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The defensive peripersonal space represents a “safety margin” advantageous for survival. Its spatial extension and the possible relationship with personality traits have never been investigated. Here, in a population of 15 healthy human participants, we show that the defensive peripersonal space has a sharp boundary, located between 20 and 40 cm from the face, and that within such space there is a thin, “highest-risk area” closest to the face (i.e., an “ultra-near” defensive space). Single-subject analysis revealed clear interindividual differences in the extension of such peripersonal space. These differences are positively related to individual variability in trait anxiety. These findings point to the potential for measuring a range of defensive behaviors in relation to individual levels of anxiety. Such measures will allow developing procedures to test risk assessment abilities, particularly in professions that require reacting quickly to aversive stimuli near the body, such as firemen, policemen, and military officers. This may also lead to possible interventions to improve their performance under pressure.

Summary The spatial resolution of sensory systems is not homogeneous across their receptive surfaces. For example, tactile acuity is greatest on the fingertips, reflecting the high innervation density and small mechanoreceptive fields in this area [1, 2]. In contrast, pain is considered to lack any equivalent to the tactile fovea on the fingertips, where the density of nociceptive fibers is remarkably low [3]. Here, by combining psychophysics with histology, we show that this established notion is incorrect. By delivering small-diameter nociceptive-specific laser pulses to human volunteers, we discovered that (1) the spatial acuity for pain is higher on the fingertips than on proximal skin regions such as the hand dorsum, and (2) this distal-proximal gradient for pain is comparable to that for touch. In contrast, skin biopsies in the same participants showed that the intraepidermal nerve fiber density is lower in the fingertips than in the hand dorsum. The increased spatial acuity for pain on the fingertips therefore cannot be explained simply by peripheral innervation density. This finding is, however, consistent with the existence of fine-grained maps of nociceptive input to individual digits in the human primary somatosensory cortex [4]. Video Abstract, Highlights ► The spatial acuity for pain is higher on the fingertips than on the hand dorsum ► The density of nociceptive fibers is lower on the fingertips than on the dorsum ► The fovea at the fingertips is not explained by nociceptive innervation density

Sustained attention to a body location results in enhanced processing of tactile stimuli presented at that location compared to another unattended location. In this paper, we review studies investigating the neural correlates of sustained spatial attention in touch. These studies consistently show that activity within modality-specific somatosensory areas (SI and SII) is modulated by sustained tactile-spatial attention. Recent evidence suggests that these somatosensory areas may be recruited as part of a larger cortical network,also including higher-level multimodal regions involved in spatial selection across modalities. We discuss, in turn, the following multimodal effects in sustained tactile-spatial attention tasks. First, cross-modal attentional links between touch and vision, reflected in enhanced processing of task-irrelevant visual stimuli at tactuallyattended locations, are mediated by common (multimodal) representations of external space. Second, vision of the body modulates activity underlying sustained tactile-spatial attention, facilitating attentional modulation of tactile processing in between-hand (when hands are sufficiently far apart) and impairing attentional modulation in within-hand selection tasks. Finally, body posture influences mechanisms of sustained tactile-spatial attention, relying, at least partly, on remapping of tactile stimuli in external, visuallydefined, spatial coordinates. Taken together, the findings reviewed in this paper indicate that sustained spatial attention in touch is subserved by both modality-specific and multimodal mechanisms. The interplay between these mechanisms allows flexible and efficient spatial selection within and across sensory modalities.

The sight of a hand can bias the distribution of spatial attention, and recently it has been shown that viewing both hands simultaneously can facilitate spatial selection between tactile events at the hands when these are far apart. Here we directly compared the electrophysiological correlates of within-hand and between-hands tactile-spatial selection to investigate whether within-hand selection is similarly facilitated by viewing the fingers. Using somatosensory event-related potentials, we have shown that effects of selection between adjacent fingers of the same hand at early somatosensory components P45 and N80 were absent when the fingers were viewed. Thus, we found a detrimental effect of vision on tactile-spatial within-body part (i.e. hand) selection. In contrast, effects of tactile-spatial selection between hands placed next to each other, which were first found at the P100 component, were unaffected by vision of the hands. Our findings suggest that (i) within-hand and between-hands selection can operate at different stages of processing, and (ii) the effects of vision on within-hand and between-hands attentional selection may reflect fundamentally different mechanisms.

