Chapter 19 Auditory startle responses as a probe of brainstem function in healthy subjects and patients with movement disorders

Are You Listening? Facilitation of the Auditory Blink Response in People with Fibromyalgia

The purpose of the current study was to determine whether auditory prepulse inhibition (PPI) and/or prepulse facilitation (PPF) were altered in people with fibromyalgia (FM) when compared with controls. Eyeblink responses were recorded from 29 females with FM and 27 controls, while they listened to 3 blocks of auditory stimuli that delivered pulses with either PPI or PPF. Using a linear mixed model, our main findings were that there was a GROUP*CONDITION interaction (F4, 1084 = 4.01, P= .0031) indicating that the difference in amplitude between FM group and control group changed depending on the condition (PPI or PPF). Post hoc tests revealed no differences between the groups in response to PPI. The FM group showed a greater reactivity of response to the PPF conditioned stimulus than the control group did (t(39.7) = 2.03, P= .0494). Augmentation of PPF, as demonstrated by the FM group is thought to be linked to alterations in information processing mediated by an autonomically driven general orienting process. Activities that decrease autonomic drive or rebalance autonomic and parasympathetic tone such as vagal stimulation might be pursued as effective interventions for people with FM. PERSPECTIVE: This article presents evidence of preservation of neural circuitry that underpins response suppression and evidence of neural circuit disturbance mediated by autonomic drive-in people with FM. These results are important because intact circuitry underpins the effectiveness of therapies and may be harnessed, and rebalancing autonomic drive may be indicated.

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.

Modulation of the excitatory phase following the cutaneous silent period by vibration

INTRODUCTION

The post-inhibition excitatory phase (E3) of the cutaneous silent period (CSP) is attributed to the resynchronization of motoneuron activity following the inhibitory period but there is also evidence that a somatosensory startle reflex may contribute to this phase. We hypothesized that the startle reflex component contained in E3 will decrease during vibration.

METHODS

Sixteen healthy individuals were included in the study. CSP was recorded from slightly contracted right thenar muscles after painful index finger stimulation, before, during, and immediately after vibration. The values of the percentage change of E3 relative to pre-stimulus baseline (E3%) were compared before, during, and after vibration for each individual.

RESULTS

There was a reduction in E3% during vibration and the values returned to normal immediately after vibration (153.1 ± 43.5%, 115.2 ± 30.2%, 154.9 ± 68.2%, respectively; p = 0.030).

DISCUSSION

E3 is reduced during vibration in healthy individuals, presumably due to suppression of a reflex component, which is superimposed upon the known resynchronization of motoneurons.

Focus on the pedunculopontine nucleus. Consensus review from the May 2018 brainstem society meeting in Washington, DC, USA

The pedunculopontine nucleus (PPN) is located in the mesopontine tegmentum and is best delimited by a group of large cholinergic neurons adjacent to the decussation of the superior cerebellar peduncle. This part of the brain, populated by many other neuronal groups, is a crossroads for many important functions. Good evidence relates the PPN to control of reflex reactions, sleep-wake cycles, posture and gait. However, the precise role of the PPN in all these functions has been controversial and there still are uncertainties in the functional anatomy and physiology of the nucleus. It is difficult to grasp the extent of the influence of the PPN, not only because of its varied functions and projections, but also because of the controversies arising from them. One controversy is its relationship to the mesencephalic locomotor region (MLR). In this regard, the PPN has become a new target for deep brain stimulation (DBS) for the treatment of parkinsonian gait disorders, including freezing of gait. This review is intended to indicate what is currently known, shed some light on the controversies that have arisen, and to provide a framework for future research.

Dysphagia in cerebral hypoxia

INTRODUCTION

Dysphagia is a frequent problem in various neurological disorders. However, knowledge on swallowing function in patients with cerebral hypoxia is sparse. The objective of this study is to report the development of swallowing function in a series of adolescent and young-adult patients with cerebral hypoxia.

METHODS

We recruited eight patients (1 male) who were admitted to our institution after the acute phase following cerebral hypoxia. Each patient underwent detailed neurological evaluation, magnetic resonance imaging (MRI), standardized neurophysiological assessment and repeated clinical and fiber-endoscopic evaluation of swallowing. Furthermore, all patients received daily physical and occupational therapy and intensive logopedic therapy for swallowing.

