Labyrinthine Stimulation Research Articles

Influence of graviceptor stimulation initiated by off-vertical axis rotation on ventilation.

What is the central question of this study? It is known that respiration is affected by graviceptors, but it remains unclear to what extent labyrinthine and non-labyrinthine graviceptors are involved in this process. What is the main finding and its importance? Our results suggest that the modulation of respiration is not a result of a simple reflex arc, but that it involves a higher integration of different types of receptors with variable contributions of either type of graviceptor among subjects. It has been suggested that the otolith system is involved in the physiological response to changes in body orientation with respect to gravity. In studies on animals, an otolith-respiratory reflex has been observed, but data on humans are scarce and inconclusive, mainly because pure otolithic stimulation is difficult to produce in humans. To assess the otolithic-respiratory reflex in humans, we used an off-vertical axis rotation (OVAR) that produces periodic and pure stimulation of graviceptors. The inspiratory flow was measured during earth vertical axis rotation (EVAR, control conditions) and OVAR in 21 subjects. To distinguish the effects of the labyrinthine and non-labyrinthine graviceptors on ventilation, these measurements were repeated with two different static head positions: head turned leftward and rightward in yaw. The velocity of rotation was individually selected to match spontaneous breathing rate (mean 11.4cyclesmin-1 , 0.19Hz). Average ventilatory flow was higher in OVAR than in EVAR, as was tidal volume. In OVAR, the transition between inspiration and expiration occurred mainly in the forward pitch position. The phase of this transition in most subjects was driven mostly by the body position rather than by the head position, suggesting that respiratory modifications during OVAR mainly involved non-labyrinthine receptors. However, the study demonstrated a high intersubject variability both in the ability of OVAR to synchronize breathing and in the influence of labyrinthine stimulation. We conclude that the respiratory response to changes in orientation of the body with respect to the vertical involves labyrinthine and non-labyrinthine stimulation, with the gain of each signal varying individually.

Ocular stability and set-point adaptation.

A fundamental challenge to the brain is how to prevent intrusive movements when quiet is needed. Unwanted limb movements such as tremor impair fine motor control and unwanted eye drifts such as nystagmus impair vision. A stable platform is also necessary to launch accurate movements. Accordingly, nature has designed control systems with agonist (excitation) and antagonist (inhibition) muscle pairs functioning in push-pull, around a steady level of balanced tonic activity, the set-point Sensory information can be organized similarly, as in the vestibulo-ocular reflex, which generates eye movements that compensate for head movements. The semicircular canals, working in coplanar pairs, one in each labyrinth, are reciprocally excited and inhibited as they transduce head rotations. The relative change in activity is relayed to the vestibular nuclei, which operate around a set-point of stable balanced activity. When a pathological imbalance occurs, producing unwanted nystagmus without head movement, an adaptive mechanism restores the proper set-point and eliminates the nystagmus. Here we used 90 min of continuous 7 T magnetic field labyrinthine stimulation (MVS) in normal humans to produce sustained nystagmus simulating vestibular imbalance. We identified multiple time-scale processes towards a new zero set-point showing that MVS is an excellent paradigm to investigate the neurobiology of set-point adaptation.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Magnetic Vestibular Stimulation in Subjects with Unilateral Labyrinthine Disorders

We recently discovered that static magnetic fields from high-strength MRI machines induce nystagmus in all normal humans, and that a magneto-hydrodynamic Lorentz force, derived from ionic currents in the endolymph and pushing on the cupula, best explains this effect. Individuals with no labyrinthine function have no nystagmus. The influence of magnetic vestibular stimulation (MVS) in individuals with unilateral deficits in labyrinthine function is unknown and may provide insight into the mechanism of MVS. These individuals should experience MVS, but with a different pattern of nystagmus consistent with their unilateral deficit in labyrinthine function. We recorded eye movements in the static magnetic field of a 7 T MRI machine in nine individuals with unilateral labyrinthine hypofunction, as determined by head impulse testing and vestibular-evoked myogenic potentials (VEMP). Eye movements were recorded using infrared video-oculography. Static head positions were varied in pitch with the body supine, and slow-phase eye velocity (SPV) was assessed. All subjects exhibited predominantly horizontal nystagmus after entering the magnet head-first, lying supine. The SPV direction reversed when entering feet-first. Pitching chin-to-chest caused subjects to reach a null point for horizontal SPV. Right unilateral vestibular hypofunction (UVH) subjects developed slow-phase-up nystagmus and left UVH subjects, slow-phase-down nystagmus. Vertical and torsional components were consistent with superior semicircular canal excitation or inhibition, respectively, of the intact ear. These findings provide compelling support for the hypothesis that MVS is a result of a Lorentz force and suggest that the function of individual structures within the labyrinth can be assessed with MVS. As a novel method of comfortable and sustained labyrinthine stimulation, MVS can provide new insights into vestibular physiology and pathophysiology.

