The evolution of imitation: what do the capacities of non-human animals tell us about the mechanisms of imitation?

In this paper, we review reports and present new empirical data from studies with marmosets and dogs that address the correspondence problem of imitation research. We focus on the question of how it is possible to transform visual information into matching motor acts. Here, the important issue is not the learning of a complex skill, but determining the copying fidelity of animals at different levels of behavioural organization. As a theoretical framework, we suggest a classification in terms of movement, action and result, which shows a positive relationship between the organizational level of imitation and matching degree. While the monkey studies have provided evidence of very precise copying of movements and, to a lesser degree, of behaviours, the dog studies have provided evidence of action copying and the reproduction of results. In a Do-as-I-do study, a dog attempted to reproduce the results of demonstrated object manipulations at the expense of movement details. Transitive actions were more easily replicated than intransitive ones, and familiarity of actions had a major influence. The discussion of these findings addresses the question of the neuronal mechanisms underlying imitation and whether a single mechanism is sufficient to explain the different levels of copying fidelity.

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title: How do apes ape?
abstract: In the wake of telling critiques of the foundations on which earlier conclusions were based, the last 15 years have witnessed a renaissance in the study of social learning in apes. As a result, we are able to review 31 experimental studies from this period in which social learning in chimpanzees, gorillas, and orangutans has been investigated. The principal question framed at the beginning of this era, Do apes ape? has been answered in the affirmative, at least in certain conditions. The more interesting question now is, thus, How do apes ape? Answering this question has engendered richer taxonomies of the range of social-learning processes at work and new methodologies to uncover them. Together, these studies suggest that apes ape by employing a portfolio of alternative social-learning processes in flexibly adaptive ways, in conjunction with nonsocial learning. We conclude by sketching the kind of decision tree that appears to underlie the deployment of these alternatives.

title: Emulation, imitation, over-imitation and the scope of culture for child and chimpanzee
abstract: We describe our recent studies of imitation and cultural transmission in chimpanzees and children, which question late twentieth-century characterizations of children as imitators, but chimpanzees as emulators. As emulation entails learning only about the results of others' actions, it has been thought to curtail any capacity to sustain cultures. Recent chimpanzee diffusion experiments have by contrast documented a significant capacity for copying local behavioural traditions. Additionally, in recent ‘ghost’ experiments with no model visible, chimpanzees failed to replicate the object movements on which emulation is supposed to focus. We conclude that chimpanzees rely more on imitation and have greater cultural capacities than previously acknowledged. However, we also find that they selectively apply a range of social learning processes that include emulation. Recent studies demonstrating surprisingly unselective ‘over-imitation’ in children suggest that children's propensity to imitate has been underestimated too. We discuss the implications of these developments for the nature of social learning and culture in the two species. Finally, our new experiments directly address cumulative cultural learning. Initial results demonstrate a relative conservatism and conformity in chimpanzees' learning, contrasting with cumulative cultural learning in young children. This difference may contribute much to the contrast in these species' capacities for cultural evolution.

title: Ratcheting up the ratchet: on the evolution of cumulative culture
abstract: Some researchers have claimed that chimpanzee and human culture rest on homologous cognitive and learning mechanisms. While clearly there are some homologous mechanisms, we argue here that there are some different mechanisms at work as well. Chimpanzee cultural traditions represent behavioural biases of different populations, all within the species’ existing cognitive repertoire (what we call the ‘zone of latent solutions’) that are generated by founder effects, individual learning and mostly product-oriented (rather than process-oriented) copying. Human culture, in contrast, has the distinctive characteristic that it accumulates modifications over time (what we call the ‘ratchet effect’). This difference results from the facts that (i) human social learning is more oriented towards process than product and (ii) unique forms of human cooperation lead to active teaching, social motivations for conformity and normative sanctions against non-conformity. Together, these unique processes of social learning and cooperation lead to humans’ unique form of cumulative cultural evolution.

