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.

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.

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.

21The event-related brain potentials (ERPs) elicited by nociceptive stimuli are largely influenced by 22 vigilance, emotions, alertness and attention. Studies that have specifically investigated the effects of 23 cognition on the nociceptive ERPs support the idea that most of the components of these ERPs can 24 be regarded as the neurophysiological indexes of the processes underlying the detection and the 25 orientation of attention toward the eliciting stimulus. Such detection is determined by the salience of 26 the stimulus that makes it pop out from the environmental context (bottom-up capture of attention) 27and by its relevance according to the subject's goals and motivation (top-down attentional control). 28The fact that nociceptive ERPs are largely influenced by information from other sensory modalities 29 such as vision and proprioception, as well as from motor preparation, suggests that these ERPs 30 reflect a cortical system involved in the detection of potentially meaningful stimuli for the body, with 31 the purpose to respond adequately to potential threats. In such a theoretical framework, pain is seen 32 que par la préparation motrice suggèrent que ces PE reflètent un système cortical impliqué dans la 55 détection des stimuli potentiellement significatifs pour l'organisme dans le but de répondre 56 adéquatement aux menaces éventuelles. Dans un tel cadre théorique, la douleur est considérée 57 comme un épiphénomène des processus d'alerte, intégré dans des représentations multimodales et 58 multi-référentielles du corps dont le but est de guider la réalisation des comportements de défense. 59Les données présentées dans cet article soulignent que les PE obtenus en réponses à des 60 stimulations sélectives des nocicepteurs peuvent représenter l'activité des mécanismes de contrôle 61 du gain attentionnel permettant de coordonner de façon cohérente la perception d'événements 62 sensoriels saillants et la sélection de la réponse. 63 64