Triggering Mechanisms for Motor Actions: The Effects of Expectation on Reaction Times to Intense Acoustic Stimuli

The interplay of perceptual processing demands and practice in modulating voluntary and involuntary motor responses

Understanding how sensory processing demands affect the ability to ignore task-irrelevant, loud auditory stimuli (LAS) during a task is key to performance in dynamic environments. For example, tennis players must ignore crowd noise to perform optimally. We investigated how practice affects this ability by examining the effects of delivering LASs during preparatory phase of an anticipatory timing (AT) task on the voluntary and reflexive responses in two conditions: lower and higher visual processing loads. Twenty-four participants (mean age = 23.1, 11 females) completed the experiment. The AT task involved synchronizing a finger abduction response with the last visual stimulus item in a sequence of four Gabor grating patches briefly flashed on screen. The lower demand condition involved only this task, and the higher demand condition required processing the orientations of the patches to report changes in the final stimulus item. Our results showed that higher visual processing demands affected the release of voluntary actions, particularly in the first block of trials. When the perceptual load was lower, responses were released earlier by the LAS compared to the high-load condition. Practice reduced these effects largely, but high perceptual load still led to earlier action release in the second block. In contrast, practice led to more apparent facilitation of eyeblink latency in the second block. These findings indicate that a simple perceptual load manipulation can impact the execution of voluntary motor actions, particularly for inexperienced participants. They also suggest distinct movement preparation influences on voluntary and involuntary actions triggered by acoustic stimuli.

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.

Investigating motor preparation in synchronous hand and foot movements under reactive vs. predictive control

Synchronizing hand and foot movements under reactive versus predictive control results in differential timing structures between the responses. Under reactive control, where the movement is externally triggered, the electromyographic (EMG) responses are synchronized, resulting in the hand displacement preceding the foot. Under predictive control, where the movement is self-paced, the motor commands are organized such that the displacement onset occurs relatively synchronously, requiring the EMG onset of the foot to precede that of the hand. The current study used a startling acoustic stimulus (SAS), which can involuntarily trigger a prepared response, to investigate whether these results are due to differences in a pre-programmed timing structure of the responses. Participants performed synchronous movements of the right heel and right hand under both reactive and predictive modes of control. The reactive condition involved a simple reaction time (RT) task, whereas the predictive condition involved an anticipation-timing task. On selected trials, a SAS (114 dB) was presented 150 ms prior to the imperative stimulus. Results from the SAS trials revealed that while the differential timing structures between the responses was maintained under both reactive and predictive control, the EMG onset asynchrony under predictive control was significantly smaller following the SAS. These results suggest that the timing between the responses, which differs between the two control modes, is pre-programmed; however, under predictive control, the SAS may accelerate the internal timekeeper, resulting in a shortened between-limb delay.

Evolving changes in cortical and subcortical excitability during movement preparation: A study of brain potentials and eye-blink reflexes during loud acoustic stimulation

During preparation for action, the presentation of loud acoustic stimuli (LAS) can trigger movements at very short latencies in a phenomenon called the StartReact effect. It was initially proposed that a special, separate subcortical mechanism that bypasses slower cortical areas could be involved. We sought to examine the evidence for a separate mechanism against the alternative that responses to LAS can be explained by a combination of stimulus intensity effects and preparatory states. To investigate whether cortically mediated preparatory processes are involved in mediating reactions to LAS, we used an auditory reaction task where we manipulated the preparation level within each trial by altering the conditional probability of the imperative stimulus. We contrasted responses to non-intense tones and LAS and examined whether cortical activation and subcortical excitability and motor responses were influenced by preparation levels. Increases in preparation levels were marked by gradual reductions in reaction time (RT) coupled with increases in cortical activation and subcortical excitability - at both condition and trial levels. Interestingly, changes in cortical activation influenced motor and auditory but not visual areas - highlighting the widespread yet selective nature of preparation. RTs were shorter to LAS than tones, but the overall pattern of preparation level effects was the same for both stimuli. Collectively, the results demonstrate that LAS responses are indeed shaped by cortically mediated preparatory processes. The concurrent changes observed in brain and behavior with increasing preparation reinforce the notion that preparation is marked by evolving brain states which shape the motor system for action.

