Interactions between stretch and startle reflexes produce task-appropriate rapid postural reactions

The benefit of knowledge: postural response modulation by foreknowledge of equilibrium perturbation in an upper limb task

For whole-body sway patterns, a compound motor response following an external stimulus may comprise reflexes, postural adjustments (anticipatory or compensatory), and voluntary muscular activity. Responses to equilibrium destabilization may depend on both motor set and a subject`s expectation of the disturbing stimulus. To disentangle these influences on lower limb responses, we studied a model in which subjects (n = 14) were suspended in the air, without foot support, and performed a fast unilateral wrist extension (WE) in response to a passive knee flexion (KF) delivered by a robot. To characterize the responses, electromyographic activity of rectus femoris and reactive leg torque was obtained bilaterally in a series of trials, with or without the requirement of WE (motor set), and/or beforehand information about the upcoming velocity of KF (subject`s expectation). Some fast-velocity trials resulted in StartReact responses, which were used to subclassify leg responses. When subjects were uninformed about the upcoming KF, large rectus femoris responses concurred with a postural reaction in conditions without motor task, and with both postural reaction and postural adjustment when WE was required. WE in response to a low-volume acoustic signal elicited no postural adjustments. When subjects were informed about KF velocity and had to perform WE, large rectus femoris responses corresponded to anticipatory postural adjustment rather than postural reaction. In conclusion, when subjects are suspended in the air and have to respond with WE, the prepared motor set includes anticipatory postural adjustments if KF velocity is known, and additional postural reactions if KF velocity is unknown.

Identifying spinal tracts transmitting distant effects of trans-spinal magnetic stimulation

Estimating the state of tract-specific inputs to spinal motoneurons is critical to understanding movement deficits induced by neurological injury and potential pathways to recovery but remains challenging in humans. In this study, we explored the capability of trans-spinal magnetic stimulation (TSMS) to modulate distal reflex circuits in young adults. TSMS was applied over the thoracic spine to condition soleus H-reflexes involving sacral-level motoneurons. Three TSMS intensities below the motor threshold were applied at interstimulus intervals (ISIs) between 2 and 20 ms relative to peripheral nerve stimulation (PNS). Although low-intensity TSMS yielded no changes in H-reflexes across ISIs, the two higher stimulus intensities yielded two phases of H-reflex inhibition: a relatively long-lasting period at 2- to 9-ms ISIs, and a short phase at 11- to 12-ms ISIs. H-reflex inhibition at 2-ms ISI was uniquely dependent on TSMS intensity. To identify the candidate neural pathways contributing to H-reflex suppression, we constructed a tract-specific conduction time estimation model. Based upon our model, H-reflex inhibition at 11- to 12-ms ISIs is likely a manifestation of orthodromic transmission along the lateral reticulospinal tract. In contrast, the inhibition at 2-ms ISI likely reflects orthodromic transmission along sensory fibers with activation reaching the brain, before descending along motor tracts. Multiple pathways may contribute to H-reflex modulation between 4- and 9-ms ISIs, orthodromic transmission along sensorimotor tracts, and antidromic transmission of multiple motor tracts. Our findings suggest that noninvasive TSMS can influence motoneuron excitability at distal segments and that the contribution of specific tracts to motoneuron excitability may be distinguishable based on conduction velocities.NEW & NOTEWORTHY This study explored the capability of trans-spinal magnetic stimulation (TSMS) over the thoracic spine to modulate distal reflex circuits, H-reflexes involving sacral-level motoneurons, in young adults. TSMS induced two inhibition phases of H-reflex across interstimulus intervals (ISIs): a relatively long-lasting period at 2- to 9-ms ISIs, and a short phase at 11- to 12-ms ISIs. An estimated probability model constructed from tract-specific conduction velocities allowed the identification of potential spinal tracts contributing to the changes in motoneuron excitability.

