Interaction between startle and voluntary reactions in humans

Acoustic startle response in patients with orthostatic tremor

BACKGROUND AND AIM

Orthostatic tremor is a high frequency tremor predominantly on calf muscles during standing. Brainstem is the most probable generator in the pathogenesis since it comprises bilaterally projecting centers regulating stance or tone. We aimed to investigate the functional role of brainstem through the evaluation of acoustic startle response in primary orthostatic tremor patients.

PATIENTS AND METHOD

We included 7 (2 males) consecutive patients and 13 (5 males) healthy volunteers. Diagnosis was confirmed by polymyographic surface electromyography. All subjects underwent acoustic startle response and blink reflex investigations.

RESULTS

Presence rate (71.4% vs. 100%, p=0.042) and response rate (27.5% vs. 40.5%, p=0.047) of total acoustic startle response were lower in patient group. Similarly, probability over orbicularis oculi was lower among patients (p=0.003). However, blink reflex was observed in all patients and healthy volunteers and latencies of startle and blink reflexes were similar between groups.

CONCLUSIONS

In our patient group, normal response rate and latencies of R1 and R2 show structural integrity of at least blink reflex circuit at brainstem. On the other hand, suppressed response rates probably reflect decreased excitability of auditory startle reflex pathway.

Planning of ballistic movement following stroke: insights from the startle reflex

Following stroke, reaching movements are slow, segmented, and variable. It is unclear if these deficits result from a poorly constructed movement plan or an inability to voluntarily execute an appropriate plan. The acoustic startle reflex provides a means to initiate a motor plan involuntarily. In the presence of a movement plan, startling acoustic stimulus triggers non-voluntary early execution of planned movement, a phenomenon known as the startReact response. In unimpaired individuals, the startReact response is identical to a voluntarily initiated movement, except that it is elicited 30–40 ms. As the startReact response is thought to be mediated by brainstem pathways, we hypothesized that the startReact response is intact in stroke subjects. If startReact is intact, it may be possible to elicit more task-appropriate patterns of muscle activation than can be elicited voluntarily. We found that startReact responses were intact following stroke. Responses were initiated as rapidly as those in unimpaired subjects, and with muscle coordination patterns resembling those seen during unimpaired volitional movements. Results were striking for elbow flexion movements, which demonstrated no significant differences between the startReact responses elicited in our stroke and unimpaired subject groups. The results during planned extension movements were less straightforward for stroke subjects, since the startReact response exhibited task inappropriate activity in the flexors. This inappropriate activity diminished over time. This adaptation suggests that the inappropriate activity was transient in nature and not related to the underlying movement plan. We hypothesize that the task-inappropriate flexor activity during extension results from an inability to suppress the classic startle reflex, which primarily influences flexor muscles and adapts rapidly with successive stimuli. These results indicate that stroke subjects are capable of planning ballistic elbow movements, and that when these planned movements are involuntarily executed they can be as rapid and appropriate as those in unimpaired individuals.

Preparedness for landing after a self-initiated fall

A startling auditory stimulus (SAS) causes a faster execution of voluntary actions when applied together with the imperative signal in reaction time tasks (the StartReact effect). However, speeding up reaction time may not be the best strategy in all tasks. After a self-initiated fall, the program for landing has to be time-locked to foot contact to avoid damage, and therefore advanced execution of the program would not be convenient. We examined the effects of SAS on the landing motor program in 8 healthy subjects that were requested to let themselves fall from platforms either 50 or 80 cm high at the perception of a visual imperative signal and land on specific targets. In trials at random, SAS was applied either together with the imperative signal (SAS(IS)) or at an appropriate prelanding time (SAS(PL)). As expected, the latency of takeoff was significantly shortened in SAS(IS) trials. On the contrary, the timing of foot contact was not significantly different for SAS(PL) compared with control trials. No changes were observed in the size of the electromyograph bursts in the two experimental conditions with respect to the control condition. Our results indicate that the landing program after a self-initiated fall may in part be organized at the time of takeoff and involve precise information on timing of muscle activation. Once launched, the program is protected against interferences by external inputs.