Crossing the hands over the midline impairs the ability to correctly judge the order of a pair of tactile stimuli, delivered in rapid succession, one to each hand. This impairment, termed crossed-hands deficit, has been attributed to a mismatch between the somatotopic and body-centred frames of reference, onto which somatosensory stimuli are automatically mapped. Whether or not such crossed-hands deficit occurs also when delivering nociceptive stimuli has not been previously investigated. In this study, participants performed a temporal order judgement (TOJ) task in which pairs of either nociceptive or tactile stimuli were delivered, one to each hand, while their arms were either crossed over the body midline or uncrossed. We observed that crossing the hands over the midline significantly decreases the ability to determine the stimulus order when a pair of nociceptive stimuli is delivered to the hands, and that this crossed-hands deficit has a temporal profile similar to that observed for tactile stimuli. These findings suggest that similar mechanisms for integrating somatotopic and body-centred frames of reference underlie the ability to localise both nociceptive and tactile stimuli in space. © 2012 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.

The study tested whether nociceptive stimuli applied to a body limb can orient spatial attention in external space toward visual stimuli delivered close to that limb. Nociceptive stimuli were applied to either the left or the right hand. Task-relevant visual stimuli were delivered at the location adjacent to the stimulated hand (70% valid trials) or adjacent to the other hand (30% invalid trials). Visual stimuli were discriminated with shorter reaction times and elicited ERPs of greater magnitude in the valid as compared to the invalid trials. This enhancement affected the N1 component, suggesting that the location of the nociceptive cue modifies visual processing through a modulation of neural activity in the visual cortex. We hypothesize the existence of a common frame of reference able to coordinate the mapping of the space of the body and the mapping of the external space.

The blink reflex elicited by the electrical stimulation of the median nerve at the wrist [hand blink reflex (HBR)] is a subcortical, defensive response that is enhanced when the stimulated hand is inside the peripersonal space of the face. Such enhancement results from a tonic, top-down modulation of the excitability of the brainstem interneurons mediating the HBR. Here we aim to (1) characterize the somatotopical specificity of this top-down modulation and investigate its dependence on (2) cognitive expectations and (3) the presence of objects protecting the face, in healthy humans. Experiment 1 showed that the somatotopical specificity of the HBR enhancement is partially homosegmental, i.e., it is greater for the HBR elicited by the stimulation of the hand near the face compared with the other hand, always kept far from the face. Experiment 2 showed that the HBR is enhanced only when participants expect to receive stimuli on the hand close to the face and is thus strongly dependent on cognitive expectations. Experiment 3 showed that the HBR enhancement by hand–face proximity is suppressed when a thin wooden screen is placed between the participants' face and their hand. Thus, the screen reduces the extension of the defensive peripersonal space, so that the hand is never inside the peripersonal space of the face, even in the “near” condition. Together, these findings indicate a fine somatotopical and cognitive tuning of the excitability of brainstem circuits subserving the HBR, whose strength is adjusted depending on the context in a purposeful manner.

The event-related brain potentials (ERPs) elicited by nociceptive stimuli are largely influenced by vigilance, emotion, alertness, and attention. Studies that specifically investigated the effects of cognition on nociceptive ERPs support the idea that most of these ERP components can be regarded as the neurophysiological indexes of the processes underlying detection and orientation of attention toward the eliciting stimulus. Such detection is determined both by the salience of the stimulus that makes it pop out from the environmental context (bottom-up capture of attention) and by its relevance according to the subject's goals and motivation (top-down attentional control). The fact that nociceptive ERPs are largely influenced by information from other sensory modalities such as vision and proprioception, as well as from motor preparation, suggests that these ERPs reflect a cortical system involved in the detection of potentially meaningful stimuli for the body, with the purpose to respond adequately to potential threats. In such a theoretical framework, pain is seen as an epiphenomenon of warning processes, encoded in multimodal and multiframe representations of the body, well suited to guide defensive actions. The findings here reviewed highlight that the ERPs elicited by selective activation of nociceptors may reflect an attentional gain apt to bridge a coherent perception of salient sensory events with action selection processes.

