Rhesus monkeys (Macaca mulatta) immediately generalize the uncertain response

Rhesus monkeys (Macaca mulatta) have learned, like humans, to use an uncertain response adaptively under test conditions that create uncertainty, suggesting a metacognitive process by which human and nonhuman primates may monitor their confidence and alter their behavior accordingly. In this study, 4 rhesus monkeys generalized their use of the uncertain response, without additional training, to 2 familiar tasks (2-choice discrimination learning and mirror-image matching to sample) that predictably and demonstrably produce uncertainty. The monkeys were significantly less likely to use the uncertain response on trials in which the answer might be known. These results indicate that monkeys, like humans, know when they do not know and that they can learn to use a symbol as a generalized means for indicating their uncertainty.     

Rhesus macaques (Macaca mulatta) monitor uncertainty during numerosity judgments

Two rhesus macaques (Macaca mulatta) judged arrays of dots on a computer screen as having more or fewer dots than a center value that was never presented in trials. After learning a center value, monkeys were given an uncertainty response that let them decline to make the numerosity judgment on that trial. Across center values (3-7), errors occurred most often for sets adjacent in numerosity to the center value. The monkeys also used the uncertainty response most frequently on these difficult trials. A 2nd experiment showed that monkeys' responses reflected numerical magnitude and not the surface-area illumination of the displays. This research shows that monkeys' uncertainty-monitoring capacity extends to the domain of numerical cognition. It also shows monkeys' use of the purest uncertainty response possible, uncontaminated by any secondary motivator.     

Rhesus monkeys (Macaca mulatta) adaptively monitor uncertainty while multi-tasking

As researchers explore animals’ capacity for metacognition and uncertainty monitoring, some paradigms allow the criticism that animal participants—who are always extensively trained in one stimulus domain within which they learn to avoid difficult trials—use task-specific strategies to avoid aversive stimuli instead of responding to a generalized state of uncertainty like that humans might use. We addressed this criticism with an uncertainty-monitoring task environment in which four different task domains were interleaved randomly trial by trial. Four of five macaques (Macaca mulatta) were able to make adaptive uncertainty responses while multi-tasking, suggesting the generality of the psychological signal that occasions these responses. The findings suggest that monkeys may have an uncertainty-monitoring capacity that is like that of humans in transcending task-specific cues and extending simultaneously to multiple domains.     

Rhesus monkeys (Macaca mulatta) spontaneously compute addition operations over large numbers

Mathematics is a uniquely human capacity. Studies of animals and human infants reveal, however, that this capacity builds on language-independent mechanisms for quantifying small numbers (<4) precisely and large numbers approximately. It is unclear whether animals and human infants can spontaneously tap mechanisms for quantifying large numbers to compute mathematical operations. Moreover, all available work on addition operations in non-human animals has confounded number with continuous perceptual properties (e.g. volume, contour length) that correlate with number. This study shows that rhesus monkeys spontaneously compute addition operations over large numbers, as opposed to continuous extents, and that the limit on this ability is set by the ratio difference between two numbers as opposed to their absolute difference.     

Rhesus monkey (Macaca mulatta) cool sensitivity measured by a signal detection method

Two rhesus monkeys were trained to detect cool stimuli (decrements in skin temperature from the adapted temperature) presented to the palm of the left hand after the skin had been preadapted to a 33°C temperature. The procedure was analogous to the signal detection “yes-no” method used with human observers. Receiver operating characteristic curves were generated by varying the a priori probability of stimulus oecurrence. The proportion of hits and of false alarms increased with increases in the a priori probability of stimulation. The points of isosensitivity of both subjects for cool intensities of 0.3°C, but not at greater intensities, yielded straight lines with slopes of approximately one when plotted on normal-normal coordinates. The values of the d’_s detectability index decreased from 3.52 to 0.75 for one subject and from 2.81 1to 1.28 for the other as the stimulus intensity was decreased from 0.8° to 0.15° cooling from the adapting temperature. Cool thresholds for each subject of 0.19° and 0.12°C were computed from classical psychometric functions when threshold was defined as 50% hits at a 50% probability of stimulus occurrence. These thresholds are comparable to published reports of the cool threshold for the rhesus.     

