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Impulsive stimuli were used to evoke postural reflexes in healthy subjects (n = 10) and avestibular patients (n = 2). Electromyographic (EMG) activity was recorded with subjects standing erect, feet together with eyes closed and leaning forward to activate their leg muscles. EMG was recorded bilaterally from over the soleus muscles, rectified and a...
Citations
... The resulting head acceleration of about 0.2 g has shown to be an effective utricular stimulus, capable of evoking postural responses in the legs (Laube, Govender, & Colebatch, 2012;Govender, Dennis, & Colebatch, 2015), but without a signi cant movement artefact. For the axial stimulus the mini-shaker was applied to the spinous process of the C7 vertebra during anterior lean (Graus, Govender, & Colebatch, 2013). The visual stimuli were delivered via a colour display unit (Samsung, model C24F390FHE) at a viewing distance of about 30 cm and consisted of alternating vertical black/white bars generated by a custom software. ...
We report an experiment that tested the vestibular syncopation rhythm hypothesis, which holds that the rhythmic effect of syncopation is a form of vestibular reflexive/automated response to a postural perturbation, for example during locomotion. Electrophysiological signals were recorded from the cerebral cortex and cerebellum during processing of rhythmic sequences in a sample of experienced participants. Recordings were made using four different stimulus modalities, auditory, axial, vestibular and visual, under different rhythmic timing conditions, irregular, regular and syncopated/uncertain. Brain current activity was measured using a 10 dipole source regions of interest model in each of the participants, each modality, each timing condition, and for each beat within the bar of the rhythm. The cross-modal spectral power in frontal EEG and cerebellar ECeG was also analysed. The results show that the brain activity increases from the irregular to the regular and then from the regular to the uncertain timing conditions. However, the vestibular modality induces the greatest total brain activity across the regions of interest, and exhibits the highest sensitivity to the interaction of beat structure with the timing conditions in both source currents and spectral power. These data provide further evidence to support the primal role of the vestibular system in rhythm perception.
... Stimuli to the mastoid evoke short latency reflex responses in leg muscles similar to those seen with vestibular activation using galvanic stimulation (Laube et al. 2012). Brief perturbations of the upper trunk, using impulsive axial accelerations over the vertebra prominens (C7) and sternum also produce short latency postural responses in the leg muscles (Bötzel et al. 2001;Graus et al. 2013;Govender et al. 2015;. The evoked EMG responses are directed to the muscles most important for postural compensation and modulated by postural circumstances, the latter shown by the response becoming larger as the potential threat to postural stability increases. ...
... Since our report of the effects of mastoid stimuli (Govender et al. 2020), we have also examined axial stimuli for the possibility of recording associated CEPs over the posterior fossa (Todd et al. 2021). Axial stimuli share with vestibular stimuli the ability to evoke postural reflexes but do not depend on vestibular afferents, as postural reflexes to axial stimuli are preserved despite vestibular impairment (Bötzel et al. 2001;Graus et al. 2013). We confirmed here that these stimuli (C7 and sternal) also evoke short latency responses in electrodes overlying the cerebellum, but with a differing localisation and latency to the vestibular-evoked responses. ...
Recordings from over the posterior fossa following impulsive acceleration stimuli have shown short latency evoked potentials of presumed cerebellar origin. In this study, we investigated the effect of posture on these cerebellar evoked potentials (CEPs) and their relationship to postural reflexes recorded from the leg muscles evoked by the same stimuli. Nine healthy subjects were tested during lying (supine and prone), sitting and standing. Impulsive accelerations were applied at the mastoid and to truncal (both C7 and sternal) stimulation sites. The effect of vision, eyes open or closed, was investigated for all three stimuli. For the truncal stimuli, the effect of differing leaning conditions during standing was also recorded. CEP amplitudes were correlated for the three stimuli. For C7 stimulation during standing, both CEPs and postural reflexes scaled as the threat to postural stability increased. However, CEPs for all stimuli were present during lying, sitting and standing with amplitude and latency parameters mainly unaffected by posture or vision. In contrast, postural reflexes from the leg muscles were attenuated when not standing, with the effect being more marked for truncal stimuli. We conclude that CEPs evoked by axial and vestibular stimuli are not systematically gated by posture, in contrast to the reflex responses evoked by the same stimuli.