A new technique to investigate vestibulo-spinal reflexes.

Vestibulospinal reflexes can be elicited in humans by low amplitudes direct (galvanic) currents lasting tens of milliseconds and applied across the two mastoids bones, which can be delivered by particular stimulators. The stimulus induces a perception of body sway and a postural response appropriate to counteract the perceived sway. Both the direction of the perceived and induced body sway are modulated by the orientation of the head with respect to the body. This phenomenon is due to the fact that integration of vestibular and neck signals allows to correctly infer the direction of body sway from the labyrinthine input, which is instead related to direction of head motion. The modulation of stimulus-elicited body sway by neck rotation could be utilised for testing the effectiveness of neck proprioceptive signals in modifying the reference frame for labyrinthine signals from the head to the body. In the present experiments we showed that labyrinthine stimulation can be performed also by using train of pulses of 1 msec duration, which can be delivered by virtually all stimulators allowed for human use. Moreover, we developed a simple technique for visualising the time course of the changes in the direction of the postural response, based on the evaluation of the velocity vector of subject's centre of pressure. This method could be exploited in order to the test the efficacy of neck proprioceptive information in modifying the reference frame for processing vestibular signals in both physiological and pathological condition.

Effects of body to head rotation on the labyrinthine responses of rat vestibular neurons

Vestibulospinal reflexes elicited by head displacement in space depend on the direction of body displacement, because the neuronal responses to labyrinthine stimulation are tuned by neck displacement: a directional tuning takes place in the medial cerebellum and in spinal motoneurons, while a gain and a basal activity tuning can be observed in the reticular formation, a target structure of the medial cerebellum. In the present study, we investigated whether also the response of vestibular nuclear neurons (another target of the medial cerebellum) to labyrinthine stimulation is tuned by neck displacement and which parameters of the response are modulated by it. In urethane-anaesthetized Wistar rats, single-unit activity was recorded from the vestibular nuclei at rest and during wobble of the whole animal at 0.156Hz. This stimulus tilted the animal’s head by a constant amplitude (5°), in a direction rotating at a constant velocity over the horizontal plane, either in clockwise or counter clockwise direction. The gain and the direction of neuronal responses to wobble were evaluated through Fourier analysis, in the control position (with coincident head and body axes) and following a body-to-head rotation of 5–30° over the horizontal plane, in both directions. Most of the vestibular neurons modified their response gain and/or their basal activity following body-to-head rotation, as it occurs in the reticular formation. Only few neurons modified their response direction, as occurs in the cerebellum and in spinal motoneurons. The different behaviour of cerebellar neurons and of their vestibular and reticular target cells, suggests that the role played by the cerebellum in the neck tuning of vestibulospinal reflexes has to be reconsidered.