title: Imitative learning of actions on objects by children, chimpanzees, and enculturated chimpanzees.
abstract: In this study we compared the abilities of chimpanzees and human children to imitatively learn novel actions on objects. Of particular interest were possible differences between chimpanzees raised mostly with conspecifics (mother-reared) and chimpanzees raised in a human-like cultural environment (enculturated). Subjects were thus 3 enculturated and 3 mother-reared chimpanzees, along with 8 18-month-old and 8 30-month-old human children. Each subject was tested over a 2-day period with 16 novel objects. The introduction of each object was preceded by a baseline period in which the subject's natural proclivities toward the object were determined. For 12 objects, a human experimenter demonstrated first a simple and then a complex novel action, instructing the subject in each case to "Do what I do" (chimpanzees were prepared for the task behaviorally as well). For the other 4 objects, demonstration of a single action took place on the first day and the subject's opportunity to imitate was delayed until the second day, 48 hours later. Actions that a subject produced in baseline were excluded from further analysis. For each analyzed action, the subject's behavior was scored as to whether it successfully reproduced (1) the end result of the demonstrated action, and (2) the behavioral means used by the demonstrator. Results showed that in immediate imitation the mother-reared chimpanzees were much poorer imitators than the enculturated chimpanzees and the human children, who did not differ from one another. Surprisingly, on the delay trials, the enculturated chimpanzees significantly outperformed the other 3 groups. We conclude from these results that a human-like sociocultural environment is an essential component in the development of human-like social-cognitive and imitative learning skills for chimpanzees, and perhaps for human beings as well.

title: Action imitation in birds
abstract: Action imitation, once thought to be a behavior almost exclusively limited to humans and the great apes, surprisingly also has been found in a number of bird species. Because imitation has been viewed by some psychologists as a form of intelligent behavior, there has been interest in how it is distributed among animal species. Although the mechanisms responsible for action imitation are not clear, we are now at least beginning to understand the conditions under which it occurs. In this article, I try to identify and differentiate the various forms of socially influenced behavior (species-typical social reactions, social effects on motivation, social effects on perception, socially influenced learning, and action imitation) and explain why it is important to differentiate imitation from other forms of social influence. I also examine some of the variables that appear to be involved in the occurrence of imitation. Finally, I speculate about why a number of bird species, but few mammal species, appear to imitate.

title: Social learning affects object exploration and manipulation in keas, Nestor notabilis

abstract: Both the pattern of exploration and the manipulation efficiency of young keas attempting to open a complex food container proved to be influenced by the observation of an experienced group member. Five individuals were allowed to observe a trained conspecific that iteratively demonstrated several techniques to open a large steel box. The lid of the box could be opened only after several locking devices had been dismantled: a bolt had to be poked out, a split pin had to be pulled, and a screw had to be twisted out. The observers’ initial manipulative actions were compared with those of five nao¨ve control subjects (nonobservers). Although the observers failed to open the box completely and thus to get the reward in their first attempts, they explored more, approached the locking devices sooner and were more successful at opening them. Although their initial attempts did not match the response topography or the sequence of the model’s actions (movement or sequence imitation), their improved efficiency at unlocking the devices seemed to reflect the acquisition of some functional understanding of the task through observation (emulation learning).  2001 The Association for the Study of Animal Behaviour

title: Imitation and affordance learning by pigeons (Columba livia).
abstract: The bidirectional control procedure was used to determine whether pigeons (Columba livia) would imitate a demonstrator that pushed a sliding screen for food. One group of observers saw a trained demonstrator push a sliding screen door with its beak (imitation group), whereas 2 other groups watched the screen move independently (possibly learning how the environment works) with a conspecific either present (affordance learning with social facilitation) or absent (affordance learning alone). A 4th group could not see the screen being pushed (sound and odor control). Imitation was evidenced by the finding that pigeons that saw a demonstrator push the screen made a higher proportion of matching screen pushes than observers in 2 appropriate control conditions. Further, observers that watched a screen move without a demonstrator present made a significantly higher proportion of matching screen pushes than would be expected by chance. Thus, these pigeons were capable of affordance learning.

title: Can Young Chimpanzees (Pan Troglodytes) Imitate Arbitrary Actions? Hayes & Hayes (1952) Revisited
abstract: Contrary to recent scepticism, systematic use of an experimental approach suggested by HAYES & HAYES (1952) shows that chimpanzees can imitate novel actions. Two chimpanzees imitated 13 and 17 novel arbitrary gestures, respectively. They were first taught to reproduce 15 gestures on the command, "Do this!", and then presented with 48 novel items. Using a rigourous coding system, two independent observers correctly identified a significant number of the chimpanzees' imitations (p < 0.0001). We conclude that after a period of tuition chimpanzees can go on to imitate arbitrary actions, providing evidence of a basic cognitive capacity for perspective-taking and cross-modal matching.

title: Factors influencing imitation of manipulatory actions in chimpanzees (Pan troglodytes).
abstract: The purpose of the study was to investigate what kind of factors determine the degree of difficulty for chimpanzees (Pan troglodytes) when they imitate actions. Five adult chimpanzees were instructed to perform 48 arbitrary manipulatory actions consisting of different bodily motor patterns and object directionality. Results showed that actions in which an object is directed toward another external location (another object and one's own body) were easier to perform than those that involved manipulating a single object alone. Actions involving unfamiliar motor patterns were more difficult to perform than those involving familiar motor patterns that were already present in the subject's repertoire. Error responses were characterized as perseverative repetition of previously instructed actions. These findings suggest that chimpanzees find the directionality of manipulated objects a more salient cue than details of the demonstrator's body movements performing the manipulation.

