Somatosensory-evoked blink response: investigation of the physiological mechanisms

Neural control of blinking

Blinking is a motor act characterized by the sequential closing and opening of the eyelids, which is achieved through the reciprocal activation of the orbicularis oculi and levator palpebrae superioris muscles. This stereotyped movement can be triggered reflexively, occur spontaneously, or voluntarily initiated. During each type of blinking, the neural control of the antagonistic interaction between the orbicularis oculi and levator palpebrae superioris muscles is governed by partially overlapping circuits distributed across cortical, subcortical, and brainstem structures. This paper provides a comprehensive overview of the anatomical and physiological foundations underlying the neural control of blinking. We describe the infra-nuclear apparatus, as well as the supra-nuclear control mechanisms, i.e., how cortical, subcortical, and brainstem structures regulate and coordinate the different types of blinking.

The blink reflex and its modulation - Part 2: Pathophysiology and clinical utility

The blink reflex (BR) is integrated at the brainstem; however, it is modulated by inputs from various structures such as the striatum, globus pallidus, substantia nigra, and nucleus raphe magnus but also from afferent input from the peripheral nervous system. Therefore, it provides information about the pathophysiology of numerous peripheral and central nervous system disorders. The BR is a valuable tool for studying the integrity of the trigemino-facial system, the relevant brainstem nuclei, and circuits. At the same time, some neurophysiological techniques applying the BR may indicate abnormalities involving structures rostral to the brainstem that modulate or control the BR circuits. This is a state-of-the-art review of the clinical application of BR modulation; physiology is reviewed in part 1. In this review, we aim to present the role of the BR and techniques related to its modulation in understanding pathophysiological mechanisms of motor control and pain disorders, in which these techniques are diagnostically helpful. Furthermore, some BR techniques may have a predictive value or serve as a basis for follow-up evaluation. BR testing may benefit in the diagnosis of hemifacial spasm, dystonia, functional movement disorders, migraine, orofacial pain, and psychiatric disorders. Although the abnormalities in the integrity of the BR pathway itself may provide information about trigeminal or facial nerve disorders, alterations in BR excitability are found in several disease conditions. BR excitability studies are suitable for understanding the common pathophysiological mechanisms behind various clinical entities, elucidating alterations in top-down inhibitory systems, and allowing for follow-up and quantitation of many neurological syndromes.

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Hand-blink reflex modulation: The role of primary emotions and attachment dimensions

The hand-blink reflex (HBR) is a subcortical response elicited by the electrical stimulation of the median nerve. HBR magnitude is enhanced when the stimulated hand is close to the face and is modulated by high-level structures according to the perceived threat magnitude. Psychological factors may contribute to threat evaluation and possibly to HBR amplitude modulation. In this study, we assessed distinctively emotional and relational aspects of personality and evaluated their associations with the HBR response, or lack thereof, in healthy subjects. Seventy-one volunteers filled the Experiences in Close Relationships Scale, the Affective Neuroscience Personality Scales, and the State-Trait Anxiety Inventory Form Y questionnaires and underwent HBR recording. We found that the HBR could be evoked only in 50.7% of all subjects (responders). Non-responders subjects showed higher scores in the avoidance dimension (p = .005), and lower scores in the care dimension (p = .008), compared with responders. In responders, regression analysis showed a negative association of HBR amplitude (difference in near vs. far responses) with anger dimension and a positive association with state anxiety (R2  = 0.239). A positive association also emerged with HBR latency and fear dimension (R2  = 0.419). We conclude that primary emotional and relational factors may play an important role in the modulation of brainstem circuits mediating the HBR response. Our results may also contribute to the question about the absence of the HBR in about half of the subjects since high-level cognitive processes seem to play an important role in the differentiation between responder and non-responder.

In the spotlight: How the brainstem modulates information flow

OBJECTIVE

The blink reflex (BR) to supraorbital nerve (SON) stimulation is reduced by either a low-intensity prepulse stimulus to digital nerves (prepulse inhibition, PPI) or a conditioning SON stimulus (SON_-1) of the same intensity as the test (SON_-2) stimulus (paired-pulse paradigm). We studied how PPI affects BR excitability recovery (BRER) to paired SON stimulation.

METHODS

Electrical prepulses were applied to the index finger 100 ms before SON_-1, which was followed by SON_-2 at interstimulus intervals (ISI) of 100, 300, or 500 ms.

RESULTS

BRs to SON_-1 showed PPI proportional to prepulse intensity, but this did not affect BRER at any ISI. PPI was observed on the BR to SON_-2 only when additional prepulses were applied 100 ms before SON_-2, regardless of the size of BRs to SON_-1.

