Everything is worth when it is close to my body: How spatial proximity and stimulus valence affect visuo-tactile integration

Spatial attention in three-dimensional space: A meta-analysis for the near advantage in target detection and localization

Studies have explored how human spatial attention appears allocated in three-dimensional (3D) space. It has been demonstrated that target distance from the viewer can modulate performance in target detection and localization tasks: reaction times are shorter when targets appear nearer to the observer compared to farther distances (i.e., near advantage). Times have reached to quantitatively analyze this literature. In the current meta-analysis, 29 studies (n = 1260 participants) examined target detection and localization across 3-D space. Moderator analyses included: detection vs localization tasks, spatial cueing vs uncued tasks, control of retinal size across depth, central vs peripheral targets, real-space vs stereoscopic vs monocular depth environments, and inclusion of in-trial motion. The analyses revealed a near advantage for spatial attention that was affected by the moderating variables of controlling for retinal size across depth, the use of spatial cueing tasks, and the inclusion of in-trial motion. Overall, these results provide an up-to-date quantification of the effect of depth and provide insight into methodological differences in evaluating spatial attention.

Spatial proximity to others induces plastic changes in the neural representation of the peripersonal space

Peripersonal space (PPS) is a highly plastic "invisible bubble" surrounding the body whose boundaries are mapped through multisensory integration. Yet, it is unclear how the spatial proximity to others alters PPS boundaries. Across five experiments (N = 80), by recording behavioral and electrophysiological responses to visuo-tactile stimuli, we demonstrate that the proximity to others induces plastic changes in the neural PPS representation. The spatial proximity to someone else's hand shrinks the portion of space within which multisensory responses occur, thus reducing the PPS boundaries. This suggests that PPS representation, built from bodily and multisensory signals, plastically adapts to the presence of conspecifics to define the self-other boundaries, so that what is usually coded as "my space" is recoded as "your space". When the space is shared with conspecifics, it seems adaptive to move the other-space away from the self-space to discriminate whether external events pertain to the self-body or to other-bodies.

Peripersonal Space from a multisensory perspective: the distinct effect of the visual and tactile components of Visuo-Tactile stimuli

Peripersonal Space (PPS) is defined as the space close to the body where all interactions between the individual and the environment take place. Behavioural experiments on PPS exploit multisensory integration, using Multisensory Visuo-Tactile stimuli (MVT), whose visual and tactile components target the same body part (i.e. the face, the hand, the foot). However, the effects of visual and tactile stimuli targeting different body parts on PPS representation are unknown, and the relationship with the RTs for Tactile-Only stimuli is unclear. In this study, we addressed two research questions: (1) if the MVT-RTs are independent of Tactile-Only-RTs and if the latter is influenced by time-dependency effects, and (2) if PPS estimations derived from MVT-RTs depend on the location of the Visual or Tactile component of MVTs. We studied 40 right-handed participants, manipulating the body location (right hand, cheek or foot) and the distance of administration. Visual and Tactile components targeted different or the same body parts and were delivered respectively at five distances. RTs to Tactile-Only trials showed a non-monotonic trend, depending on the delay of stimulus administration. Moreover, RTs to Multisensory Visuo-Tactile trials were found to be dependent on the Distance and location of the Visual component of the stimulus. In conclusion, our results show that Tactile-Only RTs should be removed from Visuo-Tactile RTs and that the Visual and Tactile components of Visuo-Tactile stimuli do not necessarily have to target the same body part. These results have a relevant impact on the study of PPS representations, providing new important methodological information.

Seeming confines: Electrophysiological evidence of peripersonal space remapping following tool-use in humans

The peripersonal space (PPS) is a special portion of space immediately surrounding the body, where the integration between tactile stimuli delivered on the body and auditory or visual events emanating from the environment occurs. Interestingly, PPS can widen if a tool is employed to interact with objects in the far space. However, electrophysiological evidence of such tool-use dependent plasticity in the human brain is scarce. Here, in a series of three experiments, participants were asked to respond to tactile stimuli, delivered to their right hand, either in isolation (unimodal condition) or combined with auditory stimulation, which could occur near (bimodal-near) or far from the stimulated hand (bimodal-far). According to multisensory integration spatial rule, when bimodal stimuli are presented at the same location, we expected a response enhancement (response time - RT - facilitation and event-related potential - ERP - super-additivity). In Experiment 1, we verified that RT facilitation was driven by bimodal input spatial congruency, independently from auditory stimulus intensity. In Experiment 2, we showed that our bimodal task was effective in eliciting the magnification of ERPs in bimodal conditions, with significantly larger responses in the near as compared to far condition. In Experiment 3 (main experiment), we explored tool-use driven PPS plasticity. Our audio-tactile task was performed either following tool-use (a 20-min reaching task, performed using a 145 cm-long rake) or after a control cognitive training (a 20-min visual discrimination task) performed in the far space. Following the control training, faster RTs and greater super-additive ERPs were found in bimodal-near as compared to bimodal-far condition (replicating Experiment 2 results). Crucially, this far-near differential response was significantly reduced after tool-use. Altogether our results indicate a selective effect of tool-use remapping in extending the boundaries of PPS. The present finding might be considered as an electrophysiological evidence of tool-use dependent plasticity in the human brain.