title: Reproducing human actions and action sequences: “Do as I Do!” in a dog
abstract: We present evidence that a dog (Philip, a 4-year-old tervueren) was able to use different human actions as samples against which to match his own behaviour. First, Philip was trained to repeat nine human-demonstrated actions on command (‘Do it!’). When his performance was markedly over chance in response to demonstration by one person, testing with untrained action sequences and other demonstrators showed some ability to generalise his understanding of copying. In a second study, we presented Philip with a sequence of human actions, again using the ‘Do as I do’ paradigm. All demonstrated actions had basically the same structure: the owner picked up a bottle from one of six places; transferred it to one of the five other places and then commanded the dog (‘Do it!’). We found that Philip duplicated the entire sequence of moving a specific object from one particular place to another more often than expected by chance. Although results point to significant limitations in his imitative abilities, it seems that the dog could have recognized the action sequence, on the basis of observation alone, in terms of the initial state, the means, and the goal. This suggests that dogs might acquire abilities by observation that enhance their success in complex socio-behavioural situations.

title: BODY IMITATION IN AN ENCULTURATED ORANGUTAN (PONGO PYGMAEUS)
abstract: The mechanisms underlying the copying of others' actions in monkeys and apes are poorly understood. The author compared the ability of the 18-year-old, human-reared, language-trained orangutan Chantek to reproduce the same set of actions that Custance, Whiten, and Bard (1995) used with two chimpanzees. Previous studies (e.g., Miles, Mitchell, and Harper (1996)) had shown that Chantek was capable of imitating on command, but no direct comparison across studies had been attempted. The aim of this study was to investigate how he encoded observed behavior and what types of information he acquired from the experimenter. Chantek received the 48 test actions used by Custance et al. (1995). After the experimenter modeled the target action, Chantek was asked to do the same. Chantek's responses were scored for body part used, type of action performed, and general accuracy. In general, Chantek showed good accuracy in reproducing the experimenter's actions. Three types of errors were especially interesting. First, although he had a 90% matching accuracy between his own gross body areas (e.g., head, arms) and those of the experimenter, Chantek's accuracy decreased considerably for the body parts within those gross body areas. Second,Chantek performed better in those actions thatinvolved some contact between his body parts. Finally, Chantek may have had some difficulty interpreting what he was expected to reproduce (e.g., when asked to raise his index finger, he raised his arm). The author argues that an attentional bias toward certain results or goals and a less differentiated ability to encode observed actions may be important factors contributing to the observed error patterns.

title: Avian Movement Imitation and a New Form of Mimicry: Tracing the Evolution of a Complex Form of Learning
abstract: The spontaneous imitation of movements, previously known in two orders of mammals, is demonstrated in a psittacine bird. 2. The animal, a Grey parrot which has bonded to humans, utilizes its torso, legs, wings, head, beak, and tongue in the imitation of human movements. 3. A new form of imitation, ''non-vocal mimicry,'' is tentatively identified. It is defined by the use of skeletal movements to produce mimetic sounds. 4. Hierarchical relationships and phylogenetic patterns of occurrence suggest that imitative learning in birds may have evolved through the sequence: song/call learning → vocal mimicry → non-vocal mimicry→ movement imitation. 5. These relationships and patterns, and possible differences in function and incubation time, suggest that movement imitation in birds is not homologous to that in mammals., 1. The spontaneous imitation of movements, previously known in two orders of mammals, is demonstrated in a psittacine bird. 2. The animal, a Grey parrot which has bonded to humans, utilizes its torso, legs, wings, head, beak, and tongue in the imitation of human movements. 3. A new form of imitation, ''non-vocal mimicry,'' is tentatively identified. It is defined by the use of skeletal movements to produce mimetic sounds. 4. Hierarchical relationships and phylogenetic patterns of occurrence suggest that imitative learning in birds may have evolved through the sequence: song/call learning → vocal mimicry → non-vocal mimicry→ movement imitation. 5. These relationships and patterns, and possible differences in function and incubation time, suggest that movement imitation in birds is not homologous to that in mammals.