Altered frontoparietal activity in acoustic startle priming tasks during reticulospinal tract facilitation: An fNIRS study

Background

Because it is one of the important pathways for promoting motor recovery after cortical injury, the function of the reticulospinal tract (RST) has received increasing attention in recent years. However, the central regulatory mechanism of RST facilitation and reduction of apparent response time is not well understood.

Objectives

To explore the potential role of RST facilitation in the acoustic startle priming (ASP) paradigm and observe the cortical changes induced by ASP reaching tasks.

Methods

Twenty healthy participants were included in this study. The reaching tasks were performed with their left and right hands. Participants were instructed to get ready after the warning cue and complete the reach as soon as they heard the Go cue. Half of the testing trials were set as control trials with an 80-dB Go cue. The other half of the trials had the Go cue replaced with 114-dB white noise to evoke the StartleReact effect, inducing reticulospinal tract facilitation. The response of the bilateral sternocleidomastoid muscle (SCM) and the anterior deltoid was recorded via surface electromyography. Startle trials were labeled as exhibiting a positive or negative StartleReact effect, according to whether the SCM was activated early (30-130 ms after the Go cue) or late, respectively. Functional near-infrared spectroscopy was used to synchronously record the oxyhemoglobin and deoxyhemoglobin fluctuations in bilateral motor-related cortical regions. The β values representing cortical responses were estimated via the statistical parametric mapping technique and included in the final analyses.

Results

Separate analyses of data from movements of the left or right side revealed significant activation of the right dorsolateral prefrontal cortex during RST facilitation. Moreover, left frontopolar cortex activation was greater in positive startle trials than in control or negative startle trials during left-side movements. Furthermore, decreased activity of the ipsilateral primary motor cortex in positive startle trials during ASP reaching tasks was observed.

Conclusion

The right dorsolateral prefrontal cortex and the frontoparietal network to which it belongs may be the regulatory center for the StartleReact effect and RST facilitation. In addition, the ascending reticular activating system may be involved. The decreased activity of the ipsilateral primary motor cortex suggests enhanced inhibition of the non-moving side during the ASP reaching task. These findings provide further insight into the SE and into RST facilitation.

Engagement of the contralateral limb can enhance the facilitation of motor output by loud acoustic stimuli

When intense sound is presented during light muscle contraction, inhibition of the corticomotoneuronal pathway is observed. During action preparation, this effect is reversed, with sound resulting in excitation of the corticomotoneuronal pathway. We investigated how combined maintenance of a muscle contraction during preparation for a ballistic action impacts the magnitude of the facilitation of motor output by a loud acoustic stimulus (LAS) - a phenomenon known as the StartReact effect. Participants executed ballistic wrist flexion movements and a LAS was presented simultaneously with the imperative signal in a subset of trials. We examined whether the force level or muscle used to maintain a contraction during preparation for the ballistic response impacted reaction time and/or the force of movements triggered by the LAS. These contractions were sustained either ipsilaterally or contralaterally to the ballistic response. The magnitude of facilitation by the LAS was greatest when low force flexion contractions were maintained in the limb contralateral to the ballistic response during preparation. There was little change in facilitation when contractions recruited the contralateral extensor muscle, or when they were sustained in the same limb that executed the ballistic response. We conclude that a larger network of neurons which may be engaged by a contralateral sustained contraction prior to initiation may be recruited by the LAS, further contributing to the motor output of the response. These findings may be particularly applicable in stroke rehabilitation where engagement of the contralesional side may increase the benefits of a LAS to the functional recovery of movement.

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.

