To blink or not to blink: inhibition and facilitation of reflex blinks

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.

Target Site of Prepulse Inhibition of the Trigeminal Blink Reflex in Humans

Despite the clinical significance of prepulse inhibition (PPI), the mechanisms are not well understood. Herein, we present our investigation of PPI in the R1 component of electrically induced blink reflexes. The effect of a prepulse was explored with varying prepulse test intervals (PTIs) of 20-600 ms in 4 females and 12 males. Prepulse-test combinations included the following: stimulation of the supraorbital nerve (SON)-SON [Experiment (Exp) 1], sound-sound (Exp 2), the axon of the facial nerve-SON (Exp 3), sound-SON (Exp 4), and SON-SON with a long trial-trial interval (Exp 5). Results showed that (1) leading weak SON stimulation reduced SON-induced ipsilateral R1 with a maximum effect at a PTI of 140 ms, (2) the sound-sound paradigm resulted in a U-shaped inhibition time course of the auditory startle reflex (ASR) peaking at 140 ms PTI, (3) facial nerve stimulation showed only a weak effect on R1, (4) a weak sound prepulse facilitated R1 but strongly inhibited SON-induced late blink reflexes (LateRs) with a similar U-shaped curve, and (5) LateR in Exp 5 was almost completely absent at PTIs >80 ms. These results indicate that the principal sensory nucleus is responsible for R1 PPI. Inhibition of ASR or LateR occurs at a point in the startle reflex circuit where auditory and somatosensory signals converge. Although the two inhibitions are different in location, their similar time courses suggest similar neural mechanisms. As R1 has a simple circuit and is stable, R1 PPI helps to clarify PPI mechanisms.SIGNIFICANCE STATEMENT Prepulse inhibition (PPI) is a phenomenon in which the startle response induced by a startle stimulus is suppressed by a preceding nonstartle stimulus. This study demonstrated that the R1 component of the trigeminal blink reflex shows clear PPI despite R1 generation within a circuit consisting of the trigeminal and facial nuclei, without startle reflex circuit involvement. Thus, PPI is not specific to the startle reflex. In addition, PPI of R1, the auditory startle reflex, and the trigeminal late blink reflex showed similar time courses in response to the prepulse test interval, suggesting similar mechanisms regardless of inhibition site. R1 PPI, in conjunction with other paradigms with different prepulse-test combinations, would increase understanding of the underlying mechanisms.

Macaque monkey trigeminal blink reflex circuits targeting orbicularis oculi motoneurons

The trigeminal blink reflex plays an important role in protecting the corneal surface from damage and preserving visual function in an unpredictable environment. The closing phase of the human reflex, produced by activation of the orbicularis oculi (ObOc) muscles, consists of an initial, small, ipsilateral R₁ component, followed by a larger, bilateral R₂ component. We investigated the circuitry that underlies this reflex in macaque (Macaca fascicularis and Macaca mulatta) monkeys by the use of single and dual tracer methods. Injection of retrograde tracer into the facial nucleus labeled neurons in the principal trigeminal nucleus, and in the spinal nucleus pars oralis and interpolaris, bilaterally, and in pars caudalis, ipsilaterally. Injection of anterograde tracer into the principal trigeminal nucleus labeled axons that directly terminated on ObOc motoneurons, with an ipsilateral predominance. Injection of anterograde tracer into pars caudalis of the spinal trigeminal nucleus labeled axons that directly terminated on ipsilateral ObOc motoneurons. The observed pattern of labeling indicates that the reticular formation ventromedial to the principal and spinal nuclei also contributes extensive bilateral input to ObOc motoneurons. Thus, much of the trigeminal sensory complex is in a position to supply a monosynaptic drive for lid closure, and the adjacent reticular formation can supply a disynaptic drive. These findings indicate that the assignment of the R₁ and R₂ components of the blink reflex to different parts of the trigeminal sensory complex cannot be exclusively based on subdivision connectional relationships with facial motoneurons. The characteristics of the R₂ component may be due, instead, to other circuit properties.

Paired nociceptive blink stimuli can facilitate trigeminofacial circuit at a long inter-stimulus interval

BACKGROUNDS

Conditioned responses of paired nociceptive blink reflex (nBR) can reflect the excitability of trigeminofacial circuit. In the present study, we studied paired homotopic nBR with different inter-stimulus intervals (ISI). By monitoring different ISIs and consequential conditioned R2 of nBR, we aimed to investigate the impact of ISIs on the recovery cycle of nBR in normal individuals.

METHODS

Twelve healthy volunteers (mean age: 29.9 ± 7.0 years; M/F: 7/5) were enrolled in this study. After individuals' reflex threshold was determined, triple pulses were given in pairs with ISIs 125 to 10000 milliseconds randomly. We calculated the ratio of conditioned and unconditioned nBR area-under-curve (AUC) (defined as recovery index), and amplitude of each ISI.

RESULTS

The average latency of unconditioned nR2 is 42.6 ± 5.5 ms, with amplitude of 53.4 ± 43.9 μV and the AUC of 563.5 ± 480.6 ms·μV. The conditioned nBR/unconditioned nBR response ratio was less than 100% while the ISI is shorter than 1667 ms, suggesting an inhibited conditioned response. The recovery index and the amplitude of conditioned nBR gradually increased with increasing ISI. The recovery index was greater than 100% at ISI of 10 s (p = 0.005), implying full recovery and facilitation of conditioned nBR.

CONCLUSION

Our study established the time-dependent dynamic recovery curve of paired nBR. The facilitated nBR at ISI longer than 10 s might be associated with temporal summation to the facial motor neurons after repeated stimuli. Our study results provided potential applications for patients with pain disorders involving trigeminofacial region.