Rhesus macaques (Macaca mulatta) categorize unknown conspecifics according to their dominance relations

The authors trained 3 adult male rhesus macaques (Macaca mulatta) to categorize pairs of unknown conspecifics presented in a video according to the dominance status of the videotaped monkeys. The subjects were trained to choose the dominant monkey for a category of films (e.g., films showing 1 monkey chasing another); then, new films were presented involving different conspecifics, and the monkeys' first responses to this new category of behavior (e.g., monkeys fighting) were taken as evidence of transfer. Two subjects were able to generalize categorical judgments of dominance to new films involving new behaviors. These findings seem to indicate that monkeys can use abstract social concepts and are aware of the social relationships within their group.     

Rhesus macaques (Macaca mulatta) as living fossils of hominoid personality and subjective well-being

Personality dimensions capturing individual differences in behavior, cognition, and affect have been described in several species, including humans, chimpanzees, and orangutans. However, comparisons between species are limited by the use of different questionnaires. We asked raters to assess free-ranging rhesus macaques at two time points on personality and subjective well-being questionnaires used earlier to rate chimpanzees and orangutans. Principal-components analysis yielded domains we labeled Confidence, Friendliness, Dominance, Anxiety, Openness, and Activity. The presence of Openness in rhesus macaques suggests it is an ancestral characteristic. The absence of Conscientiousness suggests it is a derived characteristic in African apes. Higher Confidence and Friendliness, and lower Anxiety were prospectively related to subjective well-being, indicating that the connection between personality and subjective well-being in humans, chimpanzees, and orangutans is ancestral in catarrhine primates. As demonstrated here, each additional species studied adds another fold to the rich, historical story of primate personality evolution.     

Rhesus macaques (Macaca mulatta) spontaneously generalize to novel quantities in a reverse-reward contingency task

Abstracting generalities from concrete experience allows the application of acquired knowledge to novel situations, a hallmark of primate cognition. Abstraction may also enable some animals to overcome prepotent biases, by allowing them to treat prepotent stimuli and responses more flexibly. The aim of the current study was to determine whether rhesus macaques (Macaca mulatta) could generalize successful performance on an executive control task with one training exemplar to novel exemplars. Three monkeys learned a reverse-reward task in which they chose between one and four food items. They had to select the smaller quantity to receive the larger one, and so had to inhibit the prepotent selection of the larger quantity. After they learned the task, a transfer test assessed whether they had learned only about the quantities experienced or whether they could generalize to novel quantities. All three rhesus monkeys spontaneously generalized to novel quantities, showing that this species has the ability to generalize significantly beyond the immediate perceptual experience and use this ability to control lower-level, prepotent responses.     

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.     

Rhesus monkeys (Macaca mulatta), video tasks, and implications for stimulus-response spatial contiguity

Recent reports (Iwai, Yaginuma, & Mishkin, 1986; Yaginuma & Iwai, 1986) have supported the earlier conclusion by Meyer, Treichler, and Meyer (1965) and by Stollnitz (1965) that the efficiency of primate learning is compromised to the degree that there is spatial discontiguity between discriminanda and the locus of response. The research reported in this article calls for a reconsideration of this conclusion. Two rhesus monkeys (Macaca mulatta) easily mastered precise control of a joystick to respond to a variety of computer-generated targets despite the fact that the joystick was located 9 to 18 cm from the video screen. We hold that stimulus-response contiguity is a significant parameter of learning only to the degree that the monkey visually attends to the directional movements of its hand in order to displace discriminanda as in the Wisconsin General Test Apparatus. If, instead, attention is focused on the effects of the hand's movement rather than on the hand itself, stimulus-response contiguity is no longer a primary parameter of learning. The implications of this work for mirror-guided studies are discussed.     

Prototype abstraction by monkeys (Macaca mulatta)

The authors analyze the shape categorization of rhesus monkeys (Macaca mulatta) and the role of prototype- and exemplar-based comparison processes in monkeys' category learning. Prototype and exemplar theories make contrasting predictions regarding performance on the Posner-Homa dot-distortion categorization task. Prototype theory--which presumes that participants refer to-be-categorized items to a representation near the category's center (the prototype)--predicts steep typicality gradients and large prototype-enhancement effects. Exemplar theory--which presumes that participants refer to-be-categorized items to memorized training exemplars-predicts flat typicality gradients and small prototype-enhancement effects. Across many categorization tasks that, for the first time, assayed monkeys' dot-distortion categorization, monkeys showed steep typicality gradients and large prototype-enhancement effects. These results suggest that monkeys--like humans--refer to-be-categorized items to a prototype-like representation near the category's center rather than to a set of memorized training exemplars.     

