Sprague-Dawley rats (n=96) from postnatal 17-day-old to 24-day-old were compared for their exploration of 4 objects in an open-field, locomotor activity, object exploration time, habituation, and exploration time to the spatial change. As the locomotor activity increased, 23-day-old and 24-day-old rats explored the objects more than any other rats of the age groups. All rats showed habituation. It was not until the rats were 21-day-old that they re-explored the spatially changed object. 21-day-old rats more frequently investigated the object than older (22-day-old, 23-day-old, 24-day-old) rats. These results suggest that the exploration for objects precedes developmentally that of spatially changed object, and that 21-day-old rats begin to acquire informations about the spatial relationship in the environment.

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In the classical conditioning, the truly random control (TRC) procedure, in which conditioned stimuli (CS) and unconditioned stimuli (US) are presented independently, results in an excitatory conditioning in early part of training. The purpose of the present research was to examine an effect of CS intensity upon this initial excitatory conditioning effect. Both in Experiments 1 and 2, the TRC procedure, in which a tone was used as CS and an electric shock as US, was conducted using a technique of conditioned suppression of licking in rats. Suppression to the CS was assessed four times every fifth session of the TRC training. In Experiment 1, using relatively low CS-density, the group trained with a strong CS showed more suppression than the group trained with a moderate CS in the first test session. In Experiment 2, using relatively high CS-density, the group trained with a moderate CS and the group trained with a weak CS showed equivalent suppression in the first test session, but the suppression of the former group was long-lived. Although these results were consistent with the Rescorla-Wagner model as a whole, the interaction between the amount and the duration of the initial excitatory conditioning effect remained to be investigated in future research.

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One rat was exposed to sequences of pre-determined numbers of shocks under the all-day-long experimental procedure which contained six 30-min sessions daily. In Experiment 1, three 1.5 sec shocks per session were imposed on a variable-interval 30-sec reinforcement schedule of lever pressing. The rat did not show any evident suppression of lever pressings. In Experiment 2, the procedure was modified so that the lever pressing was not reinforced before the third shock ended. The rat showed lever pressing suppression before the third shock, though some responses were often observed immediately after the second shock. This response pattern was influenced either by the change in the shock duration or the temporal position of shock delivery (the range of shock distribution). Finally the number of shocks was increased from three to five. The subject soon learned to suppress response until the fifth shock was delivered. These results were interpreted as showing that though the rat used some parameters of stimuli as cues for response suppression, the number of shocks was the effective element.

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