Rhesus monkeys (Macaca mulatta) enumerate large and small sequentially presented sets of items using analog numerical representations

Two rhesus monkeys selected the larger of two sequentially presented sets of items on a computer monitor. In Experiment 1, performance was related to the ratio of set sizes, and the monkeys discriminated between sets with up to 10 items. Performance was not disrupted when 1 set had fewer than 4 items and 1 set had more than 4 items, a critical trial type for differentiating object file and analog models of numerical representation. Experiment 2 controlled the interitem rate of presentation. Experiment 3 included some trials on which number and amount (visual surface area) offered conflicting cues. Experiment 4 varied the total duration of set presentation and the duration of item visibility. In all of the experiments, performance remained high, although total set presentation duration also acted as a partial cue for the monkeys. Overall, the data indicated that rhesus monkeys estimate the approximate number of items in sequentially presented sets and that they are not relying solely on nonnumerical cues such as rate, duration, or cumulative amount.     

Information seeking by rhesus monkeys (Macaca mulatta) and capuchin monkeys (Cebus apella)

Animal metacognition is an active, growing research area, and one part of metacognition is flexible information-seeking behavior. In Roberts et al. (2009), pigeons failed an intuitive information-seeking task. They basically refused, despite multiple fostering experiments, to view a sample image before attempting to find its match. Roberts et al. concluded that pigeons’ lack of an information-seeking capacity reflected their broader lack of metacognition. We report a striking species contrast to pigeons. Eight rhesus macaques and seven capuchin monkeys passed the Roberts et al. test of information seeking—often in their first testing session. Members of both primate species appreciated immediately the lack of information signaled by an occluded sample, and the need for an information-seeking response to manage the situation. In subsequent testing, macaques demonstrated flexible/varied forms of information management. Capuchins did not. The research findings bear on the phylogenetic distribution of metacognition across the vertebrates, and on the underlying psychological requirements for metacognitive and information-seeking performances.     

Rhesus monkeys (Macaca mulatta) and capuchin monkeys (Cebus apella) remember future responses in a computerized task

Planning is an important aspect of many daily activities for humans. Planning involves forming a strategy in anticipation of a future need. However, evidence that nonhuman animals can plan for future situations is limited, particularly in relation to the many other kinds of cognitive capacities that they appear to share with humans. One critical aspect of planning is the ability to remember future responses, or what is called prospective coding. Two monkey species (Macaca mulatta and Cebus apella) performed a series of computerized tasks that required encoding a future response at the outset of each trial. Monkeys of both species showed competence in all tests that were given, providing evidence that they anticipated future responses and that they appropriately engaged in those responses when the time was right for such responses. In addition, some tests demonstrated that monkeys even remembered future responses that were not as presently motivating as were other aspects of the task environment. These results indicated that monkeys could anticipate future responses and retain and implement those responses when appropriate.     

Generalization hypothesis of abstract-concept learning: learning strategies and related issues in Macaca mulatta, Cebus apella, and Columba livia

The generalization hypothesis of abstract-concept learning was tested with a meta-analysis of rhesus monkeys (Macaca mulatta), capuchin monkeys (Cebus apella), and pigeons (Columba livia) learning a same/different (S/D) task with expanding training sets. The generalization hypothesis states that as the number of training items increases, generalization from the training pairs will increase and could explain the subjects' accurate novel-stimulus transfer. By contrast, concept learning is learning the relationship between each pair of items; with more training items subjects learn more exemplars of the rule and transfer better. Having to learn the stimulus pairs (the generalization hypothesis) would require more training as the set size increases, whereas learning the concept might require less training because subjects would be learning an abstract rule. The results strongly support concept or rule learning despite severely relaxing the generalization-hypothesis parameters. Thus, generalization was not a factor in the transfer from these experiments, adding to the evidence that these subjects were learning the S/D abstract concept.     