Monkeys (Macaca mulatta and Cebus apella) track, enumerate, and compare multiple sets of moving items

Despite many demonstrations of numerical competence in nonhuman animals, little is known about how well animals enumerate moving stimuli. In this series of experiments, rhesus monkeys (Macaca mulatta) and capuchin monkeys (Cebus apella) performed computerized tasks in which they had to enumerate sets of stimuli. In Experiment 1, rhesus monkeys compared two sets of moving stimuli. Experiment 2 required comparisons of a moving set and a static set. Experiment 3 included human participants and capuchin monkeys to assess all 3 species' performance and to determine whether responding was to the numerical properties of the stimulus sets rather than to some other stimulus property such as cumulative area. Experiment 4 required both monkey species to enumerate subsets of each moving array. In all experiments, monkeys performed above chance levels, and their responses were controlled by the number of items in the arrays as opposed to nonnumerical stimulus dimensions. Rhesus monkeys performed comparably to adult humans when directly compared although capuchin performance was lower.     

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.     

Ordinal judgments of numerical symbols by macaques (Macaca mulatta)

Two rhesus monkeys (Macaca mulatta) learned that the arabic numerals 0 through 9 represented corresponding quantities of food pellets. By manipulating a joystick, the monkeys were able to make a selection of paired numerals presented on a computer screen. Although the monkeys received a corresponding number of pellets even if the lesser of the two numerals was selected, they learned generally to choose the numeral of greatest value even when pellet delivery was made arrhythmic. In subsequent tests, they chose the numerals of greater value when presented in novel combinations or in random arrays of up to five numerals. Thus, the monkeys made ordinal judgments of numerical symbols in accordance with their absolute or relative values.     

Effects of competition on video-task performance in monkeys (Macaca mulatta)

The effects of competition on performance of a video-formatted task were examined in a series of experiments. Two rhesus monkeys (Macaca mulatta) were trained to manipulate a joystick to shoot at moving targets on a computer screen. The task was made competitive by requiring both animals to shoot at the same target and by rewarding only the animal that hit the target first each trial. The competitive task produced a significant and robust speed-accuracy trade-off in performance. The monkeys hit the target in significantly less time on contested than on uncontested trials. However, they required significantly more shots to hit the target on contested trials in relation to uncontested trials. This effect was unchanged when various schedules of reinforcement were introduced in the uncontested trials. This supports the influence of competition qua competition on performance, a point further bolstered by other findings of of behavioral contrast presented here.     

Retrospective and prospective metacognitive judgments in rhesus macaques (Macaca mulatta)

A growing body of research suggests that some non-human animals are capable of making accurate metacognitive judgments. In previous studies, non-human animals have made either retrospective or prospective judgments (about how they did on a test or how they will do on a test, respectively). These two types of judgments are dissociable in humans. The current study tested the abilities of two rhesus macaque monkeys to make both retrospective and prospective judgments about their performance on the same memory task. Both monkeys had been trained previously to make retrospective confidence judgments. Both monkeys successfully demonstrated transfer of retrospective metacognitive judgments to the new memory task. Furthermore, both monkeys transferred their retrospective judgments to the prospective task (one, immediately, and one, following the elimination of a response bias). This study is the first to demonstrate both retrospective and prospective monitoring abilities in the same monkeys and on the same task, suggesting a greater level of flexibility in animals’ metacognitive monitoring abilities than has been reported previously.     

Perceptual completion in rhesus monkeys (Macaca mulatta) and pigeons (Columbia livia)

In a two-dimensional drawing, when the narrow edge of a bar appears to touch the edge of a large rectangle, humans overestimate the length of the bar (Kanizsa, 1979). Kanizsa has suggested that this illusion occurs because humans perceive the bar as continuing behind the rectangle and complete the "occluded" portion of the bar. Rhesus monkeys and pigeons were trained to classify black target bars with a variety of lengths as "long" or "short." In training, the bar was always located at the same distance from a gray box. After learning this discrimination, the subjects were tested on novel stimuli, in which the bar was located at three new locations. Monkeys showed a consistent response bias for "long" when the bar touched the box, but pigeons did not. Monkeys appear to have completed the "occluded" part like humans, whereas pigeons failed to do so. Because this procedure does not require animals to complete the "occluded" part with any particular form, their failure suggests that pigeons do not even perceive the target bar as continuing behind the "occluding" figure. The failure of pigeons may be due to difficulty in perceiving depth from two-dimensional drawings.