StartReact effects are dependent on engagement of startle reflex circuits: support for a subcortically mediated initiation pathway

Enhanced Muscle Activation Using Robotic Assistance Within the Electromechanical Delay: Implications for Rehabilitation?

Robotic rehabilitation has been shown to match the effects of conventional physical therapy on motor function for patients with neurological diseases. Rehabilitation robots have the potential to reduce therapists' workload in time-intensive training programs as well as perform actions that are not replicable by human therapists. We investigated the effects of one such modality that cannot be achieved by a human therapist: assistance and resistance within the electromechanical delay between muscle activation and muscle contraction during arm extension. We found increased muscle activation when providing robotic assistance within this electromechanical delay. Assistance provided within this delay moves the participant's arm quicker than their own muscle and increases the subsequent peak voluntary muscle activation compared to normal arm extension by 68.97 ± 80.05 % (SE = 0.021; p = 0.007 ). This is surprising since all previous literature shows that muscle activation either decreases or does not change when participants receive robotic assistance. As a consequence, traditional robotic rehabilitation incrementally reduces assistance as the patient improves to maintain levels of muscle activation which is suggested to be important for neuronal repair. The present result may enable therapists to no longer have to choose between providing assistance or increasing muscle activation. Instead, therapists may be able to provide assistance while also increasing muscle activation.

Specificity of early motor unit adaptations with resistive exercise training

After exposure of the human body to resistive exercise, the force-generation capacity of the trained muscles increases significantly. Despite decades of research, the neural and muscular stimuli that initiate these changes in muscle force are not yet fully understood. The study of these adaptations is further complicated by the fact that the changes may be partly specific to the training task. For example, short-term strength training does not always influence the neural drive to muscles during the early phase (<100 ms) of force development in rapid isometric contractions. Here we discuss some of the studies that have investigated neuromuscular adaptations underlying changes in maximal force and rate of force development produced by different strength training interventions, with a focus on changes observed at the level of spinal motor neurons. We discuss the different motor unit adjustments needed to increase force or speed, and the specificity of some of the adaptations elicited by differences in the training tasks.

Word Repetition Paired With Startling Stimuli Decreases Aphasia and Apraxia Severity in Severe-to-Moderate Stroke: A Stratified, Single-Blind, Randomized, Phase 1 Clinical Trial

PURPOSE

This prospective, single-blinded, parallel, stratified, randomized clinical trial via telehealth aimed to investigate the impact of Startle Adjuvant Rehabilitation Therapy (START) on aphasia, apraxia of speech (AOS), and quality of life in individuals with chronic stroke. The study hypothesized that START would have a greater effect on AOS-related measures and more severe individuals.

METHOD

Forty-two participants with poststroke aphasia, AOS, or both were randomly assigned to the START or control group. Both groups received 77-dB GET READY and GO cues during a word repetition task for three 1-hr sessions on consecutive days. The START group additionally received 105-dB white noise GO cues during one third of trials. The Western Aphasia Battery-Revised, Apraxia Battery for Adults, Stroke Impact Scale, and Communication Outcomes After Stroke scale were administered at Day 1, Day 5, and 1-month follow-up.

RESULTS

START improved performance on some subtests of the Western Aphasia Battery (Comprehension, Repetition, Reading) and measures of AOS (Diadochokinetic Rate, Increasing Word Length) in individuals with moderate/severe aphasia, whereas moderate/severe controls saw no changes. Individuals with mild aphasia receiving START had improved Reading, whereas mild controls saw improved Comprehension. The START group had increased mood and perceived communication recovery by Day 5, whereas controls saw no changes in quality of life.

CONCLUSIONS

This study is the first to evaluate the impact of training with startling acoustic stimuli on clinical measures of aphasia and AOS. Our findings suggest START can enhance both nontrained speech production and receptive speech tasks in moderate/severe aphasia, possibly by reducing poststroke cortical inhibition. Our findings should be considered carefully, as our limitations include small effect sizes, within-group variability, and low completion rates for quality-of-life assessments and follow-up visits. Future studies should explore a mechanism of action, conduct larger and longer Phase 2 clinical trials, and evaluate long-term retention.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24093519.

