The startle reaction to somatosensory inputs: different response pattern to stimuli of upper and lower limbs

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).

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.

Acoustic pre-stimulation modulates startle and postural reactions during sudden release of standing support surface in aging

Falls contribute to injuries and reduced level of physical activity in older adults. During falls, the abrupt sensation of moving downward triggers a startle-like reaction that may interfere with protective response movements necessary to maintain balance. Startle reaction could be dampened by sensory pre-stimulation delivered immediately before a startling stimulus. This study investigated the neuromodulatory effects of pre-stimulation on postural/startle responses to drop perturbations of the standing support surface in relation to age. Ten younger and 10 older adults stood quietly on an elevated computer-controlled moveable platform. At an unpredictable time, participants were dropped vertically to elicit a startle-like response. Reactive drop perturbation trials without a pre-stimulus (control) were alternated with trials with acoustic pre-stimulus tone (PSI). A two-way mixed design analysis of variance comparing condition (control vs. PSI) X group (younger vs. older) was performed to analyze changes in muscle activation patterns, ground reaction force, and joint angular displacements. Compared to younger adults, older adults showed lower neck muscle electromyography amplitude reduction rate and incidence of response. Peak muscle activation in neck, upper arm, and hamstring muscles were reduced during PSI trials compared to control trials in both groups (p < 0.05). In addition, knee and hip joint flexion prior to ground contact was reduced in PSI trials compared to control (p < 0.05). During post-landing balance recovery, increased knee and hip flexion displacement and time to peak impact force were observed in PSI trials compared to control condition (p < 0.05). PSI reduced startle-induced muscle activation at proximal body segments and likely decreased joint flexion during abrupt downward vertical displacement perturbations of the body. Older adults retained the ability to modulate startle and postural responses but their neuromodulatory capacity was reduced compared with younger adults. Further research on the potential of applying PSI as a possible therapeutic tool to reduce the risk of fall-related injury is needed.

Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses

Introduction

Transcranial electrical (TES) and magnetic stimulation (TMS) are both used for assessment of the motor function of the spinal cord in horses. Muscular motor evoked potentials (mMEP) were compared intra-individually for both techniques in five healthy horses. mMEPs were measured twice at increasing stimulation intensity steps over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles. Significance was set at p < 0.05. To support the hypothesis that both techniques induce extracranially elicited mMEPs, literature was also reviewed.

Results

Both techniques show the presence of late mMEPs below the transcranial threshold appearing as extracranially elicited startle responses. The occurrence of these late mMEPs is especially important for interpretation of TMS tracings when coil misalignment can have an additional influence. Mean transcranial motor latency times (MLT; synaptic delays included) and conduction velocities (CV) of the ECR and TC were significantly different between both techniques: respectively, 4.2 and 5.5 ms (MLT _{TMS -}-MLT _TES ), and -7.7 and -9.9 m/s (CV _TMS -CV _TES ). TMS and TES show intensity-dependent latency decreases of, respectively, -2.6 (ECR) and -2.7 ms (TC)/30% magnetic intensity and -2.6 (ECR) and -3.2 (TC) ms/30V. When compared to TMS, TES shows the lowest coefficients of variation and highest reproducibility and accuracy for MLTs. This is ascribed to the fact that TES activates a lower number of cascaded interneurons, allows for multipulse stimulation, has an absence of coil repositioning errors, and has less sensitivity for varying degrees of background muscle tonus. Real axonal conduction times and conduction velocities are most closely approximated by TES.

Conclusion

Both intracranial and extracranial mMEPs inevitably carry characteristics of brainstem reflexes. To avoid false interpretations, transcranial mMEPs can be identified by a stepwise latency shortening of 15-20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. A ring block around the vertex is advised to reduce interference by extracranial mMEPs. mMEPs reflect the functional integrity of the route along the brainstem nuclei, extrapyramidal motor tracts, propriospinal neurons, and motoneurons. The corticospinal tract appears subordinate in horses. TMS and TES are interchangeable for assessing the functional integrity of motor functions of the spinal cord. However, TES reveals significantly shorter MLTs, higher conduction velocities, and better reproducibility.

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.