The Effects of Attention on the Trigeminal Blink Reflex

During top-down processing, higher cognitive processes modulate lower sensory processing. The present experiment tested the effects of directed attention on trigeminal reflex blinks in humans (n = 8). In separate sessions, participants either attended to blink-eliciting stimuli or were given no attentional instructions during stimulation of the supraorbital branch of the trigeminal nerve. Attention to blink-eliciting stimuli significantly increased reflex blink amplitude and duration and shortened blink latency compared with the no attention condition. These results suggested that higher processes such as attention can modify the trigeminal blink reflex circuit.

Effects of Caffeine on the Trigeminal Blink Reflex

The acoustic startle and trigeminal blink reflexes share the same motor output. Since caffeine has been shown to augment the startle reflex, it was proposed that caffeine would also increase the trigeminal blink reflex. In 6 humans, the effects of caffeine (100 mg) on the trigeminal blink reflex were investigated. Reflex blinks were elicited by stimulation of the supraorbital branch of the trigeminal nerve. Following ingestion of caffeinated coffee, reflex blinks increased in amplitude and duration and occurred at a shorter latency than reflex blinks following ingestion of decaffeinated coffee. Since the blink reflex is a brainstem reflex, these results suggest that the psychomotor effects of caffeine facilitate brainstem processing.

Classical Conditioning and Modification of the Rabbit's (Oryctolagus Cuniculus) Unconditioned Nictitating Membrane Response

A fundamental tenet of behavior is that a reflex is automatic, unconscious, involuntary, and relatively invariant. However, we have discovered that a reflex can change dramatically as a function of classical conditioning, and this change can be demonstrated independently of the conditioned stimulus. We have termed this phenomenon conditioning-specific reflex modification (CRM). Although the behavioral laws and neural substrates of nonassociative reflex changes have been identified, the behavioral laws and neural substrates of CRM are only now being revealed. For example, CRM is similar to classical conditioning in that (a) it is a function of both the strength of conditioning and (b) the strength of the unconditioned stimulus, (c) it can be extinguished, and (d) it can be generalized from one unconditioned stimulus to another. Preliminary analysis suggests that CRM may have some features in common with post-traumatic stress disorder and may provide insights into treatment of the disorder.

Frequency matters: beta-band subthalamic nucleus deep-brain stimulation induces Parkinsonian-like blink abnormalities in normal rats

The synchronized beta-band oscillations in the basal ganglia-cortical networks in Parkinson's disease (PD) may be responsible for PD motor symptoms or an epiphenomenon of dopamine loss. We investigated the causal role of beta-band activity in PD motor symptoms by testing the effects of beta-frequency subthalamic nucleus deep-brain stimulation (STN DBS) on the blink reflex excitability, amplitude, and plasticity in normal rats. Delivering 16 Hz STN DBS produced the same increase in blink reflex excitability and impairment in blink reflex plasticity in normal rats as occurs in rats with 6-hydroxydopamine lesions and patients with PD. These deficits were not an artifact of STN DBS because, when these normal rats received 130 Hz STN DBS, their blink characteristics were the same as without STN DBS. To demonstrate that the blink reflex disturbances with 16 Hz STN DBS were frequency specific, we tested the same rats with 7 Hz STN DBS, a theta-band frequency typical of dystonia. In contrast to beta stimulation, 7 Hz STN DBS exaggerated the blink reflex plasticity as occurs in focal dystonia. Thus, without destroying dopamine neurons or blocking dopamine receptors, frequency-specific STN DBS can be used to create PD-like or dystonic-like symptoms in a normal rat.

Assessment of the wind-up phenomenon in the equine nociceptive trigeminal system

Repeated sub-threshold nociceptive electrical stimulation resulting in temporal summation of the limb nociceptive withdrawal reflex is a well-established non-invasive model to investigate the wind-up phenomenon in horses. Due to structural similarities of the trigeminal sensory nucleus to the dorsal horn of the spinal cord, temporal summation should be evoked by repeated transcutaneous electrical stimulation of trigeminal afferents. To evaluate this hypothesis repeated transcutaneous electrical stimulation was applied to the supraorbital and infraorbital nerves of 10 horses. Stimulation intensities varied between 0.5 and 1.3 times the trigemino-cervical reflex threshold defined for single stimulation. Evoked electromyographic activity of the orbicularis oculi, splenius and cleidomastoideus muscles was recorded and the signals analysed in the previously established epochs typical to the early and late component of the blink reflex and to the trigemino-cervical reflex. Behavioural reactions were evaluated with the aid of numerical rating scale. The nociceptive late component and the trigemino-cervical reflex were not elicited by sub-threshold intensity repeated transcutaneous electrical stimulation. Furthermore, the median reflex amplitude for the 10 horses showed a tendency to decline over the stimulation train so temporal summation of afferent trigeminal inputs could not be observed. Therefore, the modulation of trigeminal nociceptive processing attributable to repeated Aδ fibre stimulations seems to differ from spinal processing of similar inputs as it seems to have an inhibitory rather than facilitatory effect. Further evaluation is necessary to highlight the underlying mechanism.

Characterizing the spontaneous blink generator: an animal model

Although spontaneous blinking is one of the most frequent human movements, little is known about its neural basis. We developed a rat model of spontaneous blinking to identify and better characterize the spontaneous blink generator. We monitored spontaneous blinking for 55 min periods in normal conditions and after the induction of mild dry eye or dopaminergic drug challenges. The normal spontaneous blink rate was 5.3 ± 0.3 blinks/min. Dry eye or 1 mg/kg apomorphine significantly increased and 0.1 mg/kg haloperidol significantly decreased the blink rate. Additional analyses revealed a consistent temporal organization to spontaneous blinking with a median 750 s period that was independent of the spontaneous blink rate. Dry eye and dopaminergic challenges significantly modified the regularity of the normal pattern of episodes of frequent blinking interspersed with intervals having few blinks. Dry eye and apomorphine enhanced the regularity of this pattern, whereas haloperidol reduced its regularity. The simplest explanation for our data is that the spinal trigeminal complex is a critical element in the generation of spontaneous blinks, incorporating reflex blinks from dry eye and indirect basal ganglia inputs into the blink generator. Although human subjects exhibited a higher average blink rate (17.6 ± 2.4) than rats, the temporal pattern of spontaneous blinking was qualitatively similar for both species. These data demonstrate that rats are an appropriate model for investigating the neural basis of human spontaneous blinking and suggest that the spinal trigeminal complex is a major element in the spontaneous blink generator.