Mapping the Cortical Representation of Paraspinal Muscles Using Transcranial Magnetic Stimulation Optimized in People With Chronic Back Pain

Introduction

Chronic low back pain (CLBP) is a global burden with an unknown etiology. Reorganization of the cortical representation of paraspinal muscles in the primary motor cortex (M1) may be related to the pathology. Single-pulse transcranial magnetic stimulation (TMS), commonly used to map the functional organization of M1, is not potent enough to stimulate the cortical maps of paraspinal muscles in M1 in CLBP patients with reduced corticospinal excitability (CSE) with intensities even as high as maximum stimulator output (100% MSO). This makes TMS mapping impractical for these patients. The aim of this study was to increase the practicality of TMS mapping for people with CLBP.

Methods

This study included eight men and ten women who had CLBP for over three months. A biphasic paired-pulse TMS paradigm, conjunct anticipatory postural adjustment (APA), and maximal voluntary activation of paraspinal muscles (MVC) were used to facilitate TMS mapping.

Results

TMS mapping was possible in all CLBP participants, with TMS intensities <50% of the MSO. Reorganization in terms of an anterior and lateral shift of the center of gravity (COG) of the cortical maps of paraspinal muscles was observed in all participants with CLBP, and a reduced number of discrete peaks was found in 33%.

Conclusion

The facilitation of the CSE to paraspinal muscles makes TMS mapping more practical and tolerable in people with CLBP, lowering the risk of seizure and discomfort associated with high-intensity TMS pulses.

Highlights

Conventional transcranial magnetic stimulation (TMS) brain mapping is not optimal for patients with Chronic low back pain (CLBP).Paired-pulse TMS dramatically lessens the energy needed for brain mapping.Maximal voluntary contraction of back muscles facilitates TMS mapping.Anticipatory postural activity of back muscles enhances the efficacy of TMS mapping.

Plain Language Summary

Chronic low back pain (CLBP) is a social, emotional, and economic burden and the leading cause of disability worldwide. Yet the etiology of the CLBP is unknown. The persistence of aberrant or antalgic movement patterns observed in people with CLBP has been suggested as a possible cause of pain chronification by inducing continuous damage to sensitive structures of the lumbar spine. It is well known that the brain is in charge of the production and planning of movements, so it is likely that abnormal movement patterns also stem from the abnormalities in the brain. However, until recently, human knowledge about the structure and function of the brain has been very limited. The invention of noninvasive and painless brain imaging and stimulating techniques such as transcranial magnetic stimulation (TMS) during the last decades has augmented our knowledge about the structure and function of the brain. Modification in terms of shift, shrinkage, or expansion of areas of the brain devoted to movement control or sensation of the back muscles has been documented in CLBP via these techniques, which are argued to relate to pain chronification but need further clarification. Yet monitoring the course of CLBP via TMS, despite its many potentials, is challenging. This could be due to the reduced cortical drive to back muscles in CLBP patients and the small area devoted to control of back muscles in the brain in general that increases the brain threshold to TMS in people with CLBP. The aim of this study was to tailor an approach to make TMS more applicable for CLBP patients by reducing the threshold to TMS. This could be achieved by engaging back muscles in anticipatory postural activity in combination with maximal voluntary activation of these muscles, along with TMS paradigms that induce intracortical facilitation.

