Motor, Social and Cognitive Development in Infant Rhesus Macaques (Macaca mulatta)

Abstract

Motor cognition develops from a mutual and functional relation between perception and action. Several studies have demonstrated that, by a action-perception matching system, we represent observed and executed actions in a corresponding manner (Jeannerod, 2001; Knoblich & Flach, 2003; Gallese, Keyser & Rizzolatti, 2004: Jackson & Decety, 2004). Electrophysiological research has demonstrated that specific neurons, called mirror neurons, found in two areas of the primate brain (ventral premotor cortex and superior temporal sulcus) are selectively activated during the observation of actions executed by conspecifics (di Pellegrino et al. 1992; Gallese et al. 1996; Rizzolatti, Fadiga, Fogassi, & Gallese, 1996), providing evidence that such an observation–execution matching is involved in our ability to recognize and understand the actions of others. Since shared representations relies on a common computational code and neural architecture, our aim was to explore two different aspects of motor cognition, such as organization of actions and imitation of the others actions. In Experiment 1, we investigated the emergence and the development of prehensile skills in infant macaques over the first month of life, to better understand how infants construct a motor representation of actions in relation to the task and environmental constraints. We found that our infants showed clear goal-directed movements towards the object as early as the second week of life and that their flexibility and selectivity of motor responses improved remarkably with age. Our data also showed that infant macaques were able to adjust their actions in relation to the perceived object size and organize their reaching-grasping responses accordingly. The intent of Experiment 2 was to explore the development of spatial perception and representation in infants and their ability to modulate actions when the object was placed in the near and far space and how infants construct a map of surrounding world in relation to the representation of their own body. The results revealed that monkey infants gradually improved their capability to perceive objects at a certain distance and direction in space, and thus to use this information in order to plan different motor strategies and to adjust the hand configuration accordingly. Infants seemed to code the peripersonal space in a arm-centered coordinate system since the onset of a successful reaching. In contrast, starting from the third week of life, infants started to move physically toward the object and to use their own body as a tool to expand their peripersonal space. By including the body in the action, infants induced changes in the body schema and expanded near space representation in a body-centered coordinate system. These changes in the reaching strategy used by the infants probably reflected an improvement in their capacity to move the whole body and to coordinate more precise movements. Furthermore, our data showed that changes in body schema can influence also infants’ perception of object in space and their flexibility in choosing a different grip configuration according to the size and the position in space of the target. Taken together, these findings suggest that several brain mechanisms of intermodal integration between visual proprioceptive information and motor movements might be already formed after birth, although only broadly defined, and that sensori-motor experience and learning might help infant to sequentially and successfully acquire new motor capabilities. The emergence of such visuo–motor integration may account also for the match between visual input and motor output in imitation tasks. The ability to recognize to be imitated requires the existence of a matching mechanism between visual and proprioceptive information. To this aim, in Experiment 3 we investigated the development of the infants ability to recognize when they were being imitated and the effect of imitiation on affiliation and social interaction. This experiment demonstrated that imitation can significantly affect the behavior of the infant monkeys and facilitate an affective connection with the caregiver: infants, in fact, looked longer and lipsmacked more at the experimenter both during the imitation and after being imitated. These results demonstrate that from early in life imitation might be used as a privileged form of communication which provides essential social inputs for the maturation of the infant’s relational and social skills. Moreover, our data suggest the existence of an early functioning recognition mechanism, possibly based on a mirror neurons system.