Startle Increases the Incidence of Anticipatory Muscle Activations but Does Not Change the Task-Specific Muscle Onset for Patients After Subacute Stroke

Objectives: To demonstrate the task-specificities of anticipatory muscle activations (AMAs) among different forward-reaching tasks and to explore the StartleReact Effect (SE) on AMAs in occurrence proportions, AMA onset latency or amplitude within these tasks in both healthy and stroke population. Methods: Ten healthy and ten stroke subjects were recruited. Participants were asked to complete the three forward-reaching tasks (reaching, reaching to grasp a ball or cup) on the left and right hand, respectively, with two different starting signals (warning-Go, 80 dB and warning-startle, 114 dB). The surface electromyography of anterior deltoid (AD), flexor carpi radialis (FCR), and extensor carpi radialis (ECR) on the moving side was recorded together with signals from bilateral sternocleidomastoid muscles (SCM), lower trapezius (LT), latissimus dorsi (LD), and tibialis anterior (TA). Proportions of valid trials, the incidence of SE, AMA incidence of each muscle, and their onset latency and amplitude were involved in analyses. The differences of these variables across different move sides (healthy, non-paretic, and paretic), normal or startle conditions, and the three tasks were explored. The ECR AMA onset was selected to further explore the SE on the incidence of AMAs. Results: Comparisons between move sides revealed a widespread AMA dysfunction in subacute stroke survivors, which was manifested as lower AMA onset incidence, changed onset latency, and smaller amplitude of AMAs in bilateral muscles. However, a significant effect of different tasks was only observed in AMA onset latency of muscle ECR (F = 3.56, p = 0.03, η ² _p = 0.011), but the significance disappeared in the subsequent analysis of the stroke subjects only (p > 0.05). Moreover, the following post-hoc comparison indicated significant early AMA onsets of ECR in task cup when comparing with reach (p < 0.01). For different stimuli conditions, a significance was only revealed on shortened premotor reaction time under startle for all participants (F = 60.68, p < 0.001, η p 2 = 0.056). Furthermore, stroke survivors had a significantly lower incidence of SE than healthy subjects under startle (p < 0.01). But all performed a higher incidence of ECR AMA onset (p < 0.05) than with normal signal. In addition, the incidence of ECR AMAs of both non-paretic and paretic sides could be increased significantly via startle (p ≤ 0.02). Conclusions: Healthy people have task-specific AMAs of muscle ECR when they perform forward-reaching tasks with different hand manipulations. However, this task-specific adjustment is lost in subacute stroke survivors. SE can improve the incidence of AMAs for all subjects in the forward-reaching tasks involving precision manipulations, but not change AMA onset latency and amplitude.

Response triggering by an acoustic stimulus increases with stimulus intensity and is best predicted by startle reflex activation

In a simple reaction time task, the presentation of a startling acoustic stimulus has been shown to trigger the prepared response at short latency, known as the StartReact effect. However, it is unclear under what conditions it can be assumed that the loud stimulus results in response triggering. The purpose of the present study was to examine how auditory stimulus intensity and preparation level affect the probability of involuntary response triggering and the incidence of activation in the startle reflex indicator of sternocleidomastoid (SCM). In two reaction time experiments, participants were presented with an irrelevant auditory stimulus of varying intensities at various time points prior to the visual go-signal. Responses were independently categorized as responding to either the auditory or visual stimulus and those with or without SCM activation (i.e., SCM+/−). Both the incidence of response triggering and proportion of SCM+ trials increased with stimulus intensity and presentation closer to the go-signal. Data also showed that participants reacted to the auditory stimulus at a much higher rate on trials where the auditory stimulus elicited SCM activity versus those that did not, and a logistic regression analysis confirmed that SCM activation is a reliable predictor of response triggering for all conditions.

Does height-induced threat modulate shortening of reaction times induced by a loud stimulus in a lateral stepping and a wrist extension task?

INTRODUCTION

The StartReact (SR) effect is the accelerated release of a prepared movement when a startling acoustic stimulus is presented at the time of the imperative stimulus (IS). SR paradigms have been used to study defective control of balance and gait in people with neurological conditions, but differences in emotional state (e.g. fear of failure) may be a potential confounder when comparing patients to healthy subjects. In this study, we aimed to gain insight in the effects of postural threat on the SR effect by manipulating surface height during a postural (lateral step) task and a non-postural (wrist extension) task.