CONCLUSIONS

In BR paired-pulse paradigms, the size of the response to SON_-2 is not determined by the size of the response to SON_-1. PPI does not leave any trace of inhibitory activity after it is enacted.

SIGNIFICANCE

Our data demonstrate that BR response size to SON_-2 depends on SON_-1 stimulus intensity and not SON_-1 response size, an observation that calls for further physiological studies and cautions against unanimous clinical applicability of BRER curves.

Eliciting blinks by transcutaneous electric nerve stimulation improves tear fluid in healthy video display terminal users: A self-controlled study

We aimed to elicit strong blinks among healthy video display terminal (VDT) users by periorbital transcutaneous electric nerve stimulation (TENS) and evaluate its impact on the tear fluid and visual task. Appropriate TENS conditions were evaluated to evoke strong blinks under minimum discomfort. Seventeen healthy VDT users with noninvasive Keratograph first breakup time (NIKf-BUT) 5-15 s and Ocular Surface Disease Index (OSDI) scores < 15 were recruited in this study. Before the trial, noninvasive Keratograph average breakup time (NIKa-BUT), tear meniscus height (TMH) and OSDI scores were evaluated. Before each TENS session, the volunteers played Tetris while the corresponding blink rate and Tetris scores were recorded. Then, the participants underwent 30 minutes of TENS, which evoked blinking of their right eye 20 times per minute. Tetris scores were evaluated again during TENS. The Tetris scores and corresponding blink rate were assessed after each TENS session while NIKa-BUT, TMH and OSDI scores were recorded after the third and sixth TENS sessions. We found that OSDI scores declined significantly after the sixth TENS (P = .003). The NIKa-BUT of the right eye was promoted after the sixth TENS (P = .02), and the TMH was higher after the third and sixth TENS in both eyes (P = .03, P = .03 for right eyes respectively, P = .01, P = .01 for left eyes respectively). There was no significant difference between the adjusted Tetris scores before and during TENS (P = .12). The blink rate before and after TENS were unaffected after 6 sessions (P = .61). The results indicated that periorbital TENS effectively ameliorated ocular irritation and improved tear secretion and tear film stability by eliciting strong blinks in healthy VDT users without disturbing the visual task.

Long latency responses in tongue muscle elicited by various stimulation sites in anesthetized humans - New insights into tongue-related brainstem reflexes

BACKGROUND

Long Latency Responses (LLR) in tongue muscles are a scarcely described phenomenon, the physiology of which is uncertain.

OBJECTIVES

The aim of this exploratory, observational study was to describe tongue-LLR elicited by direct trigeminal nerve (DTNS), dorsal column (DoColS), transcranial electric (TES) and peripheral median nerve (MNS) stimulation in a total of 93 patients undergoing neurosurgical procedures under general anesthesia.

METHODS

Bilateral tongue responses were derived concurrently after each of the following stimulations: (1) DTNS applied with single monophasic or train-of-three pulses, ≤5 mA; (2) DoColS applied with a train-of-three pulses, ≤10 mA; (3) TES consisting of an anodal train-of-five stimulation, ≤250 mA; (4) MNS at wrist consisting of single or train-of-three monophasic pulses, ≤50 mA. Polyphasic tongue muscle responses exceeding the latencies of tongue compound muscle action potentials or motor evoked potentials were classified as LLR.

RESULTS

Tongue-LLR were evoked from all stimulation sites, with latencies as follows: (1) DTNS: solely ipsilateral 20.2 ± 3.3 msec; (2) DoColS: ipsilateral 25.9 ± 1.6 msec, contralateral 25.1 ± 4.2 msec; (3) TES: contralateral 55.3 ± 10.2 msec, ipsilateral 54.9 ± 12.0 msec; (4) MNS: ipsilateral 37.8 ± 4.7 msec and contralateral 40.3 ± 3.5 msec.

CONCLUSION

The tongue muscles are a common efferent in brainstem pathways targeted by trigeminal and cervical sensory fibers. DTNS can elicit the "trigemino-hypoglossal-reflex". For the MNS elicited tongue-LLR, we propose the term "somatosensory-evoked tongue-reflex". Although the origin of the TES related tongue-LLR remains unclear, these data will help to interpret intraoperative tongue recordings.

Central nervous system physiology

This is the second chapter of the series on the use of clinical neurophysiology for the study of movement disorders. It focusses on methods that can be used to probe neural circuits in brain and spinal cord. These include use of spinal and supraspinal reflexes to probe the integrity of transmission in specific pathways; transcranial methods of brain stimulation such as transcranial magnetic stimulation and transcranial direct current stimulation, which activate or modulate (respectively) the activity of populations of central neurones; EEG methods, both in conjunction with brain stimulation or with behavioural measures that record the activity of populations of central neurones; and pure behavioural measures that allow us to build conceptual models of motor control. The methods are discussed mainly in relation to work on healthy individuals. Later chapters will focus specifically on changes caused by pathology.

