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.     

Pictorial similarity judgments and the organization of visual memory in the rhesus monkey

The organization of visual memory for pictures was studied in the rhesus monkey. Two monkeys were tested in a same/different task in which sequentially presented pictures were compared to each other in a pair-wise fashion. The resulting confusion matrixes were analyzed using a multidimensional scaling procedure to obtain two- and three-dimensional graphic representations of the stimulus space. In Experiment 1, the monkeys' confusion errors caused pictures of human and rhesus monkey faces to fall in the same region of multidimensional space, which suggested that the monkeys categorized facial stimuli. A similar effect was found for pictures of different types of fruit. Experiment 2 replicated the categorization of faces with a more diverse collection of human and nonhuman primate faces. Experiment 3 explored the fruit category by varying stimulus attributes orthogonally. The results from this experiment showed that both monkeys encoded the pictures in this category by type of fruit (apples or grapes) and color (red or yellow). Taken together, these studies indicate that rhesus monkeys will treat some classes of pictorial stimuli categorically in visual memory.     

Macaques’ (Macaca mulatta) use of numerical cues in maze trials

We tested the ability of number-trained rhesus monkeys to use Arabic numeral cues to discriminate between different series of maze trials and anticipate the final trial in each series. The monkeys prior experience with numerals also allowed us to investigate spontaneous transfer between series. A total of four monkeys were tested in two experiments. In both experiments, the monkeys were trained on a computerized task consisting of three reinforced maze trials followed by one nonreinforced trial. The goal of the maze was an Arabic numeral 3, which corresponded to the number of reinforced maze trials in the series. In experiment 1 (n=2), the monkeys were given probe trials of the numerals 2 and 4 and in experiment 2 (n=2), they were given probe trials of the numerals 2–8. The monkeys receiving the probe trials 2 and 4 showed some generalization to the new numerals and developed a pattern of performing more slowly on the nonreinforced trial than the reinforced trial before it for most series, indicating the use of the changing numeral cues to anticipate the nonreinforced trial. The monkeys receiving probe trials of the numerals 2–8 did not predict precisely when the nonreinforced trial would occur in each series, but they did incorporate the changing numerals into their strategy for performing the task. This study provides the first evidence that number-trained monkeys can use Arabic numerals to perform a task involving sequential presentations.     

Chunking during serial learning by a pigeon: I. Basic evidence

Chunking by pigeons was demonstrated by comparing performance on different types of lists. Experiment 1 showed that Groups II and IV (who learned lists in which colors and achromatic geometric forms were segregated: A----B----C----D'----E' and A----B----C----D----E', respectively) executed lists more rapidly than did Group I (who learned a homogeneous list of colors: A----B----C----D----E) or Groups II and III (who learned lists consisting of unsegregated colors and forms: A----B'----C----D'----E and A----B----C'----D----E, respectively). Experiment 2 showed that Groups II and IV tolerated interruptions of the list better than did Groups I, III, and V. The accuracy of responding of Groups I, III, and V decreased as a function of the duration of the interruption and the point in the sequence at which it occurred. The performance of Group II was unaffected by interruptions; Group IV was minimally affected. These results indicate that Groups II and IV organized their lists as ordered chunks.     

Learning by observation in rhesus monkeys

Habit memory provides us with a vast repertoire of learned rules, including stimulus-reward associations, that ensures fast and adapted decision making in daily life. Because we share this ability with monkeys, lesion and recording studies in rhesus macaques have played a key role in understanding the neural bases of individual trial-and-error habit learning. Humans, however, can learn new rules at a lower cost via observation of conspecifics. The neural properties underlying this more ecological form of habit learning remain unexplored, and it is unclear whether the rhesus macaque can be a useful model in this endeavor. We addressed this issue by testing four monkeys from the same social group in their usual semi-natural habitat using a well-established marker of habit memory, concurrent discrimination learning. Each monkey learned 24 lists of 10 object-reward associations each. For one list out of two, monkeys could observe the testing session of another member of the group prior to being tested with the same list themselves. Learning was faster for these lists than for those learned solely by trial-and-error. Errors to criterion (9/10 correct responses) were reduced by 39%, and faultless performance could be achieved for up to 5 of the 10 pairs. These data demonstrate that rhesus macaques spontaneously observe a conspecific learning new stimulus-reward associations, and substantially benefit from this observation. They ascertain that the neural underpinnings of socially-mediated forms of habit learning can be explored using the powerful tools of monkey research, including neurophysiological recordings.     