Peri-Personal Space Tracing by Hand-Blink Reflex Modulation in Patients with Chronic Disorders of Consciousness

The assessment of awareness in patients with chronic Disorders of Consciousness (DoC), including Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS), is challenging. The level of awareness impairment may depend on the degree of deterioration of the large-scale cortical-thalamo-cortical networks induced by brain injury. Electrophysiological approaches may shed light on awareness presence in patients with DoC by estimating cortical functions related to the cortical-thalamo-cortical networks including, for example, the cortico-subcortical processes generating motor responses to the perturbation of the peri-personal space (PPS). We measured the amplitude, latency, and duration of the hand-blink reflex (HBR) responses by recording electromyography (EMG) signals from both the orbicularis oculi muscles while electrically stimulating the median nerve at the wrist. Such a BR is thought to be mediated by a neural circuit at the brainstem level. Despite its defensive-response nature, HBR can be modulated by the distance between the stimulated hand and the face. This suggests a functional top-down control of HBR as reflected by HBR features changes (latency, amplitude, and magnitude). We therefore estimated HBR responses in a sample of patients with DoC (8 MCS and 12 UWS, compared to 15 healthy controls -HC) while performing a motor task targeting the PPS. This consisted of passive movements in which the hand of the subject was positioned at different distances from the participant's face. We aimed at demonstrating a residual top-down modulation of HBR properties, which could be useful to differentiate patients with DoC and, potentially, demonstrate awareness preservation. We found a decrease in latency, and an increase in duration and magnitude of HBR responses, which were all inversely related to the hand-to-face distance in HC and patients with MCS, but not in individuals with UWS. Our data suggest that only patients with MCS have preserved, residual, top-down modulation of the processes related to the PPS from higher-order cortical areas to sensory-motor integration network. Although the sample size was relatively small, being thus our data preliminary, HBR assessment seems a rapid, easy, and first-level tool to differentiate patients with MCS from those with UWS. We may also hypothesize that such a HBR modulation suggests awareness preservation.

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.

Jendrassik maneuver effect on spinal and brainstem reflexes

The effect of Jendrassik Maneuver (JM) has been extensively studied on monosynaptic reflexes in numerous muscles below the level at which the maneuver was performed. Here we hypothesize that the effect of JM could be observed also on other reflexes, indicating a widespread influence of performing a motor act such as the JM. We examined polysynaptic reflexes caudal (i.e., the withdrawal reflex of the lower extremities) and rostral (i.e., the blink reflex to supraorbital nerve stimulation) to the level of JM contraction. We have assessed soleus tendon (T) reflex; withdrawal reflex in tibialis anterior and soleus muscle; blink reflex (BR), blink reflex excitability recovery curve (BR-ER) and prepulse inhibition of the blink reflex. Our results showed that (1) T-reflex amplitude increased during JM and decreased just after and 15 min after JM; (2) no change in the withdrawal reflex; (3) R2 area of BR reduced significantly just after or 15 min after JM; (4) Prepulse inhibition in BR reduced significantly during JM; (5) no change in BR-ER. Our results indicate that JM leads to generalized effects on neural excitability at both caudal and rostral levels. Furthermore, JM has a selective effect on excitability of reflex circuitries.

Habituation of the Interlimb Reflex (ILR) Over the Biceps Brachii Muscle After Electrical Stimuli Over the Sural Nerve

Up to now relatively little is known about interlimb reflexes (ILR). Especially it is not well known whether ILR may habituate or not to subsequent stimuli. The main aim of the present investigation was to explore the short term habituation behavior of ILR. The electromyogram was recorded over the tonically active biceps brachii (BB) muscle in 11 healthy subjects contralateral and ipsilateral to supramaximum electrical stimuli (9–12 mA) that were delivered at 1.0 and 0.4 Hz over the left sural nerve. In addition, a selective averaging method was used to investigate the influence of preceding stimuli on the ILR. Thus, 30 blocks of 3 subsequent stimuli were used. All 1st ILR of each block were averaged together. Averages were also obtained for 2nd and 3rd ILR. While ILR amplitudes gained significantly both ipsilateral and contralateral to the stimulus (p < 0.05) after train stimuli as compared with single stimuli, ILR amplitudes showed a significant decrease at 1.0 Hz versus 0.4 Hz stimuli. ILR amplitudes decreased significantly after the 2nd and 3rd stimulus relative to the 1st (p < 0.05). ILR can be recorded bilaterally remote from the stimulus site. Furthermore, ILR show clear short term habituation behavior.

Influence of posture on blink reflex prepulse inhibition induced by somatosensory inputs from upper and lower limbs

BACKGROUND

Prepulse inhibition (PPI) is a neurophysiological phenomenon whereby a weak stimulus modulates the reflex response to a subsequent strong stimulus. Its physiological purpose is to avoid interruption of sensory processing by subsequent disturbing stimuli at the subcortical level, thereby preventing undesired motor reactions. An important hub in the PPI circuit is the pedunculopontine nucleus, which is also involved in the control of posture and sleep/wakefulness.

