dissociation of the two instances of response decrement indicates that accounts of both instances in terms of a common process called habituation may be unwarranted. In addition, these data, in conjunction with earlier findings, indirectly support the possibility that reflex decrements may be relatively more dependent on brain serotonin whereas decrements in exploration may be more dependent on normal cholinergic activity in brain.