Chronic (neuropathic) corneal pain and blepharospasm: five case reports

Pain and focal dystonias have been associated with chronic pain conditions such as complex regional pain syndrome. Corneal pain, frequently known as "dry eye", may be a neuropathic pain condition with abnormalities of the nerve plexus. Here we present 5 case histories of patients with defined corneal pain (with associated neuropathic features) and objective measures of changes in the nerve plexus and associated blepharospasm. A putative relationship between pain and blepharospasm suggests potential involvement of the basal ganglia in both these conditions.

Normal and abnormal lid function

This chapter on lid function is comprised of two primary sections, the first on normal eyelid anatomy, neurological innervation, and physiology, and the second on abnormal eyelid function in disease states. The eyelids serve several important ocular functions, the primary objectives of which are protection of the anterior globe from injury and maintenance of the ocular tear film. Typical eyelid behaviors to perform these functions include blinking (voluntary, spontaneous, or reflexive), voluntary eye closure (gentle or forced), partial lid lowering during squinting, normal lid retraction during emotional states such as surprise or fear (startle reflex), and coordination of lid movements with vertical eye movements for maximal eye protection. Detailed description of the neurological innervation patterns and neurophysiology of each of these lid behaviors is provided. Abnormal lid function is divided by conditions resulting in excessive lid closure (cerebral ptosis, apraxia of lid opening, blepharospasm, oculomotor palsy, Horner's syndrome, myasthenia gravis, and mechanical) and those resulting in excessive lid opening (midbrain lid retraction, facial nerve palsy, and lid retraction due to orbital disease).

Multiple Manifestations of Microstimulation in the Optic Tectum: Eye Movements, Pupil Dilations, and Sensory Priming

It is well established that the optic tectum (or its mammalian homologue, the superior colliculus) is involved in directing gaze toward salient stimuli. However, salient stimuli typically induce orienting responses beyond gaze shifts. The role of the optic tectum in generating responses such as pupil dilation, galvanic responses, or covert shifts is not clear. In the present work, we studied the effects of microstimulation in the optic tectum of the barn owl ( Tyto alba) on pupil diameter and on eye shifts. Experiments were conducted in lightly anesthetized head-restrained barn owls. We report that low-level microstimulation in the deep layers of the optic tectum readily induced pupil dilation responses (PDRs), as well as small eye movements. Electrically evoked PDRs, similar to acoustically evoked PDRs, were long-lasting and habituated to repeated stimuli. We further show that microstimulation in the external nucleus of the inferior colliculus also induced PDRs. Finally, in experiments in which tectal microstimulations were coupled with acoustic stimuli, we show a tendency of the microstimulation to enhance pupil responses and eye shifts to previously habituated acoustic stimuli. The enhancement was dependent on the site of stimulation in the tectal spatial map; responses to sounds with spatial cues that matched the site of stimulation were more enhanced compared with sounds with spatial cues that did not match. These results suggest that the optic tectum is directly involved in autonomic orienting reflexes as well as in gaze shifts, highlighting the central role of the optic tectum in mediating the body responses to salient stimuli.

Impaired prepulse inhibition and prepulse-elicited reactivity but intact reflex circuit excitability in unmedicated schizophrenia patients: a comparison with healthy subjects and medicated schizophrenia patients

Deficient sensorimotor gating as indexed by prepulse inhibition (PPI) of the startle response has been reported repeatedly in patients suffering from schizophrenia. According to the widely accepted "protective hypothesis," PPI reflects the protection of ongoing information processing against interference by other stimuli. Alternatively, it has been proposed that PPI might be regulated by startle reflex circuit excitability. In the present study, we evaluated these 2 conceptually divergent approaches underlying the regulation of PPI. To this end, we assessed sensorimotor gating as indexed by PPI, the reactivity to the prepulse-alone stimulus indexed as prepulse-elicited reactivity (PPER), and acoustic blink reflex excitability in terms of paired pulse suppression (PPS) within a single recording session in 13 unmedicated and 24 medicated (11 first break) schizophrenia patients in comparison to 43 healthy control subjects. The results showed that PPI was significantly reduced in unmedicated, but not in medicated schizophrenia patients. Furthermore, unmedicated patients could be distinguished from the medicated patients and control subjects in terms of PPER. In contrast to PPI, PPS did not differ between patients and control subjects. These findings are in line with the "protective hypothesis" of PPI and indicate that reduced sensorimotor gating in schizophrenia patients might be based on a reduced perception and/or processing of the prepulse stimulus. The extent to which PPER may or may not be causally associated with sensorimotor gating in schizophrenia has to be further investigated in human and animal studies.

Effect of slow rTMS of motor cortex on the excitability of the blink reflex: a study in healthy humans

OBJECTIVE

To evaluate the after-effects of low frequency, sub-threshold repetitive Transcranial Magnetic Stimulation (rTMS) of primary motor cortex, on the excitability of Blink Reflex (BR) in healthy subjects.