OBJECTIVE

To study the effect of posture (supine versus standing) on PPI, induced by somatosensory prepulses to either upper or lower limb. PPI was measured as the percentage inhibition of the blink reflex response to electrical supraorbital nerve (SON) stimulation.

METHODS

Sixteen healthy volunteers underwent bilateral blink reflex recordings following SON stimulation either alone (baseline) or preceded by an electrical prepulse to the median nerve (MN) or sural nerve (SN), both in supine and standing. Stimulus intensity was 8 times sensory threshold for SON, and 2 times sensory threshold for MN and SN, respectively. Eight stimuli were applied in each condition.

RESULTS

Baseline blink reflex parameters did not differ significantly between the two postures. Prepulse stimulation to MN and SN caused significant inhibition of R2. In supine but not in standing, R2 was significantly more inhibited by MN than by SN prepulses. In standing, SN stimulation caused significantly more inhibition of R2 than in supine, while the inhibition caused by MN prepulses did not differ significantly between postures.

SIGNIFICANCE

PPI induced by lower limb afferent input may contribute to postural control while standing.

Fear leads to a deficit of prepulse inhibition of blink reflex in healthy humans

OBJECTIVE

We aimed to analyze whether or not fear conditioning exerts an effect on prepulse inhibition (PPI) of blink reflex (BR). To create fear conditioning, we used fearful faces. Since fearful faces lead to a specific set of fear conditioning, we hypothesized PPI of BR would change under the observation of fearful faces.

METHOD

We included 17 healthy subjects with a mean age of 30.8 ± 6.9 years and seven healthy subjects with a mean age of 57.7 ± 7.3 years between January 2018 and June 2018 and recorded PPI of BR. The recordings were done before observation of any image, during observation of images, and immediately after observation of images. Observation of images included observation of fearful faces for 30 s and a neutral image of a white screen for 30 s (in a randomized order).

RESULTS

There was a R2-PPI deficit during observation of fearful faces in each group whereas R2-PPI fully developed at other time points. R1 amplitude and R2 magnitude were lower during observation of any image compared with baseline and post-observation time points.

CONCLUSION

In conclusion, a deficit of R2-PPI develops during observation of fearful faces in humans which is probably related to activation of the amygdala.

Video-Based Analysis of Bodily Startle and Subsequent Emotional Facial Expression in Posttraumatic Stress Disorder

Exaggerated startle is a core hyperarousal symptom of Posttraumatic Stress Disorder (PTSD). Observational studies on the characteristics of this response are strikingly lacking. Previous research has demonstrated that detailed video analysis of facial and bodily responses to startling stimuli enables the differentiation of an initial, primarily reflexive response and a secondary, primary emotional response. We exposed PTSD patients (n = 34) and trauma-exposed but healthy participants (TE group, n = 26) to startling electric stimuli while acquiring observational behavioral, psychophysiological, and experiential measures. Videographic results indicate that PTSD patients' initial bodily startle response was stronger than in the TE group, and their secondary facial expression was more negatively valenced, largely due to elevated anger expression. Videographic assessment of primary and secondary startle responses provides a new window into reflexive and emotional functioning of PTSD and may therefore complement existing startle measures. PTSD treatments should address bodily startle, negative symptom appraisals, and anger more specifically.

The nociceptive withdrawal reflex of the trunk is organized with unique muscle receptive fields and motor strategies

Noxious stimuli induce a nociceptive withdrawal reflex (NWR) to protect the tissue from injury. Although the NWR was once considered as a stereotyped response, previous studies report distinct responses depending on the stimulation site and context for limbs. We aimed to determine whether noxious stimuli over the trunk produced adaptable complex NWR. We hypothesized that organization of the NWR of the trunk muscle would vary with the site of noxious input and would differ between body and spine postures, which modify the potential for specific muscles to remove threat. Fourteen participants were tested in sitting and three lumbar spine postures in side lying (neutral, flexion and extension). Noxious electrical stimuli were applied over the sacrum, spinous process of L3 and T12, lateral side of the 8th rib and anterior midline. NWR latency and amplitude were recorded with surface electromyography (EMG) electrodes over different trunk muscles. Distinct patterns of muscle activation depended on the stimulation site and were consistent with motor strategies needed to withdraw from the noxious stimuli. The NWR pattern differed between body positions, with less modulation observed in sitting than side lying. Spine posture did not affect the NWR organisation. Our results suggest the circuits controlling trunk muscle NWR presents with adaptability as a function of stimulation site and body position by utilizing the great complexity of the trunk muscle system to produce an efficient protective response. This suggests that the central nervous system (CNS) uses multiple adaptable strategies that are unique depending on which context the noxious stimuli are applied.