Effect of Ketamine and Dexmedetomidine as Adjuvant to Total Intravenous Anesthesia on Intraoperative Cranial Nerve Monitoring in the Patients Undergoing Posterior Fossa Craniotomies-A Randomized Quadruple Blind Placebo-Controlled Study

Objectives  Total intravenous anesthesia (TIVA) is used during surgery with intraoperative neurophysiological monitoring. Addition of adjuvant may minimize suppression of potentials by reducing doses of propofol. We studied the effect of addition of ketamine or dexmedetomidine to propofol-fentanyl-based TIVA on corticobulbar motor evoked potential (CoMEP) in patients undergoing posterior fossa surgeries. Materials and Methods  Forty-two patients were assigned to three groups ( n  = 14 each), Group S-saline, Group D-dexmedetomidine (0.25 μg/kg/h), and Group K-ketamine (0.25 mg/kg/h). Patients received propofol and fentanyl infusions along with study drugs. CoMEPs were recorded from muscles innervated by cranial nerves bilaterally at predefined intervals (T baseline , T 2 , T 3 , T 4 , and T 5 ). Effect on amplitude and latency of CoMEPs was assessed. Results  A significant fall in CoMEP amplitude was observed across all analyzed muscles at time T 4 and T 5 in saline and dexmedetomidine group as compared with ketamine group, p -value less than 0.05. A significant increase in latency was observed at T4 and T5 among groups ( p -value, D vs. K = 0.239, D vs. S = 0.123, and K vs. S = 0.001). Conclusion  Both ketamine and dexmedetomidine provide and allow effective recording of CoMEPs. Ketamine emerges as a better agent especially when prolonged surgical duration is expected as even propofol-fentanyl-based TIVA adversely affects CoMEPs when used for long duration.

Testing a Novel Wearable Device for Motor Recovery of the Elbow Extensor Triceps Brachii in Chronic Spinal Cord Injury

After corticospinal tract damage, reticulospinal connections to motoneurons strengthen preferentially to flexor muscles. This could contribute to the disproportionately poor recovery of extensors often seen after spinal cord injury (SCI) and stroke. In this study, we paired electrical stimulation over the triceps muscle with auditory clicks, using a wearable device to deliver stimuli over a prolonged period of time. Healthy human volunteers wore the stimulation device for ∼6 h and a variety of electrophysiological assessments were used to measure changes in triceps motor output. In contrast to previous results in the biceps muscle, paired stimulation: (1) did not increase the StartReact effect; (2) did not decrease the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; (3) did not enhance muscle responses elicited by a TMS coil oriented to induce anterior-posterior current. In a second study, chronic cervical SCI survivors wore the stimulation device for ∼4 h every day for four weeks; this was compared with a four-week period without wearing the device. Functional and electrophysiological assessments were repeated at week 0, week 4, and week 8. No significant changes were observed in electrophysiological assessments after paired stimulation. Functional measurements such as maximal force and variability and speed of trajectories made during a planar reaching task also remained unchanged. Our results suggest that the triceps muscle shows less potential for plasticity than biceps; pairing clicks with muscle stimulation does not seem beneficial in enhancing triceps recovery after SCI.

Startle-triggered responses indicate reticulospinal drive is larger for voluntary shoulder versus finger movements

Recent primate studies have implicated a substantial role of reticulospinal pathways in the production of various voluntary movements. A novel way to assess the relative reticulospinal contributions in humans is through the use of a "StartReact" paradigm where a startling acoustic stimulus (SAS) is presented during a simple reaction time (RT) task. The StartReact response is characterized by short-latency triggering of a prepared response, which is attributed to increased reticulospinal drive associated with startle reflex activation. The current study used a StartReact protocol to examine differences in reticulospinal contributions between proximal and distal effectors by examining EMG onset latencies in lateral deltoid and first dorsal interosseous during bilateral shoulder or finger abduction. The magnitude of the StartReact effect, and thus relative reticulospinal drive, was quantified as the difference in RT between startle trials in which startle-reflex related EMG activation in the sternocleidomastoid (SCM) was present (SCM +) versus absent (SCM -). A significantly larger StartReact effect was observed for bilateral shoulder abduction versus bimanual finger abduction and a higher incidence of SCM + trials occurred in the proximal task. Additionally, both startle reflex and response-related EMG measures were larger on SCM + trials for the shoulder versus finger task. These results provide compelling novel evidence for increased reticulospinal activation in bilateral proximal upper-limb movements.

