Sensitivity to Visual-Tactile Colocation on the Body Prior to Skilled Reaching in Early Infancy.

Two experiments examined perceptual colocation of visual and tactile stimuli in young infants. Experiment 1 compared 4- (n = 15) and 6-month-old (n = 12) infants' visual preferences for visual-tactile stimulus pairs presented across the same or different feet. The 4- and 6-month-olds showed, respectively, preferences for colocated and noncolocated conditions, demonstrating sensitivity to visual-tactile colocation on their feet. This extends previous findings of visual-tactile perceptual colocation on the hands in older infants. Control conditions excluded the possibility that both 6- (Experiment 1), and 4-month-olds (Experiment 2, n = 12) perceived colocation on the basis of an undifferentiated supramodal coding of spatial distance between stimuli. Bimodal perception of visual-tactile colocation is available by 4 months of age, that is, prior to the development of skilled reaching.

The Developing Bodily Self: How Posture Constrains Body Representation in Childhood.

Adults' body representation is constrained by multisensory information and knowledge of the body such as its possible postures. This study (N = 180) tested for similar constraints in children. Using the rubber hand illusion with adults and 6- to 7-year olds, we measured proprioceptive drift (an index of hand localization) and ratings of felt hand ownership. The fake hand was either congruent or incongruent with the participant's own. Across ages, congruency of posture and visual-tactile congruency yielded greater drift toward the fake hand. Ownership ratings were higher with congruent visual-tactile information, but unaffected by posture. Posture constrains body representation similarly in children and adults, suggesting that children have sensitive, robust mechanisms for maintaining a sense of bodily self.

The development of bodily self-consciousness: changing responses to the Full Body Illusion in childhood.

The present work investigates the development of bodily self-consciousness and its relation to multisensory bodily information, by measuring for the first time the development of responses to the full body illusion in childhood. We tested three age groups of children: 6- to 7-year-olds (n = 28); 8- to 9-year-olds (n = 21); 10- to 11-year-olds (n = 19), and a group of adults (n = 31). Each participant wore a head-mounted display (HMD) which displayed a view from a video camera positioned 2 metres behind their own back. Thus, they could view a virtual body from behind. We manipulated visuo-tactile synchrony by showing the participants a view of their virtual back being stroked with a stick at the same time and same place as their real back (synchronous condition), or at different times and places (asynchronous condition). After each period of stroking, we measured three aspects of bodily self-consciousness: drift in perceived self-location, self-identification with the virtual body, and touch referral to the virtual body. Results show that self-identification with the virtual body was significantly stronger in the synchronous condition than in the asynchronous condition even in the youngest group tested; however, the size of this effect increased with age. Touch referral to the virtual body was greater in the synchronous condition than in the asynchronous condition only for 10- to 11-year-olds and adults. Drift in perceived self-location was greater in the synchronous condition than in the asynchronous condition only for adults. Thus, the youngest age tested can self-identify with a virtual body, but the links between multisensory signals and bodily self-consciousness develop significantly across childhood. This suggests a long period of development of the bodily self and exciting potential for the use of virtual reality technologies with children.

Sensitivity to auditory-tactile colocation in early infancy.

An ability to detect the common location of multisensory stimulation is essential for us to perceive a coherent environment, to represent the interface between the body and the external world, and to act on sensory information. Regarding the tactile environment "at hand", we need to represent somatosensory stimuli impinging on the skin surface in the same spatial reference frame as distal stimuli, such as those transduced by vision and audition. Across two experiments we investigated whether 6- (n = 14; Experiment 1) and 4-month-old (n = 14; Experiment 2) infants were sensitive to the colocation of tactile and auditory signals delivered to the hands. We recorded infants' visual preferences for spatially congruent and incongruent auditory-tactile events delivered to their hands. At 6 months, infants looked longer toward incongruent stimuli, whilst at 4 months infants looked longer toward congruent stimuli. Thus, even from 4 months of age, infants are sensitive to the colocation of simultaneously presented auditory and tactile stimuli. We conclude that 4- and 6-month-old infants can represent auditory and tactile stimuli in a common spatial frame of reference. We explain the age-wise shift in infants' preferences from congruent to incongruent in terms of an increased preference for novel crossmodal spatial relations based on the accumulation of experience. A comparison of looking preferences across the congruent and incongruent conditions with a unisensory control condition indicates that the ability to perceive auditory-tactile colocation is based on a crossmodal rather than a supramodal spatial code by 6 months of age at least.