Electrical stimulation of the median nerve at the wrist may elicit a blink reflex [hand blink reflex (HBR)] mediated by a neural circuit at brain stem level. As, in a Sherringtonian sense, the blink reflex is a defensive response, in a series of experiments we tested, in healthy volunteers, whether and how the HBR is modulated by the proximity of the stimulated hand to the face. Electromyographic activity was recorded from the orbicularis oculi, bilaterally. We observed that the HBR is enhanced when the stimulated hand is inside the peripersonal space of the face, compared with when it is outside, irrespective of whether the proximity of the hand to the face is manipulated by changing the position of the arm ( experiment 1) or by rotating the head while keeping the arm position constant ( experiment 3). Experiment 2 showed that such HBR enhancement has similar magnitude when the participants have their eyes closed. Experiments 4 and 5 showed, respectively, that the blink reflex elicited by the electrical stimulation of the supraorbital nerve, as well as the N20 wave of the somatosensory evoked potentials elicited by the median nerve stimulation, are entirely unaffected by hand position. Taken together, our results provide compelling evidence that the brain stem circuits mediating the HBR in humans undergo tonic and selective top-down modulation from higher order cortical areas responsible for encoding the location of somatosensory stimuli in external space coordinates. These findings support the existence of a “defensive” peripersonal space, representing a safety margin advantageous for survival.

Visuo-tactile integration occurs in a privileged way in peripersonal space, namely when visual and tactile stimuli are in spatial proximity. Here, we investigated whether crossmodal spatial effects (i.e. stronger crossmodal interactions for spatially congruent compared to incongruent visual and tactile stimuli) are also present when visual stimuli presented near the body are indirectly viewed in a mirror, thus appearing in far space. Participants had to attend to one of their hands throughout a block of stimuli in order to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, all tactile non-target stimuli, and any visual stimuli. Visual stimuli were presented simultaneously with tactile stimuli, in the same (congruent) or opposite (incongruent) hemispace with respect to the tactile stimuli. In one group of participants the visual stimuli were delivered near the participants’ hands and were observed as indirect mirror reflections (‘mirror’ condition), while in the other group these were presented at a distance from the hands (‘far’ condition). The main finding was that crossmodal spatial modulations of ERPs recorded over and close to somatosensory cortex were present in the ‘mirror’ condition but not the ‘far’ condition. That is, ERPs were enhanced in response to tactile stimuli coupled with spatially congruent versus incongruent visual stimuli when the latter were viewed through a mirror. These effects emerged around 190 ms after stimuli onset, and were modulated by the focus of spatial attention. These results provide evidence that visual stimuli observed in far space via a mirror are coded as near-thebody stimuli according to their known rather than to their perceived location. This suggests that crossmodal interactions between vision and touch may be modulated by previous knowledge of reflecting surfaces (i.e. top-down processing).

The sight of a hand can bias the distribution of spatial attention, and recently it has been shown that viewing both hands simultaneously can facilitate spatial selection between tactile events at the hands when these are far apart. Here we directly compared the electrophysiological correlates of within-hand and between-hands tactile-spatial selection to investigate whether within-hand selection is similarly facilitated by viewing the fingers. Using somatosensory event-related potentials, we have shown that effects of selection between adjacent fingers of the same hand at early somatosensory components P45 and N80 were absent when the fingers were viewed. Thus, we found a detrimental effect of vision on tactile-spatial within-body part (i.e. hand) selection. In contrast, effects of tactile-spatial selection between hands placed next to each other, which were first found at the P100 component, were unaffected by vision of the hands. Our findings suggest that (i) within-hand and between-hands selection can operate at different stages of processing, and (ii) the effects of vision on within-hand and between-hands attentional selection may reflect fundamentally different mechanisms.

Other people can have a significant impact on one’s pain. Although correlational data abound, causal relationships between one’s pain experience, individual traits of social relating (e.g. attachment style), and social factors (e.g. empathy) have not been investigated. Here, we studied whether the presence of others and ‘perceived empathy’ (defined as participants’ knowledge of the extent to which observers felt they understood and shared their pain) can modulate subjective and autonomic responses to pain; and whether these influences can be explained by individual traits of pain coping and social attachment. Participants received noxious thermal stimuli via a thermode attached to their forearm and were asked to rate their pain. In separate blocks they were witnessed by (a) high-empathic and (b) low-empathic unfamiliar observers, and in a third condition (c) no observer was present (alone condition). We found that the effects of social presence and empathy on pain ratings depended on individual differences in attachment style. Higher scores on attachment anxiety predicted higher pain ratings in the low-empathy than in the high-empathy condition; and higher scores on attachment avoidance predicted lower pain ratings in the alone condition than with social presence. In addition, social presence decreased autonomic responses to pain irrespective of individual personality traits. To our knowledge this is the first time that adult attachment style has been shown to modulate the effects of social presence and ‘perceived empathy’ on experimentally induced pain. The results are discussed in relation to recent cognitive models of pain coping and attachment theory.