A new technique to investigate vestibulospinal reflexes

800x600 Vestibulospinal reflexes can be elicited in humans by low amplitudes direct (galvanic) currents lasting tens of milliseconds and applied across the two mastoids bones, which can be delivered by particular stimulators. The stimulus induces a perception of body sway and a postural response appropriate to counteract the perceived sway. Both the direction of the perceived and induced body sway are modulated by the orientation of the head with respect to the body. This phenomenon is due to the fact that integration of vestibular and neck signals allows to correctly infer the direction of body sway from the labyrinthine input, which is instead related to direction of head motion. The modulation of stimulus-elicited body sway by neck rotation could be utilised for testing the effectiveness of neck proprioceptive signals in modifying the reference frame for labyrinthine signals from the head to the body. In the present experiments we showed that labyrinthine stimulation can be performed also by using train of pulses of 1 msec duration, which can be delivered by virtually all stimulators allowed for human use. Moreover, we developed a simple technique for visualising the time course of the changes in the direction of the postural response, based on the evaluation of the velocity vector of subject’s centre of pressure. This method could be exploited in order to the test the efficacy of neck proprioceptive information in modifying the reference frame for processing vestibular signals in both physiological and pathological condition. Normal 0 14 false false false IT X-NONE X-NONE MicrosoftInternetExplorer4

Effects of Leg-to-Body Position on the Responses of Rat Cerebellar and Vestibular Nuclear Neurons to Labyrinthine Stimulation

The spatial organization of vestibulospinal (VS) reflexes, elicited by labyrinthine signals and related to head motion, depends on the direction of body tilt, due to proprioceptive neck afferents acting through the cerebellar anterior vermis. The responses of Purkinje cells located within this region to labyrinthine stimulation are modulated by the head-to-body position. We investigated, in urethane-anesthetized rats, whether a 90° leg-to-trunk displacement modifies the responses of corticocerebellar and vestibular nuclear neurons to the labyrinthine input, which would indicate that VS reflexes are tuned by the leg-to-trunk position. With this aim, unit activity was recorded during "wobble" stimuli that allow evaluating the gain and spatiotemporal properties of neuronal responses. The response gain of corticocerebellar units showed a significant drop in the leg-rotated position with respect to the control one. Following a change in leg position, a proportion of the recorded neurons showed significant changes in the direction and phase of the response vector. In contrast, vestibular nuclear neurons did not show significant modifications in their response gain and direction. Thus, proprioceptive afferents signaling leg-to-trunk position seem to affect the processing of directional labyrinthine signals within the cerebellar cortex.

Tuning of human vestibulospinal reflexes by leg rotation

Changing the foot position modifies the mechanical action exerted by the ankle extensor and flexor muscles over the body. We verified, in two groups of healthy subjects standing with the heels touching or apart, whether a 90° external rotation of the right leg and foot also changes the pattern of vestibulospinal reflexes elicited by electrical stimulation of the labyrinth. With the head oriented forward, leg rotation did not modify the labyrinthine-driven displacements of the center of pressure (CoP). When the head was rotated in the horizontal plane, either to the right or to the left, the CoP displacement increased along the y axis in all subjects. Changes in the x component in most instances appropriate to preserve unmodified the direction of body sway elicited by the stimulus were observed. Right leg rotation increased the basal EMG activity of ankle extensors and flexors on the left side, while the right side activity was unaffected. The EMG responses to labyrinthine stimulation were modified only on the left side, in a way appropriate to correct the effects of the altered torque pattern exerted on the body by right leg muscles. It appears, therefore, that somatosensory signals related to leg rotation and/or copy of the corresponding voluntary motor commands modify the pattern of vestibulospinal reflexes and maintain the postural response appropriate to counteract a body sway in the direction inferred by labyrinthine signals.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

Neuronal responses to tilt within the rat cerebellar vermis

Cats corticocerebellar units within the anterior vermis are affected by the vestibular input and show directionally tuned responses. The aim of the study was investigating whether a similar representation of labyrinth signals was present in the rat cerebellar vermis by recording units activity during tilt and wobble rotations. The analysis of the neuronal discharge during both clockwise (CW) and counterclockwise (CCW) wobble allowed to determine the spatial (preferred direction) and temporal (response phase) response properties of the recorded neurons. Many units were affected by labyrinthine stimulation. “Bi-directional” units responded to both CW and CCW stimuli, being characterized by a direction of maximal sensitivity ( θ max), the distribution of which covered all the sectors of the horizontal plane, with contralaterally pointing vectors more represented within the caudal part of the explored region. Differences in the amplitude of the CW and CCW responses indicated that neurons received a convergence of vestibular signals endowed with different spatial and temporal properties, a process that is expected to link their response phase with the tilt direction. Population vector analysis showed that recorded neurons coded both the amplitude and direction of head tilt during different types of rotational stimuli. In conclusion, the present results show that the processing of vestibular signals with complex spatiotemporal properties represents a general function of the mammalian cerebellar vermis, allowing accurate monitoring of head rotational movements (of the head) in vertical planes. Finally, in rats, different cerebellar regions seem to receive specific vestibular inputs.