title: Dogs, Canis familiaris, fail to copy intransitive actions in third-party contextual imitation tasks
abstract: Contextual imitation of intransitive actions within a third-party copying setting was tested in 180 household dogs. A demonstrator dog, in sight of observer dogs, performed one of two trained actions upon hearing verbal commands given by an experimenter. Observer dogs were later tested to see whether they produced more target actions than control groups who had not seen demonstrations of those actions. In all conditions, except one, the demonstrated actions had been previously trained in the observer dogs by their owners, but had been attached to a different verbal command to that used in our study. For these ‘pretrained’ actions we further investigated whether reward contingency, the relationship of the dog to the human experimenter and ostensive cues had an effect. In no condition did we find any contextual imitation. A subsample of all tested dogs had, during their lifetimes, received extensive training in a diverse range of tasks outside of social learning, but this background similarly did not lead to contextual imitation. We conclude that dogs, even when highly trained, fail to show evidence of contextual imitation for intransitive actions within a third-party copying paradigm.

title: Selective Imitation in Domestic Dogs
abstract: The transmission of cultural knowledge requires learners to identify what relevant information to retain and selectively imitate when observing others' skills. Young human infants--without relying on language or theory of mind--already show evidence of this ability. If, for example, in a communicative context, a model demonstrates a head action instead of a more efficient hand action, infants imitate the head action only if the demonstrator had no good reason to do so, suggesting that their imitation is a selective, interpretative process . Early sensitivity to ostensive-communicative cues and to the efficiency of goal-directed actions is thought to be a crucial prerequisite for such relevance-guided selective imitation . Although this competence is thought to be human specific , here we show an analog capacity in the dog. In our experiment, subjects watched a demonstrator dog pulling a rod with the paw instead of the preferred mouth action. In the first group, using the "inefficient" action was justified by the model's carrying of a ball in her mouth, whereas in the second group, no constraints could explain the demonstrator's choice. In the first trial after observation, dogs imitated the nonpreferred action only in the second group. Consequently, dogs, like children, demonstrated inferential selective imitation.

title: Imitation in autism. A preliminary research note.
abstract: Previous studies have claimed that children with autism are poor imitators and a lack of imitative capacity has been included by some investigators as one early sign of autism. Presented here are results from a pilot study focusing on observed imitation after presenting 15 tasks to five children with autism (mental age 25-51 months). Imitation tasks involving simple object manipulation, vocal responses, facial and manual gestures, and object substitution were presented to each child. The performance of the children with autism is compared with (1) three normal 4-year-old children (for all 15 tasks), and (2) observations from 28 healthy 1-year-olds (for 10 of the tasks used). The findings indicate that the autistic group displayed the highest level of imitation on object manipulation and vocal tasks while object substitution, facial, and motor imitation acts seemed to be difficult for children with autism. However, the small number of children included as well as the individual variation observed among the autistic subjects precludes any definite conclusions from these pilot observations. It is hypothesized that imitation in children with autism has to be studied separately for different domains and probably also for different subgroups within the autistic population.

title: Imitation of novel complex actions: What does the evidence from animals mean?
abstract: Underlying the various behaviors that are classified as imitation, there may be several distinct mechanisms, differing in adaptive function, cognitive basis, and computational power. Experiments reporting “true motor imitation” in animals do not as yet give evidence of production learning by imitation; instead, contextual imitation can explain their data, and this can be explained by a simple mechanism (response facilitation) which matches known neural findings. When imitation serves a function in social mimicry, which applies to a wide range of phenomena from neonatal imitation in humans and great apes to pair-bonding in some bird species, the fidelity of the behavioral match is crucial. Learning of novel behavior can potentially be achieved by matching the outcome of a model's action, and it is argued that vocal imitation by birds is a clear example of this method (which is sometimes called emulation ). Alternatively, the behavior itself may be perceived in terms of actions that the observer can perform, and thus it may be copied. If the imitation is linear and stringlike (action level), following the surface form rather than the underlying plan, then its utility for learning new instrumental methods is limited. However, the underlying plan of hierarchically organized behavior is visible in output behavior, in subtle but detectable ways, and imitation could instead be based on this organization (program level), extracted automatically by string parsing. Currently, the most likely candidates for such capacities are all great apes. It is argued that this ability to perceive the underlying plan of action, in addition to allowing highly flexible imitation of novel instrumental methods, may have resulted in the competence to understand the intentions (theory of mind) of others.

title: Imitative learning of stimulus-response and response-outcome associations in pigeons.
abstract: A novel automated procedure was used to study imitative learning in pigeons. In Experiments 1 and 2, observer pigeons witnessed a demonstrator pigeon successfully performing an instrumental discrimination in which different discriminative stimuli indicated which of 2 topographically distinct responses (R1 and R2) resulted in the delivery of seed. The observers were then presented with the discriminative stimuli and given access to the response panel. Observer pigeons' behavior during the discriminative stimuli was influenced by how the demonstrator had responded during these stimuli. In Experiment 3, observers witnessed demonstrator pigeons performing R1 for Outcome 1 and R2 for Outcome 2. Observers then received a procedure designed to devalue Outcome 1 relative to Outcome 2 and were subsequently less likely to perform R1 than R2. These results suggest that pigeons can learn both stimulus response and response-outcome associations by observation.