Spontaneous eye blinking as a diagnostic marker in prolonged disorders of consciousness

Clinical diagnosis of patients with prolonged disorders of consciousness is very challenging. As spontaneous eye blink rate (EBR) is reliably correlated with cognitive activity in healthy individuals, we investigated whether EBR could serve as a marker of patients' level of consciousness. We assessed ten patients in prolonged Vegetative State/Unresponsive Wakefulness Syndrome (VS/UWS; three females; mean age = 50.3 ± 17.8 years) and fourteen patients in Minimally Conscious State (MCS; three females; mean age = 52.9 ± 17.5 years) at their admission to a rehabilitation unit after the acute phase. During two separate 3-min rest conditions, we recorded patients' EBR by integrating on-line visual and off-line electro-oculographic count. We also assessed EBR during two auditory oddball tasks, i.e. passive listening and active counting of target tones in a sub-group of patients. EBR was significantly higher in MCS than in VS/UWS; moreover, EBR positively correlated with a validated index of responsiveness derived from the Coma Recovery Scale-Revised. Patients' mean EBR showed no significant differences within sessions and across experimental conditions of the oddball task, in both VS/UWS and MCS. Our findings suggest that, at least in the post-acute phase, observing patients' EBR for 3 min at rest could help to discriminate between VS/UWS and MCS, improving accuracy of clinical diagnosis.

Prepulse inhibition vs cognitive modulation of the hand-blink reflex

The excitability of brainstem circuitries mediating defensive blinking in response to abrupt sensory inputs is continuously modulated by cortical areas, e.g., the hand-blink reflex (HBR), elicited by intense electrical median nerve stimulation, is enhanced when the stimulated hand is close to the face, with the behavioural purpose to optimize self-protection from increased threat. Here we investigated whether such cortically mediated HBR facilitation can be influenced by prepulse inhibition (PPI), which is known to occur entirely at the subcortical level. Twenty healthy volunteers underwent HBR recordings in five experimental conditions. In conditions 1 and 2, the stimulated hand was held either near (1) or far (2) from the face, respectively. In conditions 3 and 4, stimulation of the hand near the face was preceded by a peri-liminal prepulse to the index finger of the contralateral hand held either near (3) or far from the face (4). In condition 5, participants self-triggered the stimulus eliciting the HBR. We observed a reproducible HBR in 14 out of 20 participants and measured onset latency and area of the HBR in orbicularis oculi muscles bilaterally. HBR area decreased and latency increased in condition 2 relative to condition 1; HBR area decreased and latency increased markedly in condition 3, and somewhat less in condition 4, relative to conditions 1 and 2; self-stimulation (condition 5) also suppressed HBRs, but less than prepulses. These findings indicate that PPI of the HBR is more robust than the cognitive modulation exerted by top-down cortical projections. Possibly, an attentional shift to a prepulse may serve to reduce blinking in response to perturbation when it is convenient, in a given situation, not to interrupt ongoing visual processing.

Peripersonal Space and Bodily Self-Consciousness: Implications for Psychological Trauma-Related Disorders

Peripersonal space (PPS) is defined as the space surrounding the body where we can reach or be reached by external entities, including objects or other individuals. PPS is an essential component of bodily self-consciousness that allows us to perform actions in the world (e.g., grasping and manipulating objects) and protect our body while interacting with the surrounding environment. Multisensory processing plays a critical role in PPS representation, facilitating not only to situate ourselves in space but also assisting in the localization of external entities at a close distance from our bodies. Such abilities appear especially crucial when an external entity (a sound, an object, or a person) is approaching us, thereby allowing the assessment of the salience of a potential incoming threat. Accordingly, PPS represents a key aspect of social cognitive processes operational when we interact with other people (for example, in a dynamic dyad). The underpinnings of PPS have been investigated largely in human models and in animals and include the operation of dedicated multimodal neurons (neurons that respond specifically to co-occurring stimuli from different perceptive modalities, e.g., auditory and tactile stimuli) within brain regions involved in sensorimotor processing (ventral intraparietal sulcus, ventral premotor cortex), interoception (insula), and visual recognition (lateral occipital cortex). Although the defensive role of the PPS has been observed in psychopathology (e.g., in phobias) the relation between PPS and altered states of bodily consciousness remains largely unexplored. Specifically, PPS representation in trauma-related disorders, where altered states of consciousness can involve dissociation from the body and its surroundings, have not been investigated. Accordingly, we review here: (1) the behavioral and neurobiological literature surrounding trauma-related disorders and its relevance to PPS; and (2) outline future research directions aimed at examining altered states of bodily self-consciousness in trauma related-disorders.

