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.

Erratum to: Chimpanzees (Pan troglodytes) accurately compare poured liquid quantities

Although many studies have shown that nonhuman animals can choose the larger of two discrete quantities of items, less emphasis has been given to discrimination of continuous quantity. These studies are necessary to discern the similarities and differences in discrimination performance as a function of the type of quantities that are compared. Chimpanzees made judgments between continuous quantities (liquids) in a series of three experiments. In the first experiment, chimpanzees first chose between two clear containers holding differing amounts of juice. Next, they watched as two liquid quantities were dispensed from opaque syringes held above opaque containers. In the second experiment, one liquid amount was presented by pouring it into an opaque container from an opaque syringe, whereas the other quantity was visible the entire time in a clear container. In the third experiment, the heights at which the opaque syringes were held above opaque containers differed for each set, so that sometimes sets with smaller amounts of juice were dropped from a greater height, providing a possible visual illusion as to the total amount. Chimpanzees succeeded in all tasks and showed many similarities in their continuous quantity estimation to how they performed previously in similar tasks with discrete quantities (for example, performance was constrained by the ratio between sets). Chimpanzees could compare visible sets to nonvisible sets, and they were not distracted by perceptual illusions created through various presentation styles that were not relevant to the actual amount of juice dispensed. This performance demonstrated a similarity in the quantitative discrimination skills of chimpanzees for continuous quantities that matches that previously shown for discrete quantities.     

Chimpanzees (Pan troglodytes) accurately compare poured liquid quantities

Although many studies have shown that nonhuman animals can choose the larger of two discrete quantities of items, less emphasis has been given to discrimination of continuous quantity. These studies are necessary to discern the similarities and differences in discrimination performance as a function of the type of quantities that are compared. Chimpanzees made judgments between continuous quantities (liquids) in a series of three experiments. In the first experiment, chimpanzees first chose between two clear containers holding differing amounts of juice. Next, they watched as two liquid quantities were dispensed from opaque syringes held above opaque containers. In the second experiment, one liquid amount was presented by pouring it into an opaque container from an opaque syringe, whereas the other quantity was visible the entire time in a clear container. In the third experiment, the heights at which the opaque syringes were held above opaque containers differed for each set, so that sometimes sets with smaller amounts of juice were dropped from a greater height, providing a possible visual illusion as to the total amount. Chimpanzees succeeded in all tasks and showed many similarities in their continuous quantity estimation to how they performed previously in similar tasks with discrete quantities (for example, performance was constrained by the ratio between sets). Chimpanzees could compare visible sets to nonvisible sets, and they were not distracted by perceptual illusions created through various presentation styles that were not relevant to the actual amount of juice dispensed. This performance demonstrated a similarity in the quantitative discrimination skills of chimpanzees for continuous quantities that matches that previously shown for discrete quantities.     

Numerical judgments by chimpanzees (Pan troglodytes) in a token economy

We presented four chimpanzees with a series of tasks that involved comparing two token sets or comparing a token set to a quantity of food. Selected tokens could be exchanged for food items on a one-to-one basis. Chimpanzees successfully selected the larger numerical set for comparisons of 1 to 5 items when both sets were visible and when sets were presented through one-by-one addition of tokens into two opaque containers. Two of four chimpanzees used the number of tokens and food items to guide responding in all conditions, rather than relying on token color, size, total amount, or duration of set presentation. These results demonstrate that judgments of simultaneous and sequential sets of stimuli are made by some chimpanzees on the basis of the numerousness of sets rather than other non-numerical dimensions. The tokens were treated as equivalent to food items on the basis of their numerousness, and the chimpanzees maximized reward by choosing the larger number of items in all situations.     

Summation and numerousness judgments of sequentially presented sets of items by chimpanzees (Pan troglodytes).