Impact of Startling Acoustic Stimuli on Word Repetition in Individuals With Aphasia and Apraxia of Speech Following Stroke

PURPOSE

The StartReact effect, whereby movements are elicited by loud, startling acoustic stimuli (SAS), allows the evaluation of movements when initiated through involuntary circuitry, before auditory feedback. When StartReact is applied during poststroke upper extremity movements, individuals exhibit increased muscle recruitment, reaction times, and reaching distances. StartReact releases unimpaired speech with similar increases in muscle recruitment and reaction time. However, as poststroke communication disorders have divergent neural circuitry from upper extremity tasks, it is unclear if StartReact will enhance speech poststroke. Our objective is to determine if (a) StartReact is present in individuals with poststroke aphasia and apraxia and (b) SAS exposure enhances speech intelligibility.

METHOD

We remotely delivered startling, 105-dB white noise bursts (SAS) and quiet, non-SAS cues to 15 individuals with poststroke aphasia and apraxia during repetition of six words. We evaluated average word intensity, pitch, pitch trajectories, vowel formants F1 and F2 (first and second formants), phonemic error rate, and percent incidence of each SAS versus non-SAS-elicited phoneme produced under each cue type.

RESULTS

For SAS trials compared to non-SAS, speech intensity increased (∆ + 0.6 dB), speech pitch increased (∆ + 22.7 Hz), and formants (F1 and F2) changed, resulting in a smaller vowel space after SAS. SAS affected pitch trajectories for some, but not all, words. Non-SAS trials had more stops (∆ + 4.7 utterances) while SAS trials had more sustained phonemes (fricatives, glides, affricates, liquids; ∆ + 5.4 utterances). SAS trials had fewer distortion errors but no change in substitution errors or overall error rate compared to non-SAS trials.

CONCLUSIONS

We show that stroke-impaired speech is susceptible to StartReact, evidenced by decreased intelligibility due to altered formants, pitch trajectories, and articulation, including increased incidence of sounds that could not be produced without SAS. Future studies should examine the impact of SAS on voluntary speech intelligibility and clinical measures of aphasia and apraxia.

The relation of neuroticism to physiological and behavioral stress responses induced by auditory startle

INTRODUCTION

The negative cognitive effects of the startle response are not yet fully understood. Ecological observations in the aviation field indicate risk for severe outcomes in complex or pressured situations, while sparse previous research suggests milder negative effects on simple cognitive tasks. Neuroticism is proposed as a factor related to the level of negative effects following startle.

METHODS

This study examined the effects of startle on performance in a choice reaction time task and analyzed relations between performance, neuroticism, and physiological stress.

RESULTS

Our results indicate that reaction time directly following startle was not affected, but reaction time in subsequent trials was significantly slower. Neuroticism and physiological stress were both unrelated to this performance effect.

DISCUSSION

We argue that higher complexity/demand tasks are necessary to complement the research on base cognitive functioning in relation to startle. If neuroticism is related to startle effects, this is more likely to be found in these higher demand situations.

Response Preparation of a Secondary Reaction Time Task is Influenced by Movement Phase within a Continuous Visuomotor Tracking Task

The simultaneous performance of two motor tasks is challenging. Currently, it is unclear how response preparation of a secondary task is impacted by the performance of a continuous primary task. The purpose of the present experiment was to investigate whether the position of the limb performing the primary cyclical tracking task impacts response preparation of a secondary reaction time task. Participants (n=20) performed a continuous tracking task with their left hand that involved cyclical and targeted wrist flexion and extension. Occasionally, a probe reaction time task requiring isometric wrist extension was performed with the right hand in response to an auditory stimulus (80 dB or 120 dB) that was triggered when the left hand passed through one of ten locations identified within the movement cycle. On separate trials, transcranial magnetic stimulation was applied over the left primary motor cortex and triggered at the same 10 stimulus locations to assess corticospinal excitability associated with the probe reaction time task. Results revealed that probe reaction times were significantly longer and motor evoked potential amplitudes were significantly larger when the left hand was in the middle of a movement cycle compared to an endpoint, suggesting that response preparation of a secondary probe reaction time task was modulated by the phase of movement within the continuous primary task. These results indicate that primary motor task requirements can impact preparation of a secondary task, reinforcing the importance of considering primary task characteristics in dual-task experimental design.