Primate translational vestibuloocular reflexes. III. Effects of bilateral labyrinthine electrical stimulation.

The effects of functional, reversible ablation and potential recruitment of the most irregular otolith afferents on the dynamics and sensitivity of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near and far targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient lateral head displacements. Short-duration (usually <2 s) anodal (inhibitory) and cathodal (excitatory) currents (50-100 microA) were delivered bilaterally during motion. In the presence of anodal labyrinthine stimulation, trVOR sensitivity and its dependence on viewing distance were significantly decreased. In addition, anodal currents significantly increased phase lags. During transient motion, anodal stimulation resulted in significantly lower initial eye acceleration and more sluggish responses. Cathodal currents tended to have opposite effects. The main characteristics of these results were simulated by a simple model where both regularly and irregularly discharging afferents contribute to the trVORs. Anodal labyrinthine currents also were found to decrease eye velocity during long-duration, constant velocity rotations, although results were generally more variable compared with those during translational motion.

Functional organization of primate translational vestibulo-ocular reflexes and effects of unilateral labyrinthectomy.

Translational vestibulo-ocular reflexes (trVORs) are characterized by distinct spatio-temporal properties and sensitivities that are proportional to the inverse of viewing distance. Anodal (inhibitory) labyrinthine stimulation (100 microA, < 2 s) during motion decreased the high-pass filtered dynamics, as well as horizontal trVOR sensitivity and its dependence on viewing distance. Cathodal (excitatory) currents had opposite effects. Translational VORs were also affected after unilateral labyrinthectomy. Animals lost their ability to modulate trVOR sensitivity as a function of viewing distance acutely after the lesion. These deficits partially recovered over time, albeit a significant reduction in trVOR sensitivity as a function of viewing distance remained in compensated animals. During fore-aft motion, the effects of unilateral labyrinthectomy were more dramatic. Both acute and compensated animals permanently lost their ability to modulate fore-aft trVOR responses as a function of target eccentricity. These results suggest that (1) the dynamics and viewing distance-dependent properties of the trVORs are very sensitive to changes in the resting firing rate of vestibular afferents and, consequently, vestibular nuclei neurons; (2) the most irregularly firing primary otolith afferents that are most sensitive to labyrinthine electrical stimulation might contribute to reflex dynamics and sensitivity; (3) inputs from both labyrinths are necessary for the generation of the translational VORs.

Stress-induced changes in the gastrointestinal motor system.

Several autonomic, hormonal, behavioural and neuropeptidergic bodily responses to stressful stimuli have been described over the past few decades. Both animal models and human paradigms have been explored. It is acknowledged that stress modulates gastrointestinal (GI) motility through central mechanisms including corticotropin-releasing-factor. This process requires the integrity of autonomic neural pathways. It has become evident that the effects of stress on GI motility vary according to the stressful stimulus, its intensity, the animal species under study and the time course of the study. Recent evidence suggests that chronic or possibly permanent changes develop in enteric smooth muscle properties in response to stress. In animals, the most consistent findings include retardation of gastric emptying in response to various stressors; acceleration of gastric emptying upon cold stress, presumably through the secretion of brain thyroglobulin-hormone; acceleration of intestinal transit; and stimulation of colonic transit and fecal output. In humans, the cold water immersion test has been associated with an inhibition of gastric emptying, while labyrinthine stimulation induces the transition from postprandial to fasting motor patterns in the stomach and the small bowel. Psychological stress has been shown to induce a reduction in the number and amplitude of intestinal migrating motor complexes and to neither affect nor stimulate colonic motility. These various responses to stress are presumably attributed to the preferential activation of specific neuronal pathways under the influence of a given stimulus or its intensity. The significance of these findings and the directions of further studies are discussed.