title: Blind imitation in pigeons, Columba livia

abstract: Pigeons that had been trained with a food reward both to peck at and to step on a horizontal plate were allowed to observe a conspecific demonstrator pecking at or stepping on the plate before a test in which the observers were not rewarded for either pecking or stepping. In experiment 1, the demonstrators were not rewarded while being observed. In spite of this, the observers provided evidence of imitation: those that had observed pecking made a greater proportion of pecking responses on test than observers of stepping. In experiment 2, each observer was exposed to a pecking or a stepping conspecific on two occasions. On one occasion, the demonstrator received a food reward for each demonstrated response (continuous reinforcement condition), and on the other the demonstrator's responses were rewarded only rarely (variable interval condition). The observers provided equally strong evidence of imitation in each of these conditions; on test, they made proportionally more of the observed response both when the demonstrators had been richly rewarded and when they had been rarely rewarded. These results show that pigeons engage in ‘blind’ imitation, that is, their imitative behaviour is not always guided by observational learning about response outcomes.

title: True imitation in marmosets
abstract: Marmosets, Callithrix jacchus, observed a demonstrator removing the lids from a series of plastic canisters to obtain a mealworm. When subsequently allowed access to the canisters, marmosets that observed a demonstrator using its hands to remove the lids used only their hands. In contrast, marmosets that observed a demonstrator using its mouth also used their mouth to remove the lids. Since hand and mouth demonstrators brought about identical changes in the canisters, the differential test behaviour of the observer groups suggests that they learned about the demonstrator's behaviour. Furthermore, marmosets that had not been given the opportunity to observe a demonstrator prior to testing had a low probability of mouth opening, even if the canisters were previously opened by a mouth-opening demonstrator in an olfactory control experiment. Corroborating Bugnyar & Huber's (1997, Animal Behaviour, 54, 817-831) earlier findings, our results provide further evidence that marmosets can imitate.

title: An fMRI study of imitation: action representation and body schema
abstract: Recent neuropsychological investigations of apraxia have led to new hypotheses about the representational defects associated with imitation impairments in neurological patients. This fMRI experiment investigated the relation between imitation and the body schema in healthy subjects. Experimental conditions were derived from a factorial plan, and participants were asked to watch a human model performing bodily gestures and then to execute either an identical or a different action, with the same or different limbs. Brain areas activated when subjects imitated the model were traced to the representation of the action (main effect of performing an identical action regardless of limb), to the body schema (using the same limb regardless of action), or to both. The first set of analyses yielded a network associated with visual perception, indicating that action representation is primarily visuospatial not motor, while the second analysis highlighted regions involved in body schema including the inferior parietal cortex and the insula. It is suggested that imitation of simple body gestures requires both a visuospatial description of the observed model, sustained by visual perception areas in the right occipitotemporal and superior parietal cortices and a visuospatial description of one's own body, supported by the left inferior parietal lobule. These results favor a model of praxis proposing that imitation deficits in left inferior parietal lobe patients with apraxia reflect primarily an impairment of the body schema, while deficits of praxis in right parietal patients are limited to gestures demanding in terms of visuospatial analysis.

title: Push or pull: an experimental study on imitation in marmosets
abstract: A laboratory experiment was conducted in order to explore the possibility of imitation, that is, response learning by observation, in marmosets, Callithrix jacchusInexperienced individuals were allowed to observe a skilful model that demonstrated one of two possible techniques (pushing or pulling a pendulum-door) to get food from inside a wooden box. Their initial manipulative actions, performed when exposed to the box in a subsequent test, were compared with those of naive control subjects (non-observers). The observers showed less exploratory behaviour than the non-observers and, more importantly, some showed a strong tendency to use the demonstrated opening technique in the initial test phase. This initial preference disappeared in the course of five test sessions and the observers converged towards the simpler, alternative solution that was generally preferred by the non-observers. Despite fundamental individual differences in the observer group and the failure to find a significant group effect, the results indicate that marmosets are capable of learning simple motor skills through conspecific observation.