Transcutaneous trigeminal nerve stimulation modulates the hand blink reflex

The hand-blink reflex (HBR) is a subcortical response, elicited by the electrical stimulation of the median nerve, whose magnitude is specifically modulated according to the spatial properties of the defensive peripersonal space (DPPS) of the face. For these reasons, the HBR is commonly used as a model to assess the DPPS of the face. Little is known on the effects induced by the activation of cutaneous afferents from the face on the DPPS of the face. Therefore, we tested the effect of non-painful transcutaneous trigeminal nerve stimulation (TNS) on the amplitude of the HBR. Fifteen healthy participants underwent HBR recording before and after 20 min of sham- and real-TNS delivered bilaterally to the infraorbital nerve in two separate sessions. The HBR was recorded bilaterally from the orbicularis oculi muscles, following non-painful median nerve stimulation at the wrist. The HBR amplitude was assessed in the “hand‐far” and “hand‐near” conditions, relative to the hand position in respect to the face. The amplitudes of the hand-far and hand-near HBR were measured bilaterally before and after sham- and real-TNS. Real-TNS significantly reduced the magnitude of the HBR, while sham-TNS had no significant effect. The inhibitory effect of TNS was of similar extent on both the hand-far and hand-near components of the HBR, which suggests an action exerted mainly at brainstem level.

Modulation of the Somatosensory Blink Reflex in the Peripersonal Space Is Defective in Episodic Migraine

OBJECTIVE

In migraine, there is an altered behavior of patients during the attack and an altered connectivity in the cortical structures modulating and encoding the sensation and pain. Thus, we hypothesized that the extent of the peripersonal space (PPS) and the responses in the PPS may change during a migraine attack. For this reason, we analyzed the modulation of somatosensory blink reflex (SBR) in the PPS during episodic migraine.

DESIGN

Cross-sectional assessment of modulation of SBR in patients with migraine.

SETTING

Headache outpatient clinic of a tertiary referral center.

SUBJECTS

We included 22 patients with episodic migraine, of whom 13 individuals were in the interictal period and nine were experiencing a headache episode. We also included 14 healthy individuals. The three groups were similar in age and gender.

METHODS

SBR was recorded when the participants were sitting with their forearm in the extrapersonal space and also when their hands were in the PPS surrounding the face. Latency, amplitude, and area under the curve (AUC) were measured and compared.

RESULTS

The amplitude and AUC of the SBR were significantly higher in patients during the attack compared with healthy subjects. The magnitude of the SBR was increased in the PPS in healthy subjects, whereas the increase was not significant in patients during the attack or in the interictal period.

CONCLUSIONS

We think that the modulation in the PPS is defective in patients with migraine both during the acute attack and in the interictal phase, suggesting diminished top-down modulation of the SBR.

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 S_4. Perceived threat close to the face facilitates TBR-R2, but knowledge about impending stimulation or self-agency overrules this effect.

Spontaneous, Voluntary, and Reflex Blinking in Clinical Practice

Blinking is one of the motor acts performed more frequently by healthy human subjects. It involves the reciprocal action of at least two muscles: the orbicularis oculi shows a brief phasic activation while the levator palpebrae shows transient inhibition. In clinical practice, noninvasive recording of the orbicularis oculi activity is sufficient to obtain useful information for electrodiagnostic testing. Blinking can be spontaneous, voluntary, or reflex. Although the analysis of spontaneous blinks can already furnish interesting data, most studies are based on reflex blinking. This article is a review of some of the alterations that can be observed in blinking, focusing in four patterns of abnormality that can be distinguished in the blink reflex: (1) afferent versus efferent, which allows characterization of trigeminal or facial lesions; (2) peripheral versus central, which distinguishes alterations in nerve conduction from those involving synaptic delay; (3) upper versus lower brainstem lesions, which indicates the lesions involving specific circuits for trigeminal and somatosensory blink reflexes; and (4) asymmetric abnormal excitability pattern, which shows a unilateral alteration in the descending control of excitability in brainstem circuits. The blink reflex excitability recovery curve to paired stimuli may provide information about other modulatory inputs to trigemino-facial circuits, such as those proposed for the connection between basal ganglia and trigeminal neurons. Finally, prepulse inhibition of blink reflex reflects the motor surrogate of subcortical gating on sensory volleys, which is still another window by which electrodiagnosis can document motor control mechanisms and their abnormalities in neurologic diseases.

Muscular effort increases hand-blink reflex magnitude

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The magnitude of hand-blink reflex is increased by tonic cortico-spinal activation.

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This effect is smaller than the commonly observed HBR increase when the stimulated hand is near the eye.

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Nonetheless, when using HBR as an indicator of behavioural relevance, this effect should be taken into account.