A Systematic Review with Meta-analysis of the StartReact Effect on Motor Responses in Stroke Survivors and Healthy Individuals

INTRODUCTION

Control of limb movements may be impaired after stroke due to the loss of connectivity between the cerebral cortex and spinal cord. A notion to improve motor function in stroke survivors is to employ alternate motor fibers, such as the reticulospinal tract (RST), which originate from the brainstem and terminate at different levels of spinal cord. One way of targeting the RST is to use a "StartReact" protocol to foster premature release of a pre-planned movement in response to a startling stimulus. Our aim was to find support for the preservation of such StartReact effect in stroke survivors.

METHODS

We conducted a systematic review with meta-analysis of literature published in English up to September 2020, to explore differences in motor responses to startling stimuli in StartReact effects. Protocol of the study was registered (PROSPERO Registration No: CRD42020191581). PubMed, Google Scholar, Web of Science, PsycINFO, and Science Direct were searched for relevant literature. The meta-analysis contained six studies involving a total of 151 stroke and healthy participants. Muscle onset latency data was extracted from the qualifying studies and compared using RevMan.

RESULTS AND CONCLUSIONS

StartReact effect was present in both stroke and healthy groups, represented by shortened muscle onset latency when startling stimulus was present. There was considerable heterogeneity of the outcome measures, which was attributed to the range of motor impairments among stroke survivors and methodologies employed. Our findings support notion of preservation of preprogramming ability and suitability of RST and StartReact effect for motor rehabilitation following stroke.

"Tricking the Brain" Using Immersive Virtual Reality: Modifying the Self-Perception Over Embodied Avatar Influences Motor Cortical Excitability and Action Initiation

To offer engaging neurorehabilitation training to neurologic patients, motor tasks are often visualized in virtual reality (VR). Recently introduced head-mounted displays (HMDs) allow to realistically mimic the body of the user from a first-person perspective (i.e., avatar) in a highly immersive VR environment. In this immersive environment, users may embody avatars with different body characteristics. Importantly, body characteristics impact how people perform actions. Therefore, alternating body perceptions using immersive VR may be a powerful tool to promote motor activity in neurologic patients. However, the ability of the brain to adapt motor commands based on a perceived modified reality has not yet been fully explored. To fill this gap, we "tricked the brain" using immersive VR and investigated if multisensory feedback modulating the physical properties of an embodied avatar influences motor brain networks and control. Ten healthy participants were immersed in a virtual environment using an HMD, where they saw an avatar from first-person perspective. We slowly transformed the surface of the avatar (i.e., the "skin material") from human to stone. We enforced this visual change by repetitively touching the real arm of the participant and the arm of the avatar with a (virtual) hammer, while progressively replacing the sound of the hammer against skin with stone hitting sound via loudspeaker. We applied single-pulse transcranial magnetic simulation (TMS) to evaluate changes in motor cortical excitability associated with the illusion. Further, to investigate if the "stone illusion" affected motor control, participants performed a reaching task with the human and stone avatar. Questionnaires assessed the subjectively reported strength of embodiment and illusion. Our results show that participants experienced the "stone arm illusion." Particularly, they rated their arm as heavier, colder, stiffer, and more insensitive when immersed with the stone than human avatar, without the illusion affecting their experienced feeling of body ownership. Further, the reported illusion strength was associated with enhanced motor cortical excitability and faster movement initiations, indicating that participants may have physically mirrored and compensated for the embodied body characteristics of the stone avatar. Together, immersive VR has the potential to influence motor brain networks by subtly modifying the perception of reality, opening new perspectives for the motor recovery of patients.