Part-based representations of the body in early childhood: evidence from perceived distortions of tactile space across limb boundaries.

Studies show that touch in adults is referenced to a representation of the body that is structured topologically according to body parts; the perceived distance between two stimuli crossing over a body part boundary is elongated relative to the perceived distance between two stimuli presented within one body part category. Here we investigate this influence of body parts on tactile space perception in children of 5, 6 and 7 years of age. We presented children with pairs of tactile stimuli on the left hand/arm, either within the hand, within the forearm, or over the wrist. With their eyes closed children were asked to adjust the distance between the thumb and forefinger of their right hand to represent the felt distance between the two tactile stimuli. Like adults, the children perceived the distance between two stimuli that cross the body part boundary to be further apart than those that were presented within the hand or arm. They also perceive tactile distance to be greater on the hand than the arm which is the first observation of Weber's illusion in young children. We propose that a topological mode of body representation is particularly advantageous during early life given that body part categories remain constant while the metric proportions of the body change substantially as the child grows.

Specialization of the motor system in infancy: from broad tuning to selectively specialized purposeful actions.

In executing purposeful actions, adults select sufficient and necessary limbs. But infants often move goal-irrelevant limbs, suggesting a developmental process of motor specialization. Two experiments with 9- and 12-month-olds revealed gradual decreases in extraneous movements in non-acting limbs during unimanual actions. In Experiment 1, 9-month-olds produced more extraneous movements in the non-acting hand/arm and feet/legs than 12-month-olds. In Experiment 2, analysis of the spatiotemporal dynamics of infants' movements revealed developmental declines in the spatiotemporal coupling of movements between acting and non-acting arms. We also showed that the degree of specialization in infants' unimanual actions is associated with individual differences in motor experience and visual attention, indicating the experience-dependent and broad functional nature of these developmental changes. Our study provides important new insights into motor development: as in cognitive domains, motor behaviours are initially broadly tuned to their goal, becoming progressively specialized during the first year of life.

Developing body representations in early life: combining somatosensation and vision to perceive the interface between the body and the world.

This article lays out the computational challenges involved in constructing multisensory representations of the body and the interface between the body and the external world. It then provides a review of the most pertinent empirical literature regarding the ontogeny of such representational abilities in early life, focussing especially on ability to make spatiotemporal links between bodily events transduced by vision and somatosensation (cutaneous touch and proprioception), and the ability to use multisensory bodily cues to locate tactile stimuli. Findings from infants, children, and blind adults point towards a trajectory of development in early life in which infants and children, as a result of sensory experience, learn new ways of combining cues concerning the body arising from vision and somatosensation, in order to best represent the layout of their limbs and sensory events occurring on their limbs in relation to the external environment.

Effects of Culture and the Urban Environment on the Development of the Ebbinghaus Illusion.

The development of visual context effects in the Ebbinghaus illusion in the United Kingdom and in remote and urban Namibians (UN) was investigated (N = 336). Remote traditional Himba children showed no illusion up until 9-10 years, whereas UK children showed a robust illusion from 7 to 8 years of age. Greater illusion in UK than in traditional Himba children was stable from 9 to 10 years to adulthood. A lesser illusion was seen in remote traditional Himba children than in UN children growing up in the nearest town to the traditional Himba villages across age groups. We conclude that cross-cultural differences in perceptual biases to process visual context emerge in early childhood and are influenced by the urban environment.