Modulation of H-reflex and V-wave responses during dynamic balance perturbations

Motoneuron excitability is possible to measure using H-reflex and V-wave responses. However, it is not known how the motor control is organized, how the H-reflex and V-wave responses modulate and how repeatable these are during dynamic balance perturbations. To assess the repeatability, 16 participants (8 men, 8 women) went through two, identical measurement sessions with ~ 48 h intervals, where maximal isometric plantar flexion (IMVC) and dynamic balance perturbations in horizontal, anterior-posterior direction were performed. Soleus muscle (SOL) neural modulation during balance perturbations were measured at 40, 70, 100 and 130 ms after ankle movement by using both H-reflex and V-wave methods. V-wave, which depicts the magnitude of efferent motoneuronal output (Bergmann et al. in JAMA 8:e77705, 2013), was significantly enhanced as early as 70 ms after the ankle movement. Both the ratio of M-wave-normalized V-wave (0.022-0.076, p < 0.001) and H-reflex (0.386-0.523, p < 0.001) increased significantly at the latency of 70 ms compared to the latency of 40 ms and remained at these levels at latter latencies. In addition, M-wave normalized V-wave/H-reflex ratio increased from 0.056 to 0.179 (p < 0.001). The repeatability of V-wave demonstrated moderate-to-substantial repeatability (ICC = 0.774-0.912) whereas the H-reflex was more variable showing fair-to-substantial repeatability (ICC = 0.581-0.855). As a conclusion, V-wave was enhanced already at 70 ms after the perturbation, which may indicate that increased activation of motoneurons occurred due to changes in descending drive. Since this is a short time-period for voluntary activity, some other, potentially subcortical responses might be involved for V-wave increment rather than voluntary drive. Our results addressed the usability and repeatability of V-wave method during dynamic conditions, which can be utilized in future studies.

Dynamic Spatial Tuning Patterns of Shoulder Muscles with Volunteers in a Driving Posture

Computational human body models (HBMs) of drivers for pre-crash simulations need active shoulder muscle control, and volunteer data are lacking. The goal of this paper was to build shoulder muscle dynamic spatial tuning patterns, with a secondary focus to present shoulder kinematic evaluation data. 8M and 9F volunteers sat in a driver posture, with their torso restrained, and were exposed to upper arm dynamic perturbations in eight directions perpendicular to the humerus. A dropping 8-kg weight connected to the elbow through pulleys applied the loads; the exact timing and direction were unknown. Activity in 11 shoulder muscles was measured using surface electrodes, and upper arm kinematics were measured with three cameras. We found directionally specific muscle activity and presented dynamic spatial tuning patterns for each muscle separated by sex. The preferred directions, i.e. the vector mean of a spatial tuning pattern, were similar between males and females, with the largest difference of 31° in the pectoralis major muscle. Males and females had similar elbow displacements. The maxima of elbow displacements in the loading plane for males was 189 ± 36 mm during flexion loading, and for females, it was 196 ± 36 mm during adduction loading. The data presented here can be used to design shoulder muscle controllers for HBMs and evaluate the performance of shoulder models.

Occlusal Splints and Exercise Performance: A Systematic Review of Current Evidence

The role of the dento-mandibular apparatus and, in particular, occlusion and jaw position, received increased attention during last years. In the present study, we aimed to systematically review, on the light of the new potential insights, the published literature covering the occlusal splint (OS) applications, and its impact on exercise performance. A structured search was carried out including MEDLINE^®/PubMed and Scopus databases with additional integration from external sources, between March and June 2021. To meet the inclusion criteria, studies published in the English language, involving humans in vivo, published from 2000 to 2021 and that investigated the role of occlusal splints on athletes' performance were selected. Starting from the 587 identified records, 17 items were finally included for the review. Four main aspects were considered and analyzed: (1) occlusal splint characteristics and occlusion experimental conditions, (2) jump performance, (3) maximal and explosive strength, and (4) exercise technique and biomechanics. The results of the systematic literature analysis depicted a wide heterogenicity in the experimental conditions and suggested the application of the OS as a way to improve athletes' or individuals' oral health, and as a potential tool to optimize marginal aspects of exercise performance.

Influence of kinesthetic motor imagery and effector specificity on the long-latency stretch response