Capuchin monkeys’ use of human and conspecific cues to solve a hidden object-choice task

Learning by watching others can provide valuable information with adaptive consequences, such as identifying the presence of a predator or locating a food source. The extent to which nonhuman animals can gain information by reading the cues of others is often tested by evaluating responses to human gestures, such as a point, and less often evaluated by examining responses to conspecific cues. We tested whether ten brown capuchin monkeys (Cebus [Sapajus] apella) were able to use cues from monkeys and a pointing cue from a human to obtain hidden rewards. A monkey could gain access to a reward hidden in one of two locations by reading a cue from a conspecific (e.g., reaching) or a human pointing. We then tested whether they could transfer this skill from monkeys to humans, from humans to monkeys, and from one conspecific to another conspecific. One group of monkeys was trained and tested using a conspecific as the cue-giver and was then tested with a human cue-giver. The second group of monkeys was trained and tested with a human cue-giver and was then tested with a monkey cue-giver. Monkeys that were successful with a conspecific cue-giver were also tested with a novel conspecific cue-giver. Monkeys learned to use a human point and conspecific cues to obtain rewards. Monkeys that had learned to use the cues of a conspecific to obtain rewards performed significantly better than expected by chance when they were transferred to the cues of a novel conspecific. Monkeys that learned to use a human point to obtain rewards performed significantly better than expected by chance when tested while observing conspecific cues. Some evidence suggested that transferring between conspecific cue-givers occurred with more facility than transferring across species. Results may be explained by simple rules of association learning and stimulus generalization; however, spontaneous flexible use of gestures across conspecifics and between different species may indicate capuchins can generalize learned social cues within and partially across species.     

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.     

Serial expertise of rhesus macaques

Here we describe the development of serial expertise in 4 experimentally naive rhesus monkeys that learned, by trial and error, the correct order in which to respond to 3-, 4-, and 7-item lists of arbitrarily selected photographs. The probabilities of guessing the correct sequence on 3-, 4-, and 7-item lists were, respectively, 1/6, 1/24, and 1/5040. Each monkey became progressively more efficient at determining the correct order in which to respond on new lists. During subsequent testing, the subjects were presented with all possible pairs of the 28 items used to construct the four 7-item lists (excluding pairs of items that occupied the same ordinal position in different lists). Subjects responded to pairs from different lists in the correct order 91% of the time on the first trials on which these pairs were presented. These features of subjects' performance, which cannot be attributed to procedural memory, satisfy two criteria of declarative memory: rapid acquisition of new knowledge and flexible application of existing knowledge to a new problem.     

Colour versus quantity as cues in reverse-reward-competent squirrel monkeys (Saimiri sciureus)

To assess the relative salience of colour and quantity cues, squirrel monkeys previously trained to reach for the smaller of two quantities of food in a reverse-reward contingency task received colour discrimination training. After initial failure to discriminate between two colours of dots under a differential reinforcement regime, they learned the task when the S- colour was associated with zero reward. The monkeys then showed good retention on the original reverse-reward task of 1 versus 4 with pairs of dots presented in S+ or S- colours. However, on "mismatch" trials of 1S- versus 4S+ , only 2 of 4 monkeys tested showed a preference--1 monkey chose based on quantity, the other based on colour. Individual differences and the possible roles of overshadowing and blocking are discussed.     

Serial list linking by macaque monkeys (Macaca mulatta): list property limitations

Four sophisticated macaque monkeys (Macaca mulatta) learned 6 different, 15-item ordinal lists (via conditional, 2-choice discriminations) as part of a study assessing some properties of serial list memory in monkeys. After assuring that the first 3 lists were well retained, the researchers attempted to link these by training only the 2 end-item pairs that ordered all 45 items into an inclusive series. A 20-day test of possible pairings among these 45 items was then conducted. Subsequently, the other 3 lists were trained and tested for retention, but no link training was provided. Then, a test like the one that had followed linking was administered. Unlike previous outcomes with three 5-item lists, linking did not yield immediate merger of three 15-item lists into a 45-item list, although 45-item lists were acquired after sufficient exposure to the testing/training regimen under both linking and nonlinking conditions. List length as a limiting factor in linking suggested processing restrictions analogous to those observed in human list memory. Results supported further investigation of list-linking characteristics.     