METHODS

The BR recovery cycle was carried out in 10 healthy volunteers in basal conditions, immediately after rTMS (30s), 15 and 60min later. A paired electric supraorbital stimulus paradigm with inter-stimulus intervals (ISI) of 100-600-1000-1500ms was used. The "real" rTMS consisted of a 200 stimuli long train delivered at 1Hz and intensity 80% of rest Motor Threshold of the FDI muscle, using a focal coil applied over the primary motor cortex region. The basal BR recovery cycle was also compared with that obtained after a "sham" rTMS.

RESULTS

The recovery of the R2 component of the BR was significantly suppressed 30s after rTMS. This effect was also observed at 15min, though of lower magnitude and only at long ISIs (1000-1500ms). No significant effect on R2 recovery was observed 60min after real rTMS as well as after sham rTMS.

CONCLUSIONS

rTMS of motor cortex modulates the excitability of BR through its action on cortical excitability and on the cortical facilitatory drive to the brainstem reflex pathways.

SIGNIFICANCE

Slow (1Hz), sub-threshold rTMS of motor cortex determines a long-lasting reduction of excitability of BR.

Experiential modification of the trigeminal reflex blink circuit

To characterize the organization and plasticity of the trigeminal reflex blink circuit, we interacted blink-evoking supraorbital (SO) and infraorbital (IO) nerve stimuli in alert rats. Stimulation of either trigeminal branch produced a short-lasting inhibition followed by a longer-lasting facilitation of blinks evoked by stimulating the other nerve. When IO stimulation evoked a smaller blink than SO stimulation (IO < SO), SO stimulation facilitated subsequent IO-evoked blinks more than IO stimulation facilitated SO-evoked blinks. When IO > SO, IO and SO stimulation exerted equivalent facilitation of subsequent reflex blinks. To investigate whether the blink circuit obeyed rules analogous to those governing the associative and spike timing-dependent plasticity exhibited by individual synapses, we compared the effects of 3600 simultaneous IO and SO pairings, asynchronous IO and SO pairings, or synchronous IO and SO pairings separated by 20 ms on temporal interactions between IO and SO inputs to the blink circuit. Simultaneous pairing of a weak IO and a strong SO strengthened the IO input to the blink circuit, whereas asynchronous pairing weakened the stronger input. When the pairing pattern made an afferent input arrive after blink circuit activity, it weakened that afferent input. Analogous to synaptic modifiability, the results revealed that blink-evoking stimuli acted as a "presynaptic input" and blink circuit activity acted as a "postsynaptic spike." These mechanisms may create the maladaptive reorganization of trigeminal inputs in diseases such as hemifacial spasm.

The three-neuron corneal reflex circuit and modulation of second-order corneal responsive neurons

Neurons located in the border region between the interpolaris and caudalis subdivisions of the spinal trigeminal nucleus (Vi/Vc) are second order neurons of the corneal reflex, receiving corneal afferents and projecting to the lid closing, orbicularis oculi (OO) motoneurons. Recordings of Vi/Vc neurons identified by antidromic activation from stimulation of the facial nucleus and non-identified Vi/Vc neurons reveal two neuron types, phasic and tonic. Corneal stimulation elicits Adelta latency action potentials that occur early enough to initiate OO contraction and C-fiber latency action potentials that can modulate the end of the blink in phasic Vi/Vc neurons. Tonic Vi/Vc neurons exhibit a constant irregular, low frequency discharge as well as the cornea-evoked activity exhibited by phasic neurons. For both phasic and tonic neurons, blink amplitude increases with the total number of spikes evoked by the corneal stimulus. Peak firing frequency predicts peak orbicularis oculi EMG activity. Paradigms that suppress cornea-evoked blinks differentially affect Vi/Vc neurons. Microstimulation of the border region between the spinal trigeminal caudalis subdivision and the C1 spinal cord (Vc/C1) significantly reduces the number of spikes evoked by corneal stimulation and suppresses blink amplitude. In the paired stimulus paradigm, a blink evoked by a corneal stimulus 150 ms after an identical corneal stimulus is significantly smaller than the blink elicited by the first stimulus. Vi/Vc neuron discharge, however, is slightly larger for the second blink. Our data indicate that second-order Vi/Vc neurons do not determine the specific pattern of OO muscle activity; rather Vi/Vc neurons initiate OO motoneuron discharge and program the activity of another circuit that generates the late phase of the blink. The Vc/C1 suppression of Vi/Vc neurons suggests that the Vc/C1 region provides an "internal model" of the intended blink.

Bell's palsy-induced blepharospasm relieved by passive eyelid closure and responsive to apomorphine

OBJECTIVE

We describe the case of a woman with Bell's Palsy-induced blepharospasm (BPIB) of the right eye that appeared simultaneously with a complete left facial nerve palsy. The involuntary spasm was relieved by passive lowering of the upper eyelid on the paretic side.

METHODS

The recovery curve of the blink reflex was evaluated on the non-paretic side in baseline conditions, after subcutaneous apomorphine and placebo administration and 8 months later, at recovery from the palsy.

RESULTS

We found increased recovery of the test-R2 responses at short interstimulus intervals at baseline, which was normalised by apomorphine but not by placebo. At recovery the blink reflex R2 recovery curve returned to normal.

CONCLUSIONS

This report demonstrates for the first time a response of BPIB to a dopamine agonist.

SIGNIFICANCE

Our findings are in agreement with an animal model of blepharospasm that suggests a combined role of weakness of the orbicularis oculi muscle and a dysfunction of the dopaminergic system in the pathogenesis of this disorder.