Evidence for Subcortical Plasticity after Paired Stimulation from a Wearable Device

Existing non-invasive stimulation protocols can generate plasticity in the motor cortex and its corticospinal projections; techniques for inducing plasticity in subcortical circuits and alternative descending pathways such as the reticulospinal tract (RST) are less well developed. One possible approach developed by this laboratory pairs electrical muscle stimulation with auditory clicks, using a wearable device to deliver stimuli during normal daily activities. In this study, we applied a variety of electrophysiological assessments to male and female healthy human volunteers during a morning and evening laboratory visit. In the intervening time (∼6 h), subjects wore the stimulation device, receiving three different protocols, in which clicks and stimulation of the biceps muscle were paired at either low or high rate, or delivered at random. Paired stimulation: (1) increased the extent of reaction time shortening by a loud sound (the StartReact effect); (2) decreased the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; (3) enhanced muscle responses elicited by a TMS coil oriented to induce anterior-posterior (AP) current, but not posterior-anterior (PA) current, in the brain. These measurements have all been suggested to be sensitive to subcortical, possibly reticulospinal, activity. Changes were similar for either of the two paired stimulus rates tested, but absent after unpaired (control) stimulation. Taken together, these results suggest that pairing clicks and muscle stimulation for long periods does indeed induce plasticity in subcortical systems such as the RST.SIGNIFICANCE STATEMENT Subcortical systems such as the reticulospinal tract (RST) are important motor pathways, which can make a significant contribution to functional recovery after cortical damage such as stroke. Here, we measure changes produced after a novel non-invasive stimulation protocol, which uses a wearable device to stimulate for extended periods. We observed changes in electrophysiological measurements consistent with the induction of subcortical plasticity. This protocol may prove an important tool for enhancing motor rehabilitation, in situations where insufficient cortical tissue survives to be a plausible substrate for recovery of function.

When reflex reactions oppose voluntary commands: The StartReact effect on eye opening

A startling auditory stimulus (SAS) induces a reflex response involving, among other reactions, a strong contraction of the orbicularis oculi muscle (OOc) and subsequent eye closure. A SAS also induces the StartReact effect, a significant shortening of reaction time in subjects ready for task execution. We examined the obvious conflict appearing when a StartReact paradigm requires participants with eyes closed to open their eyes to look for a visual target. We recorded OOc EMG activity and eyelid movements in healthy volunteers who were instructed to open their eyes at perception of a somatosensory imperative stimulus (IS) and locate the position of a Libet's clock's hand shown on a computer screen at 80 cm distance. In 6 out of 20 trials, we delivered a SAS simultaneously with the IS. The main outcome measures were reaction time at onset of eyelid movement and the time gap (TG) separating subjective assessment of the clock's hand position from real IS issuing. Control experiments included reaction time to eye closing and target location with eyes open to the same IS. Reaction time was significantly faster in SAS than in noSAS trials and slower for eye opening than for eye closing in both conditions. In the eye-opening task, TG was significantly shorter in SAS with respect to noSAS trials, despite the presence of the SAS-related burst in the OOc before EMG cessation. Our results indicate that the StartReact effect speeds up eye opening and location of a target in the visual field despite the startle reaction opposing the task.