Body-Space Interactions: Same Spatial Encoding but Different Influence of Valence for Reaching and Defensive Purposes

The space around our body, the so-called peripersonal space, is where interactions with nearby objects may occur. "Defensive space" and "Reaching space", respectively, refer to two opposite poles of interaction between our body and the external environment: protecting the body and performing a goal-directed action. Here, we hypothesized that mechanisms underlying these two action spaces are differentially modulated by the valence of visual stimuli, as stimuli with negative valence are more likely to activate protective actions whereas stimuli with positive valence may activate approaching actions. To test whether such distinction in cognitive/evaluative processing exists between Reaching and Defensive spaces, we measured behavioral responses as well as neural activations over sensorimotor cortex using EEG while participants performed several tasks designed to tap into mechanisms underlying either Defensive (e.g., respond to touch) or Reaching space (e.g., estimate whether object is within reaching distance). During each task, pictures of objects with either positive or negative valence were presented at different distances from the participants' body. We found that Defensive space was smaller for positively compared with negatively valenced visual stimuli. Furthermore, sensorimotor cortex activation (reflected in modulation of beta power) during tactile processing was enhanced when coupled with negatively rather than positively valenced visual stimuli regarding Defensive space. On the contrary, both the EEG and behavioral measures capturing the mechanisms underlying Reaching space did not reveal any modulation by valence. Thus, although valence encoding had differential effects on Reaching and Defensive spaces, the distance of the visual stimulus modulated behavioral measures as well as activity over sensorimotor cortex (reflected in modulations of mu power) in a similar way for both types of spaces. Our results are compatible with the idea that Reaching and Defensive spaces involve the same distance-dependent neural representations of sensory input, whereas task goals and stimulus valence (i.e., contextual information) are implemented at a later processing stage and exert an influence on motor output rather than sensory/space encoding.

Associative learning in peripersonal space: fear responses are acquired in hand-centered coordinates

Space coding affects perception of stimuli associated to negative valence: threatening stimuli presented within the peripersonal space (PPS) speed up behavioral responses compared with nonthreatening events. However, it remains unclear whether the association between stimuli and their negative valence is acquired in a body part-centered reference system, a main feature of the PPS coding. Here we test the hypothesis that associative learning takes place in hand-centered coordinates and can therefore remap according to hand displacement. In two experiments, we used a Pavlovian fear-learning paradigm to associate a visual stimulus [light circle, the conditioned stimulus (CS)] with an aversive stimulus (electrocutaneous shock) applied on the right hand only when the CS was displayed close (CS+) but when not far from it (CS-). Measuring the skin conductance response (SCR), we observed successful fear conditioning, with increased anticipatory fear responses associated with CS+. Crucially, experiment I showed a remapping of these responses following hand displacement, with a generalization to both types of CS. Experiment II corroborated and further extended our findings by ruling out the novelty of the experimental context as a driving factor of such modulations. Indeed, fear responses were present only for stimuli within the PPS but not for new stimuli displayed outside the PPS. By revealing a hand-centered (re)mapping of the conditioning effect, these findings indicate that associative learning can arise in hand-centered coordinates. They further suggest that the threatening valence of an object also depends on its basic spatial relationship with our body.NEW & NOTEWORTHY Associative fear learning takes place in hand-centered coordinates. Using a Pavlovian fear-learning paradigm, we show that the anticipatory skin conductance response indicating the association between the negative value and an initially neutral stimulus is acquired and then remapped in space when the stimulated body part moves to a different position. These results demonstrate the relationship between the representation of peripersonal space and the encoding of threatening stimuli. Hypotheses concerning the underlying neural network are discussed.