Narcolepsy-cataplexy: deficient prepulse inhibition of blink reflex suggests pedunculopontine involvement

Hypocretin (orexin) deficiency plays a major role in the pathophysiology of narcolepsy-cataplexy. In animal models, hypocretinergic projections to the pedunculopontine nucleus are directly involved in muscle tone regulation mediating muscle atonia - a hallmark of cataplexy. We hypothesized that pedunculopontine nucleus function, tested with prepulse inhibition of the blink reflex, is altered in human narcolepsy-cataplexy. Twenty patients with narcolepsy-cataplexy and 20 healthy controls underwent a neurophysiological study of pedunculopontine nucleus function. Blink reflex, prepulse inhibition of the blink reflex and blink reflex excitability recovery were measured. Blink reflex characteristics (R1 latency and amplitude, and R2 and R2c latency and area under the curve) did not differ between patients and controls (P > 0.05). Prepulse stimulation significantly increased R2 and R2c latencies and reduced R2 and R2c areas in patients and controls. However, the R2 and R2c area suppression was significantly less in patients than in controls (to 69.8 ± 14.4 and 74.9 ± 12.6%, respectively, versus 34.5 ± 28.6 and 43.3 ± 29.5%, respectively; each P < 0.001). Blink reflex excitability recovery, as measured by paired-pulse stimulation, which is not mediated via the pedunculopontine nucleus, did not differ between patients and controls (P > 0.05). Our data showed that prepulse inhibition is reduced in narcolepsy-cataplexy, whereas unconditioned blink reflex and its excitability recovery are normal. Because the pedunculopontine nucleus is important for prepulse inhibition, these results suggest its functional involvement in narcolepsy-cataplexy.

The effects of prepulse inhibition timing on the startle reflex and reaction time

A loud acoustic stimulus has been shown to provoke a reflexive startle response and accelerate simple reaction times. However, an auditory prepulse presented in advance of a startling stimulus can reduce the effect of the startling stimulus. The current study examined the effect of the timing of the prepulse on startle-induced reaction times and the startle reflex. The task was to perform a 30° arm extension movement in response to a visual "go" stimulus. On selected trials, an auditory prepulse (80dB) was presented either 100ms, 500ms or 1000ms prior to the "go" signal. In addition, an auditory startling stimulus (124dB) was presented in conjunction with the "go" signal on some trials. Our results indicated that an auditory prepulse presented 100ms, and to a lesser extent 500ms, significantly decreased the amplitude of the startle reflex; however, the reaction time acceleration associated with the startling acoustic stimulus (SAS) was unaffected. The differential effect of the prepulse on the startle reflex and reaction time acceleration confirm different neural pathways for these effects while the differential effect of the prepulse on the control and startle RTs suggest different mechanisms for movement initiation.

The early release of planned movement by acoustic startle can be delayed by transcranial magnetic stimulation over the motor cortex

Previous studies have shown that preplanned movements can be rapidly released when a startling acoustic stimulus (SAS) is presented immediately prior to, or coincident with, the imperative signal to initiate movement. Based on the short latency of the onset of muscle activity (typically in less than 90 ms) and the frequent co-expression of startle responses in the neck and eye muscles, it has been proposed that the release of planned movements by a SAS is mediated by subcortical, possibly brainstem, pathways. However, a role for cortical structures in mediating these responses cannot be ruled out based on timing arguments alone. We examined the role of the cortex in the mediation of these responses by testing if a suprathreshold transcranial magnetic stimulation applied over the primary motor cortex, which suppresses voluntary drive and is known to delay movement initiation, could delay the release of movement by a SAS. Eight subjects performed an instructed-delay task requiring them to make a ballistic wrist movement to a target in response to an acoustic tone (control task condition). In a subset of trials subjects received one of the following: (1) suprathreshold TMS over the contralateral primary motor cortex 70 ms prior to their mean response time on control trials (TMS(CT)), (2) SAS 200 ms prior to the go cue (SAS), (3) suprathreshold TMS 70 ms prior to the mean SAS-evoked response time (TMS(SAS)), or (4) TMS(SAS) and SAS presented concurrently (TMS+SAS). Movement kinematics and EMG from the wrist extensors and flexors and sternocleidomastoid muscles were recorded. The application of TMS(CT) prior to control voluntary movements produced a significant delay in movement onset times (P < 0.001) (average delay = 37.7 ± 12.8 ms). The presentation of a SAS alone at -200 ms resulted in the release of the planned movement an average of 71.7 ± 2.7 ms after the startling stimulus. The early release of movement by a SAS was significantly delayed (P < 0.001, average delay = 35.0 ± 12.9 ms) when TMS(SAS) and SAS were presented concurrently. This delay could not be explained by a prolonged suppression of motor unit activity at the spinal level. These findings provide evidence that the release of targeted ballistic wrist movements by SAS is mediated, in part, by a fast conducting transcortical pathway via the primary motor cortex.