Summation and numerousness judgments by 2 chimpanzees (Pan troglodytes) were investigated when 2 quantities of M&Ms were presented sequentially, and the quantities were never viewed in their totality. Each M&M was visible only before placement in 1 of 2 cups. In Experiment 1, sets of 1 to 9 M&Ms were presented. In Experiment 2, the quantities in each cup were presented as 2 smaller sets (e.g., 2 + 2 vs. 4 + 1). In Experiment 3, the quantities were presented as 3 smaller sets (e.g., 2 + 2 + 3 vs. 3 + 4 + 1). In Experiment 4, an M&M was removed from 1 set before the chimpanzees' selection. In Experiments 1 and 2, the chimpanzees selected the larger quantity on significantly more trials than would be predicted by chance. In Experiments 3 and 4, 1 chimpanzee performed at a level significantly better than chance. Therefore, chimpanzees mentally represent quantity and successfully combine and compare nonvisible, sequentially presented sets of items.     

Chimpanzees (Pan troglodytes) respond to nonvisible sets after one-by-one addition and removal of items

Two chimpanzees (Pan troglodytes) made numerousness judgments of nonvisible sets of items. In Experiment 1, 1-10 items were dropped 1 at a time into an opaque cup, and then an additional 1-10 items were dropped 1 at a time into another opaque cup. The chimpanzees' performance levels were high and were more dependent on factors indicative of an analogue-magnitude mechanism for representation of set size than on an object file mechanism. In Experiment 2, a 3rd visible set was made available after the sequential presentation of the first 2 sets. The chimpanzees again performed at high levels in selecting the largest of the 3 sets. In Experiment 3, 1 of the 2 initially presented sets was reduced in number by the sequential removal of 1, 2, or 3 items. Both chimpanzees performed above chance levels for the removal of 1, but not more than 1, item.     

Natural choice in chimpanzees (Pan troglodytes): Perceptual and temporal effects on selective value

In three experiments, four chimpanzees made choices between two visible food options to assess the validity of the selective value effect (the assignment of value to only the most preferred type of food presented in a comparison). In Experiment 1, we established that all chimpanzees preferred single banana pieces to single apple pieces before presenting the critical test. In this test two chimpanzees preferred a mix of one banana piece and one apple piece to a single banana piece when both banana pieces were approximately the same size, but two chimpanzees were indifferent between the two options, exhibiting the selective value effect. In Experiment 2, when the banana pieces in both options were more closely equated in size, the chimpanzees were biased to choose the single banana piece over the mixed array even though this was the smaller total amount of food. However, in Experiment 3, when we introduced longer intervals between each trial, the chimpanzees preferred the mixed set and thus the larger total amount of food. The results demonstrate that only some chimpanzees exhibit the choice pattern indicative of the selective value effect, and they do so only when item size is not carefully controlled and trials are presented quickly in succession. Thus, the behavior pattern originally labeled the selective value effect may actually be explained by a combination of chimpanzees’ sensitivity to small differences in preferred food amount and chimpanzees tendency to avoid less preferred foods that would delay the acquisition of further preferred food items.     

Parallel scanning of auditory and visual information

Two experiments involving memory retrieval of auditorilv and visually presented materials were performed. In Experiment I, subjects were presented with memory sets of 1, 2, or 4 stimuli and then with a test item to be classified as belonging or not belonging to the memory set. In Condition 1, each memory stimulus was a single, auditorily presented letter. In Condition 2, each memory stimulus was a visually presented letter. In Conditions 3 and 4, each memory stimulus was a pair of letters, one presented visually and the other auditorily. Mean reaction time (RT) for the classification task increased as a function of number of memory stimuli at equal rates for all four conditions. This was interpreted as evidence for a parallel scanning process in Conditions 3 and 4 where the auditory item and visual item of each memory stimulus pair can be scanned simultaneously. Experiment II compared memory retrieval for a simultaneous condition in which auditory and visual memory items were presented as pairs with a sequential condition in which mixed auditory-visual memory sets were presented one item at a time. RTs were shorter for the simultaneous condition. This was interpreted as evidence that parallel scanning may depend upon memory input parameters.     

