Stacking of blocks by chimpanzees: developmental processes and physical understanding

The stacking-block task has been used to assess cognitive development in both humans and chimpanzees. The present study reports three aspects of stacking behavior in chimpanzees: spontaneous development, acquisition process following training, and physical understanding assessed through a cylindrical-block task. Over 3 years of longitudinal observation of block manipulation, one of three infant chimpanzees spontaneously started to stack up cubic blocks at the age of 2 years and 7 months. The other two infants began stacking up blocks at 3 years and 1 month, although only after the introduction of training by a human tester who rewarded stacking behavior. Cylindrical blocks were then introduced to assess physical understanding in object-object combinations in three infant (aged 3-4) and three adult chimpanzees. The flat surfaces of cylinders are suitable for stacking, while the rounded surface is not. Block manipulation was described using sequential codes and analyzed focusing on failure, cause, and solution in the task. Three of the six subjects (one infant and two adults) stacked up cylindrical blocks efficiently: frequently changing the cylinders' orientation without contacting the round side to other blocks. Rich experience in stacking cubes may facilitate subjects' stacking of novel, cylindrical shapes from the beginning. The other three subjects were less efficient in stacking cylinders and used variable strategies to achieve the goal. Nevertheless, they began to learn the effective way of stacking over the course of testing, after about 15 sessions (75 trials).     

Monkeys perform as well as apes and humans in a size discrimination task

Whether the cognitive competences of monkeys and apes are rather similar or whether the larger-brained apes outperform monkeys in cognitive experiments is a highly debated topic. Direct comparative analyses are therefore essential to examine similarities and differences among species. We here compared six primate species, including humans, chimpanzees, bonobos, gorillas (great apes), olive baboons, and long-tailed macaques (Old World monkeys) in a task on fine-grained size discrimination. Except for gorillas, subjects of all taxa (i.e. humans, apes, and monkeys) were able to discriminate three-dimensional cubes with a volume difference of only 10 % (i.e. cubes of 50 and 48 mm side length) and performed only slightly worse when the cubes were presented successively. The minimal size discriminated declined further with increasing time delay between presentations of the cubes, highlighting the difficulty to memorize exact size differences. The results suggest that differences in brain size, as a proxy for general cognitive abilities, did not account for variation in performance, but that differential socio-ecological pressures may better explain species differences. Our study highlights the fact that differences in cognitive abilities do not always map neatly onto phylogenetic relationships and that in a number of cognitive experiments monkeys do not fare significantly worse than apes, casting doubt on the assumption that larger brains per se confer an advantage in such kinds of tests.     

Effects of element separation on perceptual grouping by humans (Homo sapiens) and chimpanzees (Pan troglodytes): perception of Kanizsa illusory figures

The processing of Kanizsa-square illusory figures was studied in two experiments with four humans and two chimpanzees. Subjects of the two species were initially trained to select a Kanizsa-square illusory figure presented in a computerized two-alternative forced choice task. After training, adding narrow closing segments to the pacman inducers that composed the Kanisza illusory figures lowered performance in both chimpanzees and humans, suggesting that the discrimination could be controlled by the perception of illusory forms. A second experiment assessed transfer of performance with five sets of figures in which the size of the inducers and their separation were manipulated. Only for chimpanzees was performance directly controlled by separation, suggesting that chimpanzees are more sensitive than humans to the separation between visual elements. Accepted after revision: 15 August 2001 Electronic Publication     

Development of combinatory manipulation in chimpanzee infants (Pan troglodytes)

I made systematic observations of three infant chimpanzees aged 2–4 years, who participated in a series of diagnostic tests of combinatory manipulation. The tasks were stacking blocks, seriating nesting cups, and inserting an object into the corresponding hole in a plate or a box. These tasks were originally devised for developmental diagnosis of human infants. The chimpanzee infants displayed combinatory manipulation comparable to that of 1-year-old human infants. Common motor characteristics were observed across the tasks, namely "repetition" of actions, "adjustment" of actions, "reversal" of actions, and "shifts" of attention. Humans and chimpanzees share these actions when manipulating multiple objects to complete a task. Repetition, adjustment, and reversal of actions and shifts of attention underlie higher levels of cognition common to both species. Accepted after revision: 5 May 2001 Electronic Publication     

Spatial construction skills of chimpanzees (Pan troglodytes) and young human children (Homo sapiens sapiens)