METHODS

Eleven healthy participants performed a lateral step perpendicular to the platform edge, and 19 participants performed a wrist extension task while standing at the platform edge. Participants initiated the movement as fast as possible in response to an IS that varied in intensity across trials (80 dB to 121 dB) at both low and high platform height (3.2 m). For the lateral step task, we determined anticipatory postural adjustments (APA) and step onset latencies. For the wrist extension task, muscle onset latencies were determined. We used Wilcoxon signed-rank tests on the relative onset latencies between both heights, to identify whether the effect of height was different for IS intensities between 103 and 118 dB compared to 121 dB.

RESULTS

For both tasks, onset latencies were significantly shortened at 121 dB compared to 80 dB, regardless of height. In the lateral step task, the effect of height was larger at 112 dB compared to 121 dB. The absolute onset latencies showed that at 112 dB there was no such stimulus intensity effect at high as seen at low surface height. In the wrist extension task, no differential effects of height could be demonstrated across IS intensities.

CONCLUSIONS

Postural threat had a significant, yet modest effect on shortening of RTs induced by a loud IS, with a mere 3 dB difference between standing on high versus low surface height. Interestingly, this effect of height was specific to the postural (i.e. lateral stepping) task, as no such differences could be demonstrated in the wrist extension task. This presumably reflects more cautious execution of the lateral step task when standing on height. The present findings suggest that applying stimuli of sufficiently high intensity (≥115 dB) appears to neutralize potential differences in emotional state when studying SR effects.

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.

Preparatory suppression and facilitation of voluntary and involuntary responses to loud acoustic stimuli in an anticipatory timing task

In this study, we sought to characterize the effects of intense sensory stimulation on voluntary and involuntary behaviors at different stages of preparation for an anticipated action. We presented unexpected loud acoustic stimuli (LAS) at-rest and at three critical times during active movement preparation (-1,192, -392, and 0 ms relative to expected voluntary movement onset) to probe the state of the nervous system, and measured their effect on voluntary and involuntary motor actions (finger-press and eye-blink startle reflex, respectively). Voluntary responses were facilitated by LAS presented during active preparation, leading to earlier and more forceful responses compared to control and LAS at-rest. Notably, voluntary responses were significantly facilitated on trials where the LAS was presented early during preparation (-1,192 ms). Eye-blink reflexes to the LAS at -392 ms were significantly reduced and delayed compared to blinks elicited at other time-points, indicating suppression of sub-cortical excitability. However, voluntary responses on these trials were still facilitated by the LAS. The results provide insight into the mechanisms involved in preparing anticipatory actions. Induced activation can persist in the nervous system and can modulate subsequent actions for a longer time-period than previously thought, highlighting that movement preparation is a continuously evolving process that is susceptible to external influence throughout the preparation period. Suppression of sub-cortical excitability shortly before movement onset is consistent with previous work showing corticospinal suppression which may be a necessary step before the execution of any voluntary response.

Cumulative distribution functions: An alternative approach to examine the triggering of prepared motor actions in the StartReact effect

There has been much debate concerning whether startling sensory stimuli can activate a fast-neural pathway for movement triggering (StartReact) which is different from that of voluntary movements. Activity in sternocleidomastoid (SCM) electromyogram is suggested to indicate activation of this pathway. We evaluated whether SCM activity can accurately identify trials which may differ in their neurophysiological triggering and assessed the use of cumulative distribution functions (CDFs) of reaction time (RT) data to identify trials with the shortest RTs for analysis. Using recent data sets from the StartReact literature, we examined the relationship between RT and SCM activity. We categorised data into short/longer RT bins using CDFs and used linear mixed-effects models to compare potential conclusions that can be drawn when categorising data on the basis of RT versus on the basis of SCM activity. The capacity of SCM to predict RT is task-specific, making it an unreliable indicator of distinct neurophysiological mechanisms. Classification of trials using CDFs is capable of capturing potential task- or muscle-related differences in triggering whilst avoiding the pitfalls of the traditional SCM activity-based classification method. We conclude that SCM activity is not always evident on trials that show the early triggering of movements seen in the StartReact phenomenon. We further propose that a more comprehensive analysis of data may be achieved through the inclusion of CDF analyses. These findings have implications for future research investigating movement triggering as well as for potential therapeutic applications of StartReact.