Modification of cornea-evoked reflex blinks in rats

Although maintaining the tear film on the cornea is the most important role of blinking, information about the organization and modification of cornea-evoked blinks is sparse. This study characterizes cornea-evoked blinks and their modification in urethane-anesthetized rats. Cornea-evoked blinks typically begin 16.2 ms after an electrical stimulus to the cornea and last an average of 50.2 ms. In anesthetized rats, the blink only occurs ipsilateral to the stimulus. In response to cornea stimulation, the orbicularis oculi EMG activity typically exhibits two bursts that correlate with the arrival of A delta and C-fiber inputs to the spinal trigeminal complex. In the paired-stimulus paradigm, suppression of the blink evoked by the second cornea stimulus occurs for interstimulus intervals less than 300 ms and is exclusively unilateral. Stimulation of the contralateral cornea does not affect subsequent blinks evoked from stimulation of the ipsilateral cornea. To determine whether activation of cornea-related neurons in the border region between the spinal trigeminal caudalis subdivision and the C1 spinal cord (Vc/C1) inhibits the second blink in the paired-stimulus paradigm, we examine the suppression of cornea-evoked blinks caused by microstimulation in this region. This suppression of orbicularis oculi EMG activity begins 8.3 ms after Vc/C1 stimulation. Activation of this region, however, is unlike suppression in the paired-stimulus paradigm because Vc/C1 activation bilaterally inhibits cornea-evoked blinks. Thus, activation of Vc/C1 is a previously unidentified mechanism for modulating cornea-evoked blinks.

Habituation of the acoustic and the tactile startle responses in mice: two independent sensory processes

To test whether habituation is specific to the stimulus modality, the authors analyzed cross-habituation between the tactile startle response' (TSR) and the acoustic startle response (ASR). The acoustic artifacts of airpuffs used to elicit the TSR were reduced by using a silencer and were effectively masked by background noise of 90-100 dB sound-pressure level. ASR was elicited by 14-kHz tones. TSR and ASR habituated in DBA and BALB mice: both the TSR and ASR habituated to a greater extent in DBA mice than in BALB mice. In both strains, habituation of the TSR did not generalize to the ASR, and vice versa. From this, the authors concluded that habituation of startle is located in the sensory afferent branches of the pathway.

Kynurenate in the pontine reticular formation inhibits acoustic and trigeminal nucleus-evoked startle, but not vestibular nucleus-evoked startle

The startle reflex is elicited by acoustic, trigeminal or vestibular stimulation, or by combinations of these stimuli. Acoustic startle is mediated largely by ibotenate-sensitive neurons in the ventrocaudal pontine reticular formation (PnC). In these studies we tested whether startle elicited by stimulation of different modalities is affected by infusion of the non-selective glutamate antagonist, kynurenate, into the PnC. In awake rats, startle responses evoked by either acoustic or spinal trigeminal nucleus stimulation were inhibited by kynurenate, but not saline, infusions, with the most effective placements nearest PnC. In chloral hydrate-anesthetized rats, kynurenate in the PnC reduced trigeminal nucleus-evoked hindlimb EMG responses, but not vestibular nucleus-evoked startle. Kynurenate in the vestibular nucleus had no effect on trigeminal nucleus-evoked startle. These results indicate that trigeminal nucleus stimulation evokes startle largely through glutamate receptors in the PnC, similarly to acoustic startle, but vestibular nucleus-evoked startle is mediated through other pathways, such as the vestibulospinal tract.

A comparison of several methods used to quantify prepulse inhibition of eyeblink responding

Several methods of quantifying prepulse inhibition (PPI) of eyeblink responding were compared in adult volunteers. Blink-eliciting stimuli were noise bursts at 85 or 100 dB, and prepulses were also noise bursts, at 55 or 70 dB and lead intervals of 60 or 120 ms. PPI was evaluated by comparing reactivity on prepulse and control trials within participants using the following methods: (1) difference between reactivity on prepulse and control trials; (2) reactivity on prepulse trials divided by that on control trials (proportion of control); (3) difference between reactivity on prepulse and control trials, divided by that on control trials (proportion of the difference from control); (4) range correction (maximum minus minimum reactivity, divided by the range of reactivity), across all control and prepulse trials; (5) z scores across all prepulse and control trials. Prepulses inhibited eyeblink response magnitude in all cases. Proportion of difference was the method least affected by differences in control reactivity, and is the preferred method to use when quantifying PPI, both from a practical and a neurophysiological perspective.

Temporal Encoding in Fear Conditioning Revealed Through Associative Reflex Facilitation

Temporal encoding in Pavlovian fear conditioning was examined through conditional facilitation of the short-latency (Rl) component of the rat eyeblink reflex. Rats were fear-conditioned to a tone conditional stimulus (CS) with either a 3- or 9-s interstimulus interval (ISI) between CS onset and the onset of the grid-shock unconditional stimulus (US). Rl facilitation was tested over 2 days, in counterbalanced order, at a latency of 3 s and 9 s from CS onset. CS-produced Rl facilitation, the conditional response (CR), was 3-4 times larger when the test latency equaled the conditioning ISI. These results, coupled with the known neurophysiology of Rl facilitation, suggest that this CR could disclose differences in the time course of CS-generated output from the amygdala when driven by cortical versus subcortical CS-CR pathways.

Central amygdala lesions block ultrasonic vocalization and freezing as conditional but not unconditional responses

Bilateral amygdala (AM) lesions prevent the acquisition of fear-related conditional responses (CRs) in rats, a result that is most commonly concluded to reflect a learning or memory deficit. An alternative hypothesis is that AM-lesioned animals fail to acquire certain fear CRs simply because they cannot perform these behaviors. This performance-deficit hypothesis is usually invoked in regard to studies in which the CR is freezing, the most commonly measured behavior. Here we explore this interpretation by measuring two different behaviors [freezing and 22 kHz ultrasonic vocalization (USV)] elicited under three conditions (during context conditioning, during subsequent retention testing, and after ejaculation) in experimental rats [that received electrolytic lesions of the central nucleus of the amygdala (ACe)] and control animals (that received a sham operation). If ACe damage produces a discrete motor deficit that specifically renders the animal unable to remain immobile, then freezing should be blocked or impaired when elicited under all three conditions, whereas USV should be spared. Alternatively, if ACe damage selectively interferes with CR formation, maintenance, or expression, then both freezing and USV should be blocked or impaired when elicited as CRs during acquisition and retention testing but spared when evoked as unconditional responses (URs) to ejaculation. ACe damage blocked or severely impaired both freezing and USV elicited as CRs but had no effect on either behavior elicited as URs. We reject the motor-deficit hypothesis and discuss some viable alternatives.