Spatial construction tasks are basic tests of visual‐spatial processing. Two studies have assessed spatial construction skills in chimpanzees (Pan troglodytes) and young children (Homo sapiens sapiens) with a block modelling task. Study 1a subjects were three young chimpanzees and five adult chimpanzees. Study 1b subjects were 30 human children belonging to five age groups (24, 30, 36, 42, 48 months). Subjects were given three model constructions to reproduce: Line, Cross‐Stack and Arch, which differed in type and number of spatial relations and dimensions, but required comparable configurational understanding. Subjects’ constructions were rated for accuracy. Our results show that: (1) chimpanzees are relatively advanced in constructing in the vertical dimension; (2) Among chimpanzees only adults make accurate copies of constructions; (3) Chimpanzees do not develop in the direction of constructing in two dimensions as human children do starting from age 30 months. The pattern of development of construction skills in chimpanzees partially diverges from that of human children and indicates that spatial analysis and spatial representation are partially different in the two species.     

Probing the limits of tool competence: Experiments with two non-tool-using species (Cercopithecus aethiops and Saguinus oedipus)

Non-human animals vary in their ability to make and use tools. The goal of the present study was to further explore what, if anything, differs between tool-users and non-tool-users, and whether these differences lie in the conceptual or motor domain. We tested two species that typically do not use tools-cotton top tamarins (Saguinus oedipus) and vervet monkeys (Cercopithecus aethiops)-on problems that mirrored those designed for prolific tool users such as chimpanzees. We trained subjects on a task in which they could choose one of two canes to obtain an out-of-reach food reward. After training, subjects received several variations on the original task, each designed to examine a specific conceptual aspect of the pulling problem previously studied in other tool-using species. Both species recognized that effective pulling tools must be made of rigid materials. Subsequent conditions revealed significant species differences, with vervets outperforming tamarins across many conditions. Vervets, but not tamarins, had some recognition of the relationship between a tool's orientation and the position of the food reward, the relationship between a tool's trajectory and the substance that it moves on, and that tools must be connected in order to work properly. These results provide further evidence that tool-use may derive from domain-general, rather than domain-specific cognitive capacities that evolved for tool use per se.     

Perception of emergent configurations in humans (Homo sapiens) and chimpanzees (Pan troglodytes)

We examined the perceptions of emergent configurations in humans and chimpanzees using a target-localization task. The stimulus display consisted of a target placed among multiple identical distractors. The target and distractors were presented either solely, within congruent contexts in which salient configurations emerge, or within incongruent contexts in which salient configurations do not emerge. We found that congruent contexts had similar facilitative effects on target localization by humans and chimpanzees, whereas similar disruptive effects emerged when the stimuli were presented within incongruent contexts. When display size was manipulated, targets under the congruent-context condition were localized in a parallel manner, but those under the no-context and incongruent-context conditions were localized in a serial manner by both species. These results suggest that both humans and chimpanzees perceive emergent configurations when targets and distractors are presented within certain congruent contexts and that they process such emergent configurations preattentively.     

Visual expertise does not predict the composite effect across species: A comparison between spider (Ateles geoffroyi) and rhesus (Macaca mulatta) monkeys

Humans are subject to the composite illusion: two identical top halves of a face are perceived as “different” when they are presented with different bottom halves. This observation suggests that when building a mental representation of a face, the underlying system perceives the whole face, and has difficulty decomposing facial features. We adapted a behavioural task that measures the composite illusion to examine the perception of faces in two nonhuman species. Specifically we had spider (Ateles geoffroyi) and rhesus monkeys (Macaca mulatta) perform a two-forced choice, match-to-sample task where only the top half of sample was relevant to the task. The results of Experiment 1 show that spider monkeys (N = 2) process the faces of familiar species (conspecifics and humans, but not chimpanzees, sheep, or sticks), holistically. The second experiment tested rhesus monkeys (N = 7) with the faces of humans, chimpanzees, gorillas, sheep, and sticks. Contrary to prediction, there was no evidence of a composite effect in the human (or familiar primate) condition. Instead, we present evidence of a composite illusion in the chimpanzee condition (an unfamiliar primate). Together, these experiments show that visual expertise does not predict the composite effect across the primate order.     

Perception of complex geometric figures in chimpanzees (Pan troglodytes) and humans (Homo sapiens): analyses of visual similarity on the basis of choice reaction time.

