Threat vs control: Potentiation of the trigeminal blink reflex by threat proximity is overruled by self-stimulation

Neural control of blinking

Blinking is a motor act characterized by the sequential closing and opening of the eyelids, which is achieved through the reciprocal activation of the orbicularis oculi and levator palpebrae superioris muscles. This stereotyped movement can be triggered reflexively, occur spontaneously, or voluntarily initiated. During each type of blinking, the neural control of the antagonistic interaction between the orbicularis oculi and levator palpebrae superioris muscles is governed by partially overlapping circuits distributed across cortical, subcortical, and brainstem structures. This paper provides a comprehensive overview of the anatomical and physiological foundations underlying the neural control of blinking. We describe the infra-nuclear apparatus, as well as the supra-nuclear control mechanisms, i.e., how cortical, subcortical, and brainstem structures regulate and coordinate the different types of blinking.

The blink reflex and its modulation - Part 2: Pathophysiology and clinical utility

The blink reflex (BR) is integrated at the brainstem; however, it is modulated by inputs from various structures such as the striatum, globus pallidus, substantia nigra, and nucleus raphe magnus but also from afferent input from the peripheral nervous system. Therefore, it provides information about the pathophysiology of numerous peripheral and central nervous system disorders. The BR is a valuable tool for studying the integrity of the trigemino-facial system, the relevant brainstem nuclei, and circuits. At the same time, some neurophysiological techniques applying the BR may indicate abnormalities involving structures rostral to the brainstem that modulate or control the BR circuits. This is a state-of-the-art review of the clinical application of BR modulation; physiology is reviewed in part 1. In this review, we aim to present the role of the BR and techniques related to its modulation in understanding pathophysiological mechanisms of motor control and pain disorders, in which these techniques are diagnostically helpful. Furthermore, some BR techniques may have a predictive value or serve as a basis for follow-up evaluation. BR testing may benefit in the diagnosis of hemifacial spasm, dystonia, functional movement disorders, migraine, orofacial pain, and psychiatric disorders. Although the abnormalities in the integrity of the BR pathway itself may provide information about trigeminal or facial nerve disorders, alterations in BR excitability are found in several disease conditions. BR excitability studies are suitable for understanding the common pathophysiological mechanisms behind various clinical entities, elucidating alterations in top-down inhibitory systems, and allowing for follow-up and quantitation of many neurological syndromes.

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Hand-blink reflex modulation: The role of primary emotions and attachment dimensions

The hand-blink reflex (HBR) is a subcortical response elicited by the electrical stimulation of the median nerve. HBR magnitude is enhanced when the stimulated hand is close to the face and is modulated by high-level structures according to the perceived threat magnitude. Psychological factors may contribute to threat evaluation and possibly to HBR amplitude modulation. In this study, we assessed distinctively emotional and relational aspects of personality and evaluated their associations with the HBR response, or lack thereof, in healthy subjects. Seventy-one volunteers filled the Experiences in Close Relationships Scale, the Affective Neuroscience Personality Scales, and the State-Trait Anxiety Inventory Form Y questionnaires and underwent HBR recording. We found that the HBR could be evoked only in 50.7% of all subjects (responders). Non-responders subjects showed higher scores in the avoidance dimension (p = .005), and lower scores in the care dimension (p = .008), compared with responders. In responders, regression analysis showed a negative association of HBR amplitude (difference in near vs. far responses) with anger dimension and a positive association with state anxiety (R2  = 0.239). A positive association also emerged with HBR latency and fear dimension (R2  = 0.419). We conclude that primary emotional and relational factors may play an important role in the modulation of brainstem circuits mediating the HBR response. Our results may also contribute to the question about the absence of the HBR in about half of the subjects since high-level cognitive processes seem to play an important role in the differentiation between responder and non-responder.

Back and front peripersonal space: behavioural and EMG evidence of top-down and bottom-up mechanisms

Previous studies have identified a 'defensive graded field' in the peripersonal front space where potential threatening stimuli induce stronger blink responses, mainly modulated by top-down mechanisms, which include various factors, such as proximity to the body, stimulus valence, and social cues. However, very little is known about the mechanisms responsible for representation of the back space and the possible role of bottom-up information. By means of acoustic stimuli, we evaluated individuals' representation for front and back space in an ambiguous environment that offered some degree of uncertainty in terms of both distance (close vs. far) and front-back egocentric location of sound sources. We aimed to consider verbal responses about localization of sound sources and EMG data on blink reflex. Results suggested that stimulus distance evaluations were better explained by subjective front-back discrimination, rather than real position. Moreover, blink response data were also better explained by subjective front-back discrimination. Taken together, these findings suggest that the mechanisms that dictate blink response magnitude might also affect sound localization (possible bottom-up mechanism), probably interacting with top-down mechanisms that modulate stimuli location and distance. These findings are interpreted within the defensive peripersonal framework, suggesting a close relationship between bottom-up and top-down mechanisms on spatial representation.

In the spotlight: How the brainstem modulates information flow

OBJECTIVE

The blink reflex (BR) to supraorbital nerve (SON) stimulation is reduced by either a low-intensity prepulse stimulus to digital nerves (prepulse inhibition, PPI) or a conditioning SON stimulus (SON_-1) of the same intensity as the test (SON_-2) stimulus (paired-pulse paradigm). We studied how PPI affects BR excitability recovery (BRER) to paired SON stimulation.