Defensive motor responses elicited by sudden environmental stimuli are finely modulated by their behavioural relevance to maximise the organism’s survival. One such response, the blink reflex evoked by intense electrical stimulation of the median nerve (Hand-Blink Reflex; HBR), has been extensively used to derive fine-grained maps of defensive peripersonal space. However, as other subcortical reflexes, the HBR might also be modulated by lower-level factors that do not bear direct relevance to the defensive value of blinking, thus posing methodological and interpretive problems. Here, we tested whether HBR magnitude is affected by the muscular effort present when holding the hand in certain postures. We found that HBR magnitude increases with muscular effort, an effect most likely mediated by the increased corticospinal drive. However, we found strong evidence that this effect is substantially smaller than the well-known effect of eye-hand proximity on HBR magnitude. Nonetheless, care should be taken in future experiments to avoid erroneous interpretations of the effects of muscular effort as indicators of behaviour relevance.

Movement of environmental threats modifies the relevance of the defensive eye-blink in a spatially-tuned manner

Subcortical reflexive motor responses are under continuous cortical control to produce the most effective behaviour. For example, the excitability of brainstem circuitry subserving the defensive hand-blink reflex (HBR), a response elicited by intense somatosensory stimuli to the wrist, depends on a number of properties of the eliciting stimulus. These include face-hand proximity, which has allowed the description of an HBR response field around the face (commonly referred to as a defensive peripersonal space, DPPS), as well as stimulus movement and probability of stimulus occurrence. However, the effect of stimulus-independent movements of objects in the environment has not been explored. Here we used virtual reality to test whether and how the HBR-derived DPPS is affected by the presence and movement of threatening objects in the environment. In two experiments conducted on 40 healthy volunteers, we observed that threatening arrows flying towards the participant result in DPPS expansion, an effect directionally-tuned towards the source of the arrows. These results indicate that the excitability of brainstem circuitry subserving the HBR is continuously adjusted, taking into account the movement of environmental objects. Such adjustments fit in a framework where the relevance of defensive actions is continually evaluated, to maximise their survival value.

Detecting peripersonal space: The promising role of ultrasonics

INTRODUCTION

The approach of an external stimulus to the peripersonal space (PPS) modifies some physiological measures, including the cerebral blood flow (CBF) in the supplementary motor area and premotor cortex. CBF measurement may be useful to assess brain activations when producing specific motor responses, likely mediated by cortical and subcortical neural circuits.

METHODS

This study investigated PPS in 15 healthy humans by characterizing the hemodynamic responses (pulsatility index, PI; and heart rate, HR) related to different directions of movements of individual's hand toward and backward his/her own face, so to perturb PPS).

RESULTS

We observed that the CBF and HR were enhanced more when the stimulated hand was inside the PPS of the face in the passive and active condition than when the hand was outside the PPS and during motor imagery task.

CONCLUSIONS

These results suggest that the modulation of PPS-related brain responses depends on specific sensory-motor integration processes related to the location and the final position of a target in the PPS. We may thus propose TCD as a rapid and easy approach to get information concerning brain responses related to stimuli approaching the PPS. Understanding the modulations of brain activations during tasks targeting PPS can help to understand the results of psychophysical and behavioral trials and to plan patient-tailored cognitive rehabilitative training.

Pattern of startle reflex to somatosensory stimuli changes after spinal cord injury

INTRODUCTION

Spinal cord injury (SCI) may cause functional changes at various levels in central and peripheral nervous systems. One of these changes is increased excitability above the lesion such as enhanced auditory startle responses (ASR). Startle response may also be obtained after somatosensory stimulus (startle reflex to somatosensory stimuli, SSS). In this study, we investigated changes of both ASR and SSS in SCI.

METHOD

We examined ASR and SSS in 14 patients with SCI and 18 age-matched healthy volunteers. SSS responses were recorded from orbicularis oculi (O.oc), sternocleidomastoid (SCM) and biceps brachii (BB) muscles by electrical stimulation of median nerve at the wrist. ASR was evoked by binaural auditory stimuli and recorded from O.oc, masseter, SCM and BB muscles. Probability, latency, amplitude and duration of responses were compared between two groups for each muscle.

RESULTS

Presence of response over O.oc after somatosensory stimuli was decreased in patients compared to controls (P = 0.004). There were no differences in SSS responses of other muscles. ASR latency was shorter in masseter, SCM and BB in patients with SCI, but only BB had significantly reduced latency (P = 0.033). The duration of O.oc response was longer and the amplitude of SCM was larger in patients with SCI (P = 0.037 and P = 0.015, respectively).

CONCLUSION

ASR is enhanced after SCI whereas SSS of eye muscles is hypoactive and pattern of SSS after median stimulation changes in SCI.