The development of multisensory body representation and awareness continues to 10 years of age: Evidence from the rubber hand illusion.

Recent research using the "rubber hand illusion" shows that the multisensory processes underlying body representations are markedly different in children of 4 to 9years and adults. In representing the position of their own hand in external space, children of this age rely more on the sight of the hand, and less on its proprioceptively felt position, than adults do. The current study investigated when during later childhood the balance between visual and proprioceptive inputs reaches an adult-like weighting. After inducing the rubber hand illusion in 10- to 13-year-olds, we asked participants to point, with eyes closed, to the perceived position of their hand. We found that pointing responses reached adult levels at 10 to 11years, showing that at this age children perceive hand location using an adult-like balance of sensory cues. We conclude that the multisensory foundations of the bodily self undergo a protracted period of development through early and mid-childhood, reaching an adult state by 10 to 11years.

Calling for a developmental perspective on action-based consciousness.

Human newborns can resolve some response conflicts in order to adapt their behaviour, suggesting that the newborn has consciousness according to Morsella et al.'s framework. However, we pose a range of developmental questions regarding Morsella et al.'s account, especially concerning the role of consciousness in the development of action.

Effects of posture on tactile localization by 4 years of age are modulated by sight of the hands: evidence for an early acquired external spatial frame of reference for touch.

Adults show a deficit in their ability to localize tactile stimuli to their hands when their arms are in the less familiar, crossed posture. It is thought that this 'crossed-hands deficit' arises due to a conflict between the anatomical and external spatial frames of reference within which touches can be encoded. The ability to localize a single tactile stimulus applied to one of the two hands across uncrossed-hands and crossed-hands postures was investigated in typically developing children (aged 4 to 6 years). The effect of posture was also compared across conditions in which children did, or did not, have visual information about current hand posture. All children, including the 4-year-olds, demonstrated the crossed-hands deficit when they did not have sight of hand posture, suggesting that touch is located in an external reference frame by this age. In this youngest age group, when visual information about current hand posture was available, tactile localization performance was impaired specifically when the children's hands were uncrossed. We propose that this may be due to an early difficulty with integrating visual representations of the hand within the body schema.

The electrophysiological time course of somatosensory spatial remapping: vision of the hands modulates effects of posture on somatosensory evoked potentials.

We investigated the electrophysiological correlates of somatosensory processing under different arm postures by recording event-related potentials at frontal, central and centroparietal sites during tactile stimulation of the hands. Short series of 200 ms vibrotactile stimuli were presented to the palms of the participants' hands, one hand at a time, in either uncrossed- or crossed-hands postures. The manipulation of posture allowed us to investigate the electrophysiological processes underlying the spatial remapping of somatosensory stimuli from anatomical into external frames of reference. To examine somatosensory spatial remapping independently of its effects on attentional processes, the stimuli were presented unpredictably in terms of location, and in temporal onset. We also examined how vision of the limbs affects the process of remapping. When participants had sight of their hands (Experiment 1) the effect of posture was observed over regions contralateral to the stimulated hand from 128 ms, whereas when their limbs were covered (Experiment 2) effects of posture influenced the ipsilateral regions from 150 ms. These findings add to an increasing body of evidence which indicates that sight of the hand modulates the way in which information in other modalities is processed. We argue that in this case, sight of the hand biases spatial encoding of touch towards an anatomical frame of reference.

Children's responses to the rubber-hand illusion reveal dissociable pathways in body representation.

The bodily self is constructed from multisensory information. However, little is known of the relation between multisensory development and the emerging sense of self. We investigated this question by measuring the strength of the rubber-hand illusion in young children (4 to 9 years old) and adults. Intermanual pointing showed that children were as sensitive as adults to visual-tactile synchrony cues for hand position, which indicates that a visual-tactile pathway to the bodily self matures by at least 4 years of age. However, regardless of synchrony cues, children's perceived hand position was closer to the rubber hand than adults' perceived hand position was. This indicates a second, later-maturing process based on visual-proprioceptive information. Furthermore, explicit feelings of embodiment were related only to the visual-tactile process. These findings demonstrate two dissociable processes underlying body representation in early life, and they call into question current models of body representation and ownership in adulthood.