Understanding the dual-task costs of walking: a StartReact study

The need to perform multiple tasks more or less simultaneously is a common occurrence during walking in daily life. Performing tasks simultaneously typically impacts task performance negatively. Hypothetically, such dual-task costs may be explained by a lowered state of preparation due to competition for attentional resources, or alternatively, by a 'bottleneck' in response initiation. Here, we investigated both hypotheses by comparing 'StartReact' effects during a manual squeezing task under single-task (when seated) and dual-task (when walking) conditions. StartReact is the acceleration of reaction times by a startling stimulation (a startling acoustic stimulus was applied in 25% of trials), attributed to the startling stimulus directly releasing a pre-prepared movement. If dual-task costs are due to a lowered state of preparation, we expected trials both with and without an accompanying startling stimulus to be delayed compared to the single-task condition, whereas we expected only trials without a startling stimulus to be delayed if a bottleneck in response initiation would underlie dual-task costs. Reaction times of the manual squeezing task in the flexor digitorum superficialis and extensor carpi radialis muscle were significantly delayed (approx. 20 ms) when walking compared to the seated position. A startling acoustic stimulus significantly decreased reaction times of the squeezing task (approx. 60 ms) both when walking and sitting. Dual-task costs during walking are, therefore, likely the result of lowered task preparation because of competition for attentional resources.

Experts, but not novices, exhibit StartReact indicating experts use the reticulospinal system more than novices

Motor skill acquisition utilizes a wide array of neural structures; however, few articles evaluate how the relative contributions of these structures shift over the course of learning. Recent evidence from rodents and songbirds suggests there is a transfer from cortical to subcortical structures following intense, repetitive training. Evidence from humans indicate that the reticulospinal system is modulated over the course of skill acquisition and may be a subcortical facilitator of learning. The objective of this study was to evaluate how reticulospinal contributions are modulated by task expertise. Reticulospinal contributions were assessed using StartReact (SR). We hypothesized that expert typists would show SR during an individuated, keystroke task but SR would be absent in novices. Expert (75.2 ± 9.8 WPM) and novice typists (41.6 ± 8.2 WPM) were evaluated during an individuated, keystroke movements. In experts, SR was present but was absent in novices. Together, these results suggest that experts use reticulospinal contributions more for movement than novices indicating that the reticular formation becomes increasingly important for movement execution in highly trained, skilled tasks even those that require individuated movement of the fingers.

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

The "StartReact" effect refers to the rapid involuntary triggering of a prepared movement in response to a loud startling acoustic stimulus (SAS). This effect is typically confirmed by the presence of short-latency electromyographic activity in startle reflex-related muscles such as the sternocleidomastoid (SCM); however, there is debate regarding the specific neural pathways involved in the StartReact effect. Some research has implicated a subcortically mediated pathway, which would predict different response latencies depending on the presence of a startle reflex. Alternatively, other research has suggested that this effect involves the same pathways responsible for voluntary response initiation and simply reflects higher preparatory activation levels, and thus faster voluntary initiation. To distinguish between these competing hypotheses, the present study assessed preparation level during a simple reaction time (RT) task involving wrist extension in response to a control tone or a SAS. Premotor RT and startle circuitry engagement (as measured by SCM activation) were determined for each trial. Additionally, preparation level at the go signal on each trial was measured using motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS). Results showed that SAS trial RTs were significantly shorter (P = 0.009) in the presence of startle-related SCM activity. Nevertheless, preparation levels (as indexed by MEP amplitude) were statistically equivalent between trials with and without SCM activation. These results indicate that the StartReact effect relates to engagement of the startle reflex circuitry rather than simply being a result of an increased level of preparatory activation.NEW & NOTEWORTHY The neural mechanism underlying the early triggering of goal-directed actions by a startling acoustic stimulus (SAS) is unclear. We show that although significant reaction time differences were evident depending on whether the SAS elicited a startle reflex, motor preparatory activation was the same. Thus, in a highly prepared state, the short-latency responses associated with the StartReact effect appear to be related to engagement of startle reflex circuitry, not differences in motor preparatory level.