Pain outside the body: defensive peripersonal space deformation in trigeminal neuralgia

Perception of space has been guiding effective therapeutic interventions in a number of unilateral chronic pain conditions. However little is known about how trigeminal neuralgia (TN), a condition in which trigeminal stimulation triggers paroxysmal facial pain, affects defensive peripersonal space (DPPS), the portion of space surrounding the body within which defensive responses are enhanced. Given that TN is unilateral, in TN patients the DPPS of the face might not be horizontally symmetric as in pain-free individuals, but instead larger around the affected side. We tested this a priori hypothesis by measuring the proximity-dependent modulation of the hand-blink reflex. Stimuli delivered to the hand ipsilateral to TN elicited a stronger blink, particularly when it was measured from the eye ipsilateral to TN and the hand was closer to the face. Geometric modelling revealed (1) that DPPS was larger on the side of space ipsilateral to TN, and (2) this asymmetry was consequent to an increased estimated potential of sensory events to cause harm when they occur ipsilaterally to TN. These observations demonstrate that neural mechanisms underlying body protection in TN are adjusted to reduce the likelihood that external events evoke the painful paroxysm typical of this condition.

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.

Bodily ownership modulation in defensive responses: physiological evidence in brain-damaged patients with pathological embodiment of other's body parts

Do conscious beliefs about the body affect defensive mechanisms within the body? To answer this question we took advantage from a monothematic delusion of bodily ownership, in which brain-damaged patients misidentify alien limbs as their own. We investigated whether the delusional belief that an alien hand is their own hand modulates a subcortical defensive response, such as the hand-blink reflex. The blink, dramatically increases when the threated hand is inside the defensive peripersonal-space of the face. In our between-subjects design, including patients and controls, the threat was brought near the face either by the own hand or by another person’s hand. Our results show an ownership-dependent modulation of the defensive response. In controls, as well as in the patients’ intact-side, the response enhancement is significantly greater when the threat was brought near the face by the own than by the alien hand. Crucially, in the patients’ affected-side (where the pathological embodiment occurs), the alien (embodied) hand elicited a response enhancement comparable to that found when the threat is brought near the face by the real hand. These findings suggest the existence of a mutual interaction between our conscious beliefs about the body and the physiological mechanisms within the body.

The blink reflex magnitude is continuously adjusted according to both current and predicted stimulus position with respect to the face

The magnitude of the hand-blink reflex (HBR), a subcortical defensive reflex elicited by the electrical stimulation of the median nerve, is increased when the stimulated hand is close to the face (‘far–near effect’). This enhancement occurs through a cortico-bulbar facilitation of the polysynaptic medullary pathways subserving the reflex. Here, in two experiments, we investigated the temporal characteristics of this facilitation, and its adjustment during voluntary movement of the stimulated hand. Given that individuals navigate in a fast changing environment, one would expect the cortico-bulbar modulation of this response to adjust rapidly, and as a function of the predicted spatial position of external threats. We observed two main results. First, the HBR modulation occurs without a temporal delay between when the hand has reached the stimulation position and when the stimulus happens (Experiments 1 and 2). Second, the voluntary movement of the hand interacts with the ‘far–near effect’: stimuli delivered when the hand is far from the face elicit an enhanced HBR if the hand is being moved towards the face, whereas stimuli delivered when the hand is near the face elicit an enhanced HBR regardless of the direction of the hand movement (Experiment 2). These results indicate that the top-down modulation of this subcortical defensive reflex occurs continuously, and takes into account both the current and the predicted position of potential threats with respect to the body. The continuous control of the excitability of subcortical reflex circuits ensures appropriate adjustment of defensive responses in a rapidly-changing sensory environment.

Blepharospasm: Update on Epidemiology, Clinical Aspects, and Pathophysiology

Blepharospasm (BSP) is a rather distressing form of focal dystonia. Although many aspects of its pathophysiological mechanisms are already known, we lack fundamental evidence on etiology, prevention, and treatment. To advance in our knowledge, we need to review what is already known in various aspects of the disorder and use these bases to find future lines of interest. Some of the signs observed in BSP are cause, while others are consequence of the disorder. Non-motor symptoms and signs may be a cue for understanding better the disease. Various cerebral sites have been shown to be functionally abnormal in BSP, including the basal ganglia, the cortex, and the cerebellum. However, we still do not know if the dysfunction or structural change affecting these brain regions is cause or consequence of BSP. Further advances in neurophysiology and neuroimaging may eventually clarify the pathophysiological mechanisms implicated. In this manuscript, we aim to update what is known regarding epidemiology, clinical aspects, and pathophysiology of the disorder and speculate on the directions of research worth pursuing in the near future.