A kinetic study of blinking responses in cats

Reflexively evoked and eye-related eyelid responses were recorded using the search coil in a magnetic field technique in alert cats. The downward phase of a blink was a large (up to 21 deg), fast (up to 2000 deg s-1) eyelid displacement in the closing direction, with an almost fixed rise time duration (15-20 ms); its maximum velocity was achieved in ~10 ms. Upward eyelid motion was separated into two phases. The first phase consisted of a fast eyelid displacement, with a short duration (approximately 30 ms) and a maximum velocity up to 900 deg s-1. The second phase had an exponential-like form, lasting for 200-400 ms, and a maximum velocity ranging between 30 and 250 deg s-1. Maximum blink velocity in the downward direction was linearly related to maximum velocity of the first upward phase. The first phase in the upward direction was never observed if the eyelid stayed closed for a long period (> 50 ms) or moved slowly in the closing direction before it started to open. In these two cases, the upswing motion of the blink reflex contained only the exponential-like movement characteristic of the second upward phase, and maximum velocity in the downward direction was not related to that of the eyelid upward displacement. Mean duration of eyelid downward saccades was approximately 130 ms, and their peak velocities ranged between 50 and 440 ms. A physiological model is presented explaining the active and passive forces involved in both reflex and saccadic eyelid responses. A second-order system seems to be appropriate to describe the postulated biomechanical model.

Sensation seeking and startle modulation by physically threatening images

The potential moderating effect of sensation seeking on anxious reactivity to threatening experiences was assessed using the affective modulation of startle-blink paradigm. Startle blinks, as measured by electromyographic (EMG) activity in response to loud (100 dB) white-noise stimuli, were elicited during the presentation of positive, neutral, and threatening visual images. Unlike participants low in sensation seeking who showed blink potentiation during threatening versus neutral images, participants high in sensation seeking showed equal magnitudes of startle to neutral and threatening images. The results suggest that individuals high compared with low on sensation seeking are less anxiously reactive to physically threatening visual stimuli. No attenuation in startle magnitude was elicited by positive images among low or high sensation seekers suggesting that the positive images employed in the current study were not arousing enough to activate the appetitive arousal system.

Eyelid movements: behavioral studies of blinking in humans under different stimulus conditions

The kinematics and neurophysiological aspects of eyelid movements were examined during spontaneous, voluntary, air puff, and electrically induced blinking in healthy human subjects, using the direct magnetic search coil technique simultaneously with electromyographic recording of the orbicularis oculi muscles (OO-EMG). For OO-EMG recordings, surface electrodes were attached to the lower eyelids. To measure the vertical lid displacement, a search coil with a diameter of 3 mm was placed 1 mm from the rim on the upper eyelid on a marked position. Blink registrations were performed from the zero position and from 28 randomly chosen positions. Blinks elicited by electrical stimulation of the supraorbital nerve had shortest duration and were least variable. In contrast, spontaneous blinks had longer duration and greater variability. Blinks induced by air puff had a slightly longer duration and similar variability as electrically induced blinks. There was a correlation between the maximal down phase amplitude and the integrated OO-EMG. Blink duration and maximal down phase amplitude were affected by eye position. Eyes positioned 30 degrees above horizontal displayed the shortest down phase duration and the largest maximal down phase amplitude and velocity. At 30 degrees below horizontal, blinks had the longest total duration, the longest down phase duration, and the lowest maximal down phase amplitude and velocity. The simultaneously recorded integrated OO-EMG was largest in the 30 degrees downward position. In four subjects, the average blinking data showed a linear relation between eye position and OO-EMG, maximal down phase amplitude, and maximal downward velocity.

Effects of voluntary blinks on saccades, vergence eye movements, and saccade-vergence interactions in humans

Blinks are known to change the kinematic properties of horizontal saccades, probably by influencing the saccadic premotor circuit. The neuronal basis of this effect could be explained by changes in the activity of omnipause neurons in the nucleus raphe interpositus or in the saccade-related burst neurons of the superior colliculus. Omnipause neurons cease discharge during both saccades and vergence movements. Because eyelid blinks can influence both sets of neurons, we hypothesized that blinks would influence the kinematic parameters of saccades in all directions, vergence, and saccade-vergence interactions. To test this hypothesis, we investigated binocular eye and lid movements in five normal healthy subjects with the magnetic search coil technique. The subjects performed conjugate horizontal and vertical saccades from gaze straight ahead to targets at 20 degrees up, down, right, or left while either attempting not to blink or voluntarily blinking. While following the same blink instruction, subjects made horizontal vergence eye movements of 7 degrees and combined saccade-vergence movements with a version amplitude of 20 degrees. The movements were performed back and forth from two targets simultaneously presented nearby (38 cm) and more distant (145 cm). Small vertical saccades accompanied most vergence movements. These results show that blinks change the kinematics (saccade duration, peak velocity, peak acceleration, peak deceleration) of not only horizontal but also of vertical saccades, of horizontal vergence eye movements, and of combined saccade-vergence eye movements. Peak velocity, acceleration, and deceleration of eye movements were decreased on the average by 30%, and their duration increased by 43% on the average when they were accompanied by blinks. The blink effect was time dependent with respect to saccade and vergence onset: the greatest effect occurred 100 ms prior to saccade onset, whereas there was no effect when the blink started after saccade onset. The effects of blinks on saccades and vergence, which are tightly coupled to latency, support the hypothesis that blinks cause profound spatiotemporal perturbations of the eye movements by interfering with the normal saccade/vergence premotor circuits. However, the measured effect may to a certain degree but not exclusively be explained by mechanical interference.