RESULTS

Mean age in this case series was 19.9±3.6 years (range 16-25). All eight patients initially displayed severe swallowing dysfunction, but the reflexive components of swallowing were intact in seven patients without brainstem lesions. The only patient with additional brainstem involvement initially suffered from absence of an intact swallowing reflex and developed silent aspiration. However, follow-up examinations revealed intact swallowing reflexes in all eight patients.

DISCUSSION

Dysphagia is common in patients with cerebral hypoxia, mainly resulting in a delayed oral phase consistent with impaired volitional execution of swallowing. Additional lesions in the brainstem may affect the integrity of the central pattern-generating circuitry for swallowing, resulting in additional dysfunction of the non-volitional reflexive component. In conclusion, dysphagia in patients with cerebral hypoxia is a common complication particularly in the early stages of remission, while long-term prognosis with respect to swallowing is often good. Swallowing function should be closely monitored in patients with acquired brain injury.

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.

Arousal and Habituation Effects (Excitability) on Startle Responses to the International Affective Picture Systems (IAPS)

The magnitude and habituation of startle reflex responses have been used to evaluate the excitability of the neural structures involved in this psychophysiological response. We analyzed the magnitude and habituation responses to startle reflex probes in 112 women. Results confirmed the modulation of eyeblink reflex by affective valence for arousing (F = 34.79, p < .001), but not for nonarousing pictures (F = 1.08, ns). Our results indicate that there is a linear adjustment for habituation in all picture groups, except for arousing unpleasant pictures where there is a quadratic adjustment that could be due to initial sensitization followed by the subsequent habituation. Multiple linear regression analysis revealed that the startle magnitude was partially determined by arousal and startle presentation order. In conclusion, our results emphasize the usefulness of arousing pictures to study startle reflex response and show evidence of different response mechanisms for pleasant and unpleasant pictures conditions.

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.

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.

Increased auditory startle reflex in children with functional abdominal pain

OBJECTIVE

To test the hypothesis that children with abdominal pain-related functional gastrointestinal disorders have a general hypersensitivity for sensory stimuli.

STUDY DESIGN

Auditory startle reflexes were assessed in 20 children classified according to Rome III classifications of abdominal pain-related functional gastrointestinal disorders (13 irritable bowel syndrome [IBS], 7 functional abdominal pain syndrome; mean age, 12.4 years; 15 girls) and 23 control subjects (14 girls; mean age, 12.3 years) using a case-control design. The activity of 6 left-sided muscles and the sympathetic skin response were obtained by an electromyogram. We presented sudden loud noises to the subjects through headphones.

RESULTS

Both the combined response of 6 muscles and the blink response proved to be significantly increased in patients with abdominal pain compared with control subjects. A significant increase of the sympathetic skin response was not found. Comorbid anxiety disorders (8 patients with abdominal pain) or Rome III subclassification did not significantly affect these results.

CONCLUSIONS

This study demonstrates an objective hyperresponsivity to nongastrointestinal stimuli. Children with abdominal pain-related functional gastrointestinal disorders may have a generalized hypersensitivity of the central nervous system.

Brainstem Reflexes Are Enhanced Following Severe Spinal Cord Injury and Reduced by Continuous Intrathecal Baclofen

Objective. Plastic changes in the human central nervous system can occur at multiple levels, including circuits rostral to the lesion level in spinal cord injury (SCI). GABA is the most important inhibitory neurotransmitter in the brain. The authors hypothesized that one of the consequences of plasticity in SCI patients could be enhancement of brainstem reflexes, and they investigated the effect of continuous intrathecal baclofen (CITB) on such enhancement. Methods. The authors studied the early ipsilateral component R1 and the late component R2 of the blink reflex (BR), jaw jerk, masseter silent period (MSP), and auditory startle response (ASR) in 9 SCI patients without baclofen and in 8 with CITB. Nine healthy volunteers served as controls. Results. The amplitude of R1 of BR was significantly smaller in patients with CITB than in the other groups. The area of R2 of BR and of the ASR recorded in the orbicularis oculi, sternocleidomastoid, and biceps brachii muscles were significantly larger in SCI patients without baclofen than in controls, whereas there was no difference between patients with CITB and controls. The MSP magnitude was significantly larger in patients with CITB as compared with those without baclofen. Conclusion. The enhancement of brainstem reflexes in SCI patients may be due to plastic changes at the brainstem level after SCI. The significant reduction in response size in patients with CITB in comparison with patients without baclofen suggests that the enhancement of brainstem reflexes may be due to decreased GABAergic activity and that CITB is effective in reducing abnormal brainstem hyperexcitability.