A temporal distinctiveness theory of recency and modality effects

A temporal distinctiveness theory of contextually cued retrieval from memory is presented and applied to recency and modality effects. According to this theory, one part of the mnemonic trace of an item is a representation of the item's time of presentation. Time of presentation may be encoded with a coarse grain (so that it is consistent with a wide range of times) or with a fine grain (so that it is consistent with a narrow range of times). Retrieval proceeds by constructing temporally defined search sets that include representations of items consistent with the temporal bounds of the search set. The temporal width of the search set increases as the retention interval increases. Recency effects arise from retrieval of recently presented items from narrow search sets that include representations of few items; within the context of the search set, these items are distinctive and recalled well. Superiority in recall of recently presented auditory information in comparison with recently presented visual information is attributed to differences in the grain of time of presentation representations for aurally (fine grain) and visually (coarse grain) presented information. Four experiments confirm qualitative and quantitative predictions of the theory, including the prediction of auditory superiority at the beginning of the list when the initial items are temporally distinct.     

Ordinal judgments and summation of nonvisible sets of food items by two chimpanzees and a rhesus macaque

Two chimpanzees and a rhesus macaque rapidly learned the ordinal relations between 5 colors of containers (plastic eggs) when all containers of a given color contained a specific number of identical food items. All 3 animals also performed at high levels when comparing sets of containers with sets of visible food items. This indicates that the animals learned the approximate quantity of food items in containers of a given color. However, all animals failed in a summation task, in which a single container was compared with a set of 2 containers of a lesser individual quantity but a greater combined quantity. This difficulty was not overcome by sequential presentation of containers into opaque receptacles, but performance improved if the quantitative difference between sizes was very large.     

How the great apes (Pan troglodytes, Pongo pygmaeus, Pan paniscus, and Gorilla gorilla) perform on the reversed contingency task: the effects of food quantity and food visibility

S. T. Boysen and G. G. Berntson (1995) found that chimpanzees performed poorly on a reversed contingency task in which they had to point to the smaller of 2 food quantities to acquire the larger quantity. The authors compared the performance of 4 great ape species (Pan troglodytes, Pongo pygmaeus, Pan paniscus, and Gorilla gorilla) on the reversed contingency task while manipulating food quantity (0-4 or 1-4) and food visibility (visible pairs or covered pairs). Results showed no systematic species differences but large individual differences. Some individuals of each species were able to solve the reversed contingency task. Both quantity and visibility of the food items had a significant effect on performance. Subjects performed better when the disparity between quantities was smaller and the quantities were not directly visible.     

Effects of Auditory/Visual and Lexical/Non-lexical Comparisons on Event-related Potentials in a Memory-scanning Task

A late parietal positivity (P3) and behavioural measures were studied during performance of a two-item memory-scanning task. Stimuli were digits presented as memorised items in one modality (auditory or visual) while the following probe, also a digit, was presented in the same or the other modality. In a separate set of experiments, P3 and behaviour were similarly studied using only visual stimuli that were either lexical (digits) or non-lexical (novel fonts with the same contours as the digits) to which subjects assigned numerical values. Reaction times (RTs) and P3 latencies were prolonged to non-lexical compared to lexical stimuli. Although RTs were longer to auditory than to visual stimuli, P3 latencies to memorised items were prolonged in response to visually compared to auditorily presented memorised items, and were further prolonged when preceding visual probes. P3 amplitudes were smaller to auditory than to visual stimuli, and were smaller for the second memorised item when lexical/non-lexical comparisons were involved. The most striking finding was scalp distribution variations indicating changes in relative contributions of brain structures involved in processing memorised items, according to the probes that followed. These findings are compatible, in general, with a phonological memorisation, but they suggest that the process is modified by memorising the item in the same terms as the expected probe that follows.     

Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) show the isolation effect during serial list recognition memory tests

The isolation effect (or von Restorff effect) occurs when one item in a to-be-remembered list is distinctive from all remaining items, and memory for that item is enhanced. Four chimpanzees were presented with a serial list of four photographs. In the homogeneous condition, all list items were from the same semantic category (e.g., four fruits). In the isolate condition, three items were from the same category, but the fourth item (the isolate) was from a different category (e.g., three fruits and one toy). Then, two photographs were presented, and the chimpanzees had to select the one that was from the list. Two of four chimpanzees were significantly more likely to select a correct isolate item than an item from the same list position in the homogeneous condition for at least some list positions. This facilitation in performance was for isolate items only, as presenting an isolate item in a list did not facilitate greater recognition of other list items compared to the homogeneous condition. These results indicated that some chimpanzees perceived the semantic categories of the photographs, and categorization of photographs led to the isolation effect. Thus, chimpanzees may share with humans some aspects of memory organization that involve spontaneously categorizing visual stimuli and recognizing categorically unique stimuli.     

More or less: spontaneous quantity discrimination in the domestic cat

We examined spontaneous quantity discrimination in untrained domestic cats in three food choice experiments. In Experiment 1, we presented the cats with two different quantities of food in eight numerical combinations. Overall, the subjects chose the larger quantity more often than the smaller one, and significantly so when the ratio between the quantities was less than 0.5. In Experiment 2, we presented the cats with two pieces of food in four different size combinations. Again, subjects chose the larger piece above chance, although not in the combination where the largest item was presented. In Experiment 3, a subset of the cats was presented multiple times with two different quantities of food, which were hidden from view. In this case, the cats did not choose the larger quantity more often than the smaller one, suggesting that in the present experiments they mainly used visual cues when comparing quantities. We conclude that domestic cats are capable of spontaneously discriminating quantities when faced with different numbers or sizes of food items, and we suggest why they may not always be motivated to choose the larger quantity. In doing so, we highlight the advantages of testing spontaneous choice behavior, which is more likely to reflect animals’ everyday manner of responding than is the case when training them in order to test their absolute limits of performance which may not always coincide with their daily needs.     

Delaying gratification for food and tokens in capuchin monkeys (Cebus apella) and chimpanzees (Pan troglodytes): when quantity is salient, symbolic stimuli do not improve performance

Capuchin monkeys have been tested for the capacity to delay gratification for accumulating rewards in recent studies and have exhibited variable results. Meanwhile, chimpanzees have consistently excelled at this task. However, neither species have ever been tested at accumulating symbolic tokens instead of food items, even though previous reports indicate that tokens sometimes facilitate performance in other self-control tasks. Thus, in the present study, we tested capuchin monkeys and chimpanzees for their capacity to delay gratification in a delay maintenance task, in which an experimenter presented items, one at a time, to within reach of an animal for as long as the animal refrained from taking them. In Experiment 1, we assessed how long capuchin monkeys could accumulate items in the delay maintenance task when items were food rewards or tokens exchangeable for food rewards. Monkeys accumulated more food rewards than they did tokens. In Experiment 2, we tested capuchin monkeys and chimpanzees in a similar accumulation test. Whereas capuchins again accumulated more food than tokens, all chimpanzees but one showed no difference in performance in the two conditions. These findings provide additional evidence that chimpanzees exhibit greater self-control capacity in this task than do capuchin monkeys and indicate that symbolic stimuli fail to facilitate delay maintenance when they do not abstract away from the quantitative dimension of the task. This is consistent with previous findings on the effects of symbols on self-control and illuminates what makes accumulation a particularly challenging task.     