Do local and global perceptual biases tell us anything about local and global selective attention?

Local, as opposed to global, perceptual bias has been linked to a lesser ability to attend globally. We examined this proposed link in Himba observers, members of a remote Namibian population who have demonstrated a strong local bias compared with British observers. If local perceptual bias is related to a lesser ability to attend globally, Himba observers, relative to British observers, should be less distracted by global information when performing a local-selection task but more distracted by local information when performing a global-selection task. However, Himba observers performed better than British observers did on both a local-selection task and a global-selection task (both of which used local/global hierarchical figures as stimuli), which suggests that they possessed greater control over attentional selection in response to task demands. We conclude that local and global perceptual biases must be distinguished from local and global selective attention.

To eat or not to eat? Kinematics and muscle activity of reach-to-grasp movements are influenced by the action goal, but observers do not detect these differences.

Recent evidence suggests that the mirror neuron system responds to the goals of actions, even when the end of the movement is hidden from view. To investigate whether this predictive ability might be based on the detection of early differences between actions with different outcomes, we used electromyography (EMG) and motion tracking to assess whether two actions with different goals (grasp to eat and grasp to place) differed from each other in their initial reaching phases. In a second experiment, we then tested whether observers could detect early differences and predict the outcome of these movements, based on seeing only part of the actions. Experiment 1 revealed early kinematic differences between the two movements, with grasp-to-eat movements characterised by an earlier peak acceleration, and different grasp position, compared to grasp-to-place movements. There were also significant differences in forearm muscle activity in the reaching phase of the two actions. The behavioural data arising from Experiments 2a and 2b indicated that observers are not able to predict whether an object is going to be brought to the mouth or placed until after the grasp has been completed. This suggests that the early kinematic differences are either not visible to observers, or that they are not used to predict the end-goals of actions. These data are discussed in the context of the mirror neuron system.

Visual objects approaching the body modulate subsequent somatosensory processing at 4 months of age.

We asked whether, in the first year of life, the infant brain can support the dynamic crossmodal interactions between vision and somatosensation that are required to represent peripersonal space. Infants aged 4 (n = 20, 9 female) and 8 (n = 20, 10 female) months were presented with a visual object that moved towards their body or receded away from it. This was presented in the bottom half of the screen and not fixated upon by the infants, who were instead focusing on an attention getter at the top of the screen. The visual moving object then disappeared and was followed by a vibrotactile stimulus occurring later in time and in a different location in space (on their hands). The 4-month-olds' somatosensory evoked potentials (SEPs) were enhanced when tactile stimuli were preceded by unattended approaching visual motion, demonstrating that the dynamic visual-somatosensory cortical interactions underpinning representations of the body and peripersonal space begin early in the first year of life. Within the 8-month-olds' sample, SEPs were increasingly enhanced by (unexpected) tactile stimuli following receding visual motion as age in days increased, demonstrating changes in the neural underpinnings of the representations of peripersonal space across the first year of life.

Cortical signatures of visual body representation develop in human infancy.

Human infants cannot report their experiences, limiting what we can learn about their bodily awareness. However, visual cortical responses to the body, linked to visual awareness and selective attention in adults, can be easily measured in infants and provide a promising marker of bodily awareness in early life. We presented 4- and 8-month-old infants with a flickering (7.5 Hz) video of a hand being stroked and recorded steady-state visual evoked potentials (SSVEPs). In half of the trials, the infants also received tactile stroking synchronously with visual stroking. The 8-month-old, but not the 4-month-old infants, showed a significant enhancement of SSVEP responses when they received tactile stimulation concurrent with the visually observed stroking. Follow-up experiments showed that this enhancement did not occur when the visual hand was presented in an incompatible posture with the infant's own body or when the visual stimulus was a body-irrelevant video. Our findings provide a novel insight into the development of bodily self-awareness in the first year of life.