... A small impulsive stimulus ("axial impulse") applied over the sternum or over the spine of C7 using a mini-shaker has been used in subsequent investigations and has been shown to evoke SL postural reflexes in leg muscles (Govender et al. 2015). Responses were largest in soleus and tibialis anterior (TA) during upright stance, and were attenuated by sitting (Graus et al. 2013). During kneeling, the responses became attenuated in soleus and TA but were evident in more proximal leg muscles which had become more functionally relevant (Govender et al. 2015). ...
... Isometric contraction of the arms produced similar levels of tonic activity to support anterior lean for the deltoid and forearm flexors, but despite this, there were no clear responses to axial impulses when standing upright unsupported, highlighting that a postural role is a key requirement, as previously established for the legs (Graus et al. 2013;Govender et al. 2015) and for triceps brachii (Teng et al. 2017). Matching tonic levels of EMG activity was not always possible, and thus, responses were normalised, because increased tonic activity should increase reflex amplitude (Matthews 1986). ...
We studied the short-latency (SL) effects of postural perturbations produced by impulses applied over the spine of the C7 vertebra or the sternum ("axial impulses") in 12 healthy subjects. EMG recordings were made bilaterally from the triceps brachii, biceps brachii, soleus, and tibialis anterior muscles, and unilaterally from the deltoid, forearm flexors, forearm extensors, and first dorsal interosseous (FDI) muscles. Sternal impulses evoked short-latency responses in the biceps when subjects leaned posteriorly to support approximately 12% of their body weight with the arms, but these responses were only modestly larger than for isometric contraction of the arms (26.3 vs. 14.7%). In contrast, clear excitatory responses could be evoked in the deltoid, triceps, forearm muscles, and FDI when leaning anteriorly to support similar amounts of body weight. These responses were significantly larger than during isometric contraction. The deltoid (42.5%) and triceps (44.7%) had the largest responses in supported anterior lean and onset latencies increased distally in this condition (mean 31.8 ms in deltoid to 53.7 ms in FDI). There was a disproportionate delay between the forearm muscles and FDI. For both directions of lean, postural reflex responses normally present in the legs were severely attenuated. SL upper limb excitatory responses were bigger in proximal muscles as well as larger and more widespread for anterior axial perturbations compared to posterior axial perturbations when using the arms to support body weight. Our findings also provide further evidence of a role for reticulospinal pathways in mediating these rapid postural responses to accelerations of the trunk.
... We have recently reported the characteristics of a response evoked by a brief pulse of acceleration applied to the upper trunk or sternum (Graus et al. 2013), first reported by Bötzel et al. (2001). The evoked response occurs at short latency, typically 50-60 ms following the stimulus onset, and has properties consistent with being a postural reflex. ...
... The response occurs only when the target muscles have a postural role (Britton et al. 1993) and is directed to the muscles relevant to the postural task (Graus et al. 2013;Govender et al. 2015). We have previously provided evidence that this reflex does not depend upon vestibular function, local cutaneous receptors, or proprioceptors around the ankles (Govender et al. 2015). ...
... Reflex latency was determined as (onset) the time at which rectified EMG levels rose consistently above the mean prestimulus level and (end) when it returned to it or below. These short latency (SL) excitability changes were normalised prior to analysis using the average level of rectified EMG during the period of excitation and expressing this as a percentage of baseline (pre-stimulus) EMG levels (Graus et al. 2013). ...
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.
... Bőtzel et al. (2001) utilised an impulsive tap stimulus to the sternum to perturb upright stance, eliciting SL responses in the soleus, a response they concluded was not dependent upon vestibular activation. A similar stimulus generated by a minishaker applied to the upper trunk (over the C7 spine or sternum) has been used in later investigations (Graus et al. 2013;Govender et al. 2015). The existence of a postural response brought about by displacements to the trunk had been suggested Abstract We studied the short-latency (SL) postural effects of axial impulses in 11 subjects (22 ± 2 years old). ...
... Gurfinkel et al. 1981;Allum et al. 1989;Bloem et al. 2002), but using the impulsive stimulus allowed the properties to be investigated systematically. In both soleus and tibialis anterior muscles, short-latency (SL) responses were present in stable stance (Graus et al. 2013) and modified by postural task, being attenuated while kneeling and sitting despite similar levels of tonic muscle activity (Govender et al. 2015). ...
... The stimulus consisted of a smoothed impulsive stimulus with a waveform having a third-order gamma distribution and a 14-ms rise time (Ross 2007;Graus et al. 2013). A hand-held minishaker device with an attached perspex rod (model 4810, Brüel and Kjaer P/L, Denmark) was used to deliver the impulsive stimulus. ...