Prepulse modulation and recovery of trigemino-cervical reflex in normal subjects

OBJECTIVE

In this study, we analyzed the inhibitory control on the trigemino-cervical reflex (TCR), and whether or not prepulse modulation (PPM) has an effect on TCR. Thus, we studied the PPM of TCR. We hypothesized that TCR would presumably be under the modulatory effect after the prepulse stimulus similar to blink reflex (BR). We also studied the recovery of TCR which was previously shown.

METHODS

We included 13 healthy individuals. All subjects underwent recordings of TCR, TCR-PPM, and recovery of TCR. For TCR-PPM, a subthreshold stimulus to second finger 50 or 100 ms before the test stimulus was applied. For recovery of TCR, two stimuli at the infraorbital nerve were applied at 300, 500, and 800 ms interstimulus intervals (ISIs).

RESULTS

There was an inhibition of bilateral late responses of TCR at the ISIs of both 50 ms and 100 ms. There was no change of latencies. Full recovery of TCR did not develop even at the ISI 800 ms.

DISCUSSION

We have provided an evidence for the TCR-PPM in healthy subjects for the first time in this study. The prepulse inhibition is attributed to the functions of the pedunculopontine tegmental nucleus. Our study provides a strong indication that there are connections between pedunculopontine tegmental nucleus and trigemino-cervical circuit, which produces TCR.

Does Valproic Acid/Na Valproate Suppress Auditory Startle Reflex in Patients With Epilepsy?

BACKGROUND AND OBJECTIVE

Auditory startle response (ASR) was normal in juvenile myoclonic epilepsy whereas it was suppressed in progressive myoclonic epilepsy. However, both groups were using valproic acid/Na valproate (VPA) in different doses. Therefore, we aimed to analyze whether VPA has an impact on ASR in a cohort of epilepsy. For this purpose, we included patients with epilepsy and analyzed ASR in patients who were using VPA.

PATIENTS AND METHOD

We included 51 consecutive patients who had epilepsy and were using VPA between January 2014 and January 2016. Two control groups of 37 epilepsy patients using other antiepileptic drugs (AEDs) and of 25 healthy subjects were also constituted. All participants underwent investigations of ASR and startle response to somatosensory inputs (SSS) under similar conditions.

RESULTS

An analysis of patients using VPA, not using VPA and healthy subjects revealed significantly longer latency and lower probability of orbicularis oculi (O.oc) and sternocleidomastoid responses after auditory stimulation, decreased total ASR probability and longer latency of O.oc response after somatosensory stimulation in patient groups compared with healthy subjects. Multivariate analysis showed type of AED had a role in the generation of abnormalities. VPA, carbamazepine, and multiple AED use caused suppression of ASR. Total ASR probability was decreased or O.oc latency got longer with longer duration of VPA use whereas serum VPA level at the time of investigation did not correlate with total ASR probability.

DISCUSSION

Both ASR and SSS are suppressed by the effect of VPA, especially in patients using for a long period and in patients using other AEDs with VPA. Given the fact that VPA leads to long-standing synaptic changes of dopaminergic transmission, abnormalities of this network may be the more likely cause.

StartReact effects in first dorsal interosseous muscle are absent in a pinch task, but present when combined with elbow flexion

Objective

To provide a neurophysiological tool for assessing sensorimotor pathways, which may differ for those involving distal muscles in simple tasks from those involving distal muscles in a kinetic chain task, or proximal muscles in both.

Methods

We compared latencies and magnitudes of motor responses in a reaction time paradigm in a proximal (biceps brachii, BB) and a distal (first dorsal interosseous, FDI) muscle following electrical stimuli used as imperative signal (IS) delivered to the index finger. These stimuli were applied during different motor tasks: simple tasks involving either one muscle, e.g. flexing the elbow for BB (FLEX), or pinching a pen for FDI (PINCH); combined tasks engaging both muscles by pinching and flexing simultaneously (PINCH-FLEX). Stimuli were of varying intensity and occasionally elicited a startle response, and a StartReact effect.