The long-latency "reflexive" response (LLR) following an upper limb mechanical perturbation is generated by neural circuitry shared with voluntary control. This feedback response supports many task-dependent behaviors and permits the expression of goal-directed corrections at latencies shorter than voluntary reaction time. An extensive body of literature has demonstrated that the LLR shows flexibility akin to voluntary control, but it has not yet been tested whether instruction-dependent LLR changes can also occur in the absence of an overt voluntary response. The present study used kinesthetic motor imagery (experiment 1) and instructed participants to execute movement with the unperturbed contralateral limb (experiment 2) to explore the relationship between the overt production of a voluntary response and LLR facilitation. Activity in stretched right wrist flexors were compared with standard "do not-intervene" and "compensate" conditions. Our findings revealed that on ~40% of imagery and ~50% of contralateral trials, a response occurred during the voluntary epoch in the stretched right wrist flexors. On these "leaked" trials, the early portion of the LLR (R2) was facilitated and displayed a similar increase to compensate trials. The latter half of the LLR (R3) showed further modulation, mirroring the patterns of voluntary epoch activity. By contrast, the LLR on "non-leaked" imagery and contralateral trials did not modulate. We suggest that even though a hastened voluntary response cannot account for all instruction-dependent LLR modulation, the overt execution of a response during the voluntary epoch in the same muscle(s) as the LLR is a prerequisite for instruction-dependent facilitation of this feedback response.NEW & NOTEWORTHY Using motor imagery and contralateral responses, we provide novel evidence that facilitation of the long-latency reflex (LLR) requires the execution of a response during the voluntary epoch. A high proportion of overt response "leaks" were found where the mentally simulated or mirrored response appeared in stretched muscle. The first half of the LLR was categorically sensitive to the appearance of leaks, whereas the latter half displayed characteristics closely resembling activity in the ensuing voluntary period.

Stabilizing stretch reflexes are modulated independently from the rapid release of perturbation-triggered motor plans

Responses elicited after the shortest latency spinal reflexes but prior to the onset of voluntary activity can display sophistication beyond a stereotypical reflex. Two distinct behaviors have been identified for these rapid motor responses, often called long-latency reflexes. The first is to maintain limb stability by opposing external perturbations. The second is to quickly release motor actions planned prior to the disturbance, often called a triggered reaction. This study investigated their interaction when motor tasks involve both limb stabilization and motor planning. We used a robotic manipulator to change the stability of the haptic environment during 2D arm reaching tasks, and to apply perturbations that could elicit rapid motor responses. Stabilizing reflexes were modulated by the orientation of the haptic environment (field effect) whereas triggered reactions were modulated by the target to which subjects were instructed to reach (target effect). We observed that there were no significant interactions between the target and field effects in the early (50–75 ms) portion of the long-latency reflex, indicating that these components of the rapid motor response are initially controlled independently. There were small but significant interactions for two of the six relevant muscles in the later portion (75–100 ms) of the reflex response. In addition, the target effect was influenced by the direction of the perturbation used to elicit the motor response, indicating a later feedback correction in addition to the early component of the triggered reaction. Together, these results demonstrate how distinct components of the long-latency reflex can work independently and together to generate sophisticated rapid motor responses that integrate planning with reaction to uncertain conditions.

Visual Feedback Processing of the Limb Involves Two Distinct Phases

Muscle responses to mechanical disturbances exhibit two distinct phases: a response starting at ~20 ms that is fairly stereotyped, and a response starting at ~60 ms modulated by many behavioral contexts including goal-redundancy and environmental obstacles. Muscle responses to disturbances of visual feedback of the hand arise within ~90 ms. However, little is known whether these muscle responses are sensitive to behavioral contexts. We had 49 human participants (27 male) execute goal-directed reaches with visual feedback of their hand presented as a cursor. On random trials, the cursor jumped laterally to the reach direction, and thus, required a correction to attain the goal. The first experiment demonstrated that the response amplitude starting at 90 ms scaled with jump magnitude, but only for jumps <2 cm. For larger jumps, the duration of the muscle response scaled with the jump size starting after 120 ms. The second experiment demonstrated that the early response was sensitive to goal redundancy as wider targets evoked a smaller corrective response. The third experiment demonstrated that the early response did not consider the presence of obstacles, as this response routinely drove participants directly to the goal even though this path was blocked by an obstacle. Instead, the appropriate muscle response to navigate around the obstacle started after 120 ms. Our findings highlight that visual feedback of the limb involves two distinct phases: a response starting at 90 ms with limited sensitivity to jump magnitude and sensitive to goal-redundancy, and a response starting at 120 ms with increased sensitivity to jump magnitude and environmental factors.SIGNIFICANCE STATEMENT The motor system can integrate proprioceptive feedback to guide an ongoing action in ~60 ms and is flexible to a broad range of behavioral contexts. In contrast, the present study identified that the motor response to a visual disturbance exhibits two distinct phases: an early response starting at 90 ms with limited scaling with disturbance size and sensitivity to goal-redundancy, and a slower response starting after 120 ms with increased sensitivity to disturbance size and sensitive to environmental obstacles. These data suggest visual feedback of the hand is processed through two distinct feedback processes.