title: Imitation as Faithful Copying of a Novel Technique in Marmoset Monkeys
abstract: Imitative learning has received great attention due to its supposed role in the development of culture and the cognitive demands it poses on the individual. Evidence for imitation in non-human primate species, therefore, could shed light on the early origins of proto-cultural traits in the primate order. Imitation has been defined as the learning of an act by seeing it done or, more specifically, as the copying of a novel or otherwise improbable act. But despite a century of research and the detection of mirror neurons the empirical basis for this most advanced form of observational learning is weak. Few, if any, studies have shown that the observer has learned the response topography, i.e., the specific action by which the response is made. In an experimental set-up we confronted marmoset monkeys (Callithrix jacchus) with a conspecific model that was previously trained to open a plastic box in a peculiar way. Employing detailed motion analyses we show that the observers precisely copied the movement patterns of the novel action demonstrated by the model. A discriminant analysis classified 13 out of 14 observer movements (92.86%) as model movements and only one as non-observer movement. This evidence of imitation in non-human primates questions the dominant opinion that imitation is a human-specific ability. Furthermore, the high matching degree suggests that marmosets possess the neuronal mechanism to code the actions of others and to map them onto their own motor repertoire, rather than priming existing motor-templates.

title: Animal Cognition: How Archer Fish Learn to Down Rapidly Moving Targets
abstract: In extremely rapid maneuvers, animals including man can launch ballistic motor patterns that cannot immediately be corrected. Such patterns are difficult to direct at targets that move in three-dimensional space, and it is presently unknown how animals learn to acquire the precision required. Archer fish live in groups and are renowned for their ballistic hunting technique in which they knock down stationary aerial insect prey with a precisely aimed shot of water. Here we report that these fish can learn to release their shots so as to hit prey that moves rapidly at great height, a remarkable accomplishment in which the shooter must take both the target's three-dimensional motion as well as that of its rising shot into account. To successfully perform in the three-dimensional task, training with horizontal motion suffices. Moreover, all archer fish of a group were able to learn the complex sensomotor skill from watching a performing group member, without having to practice. This instance of social learning in a fish is most remarkable as it could imply that observers can "change their viewpoint," mapping the perceived shooting characteristics of a distant team member into angles and target distances that they later must use to hit.

title: Enculturated chimpanzees imitate rationally.
abstract: Human infants imitate others' actions 'rationally': they copy a demonstrator's action when that action is freely chosen, but less when it is forced by some constraint (Gergely, Bekkering & Király, 2002). We investigated whether enculturated chimpanzees (Pan troglodytes) also imitate rationally. Using Gergely and colleagues' (2002) basic procedure, a human demonstrator operated each of six apparatuses using an unusual body part (he pressed it with his forehead or foot, or sat on it). In the Hands Free condition he used this unusual means even though his hands were free, suggesting a free choice. In the Hands Occupied condition he used the unusual means only because his hands were occupied, suggesting a constrained or forced choice. Like human infants, chimpanzees imitated the modeled action more often in the Hands Free than in the Hands Occupied condition. Enculturated chimpanzees thus have some understanding of the rationality of others' intentional actions, and use this understanding when imitating others.

title: Imitation as behaviour parsing.
abstract: Non-human great apes appear to be able to acquire elaborate skills partly by imitation, raising the possibility of the transfer of skill by imitation in animals that have only rudimentary mentalizing capacities: in contrast to the frequent assumption that imitation depends on prior understanding of others' intentions. Attempts to understand the apes' behaviour have led to the development of a purely mechanistic model of imitation, the 'behaviour parsing' model, in which the statistical regularities that are inevitable in planned behaviour are used to decipher the organization of another agent's behaviour, and thence to imitate parts of it. Behaviour can thereby be understood statistically in terms of its correlations (circumstances of use, effects on the environment) without understanding of intentions or the everyday physics of cause-and-effect. Thus, imitation of complex, novel behaviour may not require mentalizing, but conversely behaviour parsing may be a necessary preliminary to attributing intention and cause.

title: Associative sequence learning: the role of experience in the development of imitation and the mirror system
abstract: A core requirement for imitation is a capacity to solve the correspondence problem; to map observed onto executed actions, even when observation and execution yield sensory inputs in different modalities and coordinate frames. Until recently, it was assumed that the human capacity to solve the correspondence problem is innate. However, it is now becoming apparent that, as predicted by the associative sequence learning model, experience, and especially sensorimotor experience, plays a critical role in the development of imitation. We review evidence from studies of non-human animals, children and adults, focusing on research in cognitive neuroscience that uses training and naturally occurring variations in expertise to examine the role of experience in the formation of the mirror system. The relevance of this research depends on the widely held assumption that the mirror system plays a causal role in generating imitative behaviour. We also report original data supporting this assumption. These data show that theta-burst transcranial magnetic stimulation of the inferior frontal gyrus, a classical mirror system area, disrupts automatic imitation of finger movements. We discuss the implications of the evidence reviewed for the evolution, development and intentional control of imitation.