In a conditional-discrimination task (matching-to-sample), we assessed similarities among figures consisting of 2 elemental figures through the choice reaction time, nonmetric multidimensional scaling, and hierarchical cluster analysis data from chimpanzees (Pan troglodytes) and humans (Homo sapiens). Humans also rated similarities among figures. The results of the 3 experiments clearly indicated that the reaction time data obtained from chimpanzees' performances were useful measures of the similarities among figures. The results suggested that chimpanzees and humans perceived the complex figures similarly. The outer-contour elements were perceived most dominantly by both species, and the straight-line elements were perceived least dominantly. Both species showed the same perceptual hierarchy or dominance among perceptual categories, as determined by the similarity of simple elements, on the basis of transformational invariances.     

Acquisition and memory of sequence order in young and adult chimpanzees (Pan troglodytes)

There is no research about age difference in the process of sequential learning in non-human primates. Is there any difference between young and adults in sequential learning process? Six chimpanzees (Pan troglodytes), 3 young and 3 adults, learned the Arabic numeral sequence 1 to 9 by touching the numerals on a touch-screen monitor in ascending order. Initially, the sequence always started with the numeral 1, i.e. ‘start-fixed task’. Training began with the sequence 1–2, 1–2–3, and continued sequentially up to 1–2–3–4–5–6–7–8–9. Later, the subjects were introduced to sequences that started with a random numeral, but always ended with 9, i.e. ‘end-fixed task’. Performance in the end-fixed task was worse relative to the familiar start-fixed task. After training with various sequences of adjacent numerals, the subjects were given a transfer test for the non-adjacent numerals. The results suggested that all chimpanzees indeed mastered sequential ordering, and although there was no fundamental difference in the acquisition process between the two age groups, there was a significant age difference in memory capacity. Based on their knowledge of sequential ordering, the subjects were then asked to perform a masking task in which once a subject touched the lowest numeral, the other numeral(s) turned to white squares. Performance of the masking task by young chimpanzees was better than that of adults in accuracy and degree of difficulty (number of numerals). Taken together, these data clearly demonstrate a similarity among subjects in the way chimpanzees acquire knowledge of sequential order regardless of age differences in sequential learning. Moreover, they reveal that once knowledge of sequential order is established, it can be a good index used to evaluate memory capacity in young and adult chimpanzees.

     

Analogical reasoning and the differential outcome effect: transitory bridging of the conceptual gap for rhesus monkeys (Macaca mulatta)

Monkeys, unlike chimpanzees and humans, have a marked difficulty acquiring relational matching-to-sample (RMTS) tasks that likely reflect the cognitive foundation upon which analogical reasoning rests. In the present study, rhesus monkeys (Macaca mulatta) completed a categorical (identity and nonidentity) RMTS task with differential reward (pellet ratio) and/or punishment (timeout ratio) outcomes for correct and incorrect choices. Monkeys in either differential reward-only or punishment-only conditions performed at chance levels. However, the RMTS performance of monkeys experiencing both differential reward and punishment conditions was significantly better than chance. Subsequently when all animals experienced nondifferential outcomes tests, their RMTS performance levels were at chance. These results indicate that combining differential reward and punishment contingencies provide an effective, albeit transitory, scaffolding for monkeys to judge analogical relations-between-relations.     

Cognitive development in object manipulation by infant chimpanzees

This study focuses on the development of spontaneous object manipulation in three infant chimpanzees during their first 2 years of life. The three infants were raised by their biological mothers who lived among a group of chimpanzees. A human tester conducted a series of cognitive tests in a triadic situation where mothers collaborated with the researcher during the testing of the infants. Four tasks were presented, taken from normative studies of cognitive development of Japanese infants: inserting objects into corresponding holes in a box, seriating nesting cups, inserting variously shaped objects into corresponding holes in a template, and stacking up wooden blocks. The mothers had already acquired skills to perform these manipulation tasks. The infants were free to observe the mothers' manipulative behavior from immediately after birth. We focused on object–object combinations that were made spontaneously by the infant chimpanzees, without providing food reinforcement for any specific behavior that the infants performed. The three main findings can be summarized as follows. First, there was precocious appearance of object–object combination in infant chimpanzees: the age of onset (8–11 months) was comparable to that in humans (around 10 months old).Second, object–object combinations in chimpanzees remained at a low frequency between 11 and 16 months, then increased dramatically at the age of approximately 1.5 years. At the same time, the accuracy of these object–object combinations also increased. Third, chimpanzee infants showed inserting behavior frequently and from an early age but they did not exhibit stacking behavior during their first 2 years of life, in clear contrast to human data.     