Results

In BB, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities for non-startling trials, while for trials containing startle responses, latencies were uniformly shortened and response amplitudes similarly augmented across all IS intensities in both FLEX and PINCH-FLEX. In FDI, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities in both PINCH and PINCH-FLEX for non-startling trials, but, unlike in BB for the simple task, in PINCH for trials containing startle responses as well. In PINCH-FLEX, FDI latencies were uniformly shortened and amplitudes similarly increased across all stimulus intensities whenever startle signs were present.

Conclusions

Our results suggest the presence of different sensorimotor pathways supporting a dissociation between simple tasks that involve distal upper limb muscles (FDI in PINCH) from simple tasks involving proximal muscles (BB in FLEX), and combined tasks that engage both muscles (FDI and BB in PINCH-FLEX), all in accordance with differential importance in the control of movements by cortical and subcortical structures.

Significance

Simple assessment tools may provide useful information regarding the differential involvement of sensorimotor pathways in the control of both simple and combined tasks that engage proximal and distal muscles.

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.

Mechanical perturbations can elicit triggered reactions in the absence of a startle response

Perturbations delivered to the upper limbs elicit reflexive responses in stretched muscle at short- (M1: 25-50 ms) and long- (M2: 50-100 ms) latencies. When presented in a simple reaction time (RT) task, the perturbation can also elicit a preprogrammed voluntary response at a latency (< 100 ms) that overlaps the M2 response. This early appearance of the voluntary response following a proprioceptive stimulus causing muscle stretch is called a triggered reaction. Recent work has demonstrated that a perturbation also elicits activity in sternocleidomastoid (SCM) over a time-course consistent with the startle response and it was, therefore, proposed that the StartReact effect underlies triggered reactions (Ravichandran et al., Exp Brain Res 230:59-69, 2013). The present work investigated whether perturbation-evoked SCM activity results from startle or postural control and whether triggered reactions can also occur in the absence of startle. In Experiment 1, participants "compensated" against a wrist extension perturbation. A prepulse inhibition (PPI) stimulus (known to attenuate startle) was randomly presented before the perturbation. Rather than attenuating SCM activity, the responses in SCM were advanced by the PPI stimulus. In Experiment 2, participants "assisted" a wrist extension perturbation. The perturbation did not reliably elicit startle but despite this, two-thirds of trials had RTs of less than 100 ms and the earliest responses began at ~ 70 ms. These findings suggest that SCM activity following a perturbation is the result of postural control and is not related to startle. Moreover, an overt startle response is not a prerequisite for the elicitation of a triggered reaction.

Characterization of Volume-Based Changes in Cortical Auditory Evoked Potentials and Prepulse Inhibition

The auditory evoked startle reflex is a conserved response resulting in neurological and motor activity. The presence of a mild prepulse immediately before the main pulse inhibits startle responses, though the mechanism for this remains unknown. In this study, the electroencephalography (EEG) data recorded from 15 subjects was analyzed to study the N1 and P2 components of cortical auditory evoked potentials (CAEPs) evoked by 70, 80, 90, 100, and 110 dB stimuli both in the presence and absence of 70 dB prepulses. Results without a prepulse showed an evolution of N1 amplitudes, increasing with stimulus intensity and showing largely significant differences. Results from prepulse trials only showed noteworthy changes in peak-to-peak amplitude in the 100 dB condition. Prepulse and non-prepulse conditions were then compared using peak amplitudes and theta power. Prepulse conditions significantly decreased the amplitude for both components in the 110 dB condition, i.e., pre-pulse inhibition, but significantly increased the N1 amplitude in the 70 dB condition, i.e., pre-pulse facilitation. Similarly theta band power significantly increased in the 70 dB prepulse condition and significantly decreased in the 110 dB prepulse condition. These results expand the basis of knowledge regarding how CAEPs change and elaborate on their neural function and representation.

