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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.In a recent paper (Williams, Hamilton, & Carlton, 1974) we presented data suggesting that two instances of response decrement usually taken as indexes of a common process of habituation could be pharmacologically dissociated. In particular, we found that the decrement of an elicited reflex (a startle response) was relatively unaffected by the anticholinergic drug scopolamine, whereas the decrement of an emitted response (exploratory behavior) was profoundly attenuated by this same drug.These data indicate, among other things, that the rather general tendency to attribute various instances of response decrement to a single hypothetical process called habituation (e.g., Carlton, 1969; Thompson, Groves, Teyler, & Roemer, 1973) embodies an implicit assumption about that process that is largely unwarranted. In this article, we explore this prospect in yet another way,

Rates of habituation of an exploratory head-poke response were compared for control rats vs. rats with septal lesions or scopolamine-hydrobromide injections. Controls exhibited decreasing response rates and increasing durations per response within sessions. Response-contingent light presentation attenuated both effects; the major influence of light presentation was an activating rather than a direct reinforcing effect. Both the scopolamine-hydrobromide-injected rats and rats with septal lesions showed decreasing response rates similar to controls; however, neither increased their durations per response, suggesting a more profound impairment of habituation. Both reacted to response-contingent light presentation similarly to controls but in a more exaggerated fashion, indicating that an increased responsivity to responseproduced stimulus feedback caused the habituation deficit.

The performance of oats with septal lesions (n = 39) was compared with that of control Ss (n = 49) in 10 different behavioral tests. The effect of the lesion on the rate of acquisition of an active-avoidance task was dependent upon the testing situation: The avoidance performance of Ss with lesions was (a) inferior in three successive modifications of a shelf-jump task, (b) enhanced in a shuttle box, and (c) normal in a one-way apparatus. Disinhibitory effects were observed in some, but not all, of the behavioral tests employed: The experimental /Ss (a) returned to food more readily in a noncontingent shock situation, but performed normally on a standard passive-avoidance task, (b) required more punished responses to extinguish a two-way (but not a one-way) avoidance habit, and (c) made more errors than control Ss during the reversal of a position habit.1 This research report is based upon a dissertation submitted to the Department of Biopsychology in partial fulfillment of the requirements for the PhD degree at the University of Chicago. The author would like to thank S. P.