Two are better than one: comparison influences infants' visual recognition memory

Despite a large literature on infants’ memory for visually presented stimuli, the processes underlying visual memory are not well understood. Two studies with 4-month-olds (N = 60) examined the effects of providing opportunities for comparison of items on infants’ memory for those items. Experiment 1 revealed that 4-month-olds failed to show evidence of memory for an item presented during familiarization in a standard task (i.e., when only one item was presented during familiarization). In Experiment 2, infants showed robust memory for one of two different items presented during familiarization. Thus, infants’ memory for the distinctive features of individual items was enhanced when they could compare items.     

Inferences by exclusion in the great apes: the effect of age and species

This study investigated the ability of chimpanzees, gorillas, orangutans, and bonobos to make inferences by exclusion using the procedure pioneered by Premack and Premack (Cognition 50:347–362, 1994) with chimpanzees. Thirty apes were presented with two different food items (banana vs. grape) on a platform and covered with identical containers. One of the items was removed from the container and placed between the two containers so that subjects could see it. After discarding this item, subjects could select between the two containers. In Experiment 1, apes preferentially selected the container that held the item that the experimenter had not discarded, especially if subjects saw the experimenter remove the item from the container (but without seeing the container empty). Experiment 3 in which the food was removed from one of the containers behind a barrier confirmed these results. In contrast, subjects performed at chance levels when a stimulus (colored plastic chip: Exp. 1; food item: Exp. 2 and Exp. 3) designated the item that had been removed. These results indicated that apes made inferences, not just learned to use a discriminative cue to avoid the empty container. Apes perceived and treated the item discarded by the experimenter as if it were the very one that had been hidden under the container. Results suggested a positive relationship between age and inferential ability independent of memory ability but no species differences.This contribution is part of the special issue “Animal Logics” (Watanabe and Huber 2006).     

Discrete quantity judgments in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus): the effect of presenting whole sets versus item-by-item

The authors examined quantity-based judgments for up to 10 items for simultaneous and sequential whole sets as well as for sequentially dropped items in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), bonobos (Pan paniscus), and orangutans (Pongo pygmaeus). In Experiment 1, subjects had to choose the larger of 2 quantities presented in 2 separate dishes either simultaneously or 1 dish after the other. Representatives of all species were capable of selecting the larger of 2 quantities in both conditions, even when the quantities were large and the numerical distance between them was small. In Experiment 2, subjects had to select between the same food quantities sequentially dropped into 2 opaque cups so that none of the quantities were ever viewed as a whole. The authors found some evidence (albeit weaker) that subjects were able to select the larger quantity of items. Furthermore, the authors found no performance breakdown with the inclusion of certain quantities. Instead, the ratio between quantities was the best performance predictor. The authors conclude that quantity-based judgments rely on an analogical system, not a discrete object file model or perceptual estimation mechanism, such as subitizing.     

Chimpanzees (Pan troglodytes) transfer tokens repeatedly with a partner to accumulate rewards in a self-control task

There has been extensive research investigating self-control in humans and nonhuman animals, yet we know surprisingly little about how one’s social environment influences self-control. The present study examined the self-control of chimpanzees in a task that required active engagement with conspecifics. The task consisted of transferring a token back and forth with a partner animal in order to accumulate food rewards, one item per token transfer. Self-control was required because at any point in the trial, either chimpanzee could obtain their accumulated rewards, but doing so discontinued the food accumulation and ended the trial for both individuals. Chimpanzees readily engaged the task and accumulated the majority of available rewards before ending each trial, and they did so across a number of conditions that varied the identity of the partner, the presence/absence of the experimenter, and the means by which they could obtain rewards. A second experiment examined chimpanzees’ self-control when given the choice between immediately available food items and a potentially larger amount of rewards that could be obtained by engaging the token transfer task with a partner. Chimpanzees were flexible in their decision-making in this test, typically choosing the option representing the largest amount of food, even if it involved delayed accumulation of the rewards via the token transfer task. These results demonstrate that chimpanzees can exhibit self-control in situations involving social interactions, and they encourage further research into this important aspect of the self-control scenario.     

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.