Neurophysiological analysis of the clinical pull test

Postural reflexes are impaired in conditions such as Parkinson's disease, leading to difficulty walking and falls. In clinical practice, postural responses are assessed using the "pull test," where an examiner tugs the prewarned standing patient backward at the shoulders and grades the response. However, validity of the pull test is debated, with issues including scaling and variability in administration and interpretation. It is unclear whether to assess the first trial or only subsequent repeated trials. The ecological relevance of a forewarned backward challenge is also debated. We therefore developed an instrumented version of the pull test to characterize responses and clarify how the test should be performed and interpreted. In 33 healthy participants, "pulls" were manually administered and pull force measured. Trunk and step responses were assessed with motion tracking. We probed for the StartReact phenomenon (where preprepared responses are released early by a startling stimulus) by delivering concurrent normal or "startling" auditory stimuli. We found that the first pull triggers a different response, including a larger step size suggesting more destabilization. This is consistent with "first trial effects," reported by platform translation studies, where movement execution appears confounded by startle reflex-like activity. Thus, first pull test trials have clinical relevance and should not be discarded as practice. Supportive of ecological relevance, responses to repeated pulls exhibited StartReact, as previously reported with a variety of other postural challenges, including those delivered with unexpected timing and direction. Examiner pull force significantly affected the postural response, particularly the size of stepping. NEW & NOTEWORTHY We characterized postural responses elicited by the clinical "pull test" using instrumentation. The first pull triggers a different response, including a larger step size suggesting more destabilization. Thus, first trials likely have important clinical and ecological relevance and should not be discarded as practice. Responses to repeated pulls can be accelerated with a startling stimulus, as reported with a variety of other challenges. Examiner pull force was a significant factor influencing the postural response.

Strength improvements through occlusal splints? The effects of different lower jaw positions on maximal isometric force production and performance in different jumping types

Objective

The influence of the jaw position on postural control, body posture, walking and running pattern has been reported in the literature. All these movements have in common that a relatively small, but well controlled muscle activation is required. The induced effects on motor output through changed jaw positions have been small. Therefore, it has been questioned if it could still be observed in maximal muscle activation.

Method

Twenty-three healthy, mid age recreational runners (mean age = 34.0 ± 10.3 years) participated in this study. Three different jump tests (squat jump, counter movement jump, and drop jumps from four different heights) and three maximal strength tests (trunk flexion and extension, leg press of the right and left leg) were conducted. Four different dental occlusion conditions and an additional familiarization condition were tested. Subjects performed the tests on different days for which the four occlusion conditions were randomly changed.

Results

No familiarization effect was found. Occlusion conditions with a relaxation position and with a myocentric condylar position showed significantly higher values for several tests compared to the neutral condition and the maximal occlusion position. Significance was found in the squat jump, countermovement jump, the drop jump from 32cm and 40cm, trunk extension, leg press force and rate of force development. The effect due to the splint conditions is an improvement between 3% and 12% (min and max). No influence of the jaw position on symmetry or balance between extension and flexion muscle was found.

Conclusion

An influence of occlusion splints on rate of force development (RFD) and maximal strength tests could be confirmed. A small, but consistent increase in the performance parameters could be measured. The influence of the occlusion condition is most likely small compared to other influences as for example training status, age, gender and circadian rhythm.

Axial reflexes are present in older subjects and may contribute to balance responses

We studied the response to axial taps (mini-perturbations) of a group of 13 healthy older subjects (mean age 63 ± 12 years, 7 females, 6 males), 12 of whom were also studied using larger applied (macro-) perturbations requiring active postural responses. The mini-perturbation consisted of a brief impulsive force produced by a mini-shaker applied to the trunk at the level of the shoulders and anteriorly at the upper sternum which was perceived as a tap. Acceleration, force platform, and EMG measurements were made. The average peak accelerations for the mini-perturbations were 108 mG (anterior) and - 78.9 mG (posterior). Responses overall were very similar to those previously reported for younger subjects: the perturbation evoked short latency responses in leg muscles, modulated by degree and direction of lean, and were largest for the muscle most relevant for the postural correction. The increases in the amplitude for the main agonist were greater than the increase in tonic activity. With both anterior and posterior lean, co-contraction responses were present. The size of the EMG response to the mini-perturbations correlated with the corresponding earliest EMG responses (0-100, 100-200 ms intervals) to the larger postural perturbations, timing which corresponds to balance responses. The balance responses evoked by the larger imposed postural perturbations may, therefore, receive a contribution through the reflex pathway mediating the axial tap responses, whose efferent limb appears to be the reticulospinal tract.