title: Understanding motor events: a neurophysiological study
abstract: SummaryNeurons of the rostral part of inferior premotor cortex of the monkey discharge during goal-directed hand movements such as grasping, holding, and tearing. We report here that many of these neurons become active also when the monkey observes specific, meaningful hand movements performed by the experimenters. The effective experimenters' movements include among others placing or retrieving a piece of food from a table, grasping food from another experimenter's hand, and manipulating objects. There is always a clear link between the effective observed movement and that executed by the monkey and, often, only movements of the experimenter identical to those controlled by a given neuron are able to activate it. These findings indicate that premotor neurons can retrieve movements not only on the basis of stimulus characteristics, as previously described, but also on the basis of the meaning of the observed actions.

title: The representing brain: Neural correlates of motor intention and imagery
abstract: This paper concerns how motor actions are neurally represented and coded. Action planning and motor preparation can be studied using a specific type of representational activity, motor imagery. A close functional equivalence between motor imagery and motor preparation is suggested by the positive effects of imagining movements on motor learning, the similarity between the neural structures involved, and the similar physiological correlates observed in both imaging and preparing. The content of motor representations can be inferred from motor images at a macroscopic level, based on global aspects of the action (the duration and amount of effort involved) and the motor rules and constraints which predict the spatial path and kinematics of movements. A more microscopic neural account calls for a representation of object-oriented action. Object attributes are processed in different neural pathways depending on the kind of task the subject is performing. During object-oriented action, a pragmatic representation is activated in which object affordances are transformed into specific motor schemas (independently of other tasks such as object recognition). Animal as well as human clinical data implicate the posterior parietal and premotor cortical areas in schema instantiation. A mechanism is proposed that is able to encode the desired goal of the action and is applicable to different levels of representational organization.

title: Mirror neurons responding to the observation of ingestive and communicative mouth actions in the monkey ventral premotor cortex
abstract: In the ventral premotor cortex (area F5) of the monkey there are neurons that discharge both when the monkey performs specific motor actions and when it observes another individual performing a similar action (mirror neurons). Previous studies on mirror neurons concerned hand actions. Here, we describe the mirror responses of F5 neurons that motorically code mouth actions. The results showed that about one‐third of mouth motor neurons also discharge when the monkey observes another individual performing mouth actions. The majority of these ‘mouth mirror neurons’ become active during the execution and observation of mouth actions related to ingestive functions such as grasping, sucking or breaking food. Another population of mouth mirror neurons also discharges during the execution of ingestive actions, but the most effective visual stimuli in triggering them are communicative mouth gestures (e.g. lip smacking). Some also fire when the monkey makes communicative gestures. These findings extend the notion of mirror system from hand to mouth action and suggest that area F5, the area considered to be the homologue of human Broca's area, is also involved in communicative functions.

title: I Know What You Are Doing A Neurophysiological Study
abstract: In the ventral premotor cortex of the macaque monkey, there are neurons that discharge both during the execution of hand actions and during the observation of the same actions made by others (mirror neurons). In the present study, we show that a subset of mirror neurons becomes active during action presentation and also when the final part of the action, crucial in triggering the response in full vision, is hidden and can therefore only be inferred. This implies that the motor representation of an action performed by others can be internally generated in the observer's premotor cortex, even when a visual description of the action is lacking. The present findings support the hypothesis that mirror neuron activation could be at the basis of action recognition.

title: Hearing Sounds, Understanding Actions: Action Representation in Mirror Neurons
abstract: Many object-related actions can be recognized by their sound. We found neurons in monkey premotor cortex that discharge when the animal performs a specific action and when it hears the related sound. Most of the neurons also discharge when the monkey observes the same action. These audiovisual mirror neurons code actions independently of whether these actions are performed, heard, or seen. This discovery in the monkey homolog of Broca's area might shed light on the origin of language: audiovisual mirror neurons code abstract contents—the meaning of actions—and have the auditory access typical of human language to these contents.

title: Teleological reasoning in infancy: the naı̈ve theory of rational action
abstract: Converging evidence demonstrates that one-year-olds interpret and draw inferences about other's goal-directed actions. We contrast alternative theories about how this early competence relates to our ability to attribute mental states to others. We propose that one-year-olds apply a non-mentalistic interpretational system, the 'teleological stance' to represent actions by relating relevant aspects of reality (action, goal-state and situational constraints) through the principle of rational action, which assumes that actions function to realize goal-states by the most efficient means available. We argue that this early inferential principle is identical to the rationality principle of the mentalistic stance - a representational system that develops later to guide inferences about mental states.