前庭皮質と前庭失認に関する臨床的検討

The vestibular cortex is speculated to be involved in the perception of self-motion due to labyrinthine stimulation. In this study, we hypothesized that patients with lesions involving the vestibular cortex would show vestibular agnosia, and that nystagmus-sensation dissociation, which is demonstrated by brisk caloric nystagmus without simul-taneous sensation of vertigo, would be a feature of vestibular agnosia. We used singlephoton emission computed tomography (SPELT) to measure regional cerebral blood flow (rCBF) in two cerebrovascular patients with nystagmus-sensation dissociation. Both patients had ischemic lesions involving mainly the right parieto-temporal lobe. It is suggested that the parieto-temporal lobe contains the vestibular cortex.

The Contribution of the Vertical Semicircular Canals to High-Velocity Horizontal Vestibulo-Ocular Reflex (VOR) in Normal Subjects and Patients with Unilateral Vestibular Nerve Section

We have examined to what extent the vertical semicircular canals contribute to the nonlinearity of the horizontal VOR imposed by the driving of primary vestibular afferents into inhibitory cutoff at high velocities of head rotation (Ewald's second law). The gain (eye velocity/head velocity) of the horizontal component of the VOR with the head pitched down 30 degrees and pitched up 30 degrees was examined during constant-velocity rotations in normal subjects and patients following unilateral vestibular nerve section. In normal subjects, VOR gain decreases as chair velocity increases from 60-300 degrees/s when the head is pitched up, but VOR gain remains constant when the head is pitched down. This finding implies that the mechanism by which the gain of the horizontal VOR gain remains constant at all velocities of rotation depends upon the pattern of labyrinthine stimulation. Following unilateral nerve section, we found that the directional preponderance (DP) in horizontal VOR depends upon whether the head is pitched up 30 (mean asymmetry = 5%) or pitched down 30 degrees (mean asymmetry = 20%). This is what is expected based on the degree to which the lateral and vertical semicircular canals sense horizontal head acceleration with the head in different degrees of pitch. Hence, following unilateral vestibular lesions, the DP of horizontal VOR gain is most easily elicited at high velocities of head rotation and with the head pitched down 30 degrees. Evidence for DP at the bedside using the "head-shaking nystagmus" technique may be optimally elicited with the head pitched down 30 degrees.

Influence of vestibular organ stimulation on stomach movement and respiration.

Some previous investigators have reported controversial results in the stomach movement in response to labyrinthine stimulation (1). The purpose of the present study was to investigate the influence of vestibular stimulation on the stomach movement and respiration in anesthetized cats, and to investigate the differences between increased and decreased responses of the stomach movement. This study used an electrophysiological technique: a bipolar stainless-steel stimulating electrode with a tip diameter of 20 microns introduced chronically into the anterior and/or lateral semicircular canal (2). The intragastric pressure was recorded by using a balloon-transducer system. Respiration was recorded by measurement of the thorax. When the peripheral labyrinth was stimulated, we found conflicting results in that the pressure in the stomach increased several times and sometimes decreased. Respiration usually became irregular or intensified. It seems that forced respiration may be involved in vestibular vomiting but we were unable to induce vomiting.

Visual and vestibular projections to tongue motoneurons

The rabbit hypoglossal neurons, localized in the mediocaudal part of the XIIth nucleus, receive visual and vestibular inputs able to induce reflex responses functionally driven both to prepare the oral cavity for food reception and to ensure a correct postural arrangement of the tongue. The aim of the present research was to show a possible convergence of visual input on the hypoglossal neurons modulated by the vestibular system and, thus, demonstrate that visual input plays a part in the control of the tongue posture. It was found that 78% of tested neurons responded to both photic stimulation of the retinae, performed by a conventional strobe unit, and electrical stimulation of the labyrinth, with different patterns of response. Moreover, visual input significantly modified both the hypoglossal neuron response and the electromyographic genioglossal response to caloric stimulation of the labyrinth. Because a significant visual influence on the hypoglossal nucleus response and the genioglossal muscle response to labyrinthine stimulation was observed, it can be concluded that vision does integrate the spatial information of the labyrinth to modulate the postural tone of the tongue muscles.

