What is learned during mastery of a serial task: associations between adjacent and remote items, associations between an item and its ordinal position, or both? A clear answer to this question is lacking in the literature on human serial memory because it is difficult to control for a “naive” subject's linguistic competence and extensive experience with serial tasks. In this article, we present evidence that rhesus monkeys encode the ordinal positions of items of an arbitrary list when there is no requirement to do so. First, monkeys learned four nonverbal lists (1–4), each containing four novel items (photographs of natural objects). The monkeys then learned four 4-item lists that were derived exclusively and exhaustively from Lists 1 through 4, one item from each list. On two derived lists, each item's original ordinal position was maintained. Those lists were acquired with virtually no errors. The two remaining derived lists, on which the original ordinal position of each item was changed, were as difficult to learn as novel lists. The immediate acquisition of lists on which ordinal position was maintained shows that knowledge of ordinal position can develop without the benefit of language, extensive list-learning experience, or explicit instruction to encode ordinal information.
Two rhesus monkeys were trained to learn eight 4-item lists, each composed of 4 different photographs. Lists were trained in successive phases: A, A----B, A----B----C, and A----B----C----D. After List 4, retention, as measured by the method of savings, was, on average, 66% (range: 44-84%). Indeed, all 4 lists could be recalled reliably during a single session with neither a decrement in accuracy nor an increase in the latency of responding to each item. Response latencies on a subset test employing all possible 2- and 3-item subsets of each 4-item list support the hypothesis that monkeys form linear representations of a list. Latencies to Item 1 of a subset varied directly with the position of that item in the original list. On List 1, latencies to Item 2 varied directly with the number of intervening items between Item 1 and Item 2 in the original list. During the acquisition of Lists 5-8, both Ss mastered the A----B and A----B----C phases of training in the minimum number of trials possible.
The capacity of nonhuman primates to actively modify the acoustic structure of existing sounds or vocalizations in their repertoire appears limited. Several studies have reported population or community differences in the acoustical structure of nonhuman primate long distance calls and have suggested vocal learning as a mechanism for explaining such variation. In addition, recent studies on great apes have indicated that there are repertoire differences between populations. Some populations have sounds in their repertoire that others have not. These differences have also been suggested to be the result of vocal learning. On yet another level great apes can, after extensive human training, also learn some species atypical vocalizations. Here we show a new aspect of great ape vocal learning by providing data that an orangutan has spontaneously (without any training) acquired a human whistle and can modulate the duration and number of whistles to copy a human model. This might indicate that the learning capacities of great apes in the auditory domain might be more flexible than hitherto assumed.
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