The perception of unfamiliar faces and houses by chimpanzees: influence of rotation angle

The inversion effect, or impaired recognition of upside-down faces, is used as evidence supporting the configural processing of faces. Human studies report a linear relationship between face-discrimination performance and orientation, such that recognition is more difficult as faces are rotated away from their typical viewpoint. Previous studies on chimpanzees also support a configural bias for processing faces, particularly faces for which subjects have developed expertise. In the present study, we examined the influence of expertise and rotation angle on the visual perception of faces in chimpanzees. Six subjects were presented with unaltered and blurred conspecific faces and houses in five orientation angles. A computerized paradigm was used to further delineate the nature of configural face processing in this species. The data were consistent with those reported in humans: chimpanzees showed a significant linear impairment when discriminating conspecific faces as they rotated away from their upright orientation. No inversion effect was observed for discriminations involving houses. Thus, chimpanzees, like humans, show a face-specific inversion effect that is linearly affected by angle of orientation, suggesting that their visual processing of faces is strongly influenced by the extraction of configural cues and closely resembles the perceptual strategies of humans.     

Human gaze behaviour during action execution and observation

Gaze shifts and fixations appear to be proactive in both action execution and observation. We investigated a dependency of anticipatory gaze behaviour by using a block stacking task. Blocks were rectangles depicted on a computer screen and the stacking movements were controlled via computer mouse. Subjects either had to execute the task or had to observe it made by the experimenter, or by the computer. The dependency of gaze behaviour on the visibility of a virtual effector, the visibility of the actor, and the nature of the actor was tested by measuring eye movements. Anticipatory eye movements were predominant when the subjects themselves executed the task. During action observation, gaze behaviour did neither depend on the visibility nor depend on the nature of the actor. However, big variability was found between the subjects suggesting the use of two different strategies in action observation: some subjects were mainly tracking the blocks during stacking movements; others were strongly anticipating. We suggest that gaze behaviour during action observation is not predetermined by rigid neural circuitry, but strongly depends on the context. The possibility to explain the causal mechanism, as well as the ownership of the action may be crucial preconditions for anticipatory gaze behaviour.     

How chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) perform in a modified emotional Stroop task

The emotional Stroop task is an experimental paradigm developed to study the relationship between emotion and cognition. Human participants required to identify the color of words typically respond more slowly to negative than to neutral words (emotional Stroop effect). Here we investigated whether chimpanzees (Pan troglodytes) would show a comparable effect. Using a touch screen, eight chimpanzees were trained to choose between two simultaneously presented stimuli based on color (two identical images with differently colored frames). In Experiment 1, the images within the color frames were shapes that were either of the same color as the surrounding frame or of the alternative color. Subjects made fewer errors and responded faster when shapes were of the same color as the frame surrounding them than when they were not, evidencing that embedded images affected target selection. Experiment 2, a modified version of the emotional Stroop task, presented subjects with four different categories of novel images: three categories of pictures of humans (veterinarian, caretaker, and stranger), and control stimuli showing a white square. Because visits by the veterinarian that include anaesthetization can be stressful for subjects, we expected impaired performance in trials presenting images of the veterinarian. For the first session, we found correct responses to be indeed slower in trials of this category. This effect was more pronounced for subjects whose last anaesthetization experience was more recent, indicating that emotional valence caused the slowdown. We propose our modified emotional Stroop task as a simple method to explore which emotional stimuli affect cognitive performance in nonhuman primates.     

Asymmetries in the production of self-directed behavior by chimpanzees and gorillas during a computerized cognitive test

Self-directed behaviors (SDBs) are a commonly used behavioral indicator of arousal in nonhuman primates. Experimental manipulations, designed to increase arousal and uncertainty, have been used to elicit SDB production in primates. Beyond measuring rates of SDB production, researchers have also recorded their lateralized production by primates, thought to reflect laterality of hemispheric brain control and response to emotion. Although a handful of such studies exist, all have been conducted with chimpanzees. Expanding on this line of inquiry, we tested both chimpanzees (N = 3) and gorillas (N = 3) in a serial learning task presented on a touchscreen interface that incorporated both EASY (two-item list) and HARD (four-item list) versions of the task. Although SDB production by the apes did not differ across the two levels of task complexity, both species produced higher rates of SDB when they made an error, regardless of task difficulty. Furthermore, the apes made more SDB with the left hand—directed to the right side of their body (contralateral SDB) and left side of their body (ipsilateral SDB)—when they made an incorrect response. There was no difference in the rate of SDB produced with the right hand across correct compared to incorrect trials. The apes’ responses reflect previous reports that show humans are quicker at selecting negative emotional stimuli when using their left, compared to their right, hand (the reverse is true for positive stimuli). However, previous work has shown that chimpanzees are more likely to produce (contralateral) SDB with their right hand when aroused and so we discuss our results in relation to these findings and consider how they relate to the ‘right hemisphere’ and ‘valence’ models of emotional processing in apes.     