We studied the short-latency (SL) postural effects of axial impulses in 11 subjects (22 ± 2 years old). Recordings were made bilaterally from soleus and tibialis anterior (TA) muscles. We confirmed that with leaning anteriorly and posteriorly, reflex EMG increases occurred in both muscle groups at short latency following brief perturbations applied over C7 or the sternum (soleus mean latencies 57.5 and 66.4 ms; TA mean 51.7 and 55.4 ms, respectively). While the size of the SL reflexes was affected by the direction of lean when standing we found that light touch did not affect the amplitudes or latencies significantly. We investigated the presence of SL responses in the upper limb muscle triceps brachii during an isometric contraction and when the arm muscles had a direct role in supporting approximately 40% of the body weight. Similar levels of tonic EMG activity occurred in triceps in both conditions but significantly larger SL reflexes occurred when used posturally compared to the isometric contraction (23.0 vs 3.3%) while the reverse occurred for SL responses in soleus and TA, which were significantly attenuated. The responses were present with the head in the neutral position but with head rotation were larger contralateral to the direction of rotation. Calculations based upon the relative latencies suggest that the pathway responsible is not the corticospinal tract. We conclude that axially evoked SL postural reflexes are unaffected by light tactile input but are present in upper limb muscles when used for postural support. We propose that the pathway mediating these responses is the reticulospinal tract.
... We have recently investigated the properties of a reflex evoked by small truncal perturbations (Graus et al. 2013;Govender et al. 2015). Previous evidence existed for a reflex originating from axial structures, possibly muscle spindles within truncal muscles, and independent of any effects of ankle proprioceptors (Gurfinkel et al. 1981;Do et al. 1988;Bloem et al. 2000). ...
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.
Electronic supplementary material
The online version of this article (doi:10.1007/s00221-015-4442-2) contains supplementary material, which is available to authorized users.
... While these responses have been attributed to afferents excited by ankle movements (Fitzpatrick et al. 1994), postural reflexes arising from truncal receptors have also been reported (Gurfinkel et al. 1981;Bloem et al. 2000Bloem et al. , 2002 but not widely known or accepted. Recently, Graus et al. (2013) presented further evidence in support of an axial source of postural reflexes. These authors applied small perturbations to the head and trunk and showed that the responses in soleus were determined by the direction of the applied disturbance, that the upper trunk was the most effective site of stimulation and that the responses were not present with the Abstract Postural reflexes were recorded in healthy subjects (n = 17) using brief axial accelerations and tap stimuli applied at the vertebra prominens (C7) and manubrium sterni. ...
... Cutaneous anaesthesia applied over the C7 stimulation site had no significant subjects seated. Graus et al. (2013) also reported that a brief acceleration to the upper trunk evoked postural reflexes in the soleus muscles that inverted when the direction of trunk acceleration was changed, confirming a previous report by Bőtzel et al. (2001). Graus et al. (2013) argued that, even though a similar acceleration applied to the mastoids evoked vestibular-dependent reflexes in the legs with similar latencies, the response to truncal accelerations was not primarily mediated through vestibular receptors. ...
... Graus et al. (2013) also reported that a brief acceleration to the upper trunk evoked postural reflexes in the soleus muscles that inverted when the direction of trunk acceleration was changed, confirming a previous report by Bőtzel et al. (2001). Graus et al. (2013) argued that, even though a similar acceleration applied to the mastoids evoked vestibular-dependent reflexes in the legs with similar latencies, the response to truncal accelerations was not primarily mediated through vestibular receptors. Their arguments included the limited changes in the reflexes seen in vestibular patients. ...
Postural reflexes were recorded in healthy subjects (n = 17) using brief axial accelerations and tap stimuli applied at the vertebra prominens (C7) and manubrium sterni. Short latency (SL) responses were recorded from the soleus, hamstrings and tibialis anterior muscles and expressed as a percentage of the background EMG prior to stimulus onset. In the majority of postural conditions tested, subjects were recorded standing erect and leaning forward with their feet together. The SL response was larger for soleus than for the hamstrings during standing (soleus vs hamstrings; 70.4 vs 28.1 %), whereas the opposite occurred during kneeling (25.3 vs 127.3 %). Concordant head and trunk accelerations produced larger SL responses than discordant accelerations for soleus and hamstrings, but the evoked excitatory response was independent of head direction and as expected for the direction of truncal acceleration. Postural reflexes for soleus and tibialis anterior were strongly affected by conditions that posed a significant threat to postural stability; stimulation at C7 was associated with significant SL enhancement for soleus during anterior lean while sternal stimulation showed SL enhancement for tibialis anterior during posterior lean. Cutaneous anaesthesia applied over the C7 stimulation site had no significant effect on EMG responses, nor did vision or surface type (rigid or compliant). This study provides further evidence that postural reflexes produced by brief axial accelerations are independent of cutaneous receptors, vestibular afferents and ankle proprioceptors, and demonstrates that postural tasks and truncal orientation significantly affect the evoked response, consistent with a role in stabilising posture.
