The results from Experiment 1 imply that learning about the position of a hidden goal relative to a boundary depends upon activity within the hippocampus. The purpose of Experiments 2 and 3 is to determine if this learning takes place incidentally, as maintained by Doeller and Burgess (2008)
, or if it takes place according to well established error-correction rules of learning (e.g., Rescorla & Wagner, 1972
). The experiments were based on the design of Experiment 1, but a landmark was suspended from the ceiling directly above the platform for some of the groups. If the proposals of Doeller and Burgess (2008)
are correct, then the boundary cues will overshadow learning about the landmark, but the landmark will not overshadow learning about the boundary cues.
There were three groups in the experiment. A boundary group was trained in the same manner as rats in Experiment 1, except that the preliminary training with a beacon attached to the platform was omitted. This group, therefore, had to rely on the boundary to find the platform. A boundary + landmark group was trained in the same way, but a landmark was suspended over the platform on every trial. This group could use both the boundary and the landmark to find the platform. Finally, the landmark group was trained in a similar manner to the other two groups, except that the position of the platform was changed from trial to trial and a landmark was always suspended above the platform. This group could use only the landmark to find the platform.
Toward the end of training, two test trials were conducted in the absence of the platform. One test took place in the circular pool in the presence of the black and white curtain. The rats were released from the center of the pool, and the time spent searching in the place where the platform had previously been located was recorded. If landmarks do not overshadow boundary cues, then both the boundary and the boundary + landmark groups should spend a similar amount of time searching in the correct region of the pool. Moreover the amount of time spent by these groups in this region of the pool should be greater than for the landmark group.
The second test was conducted in the circular pool surrounded entirely by a white curtain and with the landmark suspended from the ceiling. On this occasion, the proposals of Doeller and Burgess (2008)
lead to the prediction that the landmark group will spend more time searching for the platform beneath the landmark than the boundary + landmark group. This prediction follows because the boundary cues were expected to overshadow learning about the landmark in the latter group. Finally, as the test trial will be the first occasion that the boundary group is exposed to the landmark, it would not be expected to spend much time searching beneath it for the platform.
Subjects and apparatus
The 36 rats were from the same stock of approximately the same age as the rats for Experiment 1. They were experimentally naïve prior to the start of the experiment. The apparatus was the same as for Experiment 1, but with the addition of a landmark. The landmark was a Premierlight® LED, waterproof, camping lantern (white, 60 Lumens) that was inverted for the purpose of the experiment. The lantern, which was always switched on, was 18 cm high (including base) and was cylindrical with a diameter of 14 cm. The landmark was suspended from various points above the pool by a wire attached to the base of the lantern. The lowest point of the landmark was 33 cm above the surface of the water at a distance of 50 cm from the edge of the pool.
At the start of the experiment, the rats were assigned at random to each of the three groups in equal numbers. The method of training was the same as for Experiment 1, with the following exceptions. There were 14 sessions of training. For the boundary and the boundary + landmark group, the platform was located 50 cm away from the point at which the two curtains met, with the black curtain to the left of the white curtain. The platform for the landmark group was located in the position just described for one trial of each session. For the remaining trials, it was again located 50 cm from the boundary wall, but it was rotated by 90, 180, and 270o from the position just described, with reference to the center of the pool. The sequence in which these positions were occupied varied randomly from session to session. The landmark was suspended directly above the platform for every training trial with the landmark and the landmark + boundary group.
The fourth trial of Session 12 was a boundary test with the platform removed from the pool, but with the black and white curtains surrounding the pool in the normal manner. The fourth trial of Session 14 was a landmark test. The platform was again removed from the pool, but this time the curtain surrounding the pool was entirely white, and the landmark was suspended above the water 50 cm from the edge of the pool. Rats were released from the center of the pool for both tests and allowed to swim for 60 s. The manner of recording the results from the training trials and from the boundary test was the same as for Experiment 1. For the landmark test, the time spent swimming in a 30-cm diameter search zone, with its center directly below the center of the landmark, was recorded.
Results and Discussion
shows the mean escape latencies to reach the platform across the 14 sessions of the experiment for the three groups. All groups took less time to find the platform as training progressed, but the mean escape latencies were consistently longer for the boundary group than the other two groups. To examine the asymptotic performance of the groups, a one-way ANOVA was conducted with individual mean escape latencies combined across the last three sessions before the first test trial (i.e., Sessions 10, 11, and 12). This analysis revealed a significant difference among the groups, F(2, 33) = 39.20. Newman–Keuls' tests revealed that latencies for the boundary group were significantly longer than for either of the other two groups, which did not differ. The results from the boundary group suggest that the absence of the landmark made it difficult to pin point the exact location of the platform by reference solely to the boundary cues.
Mean escape latencies for the B only, B + 1LM, and LM only groups of Experiment 2.
The left-hand panel of shows the percentage of time spent in the correct and incorrect zones for the Session-12 boundary test trial. The boundary and the boundary + landmark groups, but not the landmark group, spent more time in the correct rather than incorrect zone. In addition, the boundary group spent more time in the correct zone than the boundary + landmark group. A 3 × 2 (Group X Zone) ANOVA revealed significant main effects of group, F(2, 33) = 23.21, zone, F(1, 33) = 130.94, and a significant Group X Zone interaction, F(2, 33) = 30.26. A simple main effects analysis of the interaction revealed that the boundary and the boundary + landmark groups spent a greater proportion of time in the correct rather than incorrect zone, Fs (1, 33) > 55.89, while the landmark group did not express a preference for either zone, F < 1. There was also a significant difference among the groups in the proportion of time spent in the correct zone, F(2, 66) = 53.42, but not in the incorrect zone, F(2, 66) = 1.08. Newman–Keuls tests revealed that the boundary group spent more time in the correct zone than the boundary + landmark group, which spent more time in the correct zone than the landmark group.
Mean (+SEM) percentage of time spent in the correct and incorrect zones during the boundary test (left-hand panel) and landmark test (right-hand panel) for the three groups of Experiment 2.
The right-hand panel of shows the time spent in the correct zone for the landmark test trial conducted in Session 14 for each group. The boundary group spent the least amount of time in the correct zone, compared the other two groups, which spent approximately the same proportion of time in the correct zone. A one-way ANOVA was conducted and revealed a significant difference among the groups, F(2, 33) = 32.15. Newman–Keuls' tests revealed that the boundary group differed from the remaining groups, which did not differ from each other.
The results from both tests failed to confirm the predictions derived from the proposals of Doeller and Burgess (2008)
. On the one hand, it was anticipated that the landmark would not overshadow learning about the boundary, which led to the prediction that the boundary + landmark group and the boundary group should have performed similarly during the boundary test. In fact this test revealed evidence of overshadowing because the boundary + landmark group spent significantly less time in the correct search zone than the boundary group. On the other hand, it was anticipated that the boundary would overshadow learning about the landmark, which leads to the prediction that the boundary + landmark group will spend less time than the landmark group searching beneath the landmark during the test with this cue. On this occasion, however, there was no difference between the two groups.
A possible explanation for the outcome of the boundary test can be based on generalization decrement. The boundary test for both the boundary and the boundary + landmark groups took place in the absence of the landmark. However, this landmark had been present throughout the training trials for the boundary + landmark group but not the boundary group. It is thus conceivable that both groups learned to the same degree about the position of the platform relative to the boundary cues, but the generalization decrement consequent upon the removal of the landmark weakened responding during the test trial in the boundary + landmark group. The purpose of the next experiment was, in part, to test this explanation for the results of Experiment 2 and, in part, to confirm their reliability.