Affective sounds entering the peripersonal space influence the whole-body action preparation

The peripersonal space (PPS), the space surrounding us, is found to have enhanced multisensory-motor representation in the brain. In this study, we investigate how approaching sounds stopping at different distances within the peripersonal space, and carrying emotional content (positive, negative, and neutral), modulate the preparation of action as performing a Step. Premotor reaction times were measured by means of anticipatory forces and muscular activations to capture action preparation, the kinematics of stepping was considered for defining action performance, and for each stimulus, the individual perceived level of arousal and valence was evaluated. In general, we found a prompter premotor reaction for closer sounds compared to the farther ones and the fastest reactions detected for the neutral sound at each distance. We interpreted this time facilitation for neutral sound due to the large frequency spectrum of the stimuli and the absence of affective component and semantical content to decode. Interestingly, while at the close distance, none difference was found between positive and negative emotional stimuli, at the far distance faster reactions were present for negative compared to the positive sounds indicating that when arousal is less enhanced individuals are able to differentiate the emotional content of a sound. The kinematics observed after action initiation sustained the anticipatory results by showing that larger steps were performed when reacting to close compared to far sounds, being perceived as more arousing, and this happened particularly for neutral and negative sounds. Altogether, the results showed that action preparation is influenced by the vicinity and by the valence carried by looming auditory stimuli. For discriminating the stimuli valence, a certain distance, still within the PPS, is necessary; when instead stimuli are too close to the body valence discrimination is not performed.

Hold It Right There!

Abstract. The approach-aversion effect refers to a devaluation of approaching (vs. static) stimuli and is attributable to the fact that being approached is threatening. However, the explanation and the generalizability of this effect still remain unclear. To fill this gap, we provide a powerful test of the approach-aversion effect using Virtual Reality. Participants evaluated approaching and static virtual individuals for which we manipulated the threatening nature via their emotional facial expressions (Experiment 1), their group membership (Experiment 2), and the agency of their movements (Experiment 3). The results suggest a general approach-aversion effect which is attenuated when the self (vs. the target) initiates the movement. We thus bring convergent evidence that being approached is threatening.

Action planning and affective states within the auditory peripersonal space in normal hearing and cochlear-implanted listeners

Fast reaction to approaching stimuli is vital for survival. When sounds enter the auditory peripersonal space (PPS), sounds perceived as being nearer elicit higher motor cortex activation. There is a close relationship between motor preparation and the perceptual components of sounds, particularly of highly arousing sounds. Here we compared the ability to recognize, evaluate, and react to affective stimuli entering the PPS between 20 normal-hearing (NH, 7 women) and 10 cochlear-implanted (CI, 3 women) subjects. The subjects were asked to quickly flex their arm in reaction to positive (P), negative (N), and neutral (Nu) affective sounds ending virtually at five distances from their body. Pre-motor reaction time (pm-RT) was detected via electromyography from the postural muscles to measure action anticipation at the sound-stopping distance; the sounds were also evaluated for their perceived level of valence and arousal. While both groups were able to localize sound distance, only the NH group modulated their pm-RT based on the perceived sound distance. Furthermore, when the sound carried no affective components, the pm-RT to the Nu sounds was shorter compared to the P and the N sounds for both groups. Only the NH group perceived the closer sounds as more arousing than the distant sounds, whereas both groups perceived sound valence similarly. Our findings underline the role of emotional states in action preparation and describe the perceptual components essential for prompt reaction to sounds approaching the peripersonal space.

Bliss in and Out of the Body: The (Extra)Corporeal Space Is Impervious to Social Pleasant Touch

Slow, gentle stimulation of hairy skin is generally accompanied by hedonic sensations. This phenomenon, also known as (positive) affective touch, is likely to be the basis of affiliative interactions with conspecifics by promoting inter-individual bindings. Previous studies on healthy humans have demonstrated that affective touch can remarkably impact behavior. For instance, by administering the Rubber Hand Illusion (RHI) paradigm, the embodiment of a fake hand enhances after a slow, affective touch compared to a fast, neutral touch. However, results coming from this area are not univocal. In addition, there are no clues in the existing literature on the relationship between affective touch and the space around our body. To overcome these lacks, we carried out two separate experiments where participants underwent a RHI paradigm (Experiment 1) and a Visuo-Tactile Interaction task (Experiment 2), designed to tap into body representation and peripersonal space processing, respectively. In both experiments, an affective touch (CT-optimal, 3 cm/s) and neutral touch (CT-suboptimal, 18 cm/s) were delivered by the experimenter on the dorsal side of participants’ hand through a “skin to skin” contact. In Experiment 1, we did not find any modulation of body representation—not at behavioral nor at a physiological level—by affective touch. In Experiment 2, no visuo-tactile spatial modulation emerged depending upon the pleasantness of the touch received. These null findings are interpreted in the light of the current scientific context where the real nature of affective touch is often misguided, and they offer the possibility to pave the way for understanding the real effects of affective touch on body/space representation.