title: 'Obsessed with goals': functions and mechanisms of teleological interpretation of actions in humans.
abstract: Humans show a strong and early inclination to interpret observed behaviours of others as goal-directed actions. We identify two main epistemic functions that this 'teleological obsession' serves: on-line prediction and social learning. We show how teleological action interpretations can serve these functions by drawing on two kinds of inference ('action-to-goal' or 'goal-to-action'), and argue that both types of teleological inference constitute inverse problems that can only be solved by further assumptions. We pinpoint the assumptions that the three currently proposed mechanisms of goal attribution (action-effect associations, simulation procedures, and teleological reasoning) imply, and contrast them with the functions they are supposed to fulfil. We argue that while action-effect associations and simulation procedures are generally well suited to serve on-line action monitoring and prediction, social learning of new means actions and artefact functions requires the inferential productivity of teleological reasoning.

title: Neural mechanisms of imitation
abstract: Recent advances in our knowledge of the neural mechanisms of imitation suggest that there is a core circuitry of imitation comprising the superior temporal sulcus and the 'mirror neuron system', which consists of the posterior inferior frontal gyrus and adjacent ventral premotor cortex, as well as the rostral inferior parietal lobule. This core circuitry communicates with other neural systems according to the type of imitation performed. Imitative learning is supported by interaction of the core circuitry of imitation with the dorsolateral prefrontal cortex and perhaps motor preparation areas--namely, the mesial frontal, dorsal premotor and superior parietal areas. By contrast, imitation as a form of social mirroring is supported by interaction of the core circuitry of imitation with the limbic system.

title: A unifying computational framework for motor control and social interaction.
abstract: Recent empirical studies have implicated the use of the motor system during action observation, imitation and social interaction. In this paper, we explore the computational parallels between the processes that occur in motor control and in action observation, imitation, social interaction and theory of mind. In particular, we examine the extent to which motor commands acting on the body can be equated with communicative signals acting on other people and suggest that computational solutions for motor control may have been extended to the domain of social interaction.

title: What Is the Significance of Imitation in Animals
abstract: Publisher Summary This chapter explains the significance of imitation in animals. The greatest challenge for any theory of the cognitive mechanisms of imitation is to explain imitation of “perceptually opaque” actions, those actions that yield dissimilar sensory inputs when observed and executed. The reason of this difficulty is discussed in the chapter. The chapter also distinguishes two types of theory of imitation, transformational and associative, in terms of the way in which they attempt to meet this challenge. Transformational theories suggest that most of the information necessary to achieve a behavioral match is generated internally by complex cognitive processes, whereas associative theories claim that this information is derived principally from experience. These theories delineate plausible alternative accounts of the psychological mechanisms of imitation, but they do not provide a satisfactory framework for empirical inquiry because each theory either does not make testable predictions or is inconsistent with what is already known about the conditions of imitation. The associative sequence learning (ASL) theory is discussed and the significance of imitation in animals with respect to culture rather than cognition is also discussed.

title: Explaining Facial Imitation: A Theoretical Model.
abstract: A long-standing puzzle in developmental psychology is how infants imitate gestures they cannot see themselves perform (facial gestures). Two critical issues are: (a) the metric infants use to detect cross-modal equivalences in human acts and (b) the process by which they correct their imitative errors. We address these issues in a detailed model of the mechanisms underlying facial imitation. The model can be extended to encompass other types of imitation. The model capitalizes on three new theoretical concepts. First, organ identification is the means by which infants relate parts of their own bodies to corresponding ones of the adult's. Second, body babbling (infants' movement practice gained through self-generated activity) provides experience mapping movements to the resulting body configurations. Third, organ relations provide the metric by which infant and adult acts are perceived in commensurate terms. In imitating, infants attempt to match the organ relations they see exhibited by the adults with those they feel themselves make. We show how development restructures the meaning and function of early imitation. We argue that important aspects of later social cognition are rooted in the initial cross-modal equivalence between self and other found in newborns.

title: Precise auditory–vocal mirroring in neurons for learned vocal communication
abstract: Brain mechanisms for communication must establish a correspondence between sensory and motor codes used to represent the signal. One idea is that this correspondence is established at the level of single neurons that are active when the individual performs a particular gesture or observes a similar gesture performed by another individual. Although neurons that display a precise auditory–vocal correspondence could facilitate vocal communication, they have yet to be identified. Here we report that a certain class of neurons in the swamp sparrow forebrain displays a precise auditory–vocal correspondence. We show that these neurons respond in a temporally precise fashion to auditory presentation of certain note sequences in this songbird’s repertoire and to similar note sequences in other birds’ songs. These neurons display nearly identical patterns of activity when the bird sings the same sequence, and disrupting auditory feedback does not alter this singing-related activity, indicating it is motor in nature. Furthermore, these neurons innervate striatal structures important for song learning, raising the possibility that singing-related activity in these cells is compared to auditory feedback to guide vocal learning.