Startle responses after different stimulus modalities differ in stroke

OBJECTIVES

The auditory startle reaction (ASR) and startle reflex to somatosensory inputs (SSS) are stereotypical responses to sudden and unexpected stimuli, which are generated in the caudal brainstem reticular formation. Changes of ASR are relatively well known in stroke. Here, we aimed to investigate central pathways of SSS and plasticity changes of SSS circuits in different stages and localizations of stroke, by comparing with ASR.

METHODS

We prospectively included 39 patients with stroke between June 2009 and June 2013, and 23 age and gender-matched healthy subjects. ASR and SSS were recorded over orbicularis oculi, sternocleidomastoid, biceps brachii (BB), and abductor policis brevis muscles (APB) using surface electrodes.

RESULTS

There were supratentorial and infratentorial lesions in both acute and chronic stages. Overall, ASR probability was similar between groups (P=0.981). However, ASR probability was increased for BB and APB recordings on symptomatic sides of stroke patients with high amplitudes and long durations, most prominently on symptomatic sides of pontine strokes. Latencies and presence rates of SSS did not differ between any subgroups of stroke and healthy subjects.

CONCLUSION

ASR is facilitated in arm and hand muscles on symptomatic sides of stroke patients, whereas SSS did not show any significant changes according to stroke.

A geometric model of defensive peripersonal space

Potentially harmful stimuli occurring within the defensive peripersonal space (DPPS), a protective area surrounding the body, elicit stronger defensive reactions. The spatial features of the DPPS are poorly defined and limited to descriptive estimates of its extent along a single dimension. Here we postulated a family of geometric models of the DPPS, to address two important questions with respect to its spatial features: What is its fine-grained topography? How does the nervous system represent the body area to be defended? As a measure of the DPPS, we used the strength of the defensive blink reflex elicited by electrical stimulation of the hand (hand-blink reflex, HBR), which is reliably modulated by the position of the stimulated hand in egocentric coordinates. We tested the goodness of fit of the postulated models to HBR data from six experiments in which we systematically explored the HBR modulation by hand position in both head-centered and body-centered coordinates. The best-fitting model indicated that 1) the nervous system's representation of the body area defended by the HBR can be approximated by a half-ellipsoid centered on the face and 2) the DPPS extending from this area has the shape of a bubble elongated along the vertical axis. Finally, the empirical observation that the HBR is modulated by hand position in head-centered coordinates indicates that the DPPS is anchored to the face. The modeling approach described in this article can be generalized to describe the spatial modulation of any defensive response.

Blink reflex studies in postparalytic facial syndrome and blepharospasm: trigeminal and extratrigeminal somatosensory stimulation

PURPOSE

The somatosensory-evoked blink reflex (SBR) is one of the release phenomena of blink reflex, possibly resulting from increased excitability of brainstem reticular formation.

METHODS

The authors investigated trigeminal blink responses and SBR in 26 patients with postparalytic facial syndrome (PFS) with synkinesia, 18 patients with essential blepharospasm, and 36 healthy volunteers (control participants).

RESULTS

Trigeminal blink reflex responses were elicited in all participants, whereas SBRs were elicited in 44.4% of control participants, 38.9% of patients with essential blepharospasm, and 65.4% of patients with PFS. The mean R2 amplitude and duration and the mean amplitude and duration of SBR were highest in patients with essential blepharospasm. The mean latency of SBR was shorter on the symptomatic side of patients with PFS when compared with the asymptomatic side. The mean R2 duration on the symptomatic side of the patients with PFS was longer than the control participants.

CONCLUSIONS

These results showed that somatosensory stimulation could be used as an alternative method to demonstrate increased excitability in facial motor neuron in patients with PFS and essential blepharospasm. Disease states relating to different peripheral and/or suprasegmental structures could also influence blink reflex and change its basal excitability and manner in which the reflex responds to modulatory factors.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Better safe than sorry? The safety margin surrounding the body is increased by anxiety

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.

To blink or not to blink: fine cognitive tuning of the defensive peripersonal 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.

Defensive peripersonal space: the blink reflex evoked by hand stimulation is increased when the hand is near the face

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.