Close facial emotions enhance physiological responses and facilitate perceptual discrimination

Accumulating evidence indicates that the peripersonal space (PPS) constitutes a privileged area for efficient processing of proximal stimuli, allowing to flexibly adapt our behavior both to the physical and social environment. Whether and how behavioral and physiological signatures of PPS relate to each other in emotional contexts remains, though, elusive. Here, we addressed this question by having participants to discriminate male from female faces depicting different emotions (happiness, anger or neutral) and presented at different distances (50 cm-300 cm) while we measured the reaction time and accuracy of their responses, as well as pupillary diameter, heart rate and heart rate variability. Results showed facilitation of participants' performances (i.e., faster response time) when faces were presented close compared to far from the participants, even when controlling for retinal size across distances. These behavioral effects were accompanied by significant modulation of participants' physiological indexes when faces were presented in PPS. Interestingly, both PPS representation and physiological signals were affected by features of the seen faces such as the emotional valence, its sex and the participants' sex, revealing the profound impact of social context onto the autonomic state and behavior within PPS. Together, these findings suggest that both external and internal signals contribute in shaping PPS representation.

How ageing shapes body and space representations: A comparison study between healthy young and older adults

To efficiently interact with the external world, the brain needs to represent the size of the involved body parts - body representations (BR) - and the space around the body in which the interactions with the environment take place - peripersonal space representation (PPS). BR and PPS are both highly flexible, being updated by the continuous flow of sensorimotor signals between the brain and the body, as observed for example after tool-use or immobilization. The progressive decline of sensorimotor abilities typically described in ageing could thus influence BR and PPS representations in the older adults. To explore this hypothesis, we compared BR and PPS in healthy young and older participants. By focusing on the upper limb, we adapted tasks previously used to evaluate BR and PPS plasticity, i.e., the body-landmarks localization task and audio-tactile interaction task, together with a new task targeting explicit BR (avatar adjustment task, AAT). Results show significantly higher distortions in the older rather than young participants in the perceived metric characteristic of the upper limbs. We found significant modifications in the implicit BR of the global shape (length and width) of both upper limbs, together with an underestimation in the arm length. Similar effects were also observed in the AAT task. Finally, both young and older adults showed equivalent multisensory facilitation in the space close to the hand, suggesting an intact PPS representation. Together, these findings demonstrated significant alterations of implicit and explicit BR in the older participants, probably associated with a less efficient contribution of bodily information typically subjected to age-related decline, whereas the comparable PPS representation in both groups could be supported by preserved multisensory abilities in older participants. These results provide novel empirical insight on how multiple representations of the body in space, subserving actions and perception, are shaped by the normal course of life.

An Investigation Into Interpersonal and Peripersonal Spaces of Chinese People for Different Directions and Genders

This study explores the interpersonal space (IPS) and peripersonal space (PPS) of Chinese people and evaluates the relationship between the two spaces for different directions and genders. Seventy-one participants were recruited for this study. Participants were required to determine their IPS in eight directions (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°) when approached by male or female confederates in the comfort distance task. Each participant was also asked to judge their PPS in five directions (0°, 45°, 90°, 270°, 315°) following the same procedure. Results showed that their IPS was significantly influenced by direction (p < 0.05), with the largest distance in the front (0°) and the closest distance in the rear (135°, 180°, 225°), indicating non-circular IPS among Chinese subjects. Moreover, the PPS on the right side (90°) was larger than in other directions (0°, 45°, 270°, 315°). Participants maintained larger IPS than PPS in the front, but the IPS was closer than PPS on the right and left sides. When facing a female confederate, larger IPS was preferred than PPS, whereas the opposite held true when facing a male confederate. Comparison of participants’ arm length and PPS showed that the reachability distance was overestimated in the front but underestimated laterally. The findings of this study can be applied to environmental design, space utilization, and social interaction.

