Rostral Fastigial Nucleus Research Articles

Role of Rostral Fastigial Neurons in Encoding a Body-Centered Representation of Translation in Three Dimensions.

Many daily behaviors rely critically on estimates of our body motion. Such estimates must be computed by combining neck proprioceptive signals with vestibular signals that have been transformed from a head- to a body-centered reference frame. Recent studies showed that deep cerebellar neurons in the rostral fastigial nucleus (rFN) reflect these computations, but whether they explicitly encode estimates of body motion remains unclear. A key limitation in addressing this question is that, to date, cell tuning properties have only been characterized for a restricted set of motions across head-re-body orientations in the horizontal plane. Here we examined, for the first time, how 3D spatiotemporal tuning for translational motion varies with head-re-body orientation in both horizontal and vertical planes in the rFN of male macaques. While vestibular coding was profoundly influenced by head-re-body position in both planes, neurons typically reflected at most a partial transformation. However, their tuning shifts were not random but followed the specific spatial trajectories predicted for a 3D transformation. We show that these properties facilitate the linear decoding of fully body-centered motion representations in 3D with a broad range of temporal characteristics from small groups of 5-7 cells. These results demonstrate that the vestibular reference frame transformation required to compute body motion is indeed encoded by cerebellar neurons. We propose that maintaining partially transformed rFN responses with different spatiotemporal properties facilitates the creation of downstream body motion representations with a range of dynamic characteristics, consistent with the functional requirements for tasks such as postural control and reaching.SIGNIFICANCE STATEMENT Estimates of body motion are essential for many daily activities. Vestibular signals are important contributors to such estimates but must be transformed from a head- to a body-centered reference frame. Here, we provide the first direct demonstration that the cerebellum computes this transformation fully in 3D. We show that the output of these computations is reflected in the tuning properties of deep cerebellar rostral fastigial nucleus neurons in a specific distributed fashion that facilitates the efficient creation of body-centered translation estimates with a broad range of temporal properties (i.e., from acceleration to position). These findings support an important role for the rostral fastigial nucleus as a source of body translation estimates functionally relevant for behaviors ranging from postural control to perception.

The control of eye movements by the cerebellar nuclei: polysynaptic projections from the fastigial, interpositus posterior and dentate nuclei to lateral rectus motoneurons in primates.

Premotor circuits driving extraocular motoneurons and downstream motor outputs of cerebellar nuclei are well known. However, there is, as yet, no unequivocal account of cerebellar output pathways controlling eye movements in primates. Using retrograde transneuronal transfer of rabies virus from the lateral rectus (LR) eye muscle, we studied polysynaptic pathways to LR motoneurons in primates. Injections were placed either into the central or distal muscle portion, to identify innervation differences of LR motoneurons supplying singly innervated (SIFs) or multiply innervated muscle fibers (MIFs). We found that SIF motoneurons receive major cerebellar 'output channels' bilaterally, while oligosynaptic cerebellar innervation of MIF motoneurons is negligible and/or more indirect. Inputs originate from the fastigial nuclei di- and trisynaptically, and from a circumscribed rostral portion of the ventrolateral interpositus posterior and from the caudal pole of the dentate nuclei trisynaptically. While disynaptic cerebellar inputs to LR motoneurons stem exclusively from the caudal fastigial region involved in saccades, pursuit and convergence (via its projections to brainstem oculomotor populations), minor trisynaptic inputs from the rostral fastigial nucleus, which contributes to gaze shifts, may reflect access to vestibular and reticular eye-head control pathways. Trisynaptic inputs to LR motoneurons from the rostral ventrolateral interpositus posterior, involved in divergence (far-response), is likely mediated by projections to the supraoculomotor area, contributing to LR motoneuron activation during divergence. Trisynaptic inputs to LR motoneurons from the caudal dentate, which also innervates disynaptically the frontal and parietal eye fields, can be explained by its superior colliculus projections, and likely target saccade-related burst neurons.

Hindlimb movement modulates the activity of rostral fastigial nucleus neurons that process vestibular input

Integration of vestibular and proprioceptive afferent information within the central nervous system is a critical component of postural regulation. We recently demonstrated that labyrinthine and hindlimb signals converge onto vestibular nucleus neurons, such that hindlimb movement modulates the activity of these cells. However, it is unclear whether similar convergence of hindlimb and vestibular signals also occurs upstream from the vestibular nuclei, particularly in the rostral fastigial nucleus (rFN). We tested the hypothesis that rFN neurons have similar responses to hindlimb movement as vestibular nucleus neurons. Recordings were obtained from 53 rFN neurons that responded to hindlimb movement in decerebrate cats. In contrast to vestibular nucleus neurons, which commonly encoded the direction of hindlimb movement (81% of neurons), few rFN neurons (21%) that responded to leg movement encoded such information. Instead, most rFN neurons responded to both limb flexion and extension. Half of the rFN neurons whose activity was modulated by hindlimb movement received convergent vestibular inputs. These results show that rFN neurons receive somatosensory inputs from the hindlimb and that a subset of rFN neurons integrates vestibular and hindlimb signals. Such rFN neurons likely perform computations that participate in maintenance of balance during upright stance and movement. Although vestibular nucleus neurons are interconnected with the rFN, the dissimilarity of responses of neurons sensitive to hindlimb movement in the two regions suggests that they play different roles in coordinating postural responses during locomotion and other movements which entail changes in limb position.