Prestimulus effects on startle magnitude: sensory or motor?

Startle may be inhibited when the startling event is preceded by a stimulus; this is called prepulse inhibition (PPI) when the prestimulus is weak and nonstartling (s) and paired pulse inhibition when the prestimulus elicits startle (S1). The authors examined the relationship of these measures across species and tested whether paired pulse inhibition--like PPI--is independent of the startling effects of the prestimulus. PPI (s-S1 configuration) and paired pulse inhibition (S1-S2 configuration) were elicited in 1 test, using similar stimulus parameters in rats and humans. The amount of PPI and paired pulse inhibition was significantly correlated within subjects in both rats and humans. Paired pulse inhibition was not diminished when the startling effects of S1 were eliminated by a weak prepulse (s-S1-S2 configuration), nor was it enhanced when these prepulse effects were eliminated by the dopamine agonist apomorphine (in rats). Despite apparent differences in the inhibitory processes mediating PPI and paired pulse inhibition, both are independent of the motoric response to the prestimulus.

Nociceptive quality of the laser-evoked blink reflex in humans

Laser radiant-heat pulses selectively excite the free nerve endings in the superficial layers of the skin and activate mechano-thermal nociceptive afferents; when directed to the perioral or supraorbital skin, high-intensity laser pulses evoke a blink-like response in the orbicularis oculi muscle (the laser blink reflex, LBR). We investigated the functional properties (startle or nociceptive origin) of the LBR and sought to characterize its central pathways. Using high-intensity CO(2)-laser stimulation of the perioral or supraorbital regions and electromyographic (EMG) recordings from the orbicularis oculi muscles, we did five experiments in 20 healthy volunteers. First, to investigate whether the LBR is a startle response, we studied its habituation to expected rhythmic stimuli and to unexpected arrhythmic stimuli. To assess its possible nociceptive quality, we studied changes in the LBR and the R2 component of the electrical blink reflex after a lidocaine-induced supraorbital nerve block and after intramuscular injection of the opiate fentanyl and the opiate-antagonist naloxone. To characterize the central pathways for the LBR, we investigated the interaction between the LBR and the three components of the blink reflex (R1, R2, and R3) by delivering laser pulses to the perioral or supraorbital regions before or after electrical stimulation of the supraorbital nerve at various interstimulus intervals. Finally, to gain further information on the central LBR pathways, using two identical CO(2)-laser stimulators, we studied the LBR recovery curves with paired laser pulses delivered to adjacent forehead points at interstimulus intervals from 250 ms to 1.5 s. The LBR withstood relatively high-frequency rhythmic stimulations, and unexpected laser pulses failed to evoke larger responses. When lidocaine began to induce hypoalgesia (about 5 min after the injection), the LBR was abolished, whereas R2 was only partly suppressed 10 min after the injection. Fentanyl injection induced strong, naloxone-reversible, LBR suppression (the response decreased to 25.3% of predrug values at 10 min and to 4% at 20 min), whereas R2 remained appreciably unchanged. Whether directed to the perioral or supraorbital regions, preceding laser pulses strongly suppressed R2 and R3 though not R1. Conversely, preceding electrical stimuli to the supraorbital nerve suppressed the LBR. In response to paired stimuli, the LBR recovered significantly faster than R2. These findings indicate that the LBR is a nociceptive reflex, which shares part of the interneuron chain mediating the nonnociceptive R2 blink reflex, probably in the medullary reticular formation. The LBR may prove useful for studying the pathophysiology of orofacial pain syndromes.

Lid restraint evokes two types of motor adaptation

Unilateral reduction in eyelid motility produced two modes of blink adaptation in humans. The first adaptive modification affected both eyelids. Stimulation of the supraorbital branch of the trigeminal nerve (SO) ipsilateral to the upper eyelid with reduced motility evoked bilateral, hyperexcitable reflex blinks, whereas contralateral SO stimulation elicited normally excitable blinks bilaterally. The probability of blink oscillations evoked by stimulation of the ipsilateral SO also increased with a reduction in lid motility. The increased probability of blink oscillations correlated with the enhanced trigeminal reflex blink excitability. Thus, the trigeminal complex ipsilateral to the restrained eyelid coordinated an increase in excitability and blink oscillations independent of the eyelid experiencing reduced motility. The second type of modification appeared only in the eyelid experiencing reduced motility. When tested immediately after removing lid restraint, blink amplitude increased in this eyelid relative to the normal eyelid regardless of the stimulated SO. A patient with seventh nerve palsy exhibited the same two patterns of blink adaptation. These results were consistent with two forms of adaptation, presumably because unilateral lid restraint produced two error signals. The corneal irritation caused by reduced blink amplitude generated abnormal corneal inputs. The difference between proprioceptive feedback from the blink and expected blink magnitude signaled an error in blink amplitude. The corneal irritation appeared to drive an adaptive process organized through the trigeminal complex, whereas the proprioceptive error signal drove an adaptive process involving just the motoneurons controlling the restrained eyelid.