The spatial rule of multisensory integration holds that cross-modal stimuli presented from the same spatial location result in enhanced multisensory integration. The present study investigated whether processing within the somatosensory cortex reflects the strength of cross-modal visuotactile interactions depending on the spatial relationship between visual and tactile stimuli. Visual stimuli were task-irrelevant and were presented simultaneously with touch in peripersonal and extrapersonal space, in the same or opposite hemispace with respect to the tactile stimuli. Participants directed their attention to one of their hands to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, all tactile nontarget stimuli, and any visual stimuli. Enhancement of ERPs recorded over and close to the somatosensory cortex was present as early as 100 msec after onset of stimuli (i.e., overlapping with the P100 component) when visual stimuli were presented next to the site of tactile stimulation (i.e., perihand space) compared to when these were presented at different locations in peripersonal or extrapersonal space. Therefore, this study provides electrophysiological support for the spatial rule of visual–tactile interaction in human participants. Importantly, these early cross-modal spatial effects occurred regardless of the locus of attention. In addition, and in line with previous research, we found attentional modulations of somatosensory processing only to be present in the time range of the N140 component and for longer latencies with an enhanced negativity for tactile stimuli at attended compared to unattended locations. Taken together, the pattern of the results from this study suggests that visuotactile spatial effects on somatosensory processing occur prior and independent of tactile–spatial attention.

Cross-modal links between vision and touch have been extensively shown with a variety of paradigms. The present event-related potential (ERP) study aimed to clarify whether neural mechanisms underlying sustained tactile-spatial attention may be modulated by visual input, and the sight of the stimulated body part (i.e. hands) in particular. Participants covertly attended to one of their hands throughout a block to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, and all tactile non-targets. In different blocks, participants performed this task under three viewing conditions: full vision; hands covered from view; and blindfolded. When the participants’ hands were visible attention was found to modulate somatosensory ERPs at early latencies (i.e. in the time range of the somatosensory P100 and the N140 components), as well as at later time intervals, from 200 ms after stimulus onset. By contrast, when participants were blindfolded and, crucially, even when only their hands were not visible, attentional modulations were found to arise only at later intervals (i.e. from 200 ms post-stimulus), while earlier somatosensory components were not affected by spatial attention. The behavioural results tallied with these electrophysiological findings, showing faster response times to tactile targets under the full vision condition compared with conditions when participants’ hands were covered, and when participants were blindfolded. The results from this study provide the first evidence of the profound impact of vision on mechanisms underlying sustained tactile-spatial attention, which is enhanced by the sight of the body parts (i.e. hands).

To investigate whether the mechanisms underlying endogenous tactile spatial attention differ under pure tactile compared to mixed modality conditions event-related brain potentials (ERPs) were recorded to bilateral tactile and visual cues and tactile imperative stimuli. In the cue-stimulus interval the anterior directing attention negativity (ADAN) was present contralateral to the side of the attentional shift. Importantly, under pure tactile conditions this component persisted until imperative stimulus onset, while it diminished under intermodal conditions. Furthermore, post-tactile stimulus onset attentional modulations were present for the P100 component and later latencies under intermodal conditions. In contrast, under pure tactile conditions attentional modulations only emerged for the N140 component and later latencies. It is suggested that mechanisms underlying attentional orienting and selection are not entirely supramodal but depend in part on the modalities involved.

The study tested whether nociceptive stimuli applied to a body limb can orient spatial attention in external space toward visual stimuli delivered close to that limb. Nociceptive stimuli were applied to either the left or the right hand. Task-relevant visual stimuli were delivered at the location adjacent to the stimulated hand (70% valid trials) or adjacent to the other hand (30% invalid trials). Visual stimuli were discriminated with shorter reaction times and elicited ERPs of greater magnitude in the valid as compared to the invalid trials. This enhancement affected the N1 component, suggesting that the location of the nociceptive cue modifies visual processing through a modulation of neural activity in the visual cortex. We hypothesize the existence of a common frame of reference able to coordinate the mapping of the space of the body and the mapping of the external space.