What makes us human (Homo sapiens)? The challenge of cognitive cross-species comparison

Two major theoretical approaches have dominated the quest for uniquely human cognitive abilities: a developmentalist approach stressing the importance of environmental and social conditions, and a predominant approach in experimental and comparative psychology, the deterministic approach suggesting the effect of environmental and social conditions to be minimal. As a consequence, most claims of human cognitive uniqueness are based on comparisons of White middle class Westerner humans (Homo sapiens) with captive chimpanzees (Pan troglodytes). However, humans are much more than only White middle class Westerners, and chimpanzees are much more than only captives. A review of some data available on different populations of humans and chimpanzees reveals that only the predictions of the developmentalist approach are supported. In addition, systematic biases are too often introduced in experiment protocols when comparing humans with apes that further cast doubts on cross-species comparisons. The author argues that only with consideration of within-species population differences in the cognitive domains and the use of well-matched cross-species experimental procedures will an objective understanding of the different cognitive abilities between species emerge. This will require a shift in the theoretical approach adopted by many in experimental and comparative psychology.     

Do young chimpanzees have extraordinary working memory?

Do chimpanzees have better spatial working memory than humans? In a previous report, a juvenile chimpanzee outperformed 3 university students on memory for briefly displayed digits in a spatial array (Inoue & Matsuzawa, 2007). The authors described these abilities as extraordinary and likened the chimpanzee's performance to eidetic memory. However, the chimpanzee received extensive practice on a non-time-pressured version of the task; the human subjects received none. Here we report that, after adequate practice, 2 university students substantially outperformed the chimpanzee. There is no evidence for a superior or qualitatively different spatial memory system in chimpanzees.     

Causal knowledge and imitation/emulation switching in chimpanzees (Pan troglodytes) and children (Homo sapiens)

This study explored whether the tendency of chimpanzees and children to use emulation or imitation to solve a tool-using task was a response to the availability of causal information. Young wild-born chimpanzees from an African sanctuary and 3- to 4-year-old children observed a human demonstrator use a tool to retrieve a reward from a puzzle-box. The demonstration involved both causally relevant and irrelevant actions, and the box was presented in each of two conditions: opaque and clear. In the opaque condition, causal information about the effect of the tool inside the box was not available, and hence it was impossible to differentiate between the relevant and irrelevant parts of the demonstration. However, in the clear condition causal information was available, and subjects could potentially determine which actions were necessary. When chimpanzees were presented with the opaque box, they reproduced both the relevant and irrelevant actions, thus imitating the overall structure of the task. When the box was presented in the clear condition they instead ignored the irrelevant actions in favour of a more efficient, emulative technique. These results suggest that emulation is the favoured strategy of chimpanzees when sufficient causal information is available. However, if such information is not available, chimpanzees are prone to employ a more comprehensive copy of an observed action. In contrast to the chimpanzees, children employed imitation to solve the task in both conditions, at the expense of efficiency. We suggest that the difference in performance of chimpanzees and children may be due to a greater susceptibility of children to cultural conventions, perhaps combined with a differential focus on the results, actions and goals of the demonstrator.     

Color preferences in gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes)

Color plays an important biological role in the lives of many animals, with some species exhibiting preferences for certain colors over others. This study explored the color preferences of two species of ape, which, like humans, possess trichromatic color vision. Six western lowland gorillas, and six chimpanzees, housed in Belfast Zoological Gardens, were exposed to three stimuli (cloths, boxes, sheets of acetate) in red, blue, and green. Six stimuli of the same nature, in each of the three colors, were provided to both species for 5 days per stimulus. The amount of interest that the animals showed toward each stimulus of each color was recorded for 1 hr. Results showed that the apes, both when analyzed as two separate groups, and when assessed collectively, showed significant color preferences, paying significantly less attention to the red-, than to the blue- or green-colored stimuli. The animals' interest in the blue- and green-colored stimuli did not differ significantly. Overall, the findings suggest that gorillas and chimpanzees, our closest living relatives, may harbor color preferences comparable to those of humans and other species.