Modulation of exteroceptive electromyographic responses in defensive peripersonal space.

The cutaneous silent period (CSP) to noxious finger stimulation constitutes a robust spinal inhibitory reflex that protects the hand from injury. In certain conditions, spinal inhibition is interrupted by a brief burst-like electromyographic activity, dividing the CSP into two inhibitory phases (I1 and I2). This excitatory component is termed long-loop reflex (LLR) and is presumed to be transcortical in origin. Efficient defense from environmental threats requires sensorimotor integration between multimodal sensory afferents and planning of defensive movements. In the defensive peripersonal space (DPPS) immediately surrounding the body, we interact with objects and persons with increased alertness. We investigated whether CSP differs when the stimulated hand is in the DPPS of the face compared with a distant position. Furthermore, we investigated the possible role of vision in CSP modulation. Fifteen healthy volunteers underwent CSP testing with the handheld either within 5 cm from the nose (near) or away from the body (far). Recordings were obtained from first dorsal interosseous muscle following index (D2) or little finger (D5) stimulation with varying intensities. A subgroup of subjects underwent CSP recordings in near and far conditions, both with eyes open and with eyes closed. No inhibitory CSP parameter differed between stimulation in near and far conditions. LLRs occurring following D2 stimulation were significantly larger in near than far conditions at all stimulus intensities, irrespective of subjects seeing their hand. Similar to the hand-blink reflex, spinally organized protective reflexes may be modulated by corticospinal facilitatory input when the hand enters the DPPS of the face. NEW & NOTEWORTHY The present findings demonstrate for the first time that a spinally organized protective reflex, the cutaneous silent period (CSP), may be modulated by top-down corticospinal facilitatory input when the stimulated hand enters the defensive peripersonal space (DPPS) of the face. In particular, the cortically mediated excitatory long-loop reflex, which may interrupt the CSP, is facilitated when the stimulated hand is in the DPPS, irrespective of visual control over the hand. No spinal inhibitory CSP parameter differs significantly in or outside the DPPS.

Dynamic Shaping of the Defensive Peripersonal Space through Predictive Motor Mechanisms: When the "Near" Becomes "Far".

The hand blink reflex is a subcortical defensive response, known to dramatically increase when the stimulated hand is statically positioned inside the defensive peripersonal space (DPPS) of the face. Here, we tested in a group of healthy human subjects the hand blink reflex in dynamic conditions, investigating whether the direction of the hand movements (up-to/down-from the face) could modulate it. We found that, on equal hand position, the response enhancement was present only when the hand approached to (and not receded from) the DPPS of the face. This means that, when the hand is close to the face but the subject is planning to move the hand down, the predictive motor system can anticipate the consequence of the movement: the "near" becomes "far." We found similar results both in passive movement condition, when only afferent (visual and proprioceptive) information can be used to estimate the final state of the system, and in motor imagery task, when only efferent (intentional) information is available to predict the consequences of the movement. All these findings provide evidence that the DPPS is dynamically shaped by predictive mechanisms run by the motor system and based on the integration of feedforward and sensory feedback signals.SIGNIFICANCE STATEMENT The defensive peripersonal space (DPPS) has a crucial role for survival, and its modulation is fundamental when we interact with the environment, as when we move our arms. Here, we focused on a defensive response, the hand blink reflex, known to increase when a static hand is stimulated inside the DPPS of the face. We tested the hand blink reflex in dynamic conditions (voluntary, passive, and imagined movements) and we found that, on equal hand position, the response enhancement was present only when the hand approached to (and not receded from) the DPPS of the face. This suggests that, through the integration of efferent and afferent signals, the safety boundary around the body is continuously shaped by the predictive motor system.

Threat vs control: Potentiation of the trigeminal blink reflex by threat proximity is overruled by self-stimulation.

The magnitude of the defensive blink reflex is modulated by continuous assessment of its protective value. Here, we studied whether the trigeminal blink reflex (TBR) is modulated by a potentially offensive object close to the face, and, if so, whether self-stimulation or observation of the act of stimulus triggering counteracts such modulation. In all, 26 healthy volunteers participated in various experimental conditions. At baseline, an experimenter triggered supraorbital nerve stimuli remotely, unseen by the participants; in experimental conditions, the experimenter held a stimulation probe close to the participant's face but triggered the stimuli either remotely, "surprising" participants (S1 ), or directly on the probe, observed by participants (S2 ). In other conditions, participants triggered stimuli themselves on the probe held next to their body (S3 ) or held in front of their face (S4 ). The latter condition was repeated similarly, but pressing the button only randomly generated electrical stimuli (S5, "Russian roulette"). The size of the R2 component of the TBR (TBR-R2) was the main outcome measure. Compared to baseline, TBR-R2 area was significantly larger in S1 when the "threatening" probe was close to the face and the participant had no control over stimulation. Conversely, TBR-R2 was suppressed when participants either saw the action of triggering, thus being aware (S2 ), or had full initiative over stimulation (S3 , S4 ). Random self-generated stimuli (S5 ) inhibited TBR-R2, but to a lesser extent than S3 and S4. Perceived threat close to the face facilitates TBR-R2, but knowledge about impending stimulation or self-agency overrules this effect.

Defending the Body Without Sensing the Body Position: Physiological Evidence in a Brain-Damaged Patient With a Proprioceptive Deficit.

The ability to know where our body parts are located in space (proprioception) is fundamental for both successfully interacting with the external world and monitoring potential threats. In this case-control study, we investigated whether the absence of proprioceptive signals may affect physiological defensive responses. To this aim, a right brain-damaged patient with a left upper-limb proprioceptive deficit (P+ patient) and age-matched healthy controls, underwent the recording of the Hand-Blink Reflex (HBR). This defensive response, elicited by electrical stimulation of the median nerve and recorded from the orbicularis oculi, is modulated by the hand position: it is enhanced when the threatened hand is near to the face, inside the defensive peripersonal-space (DPPS). According to the classical neuropsychological perspective, we used P+ patient as a model to investigate the role of proprioception in HBR modulation, by manipulating the congruity/incongruity between the intended and actual positions of the stimulated hand. P+ patient, with his eyes closed, had to voluntarily place his left hand either far from or near to his face and to relieve the arm's weight over a supporting device. Then, in congruent conditions, the hand was stimulated in the actual (intended) position. In incongruent conditions, the patient's hand was moved by the examiner from the intended to the opposite (not-intended) position and then stimulated. We observed an inverse response pattern between congruent and incongruent conditions. In congruent conditions, P+ patient showed an HBR enhancement in near compared to far position, comparable to that found in healthy controls. This suggests that, even in absence of proprioceptive and visual information, the HBR modulation was still present. Conversely, in incongruent conditions, P+ patient showed a greater HBR magnitude for far position (when the hand was actually far, but the patient intended it to be near) than for near position (when the hand was actually near, but the patient intended it to be far). This result suggests that proprioceptive signals are not necessary for HBR modulation to occur. It relies more on the intended than on the actual position of the hand. The role of motor intention and planning in shaping the DPPS is discussed.