Quantity judgments of sequentially presented food items by capuchin monkeys (Cebus apella)

Recent assessments have shown that capuchin monkeys, like chimpanzees and other Old World primate species, are sensitive to quantitative differences between sets of visible stimuli. In the present study, we examined capuchins’ performance in a more sophisticated quantity judgment task that required the ability to form representations of food quantities while viewing the quantities only one piece at a time. In three experiments, we presented monkeys with the choice between two sets of discrete homogeneous food items and allowed the monkeys to consume the set of their choice. In Experiments 1 and 2, monkeys compared an entirely visible food set to a second set, presented item-by-item into an opaque container. All monkeys exhibited high accuracy in choosing the larger set, even when the entirely visible set was presented last, preventing the use of one-to-one item correspondence to compare quantities. In Experiment 3, monkeys compared two sets that were each presented item-by-item into opaque containers, but at different rates to control for temporal cues. Some monkeys performed well in this experiment, though others exhibited near-chance performance, suggesting that this species’ ability to form representations of food quantities may be limited compared to previously tested species such as chimpanzees. Overall, these findings support the analog magnitude model of quantity representation as an explanation for capuchin monkeys’ quantification of sequentially presented food items.

Ordinal judgments of symbolic stimuli by capuchin monkeys (Cebus apella) and rhesus monkeys (Macaca mulatta): the effects of differential and nondifferential reward

Ordinal learning was investigated in capuchin monkeys (Cebus apella) and rhesus monkeys (Macaca mulatta). In Experiment 1, both species were presented with pairings of the Arabic numerals 0 to 9. Some monkeys were given food rewards equal to the value of the numeral selected and some were rewarded with a single pellet only for choosing the higher numeral within the pair. Both species learned to select the larger numeral, but only rhesus monkeys that were differentially rewarded performed above chance levels when presented with novel probe pairings. In Experiment 2, the monkeys were first presented with arrays of 5 familiar numerals (from the range 0 to 9) and then arrays of 5 novel letters (from the range A to J) with the same reward outcomes in place as in Experiment 1. Both species performed better with the numerals, suggesting that an ordinal sequence of all stimuli had been learned during Experiment 1, rather than a matrix of two-choice discriminations.     

Rhesus Monkeys (Macaca mulatta) Maintain Learning Set Despite Second-Order Stimulus-Response Spatial Discontiguity

The Psychological Record - In many discrimination-learning tests, spatial separation between stimuli and response loci disrupts performance in rhesus macaques. However, monkeys are unaffected by...     

Serial learning with wild card items by monkeys (Cebus apella): implications for knowledge of ordinal position

We investigated monkeys' knowledge of the ordinal positions of stimuli that formed a 5-item serial list, ABCDE, by means of wild card items (W) that could substitute for items in the original series. In Experiment 1, training with wild cards was given on 3-, 4-, and 5-item series. In the last of these series, the wild card substitutions created five wild card sequences, WBCDE through ABCDW. During the final 10 sessions of training with each of two different wild cards (Items x and Y), the 3 subjects were able to successfully complete almost 60% of the wild card sequences. In Experiment 2, the two wild cards were presented on the same trial in 10 different double wild card sequences (e.g., AXCDY). The 2 monkey subjects correctly completed about 59% of the double wild card sequences during the final two training sessions. The performance levels achieved on single and on double wild card sequences, although well below that observed on the baseline sequence ABCDE (90% or better), support the view that the monkeys possessed some knowledge regarding the ordinal position of each baseline item. Consequently, an associative chain interpretation, which does not provide for knowledge of ordinal position, falls short as a complete account of the monkey's capacity for serial learning.     

Representation of serial order in monkeys (Cebus apella)

Cebus monkeys were trained on a five-item serial learning task, symbolized as ABCDE; the initial stages of training were on the shorter subseries AB, ABC, and ABCD. To assess the monkeys' knowledge of the sequential position of each item, pair-wise tests were given to 2 subjects after acquisition of the ABCD series and to 4 subjects after reaching criterion on the ABCDE series. In both tests, the monkeys performed at high levels on the interior pairs, which were BC for the ABCD series, and BC, BD, and CD for the ABCDE series. These results, as well as the orderly relations observed in the pair-wise tests between first-item response latency and first-item position and between second-item response latency and number of missing items, indicated that the monkeys had developed a well-organized internal representation of the four- and five-item series. Although pigeons are also capable of learning four-item and five-item series, they apparently do not develop a comparable representational structure. The disparity between the monkeys' and pigeons' representational competence for serial order is predictable from the difference in their capacities for associative transitivity.     

How capuchin monkeys (Cebus apella) quantify objects and substances

Humans and nonhuman animals appear to share a capacity for nonverbal quantity representations. But what are the limits of these abilities? Results of previous research with human infants suggest that the ontological status of an entity as an object or a substance affects infants' ability to quantify it. We ask whether the same is true for another primate species-the New World monkey Cebus apella. We tested capuchin monkeys' ability to select the greater of two quantities of either discrete objects or a nonsolid substance. Participants performed above chance with both objects (Experiment 1) and substances (Experiment 2); in both cases, the observed performance was ratio dependent. This finding suggests that capuchins quantify objects and substances similarly and do so via analog magnitude representations.     