Impaired posture, movement preparation, and execution during both paretic and nonparetic reaching following stroke

Posture and movement planning, preparation, and execution of a goal-directed reaching movement are impaired in individuals with stroke. No studies have shown whether the deficits are generally impaired or are specific to the lesioned hemisphere/paretic arm. This study utilized StartReact (SR) responses elicited by loud acoustic stimuli (LAS) to investigate the preparation and execution of anticipatory postural adjustments (APAs) and reach movement response during both paretic and nonparetic arm reaching in individuals with stroke and in age-matched healthy controls. Subjects were asked to get ready after receiving a warning cue and to reach at a "go" cue. An LAS was delivered at -500, -200, and 0 ms relative to the go cue. Kinetic, kinematic, and electromyographic data were recorded to characterize APA-reach movement responses. Individuals with stroke demonstrated systemwide deficits in posture and in movement planning, preparation, and execution of APA-reach sequence as shown by significant reduction in the incidence of SR response and impaired APA-reach performance, with greater deficits during paretic arm reaching. Use of trunk compensation strategy as characterized by greater involvement of trunk and pelvic rotation was utilized by individuals with stroke during paretic arm reaching compared with nonparetic arm reaching and healthy controls. Our findings have implications for upper extremity and postural control, suggesting that intervention should include training not only for the paretic arm but also for the nonparetic arm with simultaneous postural control requirements to improve the coordination of the APA-reach performance and subsequently reduce instability while functional tasks are performed during standing. NEW & NOTEWORTHY Our study is the first to show that nonparetic arm reaching also demonstrates impairment in posture and movement planning, preparation, and execution when performed during standing by individuals with stroke. In addition, we found compensatory trunk and pelvic rotations were used during a standing reach task for the paretic arms. The findings have clinical implications for upper extremity and postural rehabilitation, suggesting that training should include the nonparetic arms and incorporate simultaneous postural control demands.

Investigating the effect of anticipating a startling acoustic stimulus on preparatory inhibition

OBJECTIVES

Motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) show a profound suppression when elicited during the instructed-delay of reaction time (RT) tasks. One predominant hypothesis is that this phenomenon, called "preparatory inhibition", reflects the operation of processes that suppress motor activity to withhold prepared (but delayed) responses, a form of impulse control. In addition, a startling acoustic stimulus (SAS) - a loud and narrow sound - can trigger the release of prepared responses in RT tasks. We predicted that, if such premature release is clearly forbidden, then anticipating a SAS during delay periods may be associated with increased preparatory inhibition for greater impulse control.

METHODS

Subjects performed a behavioural (n=16) and TMS (n=11) experiment. Both used a choice RT task that required subjects to choose a response based on a preparatory cue but to only release it after an imperative signal. SAS and TMS pulses were elicited at the end of the delay period and subjects were asked to do their best to only release their response after the imperative signal, even in the presence of SAS. SAS could be either rare or frequent, in separate blocks.

RESULTS

Consistent with the literature, SAS shortened RTs, especially when they occurred frequently. Moreover, MEPs were suppressed when subjects delayed prepared responses but this preparatory inhibition did not depend on whether SAS were frequent or rare.

DISCUSSION

The stronger RT shortening with frequent rather than rare SAS may be due to increased attention and/or reduced reactive inhibition to SAS, leaving preparatory inhibition unaffected.