METHODS

Electrical prepulses were applied to the index finger 100 ms before SON_-1, which was followed by SON_-2 at interstimulus intervals (ISI) of 100, 300, or 500 ms.

RESULTS

BRs to SON_-1 showed PPI proportional to prepulse intensity, but this did not affect BRER at any ISI. PPI was observed on the BR to SON_-2 only when additional prepulses were applied 100 ms before SON_-2, regardless of the size of BRs to SON_-1.

CONCLUSIONS

In BR paired-pulse paradigms, the size of the response to SON_-2 is not determined by the size of the response to SON_-1. PPI does not leave any trace of inhibitory activity after it is enacted.

SIGNIFICANCE

Our data demonstrate that BR response size to SON_-2 depends on SON_-1 stimulus intensity and not SON_-1 response size, an observation that calls for further physiological studies and cautions against unanimous clinical applicability of BRER curves.

Conscious agency vs. pre-conscious sensory filtering: Disparate suppression of trigeminal blink reflex by self-stimulation and by prepulses

Modulation of the blink reflex (BR) to supraorbital nerve (SON) stimulation by a weak somatosensory prepulse (sPP) consists of inhibition of R2 and facilitation of R1. Similar BR changes occur with self-stimulation. Our aim was to compare neurophysiological processes underlying both effects. We assessed BR parameters in 18 healthy participants following right SON stimulation either performed by an experimenter (experiment 1A) or following self-stimulation (experiments 1B, 1C). In experiments 1A and 1C, sPPs to digit 2 preceded SON stimuli by 40, 100, 200 and 500 ms. In experiment 1B: self-stimulation was delayed by 40, 100, 200, and 500 ms. In experiment 2, BRs were elicited by an experimenter randomly during a 2-s period before participants applied self-stimulation. In experiment 1, as expected, sPPs caused facilitation of R1 and inhibition of R2, which peaked at 100 ms ISI, similarly in experiments 1A and 1C. Self-stimulation caused a decrease of R2, which was evident in a broad range of time intervals. In experiment 2, R2 was already inhibited at the onset of the 2-s period, while R1 began to rise significantly 1.4 s before self-stimulation. Both effects progressively increased until self-triggering. The results concur with a time-locked gating mechanism of prepulses at brainstem level, whereas self-stimulation modulates BR in a tonic manner, reflecting a cognitive influence due to self-agency.

Machine learning for exploring neurophysiological functionality in multiple sclerosis based on trigeminal and hand blink reflexes

Brainstem dysfunctions are very common in Multiple Sclerosis (MS) and are a critical predictive factor for future disability. Brainstem functionality can be explored with blink reflexes, subcortical responses consisting in a blink following a peripheral stimulation. Some reflexes are already employed in clinical practice, such as Trigeminal Blink Reflex (TBR). Here we propose for the first time in MS the exploration of Hand Blink Reflex (HBR), which size is modulated by the proximity of the stimulated hand to the face, reflecting the extension of the peripersonal space. The aim of this work is to test whether Machine Learning (ML) techniques could be used in combination with neurophysiological measurements such as TBR and HBR to improve their clinical information and potentially favour the early detection of brainstem dysfunctionality. HBR and TBR were recorded from a group of People with MS (PwMS) with Relapsing-Remitting form and from a healthy control group. Two AdaBoost classifiers were trained with TBR and HBR features each, for a binary classification task between PwMS and Controls. Both classifiers were able to identify PwMS with an accuracy comparable and even higher than clinicians. Our results indicate that ML techniques could represent a tool for clinicians for investigating brainstem functionality in MS. Also, HBR could be promising when applied in clinical practice, providing additional information about the integrity of brainstem circuits potentially favouring early diagnosis.

Prepulse inhibition vs cognitive modulation of the hand-blink reflex

The excitability of brainstem circuitries mediating defensive blinking in response to abrupt sensory inputs is continuously modulated by cortical areas, e.g., the hand-blink reflex (HBR), elicited by intense electrical median nerve stimulation, is enhanced when the stimulated hand is close to the face, with the behavioural purpose to optimize self-protection from increased threat. Here we investigated whether such cortically mediated HBR facilitation can be influenced by prepulse inhibition (PPI), which is known to occur entirely at the subcortical level. Twenty healthy volunteers underwent HBR recordings in five experimental conditions. In conditions 1 and 2, the stimulated hand was held either near (1) or far (2) from the face, respectively. In conditions 3 and 4, stimulation of the hand near the face was preceded by a peri-liminal prepulse to the index finger of the contralateral hand held either near (3) or far from the face (4). In condition 5, participants self-triggered the stimulus eliciting the HBR. We observed a reproducible HBR in 14 out of 20 participants and measured onset latency and area of the HBR in orbicularis oculi muscles bilaterally. HBR area decreased and latency increased in condition 2 relative to condition 1; HBR area decreased and latency increased markedly in condition 3, and somewhat less in condition 4, relative to conditions 1 and 2; self-stimulation (condition 5) also suppressed HBRs, but less than prepulses. These findings indicate that PPI of the HBR is more robust than the cognitive modulation exerted by top-down cortical projections. Possibly, an attentional shift to a prepulse may serve to reduce blinking in response to perturbation when it is convenient, in a given situation, not to interrupt ongoing visual processing.