Sequential responding and planning in capuchin monkeys (Cebus apella)

Previous experiments have assessed planning during sequential responding to computer generated stimuli by Old World nonhuman primates including chimpanzees and rhesus macaques. However, no such assessment has been made with a New World primate species. Capuchin monkeys (Cebus apella) are an interesting test case for assessing the distribution of cognitive processes in the Order Primates because they sometimes show proficiency in tasks also mastered by apes and Old World monkeys, but in other cases fail to match the proficiency of those other species. In two experiments, eight capuchin monkeys selected five arbitrary stimuli in distinct locations on a computer monitor in a learned sequence. In Experiment 1, shift trials occurred in which the second and third stimuli were transposed when the first stimulus was selected by the animal. In Experiment 2, mask trials occurred in which all remaining stimuli were masked after the monkey selected the first stimulus. Monkeys made more mistakes on trials in which the locations of the second and third stimuli were interchanged than on trials in which locations were not interchanged, suggesting they had already planned to select a location that no longer contained the correct stimulus. When mask trials occurred, monkeys performed at levels significantly better than chance, but their performance exceeded chance levels only for the first and the second selections on a trial. These data indicate that capuchin monkeys performed very similarly to chimpanzees and rhesus monkeys and appeared to plan their selection sequences during the computerized task, but only to a limited degree.     

Prospective memory in the formation of learning sets by rhesus monkeys (Macaca mulatta)

In conventional discrimination learning-set formation, it is possible that rhesus monkeys (Macaca mulatta) learn to lay down prospective memories by anticipating the next trial and deciding in advance what choice will be made. To test this hypothesis, the authors administered discrimination problems with 24-hr intertrial intervals, predicting that these long intervals would disrupt or prevent the putative anticipation of the next trial. Confirming their expectation, the authors found no indication of learning-set formation under these conditions.     

Self-control and tool use in tufted capuchin monkeys (Cebus apella)

Self-control is defined as forgoing immediate gratification to obtain a greater reward. Tool use may relate to self-control because both behaviors may require foresight and deliberate control over one's actions. The authors assessed 20 capuchin monkeys (Cebus apella) for the ability to delay gratification in a tool task. Subjects were given rod-shaped food items that could either be consumed immediately or be carried to an apparatus and used to extract a more preferred food. The authors found that some monkeys were able to exhibit self-control. Monkeys with relatively more tool use experience demonstrated the greatest levels of self-control. These results indicate that capuchins are capable of delaying gratification when a higher quality reinforcer is present and that tool experience can influence levels of self-control in this task.     

Hand preference in the use and manufacture of tools by tufted capuchin (Cebus apella) and lion-tailed macaque (Macaca silenus) monkeys.

The purpose of this study was to examine hand preference in the use and manufacture of probing tools by tufted capuchin and lion-tailed macaque monkeys (Cebus apella and Macaca silenus). Hand preference for the tool-using sequence was noted in 4 of 5 capuchins (a left-hand preference was noted in 3 ss) and 3 of 4 macaques (a left-hand preference was noted in 2 ss). The monkeys frequently used bimanual action in the initial and final segments of the 3-component task and typically modified tools through coordinated action of the hands and teeth. These data provide evidence consistent with the hypotheses that (a) hemisphere specialization for manual control evolved in primates prior to intensive use of tools by early hominids and (b) monkeys prefer the left hand for precise, visually guided, manipulative actions.     

Implicit and explicit category learning by capuchin monkeys (Cebus apella)

Current theories of human categorization differentiate an explicit, rule-based system of category learning from an implicit system that slowly associates regions of perceptual space with response outputs. The researchers extended this theoretical differentiation to the category learning of New World primates. Four capuchins (Cebus apella) learned categories of circular sine-wave gratings that varied in bar spatial frequency and orientation. The rule-based and information-integration tasks, respectively, had one-dimensional and two-dimensional solutions. Capuchins, like humans, strongly dimensionalized the stimuli and learned the rule-based task more easily. The results strengthen the suggestion that nonhuman primates have some structural components of humans' capacity for explicit categorization, which in humans is linked to declarative cognition and consciousness. The results also strengthen the primate contrast to other vertebrate species that may lack the explicit system. Therefore, the results raise important questions about the origins of the explicit categorization system during cognitive evolution and about its overall phylogenetic distribution.     