The effect of a prepulse stimulus on the EMG rebound following the cutaneous silent period

The cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by Adelta fibres. The postinhibitory rebound of electromyographic (EMG) activity following the CSP has been mainly attributed to resynchronization of motoneurons, but the possibility of startle reflex activity contributing to the EMG burst has also been suggested. Several types of reflexes may be suppressed by a preceding weak stimulus--a phenomenon called prepulse inhibition (PPI). Our aim was to study whether PPI would diminish the EMG rebound, thereby providing further evidence for excitatory reflex activity contained within the postinhibitory EMG rebound following the CSP. Ten healthy subjects underwent CSP testing following noxious digit II stimulation in two conditions, with and without a prepulse applied to digit III. Rectified surface EMG recordings were obtained from right orbicularis oculi, sternocleidomastoid and thenar muscles of the dominant hand during thumb abduction with 25% of maximum force. The area of the EMG rebound and the EMG reflex responses in orbicularis oculi and sternocleidomastoid were significantly smaller in recordings where a prepulse stimulus was applied 100 ms before the stimulus as compared to control responses without prepulse. CSP onset and end latency, CSP duration, and the degree of EMG suppression were not influenced. Prepulses significantly reduced subjective discomfort as based on visual analog scale scores. Inhibition of the EMG rebound by prepulse stimulation supports the hypothesis that the excitatory EMG activity following the CSP contains not only resynchronization of motoneuronal firing, but also an excitatory reflex component. The most probable type of reflex seems to be a somatosensory startle reflex, a defence reaction which is generated in structures located in the caudal brainstem following an unexpected intense stimulus. Reduction of the discomfort associated with high-intensity electrical fingertip stimulation by a prepulse without affecting CSP parameters underlines the utility of PPI in the context of CSP testing.

Auditory startle reaction in primary blepharospasm

Primary dystonia is associated with abnormal brainstem function, as shown by abnormalities of the blink reflex in blepharospasm (BSP) and of the auditory startle reaction in cervical dystonia. We examined the auditory startle reaction--a brainstem reflex elicited by an unexpected loud stimulus--in patients with primary BSP to expand knowledge on brainstem pathophysiology in primary focal dystonia. Thirteen patients with primary BSP were included and 13 age- and sex-matched healthy volunteers served as controls. Auditory startle responses (ASRs) were elicited by binaural high-intensity auditory stimuli, and reflex electromyographic activity was recorded simultaneously with surface electrodes bilaterally from masseter, orbicularis oculi, sternocleidomastoid, and biceps brachii muscles. Patients with BSP showed higher ASR probabilities (masseter, sternocleidomastoid, biceps brachii), shorter ASR onset latencies (masseter, orbicularis oculi, sternocleidomastoid), and larger ASR area-under-the-curve (masseter, sternocleidomastoid) as compared with normal controls. Habituation of ASRs did not differ significantly between patients and controls. These results corroborate previous findings of increased brainstem excitability in primary BSP but point to a different pattern of brainstem dysfunction compared to cervical dystonia, indicating that different pathophysiological mechanisms are involved in the two types of focal dystonia.

Startle and its disorders

Exaggerated startle is an uncommon feature of various neurological diseases, but is still lacking precise analysis in many of them. So far, electrophysiologic and cinematographic analyses allow discriminating two main subtypes. The prototype of primary exaggerated startle is hereditary hyperekplexia, a well-studied disorder of the inhibitory glycine receptor and thus of the neuronal Cl- channel. The involuntary jerking in hereditary hyperekplexia is considered a reticular reflex myoclonus. The prototype of primary normal startle with secondary abnormalities is startle epilepsy where a surprise stimulus typically provokes a normal startle, which in turn initiates a focal (most often frontal lobe) seizure with tonic posturing of the limbs. Clinical differential diagnosis between both subtypes may be difficult in individual cases, but there are abnormalities in clinical and neurophysiologic reflex testing, which need, however, broad validation.