Startle Response in Progressive Myoclonic Epilepsy

Cortical reflex myoclonus is a typical feature of progressive myoclonic epilepsy (PME) in which it is accompanied by other types of mostly drug-resistant seizures and progressive neurological signs. Although PME is characterized by cortical hyperexcitability, studies have demonstrated atrophy and degenerative changes in the brainstem in various types of PME. Thus, we have questioned whether any stimuli may trigger a hyperactive response of brainstem reticular formation in PME and investigated the startle reflex in individuals with PME. We recorded the auditory startle response (ASR) and the startle response to somatosensory inputs (SSS) in patients with PME, and compared the results with healthy volunteers and patients with other types of drug-resistant epilepsy. All patients were using antiepileptic drugs (AEDs), 12 were on multiple AEDs. The probability of ASR was significantly lower and mean onset latency was longer in patients with PME compared with other groups. SSS responses over all muscles were low in both the PME and drug-resistant epilepsy groups; however, the differences were not statistically significant. The presence of a response over the biceps brachii muscle was zero in the PME group and showed a borderline difference compared with the other groups. Decreased probability and prolonged latencies of ASR in PME indicate inhibition of reflex circuit. A trend for decreased responses of SSS suggests hypoactive SSS in both PME and other epilepsy groups. Hypoactive ASR in PME and hypoactive SSS in both PME and other epilepsies may be attributed to the degeneration of pontine reticular nuclei in PME and functional inhibition by AEDs in both disorders.

An examination of the startle response during upper limb stretch perturbations

Unexpected presentation of a startling auditory stimulus (SAS>120 decibels) in a reaction time (RT) paradigm results in the startle reflex and an early release (<100ms) of the preplanned motor response (StartReact effect). Mechanical perturbations applied to the upper limbs elicit short- (M1) and long-latency (M2) stretch reflexes and have also been shown to initiate intended motor responses early (<100ms). Ravichandran et al. (2013) recently proposed that unexpected delivery of a perturbation could also elicit a startle response and therefore the StartReact effect may be responsible for the early trigger of a preplanned response. To investigate this further, we examined startle incidence, RT, and stretch reflex modulation for both expected and unexpected perturbations. In Experiment 1, participants performed active (ACT) and passive (DNI) conditions to an expected large perturbation (similar to previous studies examining M2). The startle response was not observed; however, the perturbation still elicited the voluntary response at short latency (<100ms) and goal-dependent modulation of the M2 response was observed. In Experiment 2, participants performed ACT and DNI conditions to a weak auditory stimulus or a small wrist perturbation. On unexpected trials we probed startle circuitry with a large perturbation or SAS. The SAS consistently elicited a startle response in both ACT and DNI conditions, but startle-like activity was only observed on 17.4% of ACT perturbation probe trials. Our findings suggest that while unexpected upper limb perturbations can be startling, startle triggering of the preplanned voluntary response is not the primary mechanism responsible for goal-dependent modulation of the M2 response.

Calibration of the Leg Muscle Responses Elicited by Predictable Perturbations of Stance and the Effect of Vision

Motor adaptation due to task practice implies a gradual shift from deliberate control of behavior to automatic processing, which is less resource- and effort-demanding. This is true both for deliberate aiming movements and for more stereotyped movements such as locomotion and equilibrium maintenance. Balance control under persisting critical conditions would require large conscious and motor effort in the absence of gradual modification of the behavior. We defined time-course of kinematic and muscle features of the process of adaptation to repeated, predictable perturbations of balance eliciting both reflex and anticipatory responses. Fifty-nine sinusoidal (10 cm, 0.6 Hz) platform displacement cycles were administered to 10 subjects eyes-closed (EC) and eyes-open (EO). Head and Center of Mass (CoM) position, ankle angle and Tibialis Anterior (TA) and Soleus (Sol) EMG were assessed. EMG bursts were classified as reflex or anticipatory based on the relationship between burst amplitude and ankle angular velocity. Muscle activity decreased over time, to a much larger extent for TA than Sol. The attenuation was larger for the reflex than the anticipatory responses. Regardless of muscle activity attenuation, latency of muscle bursts and peak-to-peak CoM displacement did not change across perturbation cycles. Vision more than doubled speed and the amount of EMG adaptation particularly for TA activity, rapidly enhanced body segment coordination, and crucially reduced head displacement. The findings give new insight on the mode of amplitude- and time-modulation of motor output during adaptation in a balancing task, advocate a protocol for assessing flexibility of balance strategies, and provide a reference for addressing balance problems in patients with movement disorders.

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.

Startle and blink reflex in high functioning autism

An important clinical feature of autism is the presence of atypical responses to sensory stimuli. In this study, we investigated if high functioning autistic patients had abnormalities in the blink reflex and the startle reaction to auditory or somatosensory stimuli. Fourteen patients aged between 7 and 16 years were included in the study. We found a longer latency of the blink reflex, an increased duration and amplitude of the auditory startle reaction and a lower presence rate of the somatosensorial startle reaction in autistic patients. To better define the sensorial characteristics of the disease could improve the therapeutic management of children with autism spectrum disorder.

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.

