In the free responding situation in operant conditioning, various reinforcement schedules have been used for the partial reinforcement training. Differential reinforcement of low rates (DRL) schedule is one such and is defined as follows : under this schedule, only operant responses that terminated within inter-response times (IRT) equal or longer than a given time are reinforced. In general, the procedures of partial reinforcement are classified into two major types. In one of them, the condition of reinforcement is depended upon the passed time after reinforced responses, while in another type it is depended upon the number of operant responses after reinforced responses. Although the DRL schedule should be considered to belong to the former since the reinforcible condition has a critical relation to the passed time after operant responses, it is a unique procedure in that the initiator of reinforcement is started with every operant response and the reinforcible condition is heavily dependent upon the
S's operant responses.
The present series of studies was carried out to investigate how the mechanism for inhibition of operant responses under the DRL training was formed and how it was modified under treatments of psychotropic drugs, observing operant responses and various behaviors deemed apparently irrelevant to the particular reinforcement schedule used.
In the experiment reported here, the effects of chlorpromazine (CPZ) on the development and the modification of the operant responses and some of the irrelevant responses in the DRL training were examined using 16 male albino rats of Wister strain as experimental subjects.
Under the DRL schedule of 12 sec, the bar-pressing behavior of the
S's was reinforced with a pellet (0.05 g) in a turntable exerciser (6, 7, 21) having a bar for the operant responses and a food tray connected to a food magazine through a vinyl chloride tube. This continued for 6 consecutive days, and the running behavior during the DRL schedule was observed simultaneously. The daily training session was terminated when each S obtained 72 food reinforcements. On 7 th day of the DRL training, the Ss were divided into two groups, the T group (
N=8) and the O group (
N=8). The
Ss in the T group were administered CPZ intraperitoneally (2.0 mg/kg) in a physiological saline solution (10 cc/kg) 30 min prior to the training session on the day, whereas the
Ss in the O group were administered only the physiological saline (10 cc/kg). Except for the administration of these drugs, the procedure used was the same as that in the previous days. The next day, each group was again sub-divided into two groups of equal size, the E groups (given CPZ) and the O groups (given saline). In this session, the training procedure was changed from 72 to only 12 reinforcements, and immediately afterwards the extinction session was introduced. The criterion for extinction was the non-occurrence of the bar-pressing for at least 5 min. Records of extinction were taken, however, at least 30 min after it had begun. Main measurements taken in this experiment were as follows : the required time in the daily training session; the number of total bar-pressing responses; the number of total bar-pressing responses/the number of total reinforced responses; the distributions of the IRT's; the amount of running activities. A summary of experimental procedure is shown in Table 1.
The main findings were as follows. As the DRL training sessions progressed, the number of total bar-pressing responses and the number of required responses per reinforcement deceased gradually (Fig. 2), and the distributions of IRT classes were gradually changed to increments of reinforcible IRT's (Fig. 3).
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