Bimanual but not unimanual finger movements are triggered by a startling acoustic stimulus: evidence for increased reticulospinal drive for bimanual responses

The relative contributions of reticulospinal versus corticospinal pathways for movement production are thought to be dependent on the type of response involved. For example, unilateral distal movements involving the hand and finger have been thought to be primarily driven by corticospinal output, whereas bilateral responses are considered to have greater reticulospinal drive. The current study investigated whether a difference in the relative contribution of reticulospinal drive to a bimanual versus unimanual finger movement could be assessed using a StartReact protocol. The StartReact effect refers to the early and involuntary initiation of a prepared movement when a startle reflex is elicited. A decreased response latency on loud stimulus trials where a startle reflex is observed in sternocleidomastoid (SCM+ trials) confirms the StartReact effect, which is attributed to increased reticulospinal drive associated with engagement of the startle reflex circuitry. It was predicted that a StartReact effect would be absent for the predominantly corticospinal-mediated unimanual finger movement but present for the bimanual finger movement due to stronger reticulospinal drive. Results supported both predictions as reaction time was statistically equivalent for SCM+ and SCM- trials during unimanual finger movements but significantly shorter for SCM+ trials during bimanual finger movements. These results were taken as strong and novel evidence for increased reticulospinal output for bimanual finger movements.NEW & NOTEWORTHY The relative contributions of reticulospinal and corticospinal pathways to movement initiation are relatively unknown but appear to depend on the involved musculature. Here, we show that unimanual finger movements, which are predominantly initiated via corticospinal pathways, are not triggered at short latency by a startling acoustic stimulus (SAS), while bimanual finger movements are triggered by the SAS. This distinction is attributed to increased reticulospinal drive for bilateral responses.

Temporal binding and agency under startle

Forward models are a component of the motor system that predicts the sensory consequences of our actions. These models play several key roles in motor control and are hypothesized to underlie (among other things) the two phenomena under investigation in this experiment: The feeling of agency that we have over self-initiated actions (as opposed to reflexes), and "temporal binding", in which self-caused sensations are judged to have occurred earlier in time than they actually did. This experiment probes the connection between forward models and both of these phenomena using the "Startle" paradigm. In the Startle paradigm, a startlingly loud sound causes people to initiate a prepared action at a very short latency. It is hypothesized that the latency of a startle-initiated action is so short that normal cortical operations (including forward models) are circumvented. This experiment replicates the temporal-binding effect and simultaneously measures participants' sense of agency over their actions. The results show that both the temporal-binding effect and the sense of agency we have over our own actions is disrupted under the startle paradigm in line with the theory that these phenomena both rely on forward models. Furthermore, this experiment provides evidence in support of the claim that a startle-induced action is qualitatively different from other actions.

Neck rotation modulates motor-evoked potential duration of proximal muscle cortical representations in healthy adults

Transcranial magnetic stimulation (TMS) produces motor-evoked potentials (MEP) used to infer changes in corticomotor excitability. In humans, neck rotation can probe reticulospinal input on corticomotor output. This study investigated the effect of neck rotation on MEP duration in a proximal and distal upper limb muscle and compared responses between rest and preactivation. Single-pulse TMS to motor cortex was used to evoke MEPs at two stimulus intensities in 18 healthy adults (20-40 years). Surface electromyography recorded MEPs from the non-dominant biceps brachii (BB) and first dorsal interosseous (FDI). Participants were seated with the target muscle at rest or 10% preactivated, and head rotated ipsilateral, contralateral, or in neutral position. The primary outcome was MEP tail, defined as the mean difference in MEP duration between active and rest trials. Secondary outcomes were MEP duration and amplitude. MEP tail was modulated by neck rotation in the proximal BB (P = 0.03) but not distal FDI (P > 0.19), with shorter duration during ipsilateral or contralateral rotation relative to neutral. In a neutral neck position, MEP duration was prolonged by muscle preactivation and higher TMS intensities in the FDI and BB (P < 0.03). Neck rotation attenuated the prolongation of MEP duration during preactivation in the BB, but not the FDI. Neck rotation had no effect on MEP amplitude for either muscle (P > 0.05). Modulation of the late portion of the MEP by rotation of the neck could indicate subcortical projections to alpha-motoneuron pools are stronger in proximal than distal upper limb muscles. These findings may have relevance for using MEP duration as a neural biomarker in neurological diseases.