Mapping multidimensional pain experience onto electrophysiological responses to noxious laser heat stimuli

The origin of the conscious experience of pain in the brain is a continuing enigma in neuroscience. To shed light on the brain representation of a multifaceted pain experience in humans, we combined multivariate analysis of subjective aspects of pain sensations with detailed, single-trial analysis of electrophysiological brain responses. Participants were asked to fully focus on any painful or non-painful sensations occurring in their left hand during an interval surrounding the onset of noxious laser heat stimuli, and to rate their sensations using a set of visual analogue scales. Statistical parametric mapping was used to compute a multivariate regression analysis of subjective responses and single-trial laser evoked potentials (LEPs) at subject and group levels. Standardized Low Resolution Electromagnetic Tomography method was used to reconstruct sources of LEPs. Factor analysis of subjective responses yielded five factors. Factor 1, representing pain, mapped firstly as a negative potential at the vertex and a positive potential at the fronto-temporal region during the 208-260ms interval, and secondly as a strong negative potential in the right lateral frontal and prefrontal scalp regions during the 1292-1340ms interval. Three other factors, labelled "anticipated pain", "stimulus onset time", and "body sensations", represented non-specific aspects of the pain experience, and explained portions of LEPs in the latency range from 200ms to 700ms. The subjective space of pain during noxious laser stimulation is represented by one large factor featuring pain intensity, and by other factors accounting for non-specific parts of the sensory experience. Pain is encoded in two separate latency components with different scalp and brain representations.

Mentalis muscle related reflexes

The mentalis muscle (MM) arises from the incisive fossa of the mandible, raises and protrudes the lower lip. Here, we aim to characterize responses obtained from MM by supraorbital and median electrical as well as auditory stimuli in a group of 16 healthy volunteers who did not have clinical palmomental reflex. Reflex activities were recorded from the MM and orbicularis oculi (O.oc) after supraorbital and median electrical as well as auditory stimuli. Response rates over MM were consistent after each stimulus, however, mean latencies of MM response were longer than O.oc responses by all stimulation modalities. Shapes and amplitudes of responses from O.oc and MM were similar. Based on our findings, we may say that MM motoneurons have connections with trigeminal, vestibulocochlear and lemniscal pathways similar to other facial muscles and electrophysiological recording of MM responses after electrical and auditory stimulation is possible in healthy subjects.

Voluntary reaction time and long-latency reflex modulation

Stretching a muscle of the upper limb elicits short (M1) and long-latency (M2) reflexes. When the participant is instructed to actively compensate for a perturbation, M1 is usually unaffected and M2 increases in size and is followed by the voluntary response. It remains unclear if the observed increase in M2 is due to instruction-dependent gain modulation of the contributing reflex mechanism(s) or results from voluntary response superposition. The difficulty in delineating between these alternatives is due to the overlap between the voluntary response and the end of M2. The present study manipulated response accuracy and complexity to delay onset of the voluntary response and observed the corresponding influence on electromyographic activity during the M2 period. In all active conditions, M2 was larger compared with a passive condition where participants did not respond to the perturbation; moreover, these changes in M2 began early in the appearance of the response (∼ 50 ms), too early to be accounted for by voluntary overlap. Voluntary response latency influenced the latter portion of M2, with the largest activity seen when accuracy of limb position was not specified. However, when participants aimed for targets of different sizes or performed movements of various complexities, reaction time differences did not influence M2 period activity, suggesting voluntary activity was sufficiently delayed. Collectively, our results show that while a perturbation applied to the upper limbs can trigger a voluntary response at short latency (<100 ms), instruction-dependent reflex gain modulation remains an important contributor to EMG changes during the M2 period.

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.

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.

Facilitation and habituation of the startle reflex over the tonically active biceps brachii muscle contralateral to electrical stimuli

The aim of the present investigation was to explore the impact of muscle contraction on startle reflex responses after electrical stimuli (single or trains of 3) and to study startle reflex habituation. The electromyogram was recorded over the tonically active biceps brachii muscle in 19 healthy subjects contralateral to electrical stimuli (9-12mA) that were delivered at 1.0 and 0.4Hz over the superficial radial nerve. The muscle contraction level was varied by loading weight on the subject's bent arm (0.0, 1.0 or 1.5kg). Furthermore, short term reflex habituation was investigated using 30 blocks of 5 subsequent stimuli. Startle response amplitudes gained significantly (p<0.05) after (i) train stimuli as compared with single stimuli, during (ii) high versus low levels of muscle contraction, and at (iii) 0.4Hz versus 1.0Hz stimuli. Startle reflex amplitudes decreased significantly by the influence of preceding stimuli (p<0.05). This study provides evidence that the startle reflex can be significantly influenced by weight load, i.e. by volitional influences. Startle reflex investigation over a contracted limb muscle results in a high probability of startle release and thereby improved detection of SR habituation following preceding stimuli.