Colour versus quantity as cues in reverse-reward-competent squirrel monkeys (Saimiri sciureus)

To assess the relative salience of colour and quantity cues, squirrel monkeys previously trained to reach for the smaller of two quantities of food in a reverse-reward contingency task received colour discrimination training. After initial failure to discriminate between two colours of dots under a differential reinforcement regime, they learned the task when the S? colour was associated with zero reward. The monkeys then showed good retention on the original reverse-reward task of 1 versus 4 with pairs of dots presented in S + or S? colours. However, on “mismatch” trials of 1S? versus 4S + , only 2 of 4 monkeys tested showed a preference—1 monkey chose based on quantity, the other based on colour. Individual differences and the possible roles of overshadowing and blocking are discussed.     

Squirrel monkeys (Saimiri sciureus) choose smaller food arrays: long-term retention, choice with nonpreferred food, and transposition

Squirrel monkeys (Saimiri sciureus) that had learned to reach toward 1 piece of food instead of 4 in a reverse-reward contingency were tested after an 8-month delay with no intervening relevant experiences. All monkeys except 1 continued to show inhibitory control by reliably reaching toward the smaller quantity, most of them doing so within 2 sessions. Performance was maintained when a low-preference food replaced prized foods as arrays and rewards. When the quantities 1 and 4 were replaced with different ones, there was strong evidence of transposition at group level, although individual differences in bias toward the smaller quantities became apparent. Individual differences in mastering the original task more than 8 months previously were quite stable, suggesting robustness in the operations required for this form of self-control.     

Judgment of conceptual identity in monkeys

Baboons (Papio anubis) were tested on categorization tasks at two different conceptual levels. The monkeys showed their ability (1) to judge as identical or different the objects belonging to two categories, on a perceptual basis, and (2) to perform a judgment of conceptual identity—that is, to use the same/different relation between two previously learned categories. This latter experiment represents the first demonstration of judgment of conceptual identity in a monkey species.     

Mapping Among Number Words,Numerals, and Nonsymbolic Quantities in Preschoolers

In mathematically literate societies, numerical information is represented in 3 distinct codes: a verbal code (i.e., number words); a digital, symbolic code (e.g., Arabic numerals); and an analogical code (i.e., quantities; Dehaene, 1992). To communicate effectively using these numerical codes, our understanding of number must involve an understanding of each representation as well as how they map to other representations. In the current study, we looked at 3- and 4-year-old children’s understanding of Arabic numerals in relation to both quantities and number words. The results suggest that the mapping between quantities and numerals is more difficult than the mapping between numerals and number words and between number words and quantities. Thus, we compared 2 competing models designed to investigate how children represent the meanings of Arabic numbers—whether numerals are mapped directly to the quantities they represent or instead if numerals are mapped to quantities indirectly via a direct mapping to number words. We found support for the latter suggesting that children may first map numerals to number words (another symbolic representation) and only through this mapping are numerals subsequently tied to the quantities they represent. In addition, unlike both mappings involving quantity, the mapping between the 2 symbolic representations of number (numerals and number words) was not set-size-dependent, therefore providing further evidence that children may map symbols to other symbols in the absence of a quantity referent. Together, the results provide new insight into the important processes involved in how children acquire an understanding of symbolic representations of number.     

A Capuchin monkey (<Emphasis Type="Italic">Cebus apella</Emphasis>) recognizes when people do and do not know the location of food

In a previous study, Kuroshima and colleagues demonstrated that capuchin monkeys (Cebus apella) learned to discriminate between a "knower" who inspected a box for food, and a "guesser" who did not. The aim of the present study was to specify whether the subjects learned a simple conditional discrimination or a causal relationship that seeing leads to knowing. In experiment 1, we introduced five types of novel containers to two subjects. Each container was of different shape and color. The subjects gradually learned to reach toward the container the knower suggested. In experiment 2, we diversified the behavior of the knower and the guesser. In experiment 3, in order to eliminate the possibility of discrimination based on differences in the magnitude and the complexity of two trainers, we equated their behaviors. One subject adapted to the novel behaviors of the knower and the guesser, successfully discriminating the two trainers. Thus this monkey clearly learned to use the inspecting action of the knower and the non-inspecting action of the guesser as a discriminative cue to recognize the baited container. This result suggests that one capuchin monkey learned to recognize the relationship between seeing and knowing.     

On the limits of the matching concept in monkeys (Cebus apella).

Two cebus monkeys, with many years of experience matching a variety of static visual stimuli (forms and colors) within a standard matching-to-sample paradigm, were trained to press a left lever when a pair of displayed static stimuli were the same and to press a right lever when they were different. After learning the same/different task, the monkeys were tested for transfer to dynamic visual stimuli (flashing versus steady green disks), with which they had no previous experience. Both failed to transfer to the dynamic stimuli. A third monkey, also with massive past experience matching static visual stimuli, was tested for transfer to the dynamic stimuli within our standard matching paradigm, and it, too, failed. All 3 subjects were unable to reach a moderate acquisition criterion despite as many as 52 sessions of training with the dynamic stimuli. These results provide further evidence that, in monkeys, the matching (or identity) concept has a very limited reach; they consequently do not support the view held by some theorists that an abstract matching concept based on physical similarity is a general endowment of animals.     