Tuning of the excitability of transcortical cutaneous reflex pathways during mirror-like activity

Voluntary contraction of a muscle generates electromyographic (EMG) activity in the homologous muscle on the opposite side (mirror-like activity), not only in pathological states and in infants but also in healthy adults. Few studies have examined whether the cutaneous reflexes during the preparatory period of a reaction time task are affected by mirror-like activity. In the present study, we investigated the modulation of the cutaneous reflexes in the left first interosseous (FDI) muscle in 9 healthy subjects while they performed a quick abduction of the right index finger during a reaction time task. Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the left index finger. We found that mirror-like activity occurred in the left FDI at approximately the onset of EMG activity in the right FDI. The excitatory E2 component was selectively increased at ~75 ms after the "Go" signal, which corresponded to the onset of mirror-like activity. The inhibitory I2 (~90 ms) component was tuned consistently into excitation after the "Go" signal. These findings suggest that long latency reflexes, possibly transcortical cutaneous reflexes, are finely tuned in relation to mirror-like activity.

Preparation for voluntary movement in healthy and clinical populations: evidence from startle

In this review we provide a summary of the observations made regarding advance preparation of the motor system when presenting a startling acoustic stimulus (SAS) during various movement tasks. The predominant finding from these studies is that if the participant is prepared to make a particular movement a SAS can act to directly and quickly trigger the prepared action. A similar effect has recently been shown in patients with Parkinson's disease. This "StartReact" effect has been shown to be a robust indicator of advance motor programming as it can involuntarily release whatever movement has been prepared. We review the historical origins of the StartReact effect and the experimental results detailing circumstances where advance preparation occurs, when it occurs, and how these processes change with practice for both healthy and clinical populations. Data from some of these startle experiments has called into question some of the previously held hypotheses and assumptions with respect to the nature of response preparation and initiation, and how the SAS results in early response expression. As such, a secondary focus is to review previous hypotheses and introduce an updated model of how the SAS may interact with response preparation and initiation channels from a neurophysiological perspective.

A block to pre-prepared movement in gait freezing, relieved by pedunculopontine nucleus stimulation

Gait freezing and postural instability are disabling features of Parkinsonian disorders, treatable with pedunculopontine nucleus stimulation. Both features are considered deficits of proximal and axial musculature, innervated predominantly by reticulospinal pathways and tend to manifest when gait and posture require adjustment. Adjustments to gait and posture are amenable to pre-preparation and rapid triggered release. Experimentally, such accelerated release can be elicited by loud auditory stimuli—a phenomenon known as ‘StartReact’. We observed StartReact in healthy and Parkinsonian controls. However, StartReact was absent in Parkinsonian patients with severe gait freezing and postural instability. Pedunculopontine nucleus stimulation restored StartReact proximally and proximal reaction times to loud stimuli correlated with gait and postural disturbance. These findings suggest a relative block to triggered, pre-prepared movement in gait freezing and postural instability, relieved by pedunculopontine nucleus stimulation.

Postural preparation prior to stepping in patients with Parkinson's disease

People with Parkinson's disease (PD) frequently have difficulties with generating anticipatory postural adjustments (APAs) for forward propulsion and lateral weight transfer when initiating gait. This impairment has been attributed to deficits in motor planning and preparation. This study examined the preparation of APAs prior to an imperative cue to initiate forward stepping. A startling acoustic stimulus (SAS) was used to probe the state of preparation of the APA in eight PD (off medication) and seven matched control subjects. Subjects performed visually cued trials involving a pre-cue light instructing them to prepare to step, followed 3.5 s later by a go-cue light to rapidly initiate stepping. In random trials, a SAS (124 dB) was presented at −1,500, −1,000, −500, −250, −100, or 0 ms before the go-cue. Subjects also performed self-initiated steps. Ground reaction forces (GRFs), center of pressure (CoP) changes, and electromyographic (EMG) signals were recorded. The SAS triggered APAs in 94 ± 11% (PD) and 96 ± 8% (control) of trials at latencies 89 ± 4 ms (PD) and 97 ± 3 ms (control) earlier than Control trials. The temporal profile of APA preparation was similar between groups. However, peak EMG, GRF, and mediolateral CoP amplitudes were reduced in PD. SAS-evoked APAs at 0 ms matched Control trial APAs and were enhanced compared with self-initiated stepping. These results demonstrate that people with mild to moderate PD can plan and prepare the appropriate APA sequence prior to the expected cue to initiate gait; however, the prepared APAs are underscaled in magnitude.