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.

A reappraisal of long-latency abdominal muscle reflexes in patients with propriospinal myoclonus

BACKGROUND

We report 3 patients with typical clinical and electrophysiological characteristics of propriospinal myoclonus propagating from a thoracic spine generator.

METHODS

In these patients, the pattern of recruitment of long-latency electromyographic reflexes in abdominal muscles was studied in response to various stimuli.

RESULTS

Abdominal reflex latency varied from 60 to 140 ms depending on stimulus location. Latency increased from magnetic stimulation of the thoracic spine to electrical stimulation of the supraorbital nerve, electrical stimulation of the median nerve, and magnetic stimulation of the motor cortex.

CONCLUSIONS

Long-latency abdominal reflex jerks are probably controlled by the brain stem to propriospinal system projections in patients with propriospinal myoclonus. The stereotyped pattern of recruitment of these reflexes could be of clinical utility to differentiate organic propriospinal myoclonus from psychogenic or mimicked jerks.

First trial postural reactions to unexpected balance disturbances: a comparison with the acoustic startle reaction

Unexpected support-surface movements delivered during stance elicit "first trial" postural reactions, which are larger and cause greater instability compared with habituated responses. The nature of this first trial reaction remains unknown. We hypothesized that first trial postural reactions consist of a generalized startle reaction, with a similar muscle synergy as the acoustic startle response, combined with an automatic postural reaction. Therefore we compared acoustic startle responses to first trial postural reactions. Eight healthy subjects stood on a support surface that unexpectedly rotated backwards 10 times, followed by 10 startling acoustic stimuli, or vice versa. Outcome measures included full body kinematics and surface EMG from muscles involved in startle reactions or postural control. Postural perturbations and startling acoustic stimuli both elicited a clear first trial reaction, as reflected by larger kinematic and EMG responses. The ensuing habituation rate to repeated identical stimuli was comparable for neck and trunk muscles in both conditions. Onset latencies in neck muscles occurred significantly later for first trial perturbations compared with startle responses, but earlier in trunk muscles. Our results show that platform tilting initially induces reactions larger than needed to maintain equilibrium. For neck and trunk muscles, these first trial postural reactions resembled acoustic startle reflexes. First trial postural reactions may be triggered by interaction of afferent volleys formed by somatosensory and vestibular inputs. Acoustic startle reactions may also be partially triggered by vestibular inputs. Similar muscle activation driven by vestibular inputs may be the common element of first trial postural responses and acoustic startle reactions.

The effect of a prepulse stimulus on the EMG rebound following the cutaneous silent period

The cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by Adelta fibres. The postinhibitory rebound of electromyographic (EMG) activity following the CSP has been mainly attributed to resynchronization of motoneurons, but the possibility of startle reflex activity contributing to the EMG burst has also been suggested. Several types of reflexes may be suppressed by a preceding weak stimulus--a phenomenon called prepulse inhibition (PPI). Our aim was to study whether PPI would diminish the EMG rebound, thereby providing further evidence for excitatory reflex activity contained within the postinhibitory EMG rebound following the CSP. Ten healthy subjects underwent CSP testing following noxious digit II stimulation in two conditions, with and without a prepulse applied to digit III. Rectified surface EMG recordings were obtained from right orbicularis oculi, sternocleidomastoid and thenar muscles of the dominant hand during thumb abduction with 25% of maximum force. The area of the EMG rebound and the EMG reflex responses in orbicularis oculi and sternocleidomastoid were significantly smaller in recordings where a prepulse stimulus was applied 100 ms before the stimulus as compared to control responses without prepulse. CSP onset and end latency, CSP duration, and the degree of EMG suppression were not influenced. Prepulses significantly reduced subjective discomfort as based on visual analog scale scores. Inhibition of the EMG rebound by prepulse stimulation supports the hypothesis that the excitatory EMG activity following the CSP contains not only resynchronization of motoneuronal firing, but also an excitatory reflex component. The most probable type of reflex seems to be a somatosensory startle reflex, a defence reaction which is generated in structures located in the caudal brainstem following an unexpected intense stimulus. Reduction of the discomfort associated with high-intensity electrical fingertip stimulation by a prepulse without affecting CSP parameters underlines the utility of PPI in the context of CSP testing.