Reverse-reward learning in squirrel monkeys (Saimiri sciureus): retesting after 5 years, and assessment on qualitative transfer

Seven squirrel monkeys (Saimiri sciureus) previously trained on reverse-reward tasks were presented with the original "1-versus-4" task after a 5-year interval without reverse-reward experience (Experiment 1). None of them reliably selected the smaller food array; however, at around chance level, their performance was superior to when they were first exposed to the task almost 6 years previously, suggesting some long-term memory retention. One naive monkey consistently selected the larger array, as expected. In Experiment 2, trials consisting of 1 versus 1 piece of two qualitatively different types of food were interspersed among familiar 1-versus-4 trials. None of five monkeys tested reliably selected the less-preferred food to get the more preferred food as the reward, and one monkey scored below chance. However, when one piece of low-preference food was paired with four pieces of high-preference food (Experiment 3), all four monkeys tested avoided reaching for the latter and thereby obtained it as the reward; two monkeys obtained perfect scores on these trials. These two monkeys were trained on a specific qualitative reverse-reward pairing and then again tested on new pairings (Experiment 4), but transfer was incomplete. Compound trials that pit quantity against quality in novel ways appear taxing for squirrel monkeys, despite competence in reverse-reward on both dimensions separately.     

Is two a plural marker in early child language?

Is two ever a plural marker in child language? By some accounts, children bootstrap the distinction between the words one and two by observing their use with singular-plural marking (one ball/two balls). Others argue that the numeral two marks plurality before children begin using numerals to denote precise quantities. We tested the relation between numerals and singular-plural marking in English-speaking 2- and 3-year-olds by asking them to label sets of objects. When children were not prompted to use numerals they hardly ever did so, although they did frequently use plural marking. Thus, it does not appear that children spontaneously use numerals like two as plural markers. Also, children who used numerals when labeling sets were significantly more likely to use a plural marker than children who did not use numerals, suggesting that most children view plural marking as obligatory when numerals are used, rather than viewing the 2 forms as alternative markers of plurality. Finally, two was no more likely than other numerals to be used with unmarked nouns (i.e., as an alternative to the plural), suggesting that it does not have a special status as a plural marker. We conclude that two is not a plural marker in early child language.     

Representation of the numerosities 1-9 by rhesus macaques (Macaca mulatta)

Three rhesus monkeys (Macaca mulatta) were trained to respond to exemplars of 1, 2, 3, and 4 in an ascending, descending, or a nonmonotonic numerical order (1-->2-->3-->4, 4-->3-->2--1, 3-->1-->4-->2). The monkeys were then tested on their ability to order pairs of the novel numerosities 5-9. In Experiment 1, all 3 monkeys ordered novel exemplars of the numerosities 1-4 in ascending or descending order. The attempt to train a nonmonotonic order (3-->1-->4-->2) failed. In Experiment 2A, the 2 monkeys who learned the ascending numerical rule ordered pairs of the novel numerosities 5-9 on unreinforced trials. The monkey who learned the descending numerical rule failed to extrapolate the descending rule to new numerosities. In Experiment 2B all 3 monkeys ordered novel exemplars of pairs of the numerosities 5-9. Accuracy and latency of responding revealed distance and magnitude effects analogous to previous findings with human participants (R. S. Moyer & T. K. Landaeur, 1967). Collectively these studies show that monkeys represent the numerosities 1-9 on at least an ordinal scale.     

Ordinal representation of numeric quantities by brown capuchin monkeys (Cebus apella)

Using techniques established by E. M. Brannon and H. S. Terrace (2000) with rhesus macaques (Macaca mulatta), the authors tested the ability of brown capuchins (Cebus apella) to order arrays of items ranging in quantity from 1 to 9. Three monkeys were trained on a touch screen to select the quantities 1-4 in ascending order. The monkeys exhibited successful transfer of this ability to novel representations of the quantities 1-4 and to pairs of the novel quantities 5-9. Patterns of responding with respect to numeric distance and magnitude were similar to those seen in human subjects, suggesting the use of similar psychological processes. The capuchins demonstrated an ordinal representation of quantity equivalent to that shown in Old World monkeys.