Assessment of excitability in brainstem circuits mediating the blink reflex and the startle reaction

Excitability is probably the concept that fits better with the definition of the role of neurophysiology in the study of brainstem functions and circuits. Neurophysiological techniques are likely the best suited of all paraclinical tests for documenting the eventual excitability changes that may occur in certain physiological states and in many neurological disorders. The best known test of brainstem excitability is the blink reflex. While a single stimulus can already indicate the readiness of the interneuronal path and the facial motoneurons to fire, pairs of stimuli (conditioning and test) are suited to analyze the degree of excitability recovery after a single discharge. Another brainstem reflex circuit, which excitability testing can be of interest for physiological and clinical exams is the one involved in the startle reaction. The size of the responses and their habituation are the typical measures of excitability of the startle reflex circuit. Prepulse inhibition is a method to modulate both, the blink reflex and the startle reaction. It is defined as the inhibitory effect caused by a stimulus of an intensity low enough not to induce a response by itself on the response elicited by a subsequent stimulus. The circuits of the blink reflex, startle reaction and prepulse inhibition share some commonalities but they are different enough for the three techniques to provide unique, clinically relevant, information in certain conditions. The role of neurophysiology is not limited to testing those functions. It is important also for the assessment of many other circuits, such as those implicated in eye movements, vestibular reflexes, arousal, sleep, breathing, or autonomic reactions, which are not considered in this review.

Considerations for the use of a startling acoustic stimulus in studies of motor preparation in humans

Recent studies have used a loud (> 120 dB) startle-eliciting acoustic stimulus as a probe to investigate early motor response preparation in humans. The use of a startle in these studies has provided insight into not only the neurophysiological substrates underlying motor preparation, but also into the behavioural response strategies associated with particular stimulus-response sets. However, as the use of startle as a probe for preparation is a relatively new technique, a standard protocol within the context of movement paradigms does not yet exist. Here we review the recent literature using startle as a probe during the preparation phase of movement tasks, with an emphasis on how the experimental parameters affect the results obtained. Additionally, an overview of the literature surrounding the startle stimulus parameters is provided, and factors affecting the startle response are considered. In particular, we provide a review of the factors that should be taken into consideration when using a startling stimulus in human research.

Brainstem reflexes: electrodiagnostic techniques, physiology, normative data, and clinical applications

An overview is provided on the physiological aspects of the brainstem reflexes as they can be examined by use of clinically applicable neurophysiological tests. Brainstem reflex studies provide important information about the afferent and efferent pathways and are excellent physiological tools for the assessment of cranial nerve nuclei and the functional integrity of suprasegmental structures. In this review, the blink reflex after trigeminal and nontrigeminal inputs, corneal reflex, levator palpebrae inhibitory reflex, jaw jerk, masseter inhibitory reflex, and corneomandibular reflex are discussed. Following description of the recording technique, physiology, central pathways, and normative data of these reflexes, including an account of the recording of recovery curves, the application of these reflexes is reviewed in patients with various neurological abnormalities, including trigeminal pain and neuralgia, facial neuropathy, and brainstem and hemispherical lesions. Finally, simultaneous electromyographic recording from the orbicularis oculi and the levator palpebrae muscles is discussed briefly in different eyelid movement disorders.

A neurophysiological approach to brainstem reflexes. Blink reflex

The blink reflex (BR) is a generalised phenomenon in mammals. Its teleological protective eye function is perhaps the reason why the BR can be provoked by a multitude of stimuli. As corneal and glabellar reflexes, BR has an inveterate use in the neurological exploration. Some of its physiopathological aspects were discussed more than 100 years ago, and soon half a century will have passed since the first electrophysiological study was published. This review focuses on the BR elicited by the electrical stimulation of the trigeminal supraorbital nerve, a controlled and reliable model in clinical neurophysiology. The electrically elicited BR is an exteroceptive-nociceptive reflex recorded on the orbicularis oculi muscle and formed by three components: the two principal ones, R1 and R2, of well-known characteristics, and a third, R3, of increasing interest, to which there is wide mention. The trigeminal afferent limb reaches the facial efferent one by means of a long and quite complex central pathway located at the brainstem bulbopontine level. The anatomical substrate and criteria of the rich topographical lesional semiology of the BR are established. The importance of the suprasegmental influences upon the reflex, coming mainly from the cerebral cortex and basal ganglia, as well as the impairment caused by their damage, will be emphasised. Special attention is paid to the relationship between the reflex and the dopaminergic system, and the consequences of its derangement. The methods of habituation and suppression-recovery of the BR are extensively and critically reviewed. These methods measure its excitability and serve in practice for the pathophysiological study of numerous diseases. The relationship of the BR with the spontaneous blinking is considered, and the existence of a primary inhibitory reflex on levator palpebrae muscles, previous to the active reflex response of the orbicularis, is proposed. The electrophysiological characteristics of the glabellar reflex, the corneal reflex, the acoustic, photic and somatosensory provoked BR, the ontogeny, and some of the common factors influencing the reflex, such as sleep, are also discussed. The strategic position of the neural structures of the BR, in an area involved in the gating of the various sensory-motor systems and the relative ease to its evaluation with common methodology used in clinical neurophysiology, makes the BR an essential tool for the diagnosis and pathophysiological insight into an important number of human neurological disorders.