Capuchin monkeys (Cebus apella) respond to video images of themselves

Many studies have used mirror-image stimulation in attempts to find self-recognition in monkeys. However, very few studies have presented monkeys with video images of themselves; the present study is the first to do so with capuchin monkeys. Six tufted capuchin monkeys were individually exposed to live face-on and side-on video images of themselves (experimental Phase 1). Both video screens initially elicited considerable interest. Two adult males looked preferentially at their face-on image, whereas two adult females looked preferentially at their side-on image; the latter elicited lateral movements and head-cocking. Only males showed communicative facial expressions, which were directed towards the face-on screen. In Phase 2 monkeys discriminated between real-time, face-on images and identical images delayed by 1 s, with the adult females especially preferring real-time images. In this phase both screens elicited facial expressions, shown by all monkeys. In Phase 3 there was no evidence of discrimination between previously recorded video images of self and similar images of a familiar conspecific. Although they showed no signs of explicit self-recognition, the monkeys’ behaviour strongly suggests recognition of the correspondence between kinaesthetic information and external visual effects. In species such as humans and great apes, this type of self-awareness feeds into a system that gives rise to explicit self-recognition.     

Extent and limits of the matching concept in monkeys (Cebus apella)

In Experiment 1, 8 monkeys, experimentally naive with regard to visual stimuli, were trained on identity matching with a two-sample set based on two-dimensional stimuli. On a subsequent test employing two new samples, 4 of the 8 applied the matching rule to the new sample stimuli (as defined by our transfer criterion), and 3 showed substantial savings in learning to match the new samples. Two of these 3 transferred the matching rule when given a second test with two new samples, and the third showed immediate and complete transfer when tested with a third pair of new stimuli. These results indicate a much stronger representation of the matching concept in monkeys than in pigeons, even when the conditions of assessment are reasonably comparable. In Experiment 2, however, 4 monkeys from Experiment 1 failed to transfer the matching rule to steady versus flashing green samples, indicating that the matching concept did not immediately extend beyond the general class of visual stimuli with which it was developed. These and related results in the literature suggest that representation of the matching concept in animals varies along a specificity-abstractness dimension, reflecting the degree to which the concept is tied to the conditions and context of its development.     

An assessment of memory awareness in tufted capuchin monkeys (Cebus apella)

Humans, apes, and rhesus monkeys demonstrate memory awareness by collecting information when ignorant and acting immediately when informed. In this study, five capuchin monkeys searched for food after either watching the experimenter bait one of four opaque tubes (seen trials), or not watching (unseen trials). Monkeys with memory awareness should look into the tubes before making a selection only on unseen trials because on seen trials they already know the location of the food. In Experiment 1, one of the five capuchins looked significantly more often on unseen trials. In Experiment 2, we ensured that the monkeys attended to the baiting by interleaving training and test sessions. Three of the five monkeys looked more often on unseen trials. Because monkeys looked more often than not on both trial types, potentially creating a ceiling effect, we increased the effort required to look in Experiment 3, and predicted a larger difference in the probability of looking between seen and unseen trials. None of the five monkeys looked more often on unseen trials. These findings provide equivocal evidence for memory awareness in capuchin monkeys using tests that have yielded clear evidence in humans, apes, and rhesus monkeys.     

A comparison of visual and auditory short-term memory in monkeys (Cebus apella)

Using a visual and an acoustic sample set that appeared to favour the auditory modality of the monkey subjects, in Experiment 1 retention gradients generated in closely comparable visual and auditory matching (go/no-go) tasks revealed a more durable short-term memory (STM) for the visual modality. In Experiment 2, potentially interfering visual and acoustic stimuli were introduced during the retention intervals of the auditory matching task. Unlike the case of visual STM, delay-interval visual stimulation did not affect auditory STM. On the other hand, delay-interval music decreased auditory STM, confirming that the monkeys maintained an auditory trace during the retention intervals. Surprisingly, monkey vocalizations injected during the retention intervals caused much less interference than music. This finding, which was confirmed by the results of Experiments 3 and 4, may be due to differential processing of “arbitrary” (the acoustic samples) and species-specific (monkey vocalizations) sounds by the subjects. Although less robust than visual STM, auditory STM was nevertheless substantial, even with retention intervals as long as 32 sec.