The role of hand size in body representation: a developmental investigation.

Knowledge of one's own body size is a crucial facet of body representation, both for acting on the environment and perhaps also for constraining body ownership. However, representations of body size may be somewhat plastic, particularly to allow for physical growth in childhood. Here we report a developmental investigation into the role of hand size in body representation (the sense of body ownership, perception of hand position, and perception of own-hand size). Using the rubber hand illusion paradigm, this study used different fake hand sizes (60%, 80%, 100%, 120% or 140% of typical size) in three age groups (6- to 7-year-olds, 12- to 13-year-olds, and adults; N = 229). We found no evidence that hand size constrains ownership or position: participants embodied hands which were both larger and smaller than their own, and indeed judged their own hands to have changed size following the illusion. Children and adolescents embodied the fake hands more than adults, with a greater tendency to feel their own hand had changed size. Adolescents were particularly sensitive to multisensory information. In sum, we found substantial plasticity in the representation of own-body size, with partial support for the hypothesis that children have looser representations than adults.

Multisensory spatial perception in visually impaired infants.

Congenitally blind infants are not only deprived of visual input but also of visual influences on the intact senses. The important role that vision plays in the early development of multisensory spatial perception1-7 (e.g., in crossmodal calibration8-10 and in the formation of multisensory spatial representations of the body and the world1,2) raises the possibility that impairments in spatial perception are at the heart of the wide range of difficulties that visually impaired infants show across spatial,8-12 motor,13-17 and social domains.8,18,19 But investigations of early development are needed to clarify how visually impaired infants' spatial hearing and touch support their emerging ability to make sense of their body and the outside world. We compared sighted (S) and severely visually impaired (SVI) infants' responses to auditory and tactile stimuli presented on their hands. No statistically reliable differences in the direction or latency of responses to auditory stimuli emerged, but significant group differences emerged in responses to tactile and audiotactile stimuli. The visually impaired infants showed attenuated audiotactile spatial integration and interference, weighted more tactile than auditory cues when the two were presented in conflict, and showed a more limited influence of representations of the external layout of the body on tactile spatial perception.20 These findings uncover a distinct phenotype of multisensory spatial perception in early postnatal visual deprivation. Importantly, evidence of audiotactile spatial integration in visually impaired infants, albeit to a lesser degree than in sighted infants, signals the potential of multisensory rehabilitation methods in early development. VIDEO ABSTRACT.

Crossmodal spatial distraction across the lifespan.

The ability to resist distracting stimuli whilst voluntarily focusing on a task is fundamental to our everyday cognitive functioning. Here, we investigated how this ability develops, and thereafter declines, across the lifespan using a single task/experiment. Young children (5-7 years), older children (10-11 years), young adults (20-27 years), and older adults (62-86 years) were presented with complex visual scenes. Endogenous (voluntary) attention was engaged by having the participants search for a visual target presented on either the left or right side of the display. The onset of the visual scenes was preceded - at stimulus onset asynchronies (SOAs) of 50, 200, or 500 ms - by a task-irrelevant sound (an exogenous crossmodal spatial distractor) delivered either on the same or opposite side as the visual target, or simultaneously on both sides (cued, uncued, or neutral trials, respectively). Age-related differences were revealed, especially in the extreme age-groups, which showed a greater impact of crossmodal spatial distractors. Young children were highly susceptible to exogenous spatial distraction at the shortest SOA (50 ms), whereas older adults were distracted at all SOAs, showing significant exogenous capture effects during the visual search task. By contrast, older children and young adults' search performance was not significantly affected by crossmodal spatial distraction. Overall, these findings present a detailed picture of the developmental trajectory of endogenous resistance to crossmodal spatial distraction from childhood to old age and demonstrate a different efficiency in coping with distraction across the four age-groups studied.