COMT Genotype and Sensory and Sensorimotor Gating in High and Low Hypnotizable Subjects

We investigated the association between hypnotizability, COMT polymorphism, P50 suppression ratio, and prepulse inhibition of acoustic startle response (ASR) in 21 high (HH) and 19 low (LH) hypnotizable subjects. The frequency of Met/Met carriers of COMT polymorphysm was higher in HH than in LH group (33.3% versus 10.6%, p = .049). Increased ASR amplitude and latency and decreased prepulse inhibition at 120 ms lead interval were found in the HH compared to the LH group. The effect of COMT genotype on prepulse inhibition was observed in LH group only. No between-group differences in P50 measures were found. The obtained results suppose the participation of dopamine system in mechanisms of hypnotizability and different allocation of attentional resources in HH and LH subjects.

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.

Evidence for Startle Effects due to Externally Induced Lower Limb Movements: Implications in Neurorehabilitation

Passive limb displacement is routinely used to assess muscle tone. If we attempt to quantify muscle stiffness using mechanical devices, it is important to know whether kinematic stimuli are able to trigger startle reactions. Whether kinematic stimuli are able to elicit a startle reflex and to accelerate prepared voluntary movements (StartReact effect) has not been studied extensively to date. Eleven healthy subjects were suspended in an exoskeleton and were exposed to passive left knee flexion (KF) at three intensities, occasionally replaced by fast right KF. Upon perceiving the movement subjects were asked to perform right wrist extension (WE), assessed by extensor carpi radialis (ECR) electromyographic activity. ECR latencies were shortest in fast trials. Startle responses were present in most fast trials, yet being significantly accelerated and larger with right versus left KF, since the former occurred less frequently and thus less expectedly. Startle responses were associated with earlier and larger ECR responses (StartReact effect), with the largest effect again upon right KF. The results provide evidence that kinematic stimuli are able to elicit both startle reflexes and a StartReact effect, which depend on stimulus intensity and anticipation, as well as on the subjects' preparedness to respond.

Soleus H-reflex modulation during balance recovery after forward falling

INTRODUCTION

Our purpose was to examine the Hoffmann reflex (H-reflex) during balance recovery after a simulated forward fall from 2 different inclination angles.

METHODS

The soleus H-reflex of 15 healthy adults was measured in 2 different leaning positions (exerting a horizontal force at 15% and 30% of body weight, respectively), with no release (Int0) and at 2 different intervals (Int1, Int2) after the release (∼45 and ∼65 ms, respectively).

RESULTS

During Int2, the H-reflex, which was evoked before the onset of the soleus electromyography, was significantly higher than the H-reflex induced 20 ms earlier (Int1). No significant difference was observed between Int0 and Int1 and between the 2 leaning positions.

CONCLUSIONS

These findings indicate that Ia afferent input is facilitated before muscle activation during forward falling. This could be important for the timely activation and increased rate of force development required during this task. Muscle Nerve 54: 952-958, 2016.

Postural responses to anterior and posterior perturbations applied to the upper trunk of standing human subjects

This study concerned the effects of brisk perturbations applied to the shoulders of standing subjects to displace them either forwards or backwards, our aim being to characterise the responses to these disturbances. Subjects stood on a force platform, and acceleration was measured at the level of C7, the sacrum and both tibial tuberosities. Surface EMG was measured from soleus (SOL), tibialis anterior (TA), the hamstrings (HS), quadriceps (QUAD), rectus abdominis (RA) and lumbar paraspinal (PS) muscles. Trials were recorded for each of four conditions: subjects' eyes open (reference) or closed and on a firm (reference) or compliant surface. Observations were also made of voluntary postural reactions to a tap over the deltoid. Anterior perturbations (mean C7 acceleration 251.7 mg) evoked activity within the dorsal muscles (SOL, HS, PS) with a similar latency to voluntary responses to shoulder tapping. Responses to posterior perturbations (mean C7 acceleration -240.4 mg) were more complex beginning, on average, at shorter latency than voluntary activity (median TA 78.0 ms). There was activation of TA, QUAD and SOL associated with initial forward acceleration of the lower legs. The EMG responses consisted of an initial phasic discharge followed by a more prolonged one. These responses differ from the pattern of automatic postural responses that follow displacements at the level of the ankles, and it is unlikely that proprioceptive afferents excited by ankle movement had a role in the initial responses. Vision and surface properties had only minor effects. Perturbations of the upper trunk evoke stereotyped compensatory postural responses for each direction of perturbation. For posterior perturbations, EMG onset occurs earlier than for voluntary responses.