Reflex excitability regulates prepulse inhibition

Presentation of a weak stimulus, a prepulse, before a reflex-evoking stimulus decreases the amplitude of the reflex response relative to reflex amplitude evoked without a preceding prepulse. For example, presenting a brief tone before a trigeminal blink-eliciting stimulus significantly reduces reflex blink amplitude. A common explanation of such data are that sensory processing of the prepulse modifies reflex circuit behavior. The current study investigates the converse hypothesis that the intrinsic characteristics of the reflex circuit rather than prepulse processing determine prepulse modification of trigeminal and acoustic reflex blinks. Unilateral lesions of substantia nigra pars compacta neurons created rats with hyperexcitable trigeminal reflex blinks but normally excitable acoustic reflex blinks. In control rats, presentation of a prepulse reduced the amplitude of both trigeminal and acoustic reflex blinks. In 6-OHDA-lesioned rats, however, the same acoustic prepulse facilitated trigeminal reflex blinks but inhibited acoustic reflex blinks. The magnitude of prepulse modification correlated with reflex excitability. Humans exhibited the same pattern of prepulse modification. An acoustic prepulse facilitated the trigeminal reflex blinks of subjects with hyperexcitable trigeminal reflex blinks caused by Parkinson's disease. The same prepulse inhibited trigeminal reflex blinks of age-matched control subjects. Prepulse modification also correlated with trigeminal reflex blink excitability. These data show that reflex modification by a prepulse reflects the intrinsic characteristics of the reflex circuit rather than an external adjustment of the reflex circuit by the prepulse.

Role of proprioception in the control of lid position during reflex and conditioned blink responses in the alert behaving cat

The contribution of the orbicularis oculi muscle to the determination of lid position, and the putative role of eyelid proprioception in the control of reflex and conditioned eye blinks, were studied in alert behaving cats. Upper lid movements and the electromyographic activity of the orbicularis oculi muscle were recorded during reflexively evoked blinks and during the classical conditioning of the eyelid response. Blinks were evoked by air puffs, flashes and electrical stimulation of the supraorbitary branch of the trigeminal nerve. Eyelid responses were conditioned with a trace classical conditioning paradigm consisting of a short, weak air puff, followed 250 ms later by a long, strong air puff. Orbicularis oculi muscle activation during reflex blinks was independent of lid position and was not modified by the presence of weights acting in the upward or downward directions. Local anesthesia of the supraorbital nerve reduced blinks evoked by air puffs applied to the lower jaw, but did not affect flash-evoked blinks. No relationship was established between initial lid position and the first downward component of conditioned eyelid responses. In contrast, initial lid position was related to the first upward component of the same conditioned response. It is concluded that orbicularis oculi motor units receive no feedback proprioceptive signals from the eyelid, other than those coming from cutaneous receptors, and that lid position is determined by the activity of the levator palpebrae superioris muscle. The lack of sensory information about lid position in facial motoneurons probably has some functional implications on the central control of cognitive and emotional facial expressions.

Discharge profiles of abducens, accessory abducens, and orbicularis oculi motoneurons during reflex and conditioned blinks in alert cats

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

Conditioning of the masseter inhibitory reflex by homotopically applied painful heat in humans

During contraction of the jaw-closing muscles, afferent input from the intraoral and perioral region can elicit two bilateral suppression periods (SP1 and SP2, respectively) in the masseter electromyogram (EMG). Non-painful electrical stimulation 2 cm from the left labial commissure was used in the present study to evoke these trigeminal inhibitory reflexes. The subjects maintained a level of 50% of their maximum masseter EMG. The degree of suppression was quantified as the percentage suppression of the mean EMG activity in a fixed post-stimulus interval (SP2, 40-90 ms). Further, brief (200 ms) painful radiant heat conditioning stimuli were delivered to the ipsilateral cheek, in order to investigate the influence of nociceptive input on the (non-nociceptive) trigeminal masseter inhibitory reflex. Nine different conditions combining radiant heat and electrical stimuli were used. Twelve stimuli were presented for each condition. The radiant heat preceded the electrical test stimuli by fixed inter-stimulus intervals (ISI), ranging from 100 ms to 500 ms. At 250-350 ms ISIs, the bilateral SP2 suppression was significantly reduced to less than 10%, in comparison to an average suppression degree of 32.5% without conditioning stimuli. The subjects perceived the heat stimulus before the electrical stimulus for a majority of the 12 pairs of stimuli at these ISIs. No differences were found in the VAS ratings for the different conditions. For the contralateral SP1, larger suppression was seen for the 300 ms ISI compared with stimulation without conditioning heat stimuli. Onset and offset for the SP1 was, however, only detected in three subjects using a criteria of 20% suppression of the pre-stimulus activity. A pre-pulse inhibitory effect onto inter-neurons in the SP2 pathways or habituation of the same inter-neurons by the heat stimuli are suggested as possible explanations for the interaction between the non-nociceptive and nociceptive input in the present study.

Role of cerebellum in adaptive modification of reflex blinks

We investigated the involvement of the cerebellar cortex in the adaptive modification of corneal reflex blinks and the regulation of normal trigeminal reflex blinks in rats. The ansiform Crus I region contained blink-related Purkinje cells that exhibited a complex spike 20.4 msec after a corneal stimulus and a burst of simple spike activity correlated with the termination of orbicularis oculi activity. This occurrence of the complex spike correlated with trigeminal sensory information associated with the blink-evoking stimulus, and the burst of simple spike activity correlated with sensory feedback about the occurrence of a blink. Inactivation of the inferior olive with lidocaine prevented all complex and significantly reduced simple spike modulation of blink-related Purkinje cells, but did not alter orbicularis oculi activity evoked by corneal stimulation. In contrast, both acute and chronic lesions of the cerebellar cortex containing blink-related Purkinje cells blocked adaptive increases in orbicularis oculi activity of the lid ipsilateral but not contralateral to the lesion. These data are consistent with the hypothesis that the cerebellum is part of a trigeminal reflex blink circuit. Changes in trigeminal signals produce modifications of the cerebellar cortex, which in turn, reinforce or stabilize modifications of brainstem blink circuits. When the trigeminal system does not attempt to alter the magnitude of trigeminal reflex blinks, cerebellar input has little or no effect on reflex blinks.