Immersive virtual reality reveals that visuo-proprioceptive discrepancy enlarges the hand-centred peripersonal space

Vision and proprioception, informing the system about the body position in space, seem crucial in defining the boundary of the peripersonal space (PPS). What happens to the PPS representation when a conflict between vision and proprioception arises? We capitalize on the Immersive Virtual Reality to dissociate vision and proprioception by presenting the participants' 3D hand image in congruent/incongruent positions with respect to the participants' real hand. To measure the hand-centred PPS, we exploit multisensory integration occurring when visual stimuli are delivered simultaneously with tactile stimuli applied to a body district; i.e., visual enhancement of touch (VET). Participants are instructed to respond to tactile stimuli while ignoring visual stimuli (red LED), which can appear either near to or far from the hand receiving tactile (electrical) stimuli. The results show that, when vision and proprioception are congruent (i.e., real and virtual hand coincide), a space-dependent modulation of the VET effect occurs (with faster responses when visual stimuli are near to than far from the stimulated hand). Contrarily, when vision and proprioception are incongruent (i.e., a discrepancy between real and virtual hand is present), a comparable VET effect is observed when visual stimuli occur near to the real hand and when they occur far from it, but close to the virtual hand. These findings, also confirmed by the independent estimate of a Bayesian Causal Inference model, suggest that, when the visuo-proprioceptive discrepancy makes the coding of the hand position less precise, the hand-centred PPS is enlarged, likely to optimize reactions to external events.

Sensory- and Action-Oriented Embodiment of Neurally-Interfaced Robotic Hand Prostheses

Embodiment is the percept that something not originally belonging to the self becomes part of the body. Feeling embodiment for a prosthesis may counteract amputees' altered image of the body and increase prosthesis acceptability. Prosthesis embodiment has been studied longitudinally in an amputee receiving feedback through intraneural and perineural multichannel electrodes implanted in her stump. Three factors-invasive (vs non-invasive) stimulation, training, and anthropomorphism-have been tested through two multisensory integration tasks: visuo-tactile integration (VTI) and crossing-hand effect in temporal order judgment (TOJ), the former more sensible to an extension of a safe margin around the body and the latter to action-oriented remapping. Results from the amputee participant were compared with the ones from healthy controls. Testing the participant with intraneural stimulation produced an extension of peripersonal space, a sign of prosthesis embodiment. One-month training extended the peripersonal space selectively on the side wearing the prostheses. More and less-anthropomorphic prostheses benefited of intraneural feedback and extended the peripersonal space. However, the worsening of TOJ performance following arm crossing was present only wearing the more trained, despite less anthropomorphic, prosthesis, suggesting that training was critical for our participant to achieve operative tool-like embodiment.

Peripersonal space is diversely sensitive to a temporary vs permanent state of anxiety

Peripersonal Space (PPS) is the multisensory space immediately surrounding our body. Visual and tactile stimuli here are promptly processed, since their interaction gradually strengthens as the distance between visual stimulus and the body decreases. Recently, a modified version of the Temporal Order Judgment (TOJ) task was proposed to assess PPS based on the spatial congruence between somatosensory and visual stimuli. Here, we used this paradigm to explore how a temporary vs a permanent state of anxiety can alter PPS. Indeed, previous research showed that PPS boundaries are not fixed, but they can be enlarged by contingent factors (i.e. emotional features). Participants performed the TOJ paradigm twice, just before and after completing an anxiety-inducing task (experimental breathing condition) or a neutral one (control breathing condition), while their trait and state anxiety levels were repeatedly measured. We found that the pattern of visuo-tactile integration in PPS changes in the very opposite way following the two breathing tasks for participants with high levels of temporary anxiety, by strengthening and weakening its power after the experimental and control conditions, respectively. On the contrary, both the breathing tasks are capable of reducing the cross-modal interplay as compared to baseline for high trait-anxious participants, who show an overall stronger visuo-tactile integration inside the PPS than low trait anxious individuals. These results are discussed in the light of the double dissociation between orienting and alerting attentional network over-functioning, reported in state anxiety participants, and impoverished prefrontal attentional control shown by trait anxiety individuals.

Sensory-specific predictive models in the human anterior insula

Expectations affect the subjective experience of pain by increasing sensitivity to noxious events, an effect underlain by brain regions such as the insula. However, it has been debated whether these neural processes operate on pain-specific information or on more general signals encoding expectation of unpleasant events. To dissociate these possibilities, two independent studies ( Sharvit et al., 2018, Pain; Fazeli and Büchel, 2018, J. Neurosci) implemented a cross-modal expectancy paradigm, testing whether responses to pain could also be modulated by the expectation of similarly unpleasant, but painless, events. Despite their differences, the two studies report remarkably convergent (and in some cases complementary) findings. First, the middle-anterior insula response to noxious stimuli is modulated only by expectancy of pain but not of painless adverse events, suggesting coding of pain-specific information. Second, sub-portions of the middle-anterior insula mediate different aspects of pain predictive coding, related to expectancy and prediction error. Third, complementary expectancy effects are also observed for other negative experiences (i.e., disgust), suggesting that the insular cortex holds prospective models of a wide range of events concerning their sensory-specific features. Taken together, these studies have strong theoretical implications on the functional properties of the insular cortex.