The purpose of the present experiment was to identify the period when the ability of place learning begins to be ontogenically functional in rats. We gave Morris water-maze task for 3 days to 4 groups of rats : 2 conditions that differ in the starting day of the experiment (18 days old and 21 days old) × 2 stimulus conditions (open and closed). Under the open condition, external stimuli could be utilized, but not under the closed condition. It was not until 20 days old that the search time in the training quadrant (where the platform had been placed during the training phase) was over the expected value in the open condition. But during this period, the escape time to the platform was same as that of the closed condition. On the 21 days and after, the escape time under the open condition was shorter than under closed condition. Moreover, in the open condition for the 22 and 23 days old rats, the number of crossings on the annulus where the platform had been located during the training phase was significantly increased, and the search time in the training quadrant was longer than the expected value. These results demonstrate that 20 days and older rats have the ability of place learning. We discussed that the development of the ability of place learning mainly depends on the functional maturity of neural systems, especially hippocampal system.

View full abstract

The present research investigated the effects of the rule structure of serial patterns and the amount of training on resistance to extinction in rats. Rats were trained in a runway on either of following serial patterns, decreasing (D : 0-14-0-7-0-3-0-1-0 : the numbers indicate the number of food pellets), increasing (I : 0-1-0-3-0-7-0-14-0), or random (R : 0-R-0-R-0-R-0-R-0) series for five days (small training condition : S) or 20days (large training condition : L). These conditions made Groups DS, IS, RS, DL, IL, and RL. Following the acquisition phase, extinction trials were administered. While Groups DS, IS, and RS showed no patterning responses in the acquisition phase and the same degree of resistance to extinction, Groups DL, IL, and RL showed the same degree of remarkable patterning response in acquisition and a different degree of resistance to extinction (RL > DL = IL). The results, especially obtained in the large training condition, are not explained by Capaldi's stimulus-after effect theory that is accepted as an explanation for resistance to extinction. Considering the results mentioned above, it is concluded that the rule structure of serial patterns affects resistance to extinction as well as acquisition performance.

View full abstract

The present experiment examined effects of interrun interval (IRI) shift and pattern length on serial pattern learning in rats. Rats in a runway received varying number of 45mg food pellets as items of a series. In Phase 1, two groups were given long series (18-10-6-3-1-0 pellets), and the other two groups were given short series (18-1-0 pellets), under short (15-20s) or long (5-7min) IRI. All of the groups were presented their series at one time per day. All the groups developed reliable anticipation of 0 pellet. In Phase 2, each IRI was shifted from short to long or vice versa. The shift in IRI deteriorated the 0 pellet anticipation only in the group which was given 6 item series under short IRI-in Phase 1 and long IRI in Phase 2. No reliable deterioration of 0 pellet anticipation was observed in any other three groups. The interactive effect between direction of shift in IRI and pattern length on pattern tracking performance is explained most appropriately by Hulse's dual theoretical approach which suggests that either associative learning or rule learning mediate serial pattern learning depending on IRI and pattern length.

View full abstract

This study aimed to describe the within-session decreases of rat feeding behavior as a function of cumulative amount of food intake. Rats (N = 16) were trained to obtain food pellets by nose-poke responses into a recessed food tray. Session lengths were 45 minutes. Half of the rats were reinforced by CRF schedule and the other half were reinforced by VI 20-s. Three levels of hunger drive were operated by changing duration of food deprivation. The following results were obtained : 1) Within-session decreases of response rates were well described as liner functions of cumulative amount of food intake under both reinforcement schedules. This means that within-session decreases of response rates could be regarded as a satiation process that progressed proportionally to the cumulative amount of food intake. 2) Effects of drive operation appeared in the differences in slopes of regression lines. This suggests that drive level affects the progress of the satiation process.

View full abstract