Motor preparation of spatially and temporally defined movements: evidence from startle

Previous research has shown that the preparation of a spatially targeted movement performed at maximal speed is different from that of a temporally constrained movement ( Gottlieb et al. 1989b ). In the current study, we directly examined preparation differences in temporally vs. spatially defined movements through the use of a startling stimulus and manipulation of the task goals. Participants performed arm extension movements to one of three spatial targets (20°, 40°, 60°) and an arm extension movement of 20° at three movement speeds (slow, moderate, fast). All movements were performed in a blocked, simple reaction time paradigm, with trials involving a startling stimulus (124 dB) interspersed randomly with control trials. As predicted, spatial movements were modulated by agonist duration and timed movements were modulated by agonist rise time. The startling stimulus triggered all movements at short latencies with a compression of the kinematic and electromyogram (EMG) profile such that they were performed faster than control trials. However, temporally constrained movements showed a differential effect of movement compression on startle trials such that the slowest movement showed the greatest temporal compression. The startling stimulus also decreased the relative timing between EMG bursts more for the 20° movement when it was defined by a temporal rather than spatial goal, which we attributed to the disruption of an internal timekeeper for the timed movements. These results confirm that temporally defined movements were prepared in a different manner from spatially defined movements and provide new information pertaining to these preparation differences.

Reaction time effects due to imperative stimulus modality are absent when a startle elicits a pre-programmed action

When an acoustic stimulus that is sufficiently intense to elicit a startle response is delivered in conjunction with the "go" signal in a simple reaction time (RT) task, RT is greatly reduced. It has been suggested that this effect is due to the startle interacting with voluntary response channels to directly trigger the pre-programmed action. Alternatively, it may be that the startling stimulus simply increases activation along the sensory and motor pathways allowing for faster stimulus-response processing. In the present study a startling acoustic stimulus (SAS) was presented in addition to a visual or an auditory imperative stimulus (IS) in a simple RT task. Results showed that the pre-programmed response was initiated much faster when participants were startled. However, while differences in RT due to IS modality were observed in control trials, this difference was absent for startle trials. This result indicates that the SAS does not simply speed processing along the normal stimulus-response channels, but acts to release the pre-planned movement via a separate, faster neural pathway.

Review of first trial responses in balance control: influence of vestibular loss and Parkinson's disease

The reaction to an unexpected balance disturbance is unpracticed, often startling and frequently associated with falls. This everyday situation can be reproduced in an experimental setting by exposing standing humans to sudden, unexpected and controlled movements of a support surface. In this review, we focus on the responses to the very first balance perturbation, the so-called first trial reactions (FTRs). Detailed analysis of FTRs may have important implications, both for clinical practice (providing new insights into the pathophysiological mechanisms underlying accidental falls in real life) and for understanding human physiology (what triggers and mediates these FTRs, and what is the relation to startle responses?). Several aspects of the FTRs have become clear. FTRs are characterized by an exaggerated postural reaction, with large EMG responses and co-contracting muscles in multiple body segments. This balance reaction is associated with marked postural instability (greater body sway to the perturbation). When the same perturbation is repeated, the size of the postural response habituates and the instability disappears. Other issues about FTRs remain largely unresolved, and these are addressed here. First, the functional role of FTRs is discussed. It appears that FTRs produce primarily increased trunk flexion during the multi-segmental response to postural perturbations, thus producing instability. Second, we consider which sensory signals trigger and modulate FTRs, placing specific emphasis on the role of vestibular signals. Surprisingly, vestibular signals appear to have no triggering role, but vestibular loss leads to excessive upper body FTRs due to loss of the normal modulatory influence. Third, we address the question whether startle-like responses are contributing to FTRs triggered by proprioceptive signals. We explain why this issue is still unresolved, mainly because of methodological difficulties involved in separating FTRs from 'pure' startle responses. Fourth, we review new work about the influence of perturbation direction on FTRs. Recent work from our group shows that the largest FTRs are obtained for toe-up support surface rotations which perturb the COM in the posterior direction. This direction corresponds to the directional preponderance for falls seen both in the balance laboratory and in daily life. Finally, we briefly touch upon clinical diagnostic issues, addressing whether FTRs (as opposed to habituated responses) could provide a more ecologically valid perspective of postural instability in patients compared to healthy subjects. We conclude that FTRs are an important source of information about human balance performance, both in health and disease. Future studies should no longer discard FTRs, but routinely include these in their analyses. Particular emphasis should be placed on the link between FTRs and everyday balance performance (including falls), and on the possible role played by startle reactions in triggering or modulating FTRs.