Electronic supplementary material
The online version of this article (doi:10.1007/s00221-014-4105-8) contains supplementary material, which is available to authorized users.
We present an initial report using 5 subjects, of short and long latency collic evoked responses following a half cycle of 100 Hz vibration (5 ms) applied to the sternocleidomastoid (SCM) tendon. These were detected in EEG and extraocular and leg muscles and compared with vestibular-dependent responses from direct mastoid stimulation. The responses from the extraocular recording site are likely to be evoked myogenic potentials, thus “collic evoked myogenic potentials” (CEMPs). An n19/p24 presumed ocular CEMP (oCEMP) was followed by a P22/N28 response over the posterior fossa, referred to as a collic cerebellar evoked potential (CoCEP), with responses in leg muscles starting around 55 ms. In contrast to their vestibular analogues, the oCEMP and CoCEP were predominantly ipsilateral to the side of stimulation, consistent with a double-crossed projection. In addition, their thresholds were just above the threshold of vibrotactile sensation, implying a low threshold, oligo-synaptic projection of SCM afferents to both extraocular and cerebellar targets. Following these short latency responses, SCM tendon stimulation evoked prolonged EMG responses in postural muscles of the legs, consistent with a role in the afferent limb of a short latency, spino-bulbar-spinal postural response to sternal perturbations. These collic evoked responses are likely to be of value in understanding the functions of cervical muscle afferents and have clinical value, for example in monitoring compensation after vestibular loss.
In this work we examine the possible neural basis for two brainstem-spinal reflexes using source analyses of brain activity recorded over the cortex and posterior fossa. In a sample of 5 healthy adult subjects, using axial and vestibular stimulation by means of applied impulsive forces, evoked potentials were recorded with 63 channels using a 10% cerebellar extension montage. In parallel, EMG was recorded from soleus and tibialis anterior muscles and accelerometry from the lower leg. Recordings over the cerebellum (ECeG) confirmed the presence of short latency (SL) potentials and these were associated with changes in high-frequency power. The SL responses to the two stimulus modalities differed in that the axial stimulation produced an initial pause and then a burst in the high-frequency ECeG, followed by excitation/inhibition in soleus while vestibular stimulation produced an initial burst then a pause, followed by inhibition/excitation in soleus. These short latency responses were followed by longer latency N1/P2/N2 responses in the averaged EEG, which were maximal at FCz. Brain Electrical Source Analysis (BESA) demonstrated both cerebellar and cerebral cortical contributions to the short-latency responses and primarily frontal cortex contributions to the long-latency EPs. The latency and polarity of the SL EPs, in conjunction with changes in high-frequency spontaneous activity, are consistent with cerebellar involvement in the control of brainstem-spinal reflexes. The early involvement of frontal cortex and subsequent later activity may be an indicator of the activation of the cortical motor-related system for rapid responses which may follow the reflexive components. These findings provide evidence of the feasibility of non-invasive electrophysiology of the human cerebellum and have demonstrated cerebellar and frontal activations associated with postural-related stimuli.
Objective:
To measure axially-evoked postural reflexes in 11 Parkinson's disease (PD) subjects, both stable and unstable, and to compare these with 13 age-matched controls.
Methods:
We measured the short-latency electromyography (EMG) reflex effects of brief impulsive displacements applied to the upper sternum or C7 for tibialis anterior (TA) and soleus. Our subjects were studied standing normally and when leaning both forwards and backwards.
Results:
The initial mechanical effects of the stimuli were similar but the reflex responses for the unstable PD group were increased, even after allowing for the increased levels of tonic activation. For TA, unstable PD subjects had significantly larger responses than the stable PD group whose responses were in turn significantly larger than controls. For soleus, unstable PD subjects had significantly greater responses than controls.
Conclusions:
These findings are consistent with previous evidence that exaggerated postural responses are characteristic of unstable PD subjects.
Significance:
Increased postural reflexes are characteristic of unstable PD subjects and may contribute to the instability seen for these patients in response to larger perturbations.