Perturbation Predictability Can Influence the Long-Latency Stretch Response

Perturbations applied to the upper limbs elicit short (M1: 25–50 ms) and long-latency (M2: 50–100 ms) responses in the stretched muscle. M1 is produced by a spinal reflex loop, and M2 receives contribution from multiple spinal and supra-spinal pathways. While M1 is relatively immutable to voluntary intention, the remarkable feature of M2 is that its size can change based on intention or goal of the participant (e.g., increasing when resisting the perturbation and decreasing when asked to let-go or relax following the perturbation). While many studies have examined modulation of M2 between passive and various active conditions, through the use of constant foreperiods (interval between warning signal and a perturbation), it has also been shown that the magnitude of the M2 response in a passive condition can change based on factors such as habituation and anticipation of perturbation delivery. To prevent anticipation of a perturbation, most studies have used variable foreperiods; however, the range of possible foreperiod duration differs between experiments. The present study examined the influence of different variable foreperiods on modulation of the M2 response. Fifteen participants performed active and passive responses to a perturbation that stretched wrist flexors. Each block of trials had either a short (2.5–3.5 seconds; high predictability) or long (2.5–10.5 seconds; low predictability) variable foreperiod. As expected, no differences were found between any conditions for M1, while M2 was larger in the active rather than passive conditions. Interestingly, within the two passive conditions, the long variable foreperiods resulted in greater activity at the end of the M2 response than the trials with short foreperiods. These results suggest that perturbation predictability, even when using a variable foreperiod, can influence circuitry contributing to the long-latency stretch response.

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.

Does the StartReact Effect Apply to First-Trial Reactive Movements?

Introduction

StartReact is the acceleration of reaction time by a startling acoustic stimulus (SAS). The SAS is thought to release a pre-prepared motor program. Here, we investigated whether the StartReact effect is applicable to the very first trial in a series of repeated unpractised single-joint movements.

Methods

Twenty healthy young subjects were instructed to perform a rapid ankle dorsiflexion movement in response to an imperative stimulus. Participants were divided in two groups of ten. Both groups performed 17 trials. In one group a SAS (116 dB) was given in the first trial, whereas the other group received a non-startling sound (70 dB) as the first imperative stimulus. In the remaining 16 trials, the SAS was given as the imperative stimulus in 25% of the trials in both groups. The same measurement was repeated one week later, but with the first-trial stimuli counterbalanced between groups.

Results

When a SAS was given in the very first trial, participants had significantly shorter onset latencies compared to first-trial responses to a non-startling stimulus. Succeeding trials were significantly faster compared to the first trial, both for trials with and without a SAS. However, the difference between the first and succeeding trials was significantly larger for responses to a non-startling stimulus compared to responses triggered by a SAS. SAS-induced acceleration in the first trial of the second session was similar to that in succeeding trials of session 1.

Discussion

The present results confirm that the StartReact phenomenon also applies to movements that have not yet been practiced in the experimental context. The excessive SAS-induced acceleration in the very first trial may be due to the absence of integration of novel context-specific information with the existing motor memory for movement execution. Our findings demonstrate that StartReact enables a rapid release of motor programs in the very first trial also without previous practice, which might provide a behavioural advantage in situations that require a rapid response to a potentially threatening environmental stimulus.

Long-Latency Feedback Coordinates Upper-Limb and Hand Muscles during Object Manipulation Tasks

Suppose that someone bumps into your arm at a party while you are holding a glass of wine. Motion of the disturbed arm will engage rapid and goal-directed feedback responses in the upper-limb. Although such responses can rapidly counter the perturbation, it is also clearly desirable not to destabilize your grasp and/or spill the wine. Here we investigated how healthy humans maintain a stable grasp following perturbations by using a paradigm that requires spatial tuning of the motor response dependent on the location of a virtual target. Our results highlight a synchronized expression of target-directed feedback in shoulder and hand muscles occurring at ∼60 ms. Considering that conduction delays are longer for the more distal hand muscles, these results suggest that target-directed responses in hand muscles were initiated before those for the shoulder muscles. These results show that long-latency feedback can coordinate upper limb and hand muscles during object manipulation tasks.

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.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.