Upper and lower extremity proprioceptive inputs modulate EMG activity of the trapezius

Axial muscles like the trapezius have different reflexive and functional properties. The aim of this study was to analyze the long latency reflexes obtained from the trapezius by the electrical stimulation of upper and lower extremity peripheral nerves. Thirty-one healthy volunteers were included in the study. Surface EMG activity of both trapezius muscles was recorded and averaged after electrical stimulation of the median and peroneal mixed nerves. The recordings were performed during supine and erect posture in nine subjects to evaluate of the effect of postural differences on reflex response. Reflex recordings were also performed in six subjects from some other muscles together with the trapezius by the stimulation of the peroneal nerve. Reflex responses including three components were recorded from the trapezius muscle (unilateral or bilateral) by electrical stimulation of the peroneal nerve. The most stable of them was the second component (23/31) which had a latency of 72.6 ± 7.9 ms for the ipsilateral, and 74.2 ± 8.5 ms for the contralateral trapezius (15/31). For median stimulation, the first component recorded at 32.0 ± 6.7 ms was the most stable (25/31). The second component was more frequently recorded on the contralateral side (14/31). Erect posture increased the amplitude of these components. Upper and lower extremity proprioceptive inputs modulate the EMG activity of the trapezius. This modulation probably related with postural adjustments.

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

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

Voluntary activation of ankle muscles is accompanied by subcortical facilitation of their antagonists

Flexion and extension movements are organized reciprocally, so that extensor motoneurones in the spinal cord are inhibited when flexor muscles are active and vice versa. During and just prior to dorsiflexion of the ankle, soleus motoneurones are thus inhibited as evidenced by a depression of the soleus H-reflex. It is therefore surprising that soleus motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) have been found not to be reduced and even facilitated during a voluntary dorsiflexion. The objective of this study was to investigate if MEPs, evoked by TMS, show a similar facilitation prior to and at the onset of contraction of muscles that are antagonists to the muscle in which the MEP is evoked and if so, examine the origin of such a facilitatory motor programme. Eleven seated subjects reacted to an auditory cue by contracting either the tibialis anterior (TA) or soleus muscle of the left ankle. TMS was applied to the hotspot of TA and soleus muscles on separate days. Stimuli were delivered prior to and at the beginning of contraction. Soleus MEPs were significantly facilitated when TMS was applied 50 ms prior to onset of plantar flexion. Surprisingly, soleus MEPs were also facilitated (although to a lesser extent) at a similar time in relation to the onset of dorsiflexion. TA MEPs were facilitated 50 ms prior to onset of dorsiflexion and neither depressed nor facilitated prior to plantar flexion. No difference was found between the facilitation of the soleus MEP and motor evoked responses to cervicomedullary stimulation prior to dorsiflexion, suggesting that the increased soleus MEPs were not caused by changes at a cortical level. This was confirmed by the observation that short-latency facilitation of the soleus H-reflex by subthreshold TMS was increased prior to plantar flexion, but not prior to dorsiflexion. These findings suggest that voluntary contraction at the ankle is accompanied by preceding facilitation of antagonists by a subcortical motor programme. This may help to ensure that the direction of movement may be changed quickly and efficiently during functional motor tasks.

Motor preparation is modulated by the resolution of the response timing information

In the present experiment, the temporal predictability of response time was systematically manipulated to examine its effect on the time course of motor pre-programming and release of the intended movement by an acoustic startle stimulus. Participants performed a ballistic right wrist extension task in four different temporal conditions: 1) a variable foreperiod simple RT task, 2) a fixed foreperiod simple RT task, 3) a low resolution countdown anticipation-timing task, and 4) a high resolution anticipation-timing task. For each task, a startling acoustic stimulus (124dB) was presented at several intervals prior to the "go" signal ("go" -150ms, -500ms, and -1500ms). Results from the startle trials showed that the time course of movement pre-programming was affected by the temporal uncertainty of the imperative "go" cue. These findings demonstrate that the resolution of the timing information regarding the response cue has a marked effect on the timing of movement preparation such that under conditions of low temporal resolution, participants plan the movement well in advance in accordance with the anticipated probability of onset of the cue, whereas movement preparation is delayed until less than 500ms prior to response time when continuous temporal information is provided.