Influence of caloric labyrinthine stimulation on oesophageal motor function

AbstractThere is incomplete understanding of the factors regulating smooth muscle primary peristalsis and of the reflux‐associated transient lower oesophageal sphincter relaxations which are under postural control and which occur independently of swallowing. We examined the effects of labyrinthine stimulation on basal lower oesophageal sphincter tone, the frequency of non‐swallow associated lower oesophageal sphincter relaxation and on the amplitude of primary peristalsis in the distal oesophagus. In 13 healthy volunteers, labyrinthine stimulation was produced by infusion of water at 10°C into the external auditory canal for 5 minutes, or until nausea ensued. A manometric sleeve catheter assembly monitored lower oesophageal sphincter pressure while side holes recorded pharyngeal, mid and lower oesophageal, and gastric pressures. Recordings were made during labyrinthine stimulation and during matched control periods. Caloric stimulation caused a minor but significant fall in mean basal lower oesophageal sphincter pressure of 14% (P = 0.04) and had no detectable effect on oesophageal peristalsis. In association with vomiting induced by labyrinthine stimulation in 3 subjects, lower oesophageal sphincter pressure fell rapidly by 61% (P = 0.03) from control levels and this inhibition persisted for up to 20 minutes after vomiting. For 10 minutes post‐emesis, primary peristaltic amplitude was reduced by 50% (P &lt; 0.001). Swallowing triggered a completely propagated peristaltic wave significantly less often during the post‐emetic period. The rate of occurrence of transient lower oesophageal sphincter relaxations was not influenced significantly by labyrinthine stimulation. These findings are consistent with a functionally important role for the brainstem in the control of motor function of the smooth muscle oesophagus but do not permit distinction of a direct effect from a secondary humoral effect as part of the vomiting reflex.

Effects of inferior olive inactivation and lesion on the activity of medial vestibular neurons in the rat

In anaesthetized rats, the unitary activity from the medial vestibular nucleus had been recorded during horizontal sinusoidal rotation in the absence of visual stimulation. In the first series of experiments, the inferior olivary nuclei were selectively destroyed by means of 3-acetylpyridine. Unitary activity was recorded three to five days or one month after the lesion. A few days after the lesion, the average spontaneous activity, as well as the peak-to-peak amplitude of the modulation of the medial vestibular neurons during sinusoidal rotation, were significantly lower compared to those recorded in intact rats, and to those recorded one month after the lesion. In the second series of experiments, during reversible cooling of the inferior olive region of one side, in the contralateral medial vestibular nuclei 57% of units underwent a clear decrease in firing rate accompanied by a decrease in the amplitude of modulation. In rats whose inferior olivary nuclei had been destroyed by means of 3-acetylpyridine one month before, or whose cerebellum had been removed, there were few units that showed a decrease of the firing rate and modulation amplitude on cooling the same olivary region. Our experiments show that silencing the activity of the inferior olive causes a decrease both in the spontaneous firing rate and in the amplitude of the response of the vestibular neurons to natural labyrinthine stimulation. These results support the hypothesis that the inferior olive, by changing its firing rate, may regulate on-line the gain of reflexes which are under cerebellar control.

Air righting without the cervical righting reflex in adult rats

The current explanation of air righting in animals is that when falling supine in the air, labyrinthine stimulation triggers head rotation. The head rotation involves neck rotation which, via the cervical righting reflex, triggers rotation of the body. (In cats and monkeys, when the labyrinths are absent, visual stimulation when falling supine can also trigger this righting sequence.) In the present paper, a descriptive analysis of air righting in the rat shows that the shoulders rotate, carrying the unmoving head and neck passively along. Thus, for this species, labyrinthine input appears to trigger shoulder rotation directly, independently of the cervical righting reflex. This suggests that at least two physiological mechanisms exist for labyrinthine control of head rotation during air righting, one via the neck and the other via the shoulder girdle.