Many characteristics of a series of discrete independent hedonic events may be remembered by rats in terms of, for example, how many events were rewarded and how many were nonrewarded. Such memory for multiple hedonic events, which has been shown to be a potent factor controlling instrumental responding, was examined here in five investigations employing serial anticipation learning in a runway. It was found that the ability of rats to remember the hedonic events reward and nonreward is highly developed, accurate, and quite resistant to forgetting and interference. Rats not only remembered a rewarded event and a nonrewarded event, but they also remembered the order in which the two events occurred. Rats remembered how many nonrewarded events there had been accurately enough to suggest that they were using some form of a counting mechanism. Rats exhibited little forgetting of eight prior discrete hedonic events, one rewarded followed by seven nonrewarded, even when these occurred over an interval of 20 min and involved considerable potential interference. In the serial learning situation employed here, marked primacy effects were obtained, earlier nonrewarded trials in a series being better anticipated than later ones. The primacy effect was found to depend upon the type of series employed. By assuming that stimulus generalizations occur between the multiple hedonic events remembered by rats, all anticipatory learning obtained here could be explained in considerable detail.
Two experiments indicated that two approaches to serial learning are too extreme-the classical view that it consists only of interitem associations and various recent views that it involves no interitem associations. The novel assumption introduced here was that phrasing cues, normally conceptualized as merely segregating long series into smaller units or chunks, may also enter into associations with items, thereby reducing interitem interference and facilitating serial learning. It was found that one item could become a signal for another item, an interitem association, or be overshadowed by a phrasing cue, such as a brightness and temporal cue, also signaling that item. The items were ,045-g pellets. Rats traversed a runway for items arranged in ordered series, 14-7-3-1-0 pellets (Experiment 1) or 10-2-0-10 (Experiment 2), Complete tracking of, for example, the 10-2-0-10 series would consist of fastest running to 10 pellets and slowest running to 0 pellets. In both investigations, the interitem association overshadowed was that between 0 pellets and the subsequent rewarded item, 0-14 (Experiment 1) or 0-10 (Experiment 2). Either repetitions of the 14·7·3-1-0 subpattem (Experiment 1) or merely the terminall0·pellet item (Experiment 2) were phrased, both methods producing identical results. Overshadowing the O-pellet item produced superior serial learning, more rapid extinction, and, in Experiment 1, considerable elevation of responding when the brightness phrasing cue was introduced in extinction, an effect said to be conceptually identical to spontaneous recovery and one demonstrating directly that phrasing cues are in reality overshadowing cues. It was suggested that many effects attributed to forgetting may be due to unrecognized overshadowing of memory cues by phrasing cues, giving rise to exaggerated estimates of forgetting.
In these experiments, each rat received two different series of three runs each. The lone group in Experiment 1 received the series 10-0·10 and 10-0-0, where, for example, 10-0·10 means that the rat received three discrete runs (in a runway) that terminated in 10, 0, and 10 pellets, respectively. In Experiment 2, the series were 20-0-0and 0-0·20 for one group and 20-0·20 and 0·0-ofor another. Of primary concern in both experiments was the rat's anticipation, as measured by running speed, of 0 pellets on the middle, or second, run of each series. In each experiment, running speed to this O·pellet event was independent of reinforcement magnitude on the first run of each series and was greater, the greater the reinforcement magnitude on the third run of each series. These results indicate that on the second run of each series the rat was anticipating not merely the 0 pellets associated with that run (intraevent anticipation), but also the reinforcement magnitude associated with the future, third run of each series (interevent anticipation). These results are shown to be consistent with an 80Scognitive view of anticipation and inconsistent with an 8-R serial-chaining view of serial learning.
The hypothesis tested here was that the time elapsing between events may be a potent cue that accompanies the storage and retrieval of memories. If so, it follows that as the retention interval increases, recall may either increase or decrease, depending upon whether the retrieval interval is becoming more similar or less similar to the storage interval. This hypothesis was tested here employing rats in a runway. The target memory to be recalled was that of nonreward in an initial acquisition phase. How well the memory of nonreward was recalled in acquisition was measured in a subsequent extinction phase in all six experiments reported; faster extinction indicated poorer recall in acquisition. Consistent with the present hypothesis, it was found that regardless of whether the retention interval in extinction was long (10-20 min) or short (about 30 s), resistance to extinction was greater when in acquisition the storage and retrieval intervals were alike (both 30 s or 10-20 min) rather than different. The results obtained here ruled out four alternative explanations to the present hypothesis: a temporal discrimination view, time tagging the memory of nonreward, generalization decrement occasioned by a change in retention interval from acquisition to extinction, and what was called the last-trial hypothesis.Recall, as was first demonstrated experimentally by Ebbinghaus (1885Ebbinghaus ( /1964, may decrease as the retention interval increases. But recall, as recent studies of hypermnesia demonstrate, may also increase as retention interval increases (e.g.,
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