Effect of an emetic gastrointestinal input on the processing of labyrinthine inputs by cerebellar rostral fastigial nucleus neurons (1130.7)

Multiple inputs can elicit nausea and vomiting, including ingested toxins and vestibular signals. Amongst the nervous system regions believed to participate in generating motion sickness is the cerebellar rostral fastigial nucleus (rFN). In addition, there is evidence that gastrointestinal inputs are transmitted to rFN through a variety of pathways, including projections from the dorsal motor nucleus of the vagus and area postrema. These observations raise the hypothesis that GI inputs could change motion sickness susceptibility by altering the processing of labyrinthine inputs by rFN neurons. To test this hypothesis, we recorded activity from rFN neurons in decerebrate cats, and compared their responses to whole‐body rotations before and after the intragastric infusion of the emetic compound copper sulfate (CuSO4). Some (18%) rFN neurons had complex responses to vestibular stimulation, particularly responses to only one direction of rotation (known as STC behavior). Such complex responses would be expected in a brain area that participates in generating motion sickness. The spatial and temporal characteristics of these responses changed little after injection of CuSO4. Although infusion of CuSO4 affected gains of responses to rotations of ~25% rFN neurons, the effects were inconsistent (CuSO4 augmented responses in some cells and decremented responses in others). CuSO4 resulted in pronounced changes in responses to rotations of a significantly larger fraction (Chi‐squared test, p<0.05) of neurons in other brain regions that mediate nausea and vomiting, including the parabrachial nucleus and the “vomiting center” (caudal medullary lateral tegmental field). These data suggest that integration of vestibular and GI inputs by rFN neurons plays a limited role in affecting motion sickness susceptibility.Grant Funding Source: NIH grant R01‐DC03732

Integration of vestibular and gastrointestinal inputs by cerebellar fastigial nucleus neurons: multisensory influences on motion sickness

Previous studies demonstrated that ingestion of the emetic compound copper sulfate (CuSO4) alters the responses to vestibular stimulation of a large fraction of neurons in brainstem regions that mediate nausea and vomiting, thereby affecting motion sickness susceptibility. Other studies suggested that the processing of vestibular inputs by cerebellar neurons plays a critical role in generating motion sickness and that neurons in the cerebellar fastigial nucleus receive visceral inputs. These findings raised the hypothesis that stimulation of gastrointestinal receptors by a nauseogenic compound affects the processing of labyrinthine signals by fastigial nucleus neurons. We tested this hypothesis in decerebrate cats by determining the effects of intragastric injection of CuSO4 on the responses of rostral fastigial nucleus to whole-body rotations that activate labyrinthine receptors. Responses to vestibular stimulation of fastigial nucleus neurons were more complex in decerebrate cats than reported previously in conscious felines. In particular, spatiotemporal convergence responses, which reflect the convergence of vestibular inputs with different spatial and temporal properties, were more common in decerebrate than in conscious felines. The firing rate of a small percentage of fastigial nucleus neurons (15%) was altered over 50% by the administration of CuSO4; the firing rate of the majority of these cells decreased. The responses to vestibular stimulation of a majority of these cells were attenuated after the compound was provided. Although these data support our hypothesis, the low fraction of fastigial nucleus neurons whose firing rate and responses to vestibular stimulation were affected by the administration of CuSO4 casts doubt on the notion that nauseogenic visceral inputs modulate motion sickness susceptibility principally through neural pathways that include the cerebellar fastigial nucleus. Instead, it appears that convergence of gastrointestinal and vestibular inputs occurs mainly in the brainstem.

Anatomical Characterization of a Rabbit Cerebellar Eyeblink Premotor Pathway Using Pseudorabies and Identification of a Local Modulatory Network in Anterior Interpositus

Rabbit eyeblink conditioning is a well characterized model of associative learning. To identify specific neurons that are part of the eyeblink premotor pathway, a retrograde transsynaptic tracer (pseudorabies virus) was injected into the orbicularis oculi muscle. Four time points (3, 4, 4.5, and 5 d) were selected to identify sequential segments of the pathway and a map of labeled structures was generated. At 3 d, labeled first-order motor neurons were found in dorsolateral facial nucleus ipsilaterally. At 4 d, second-order premotor neurons were found in reticular nuclei, and sensory trigeminal, auditory, vestibular, and motor structures, including contralateral red nucleus. At 4.5 d, labeled third-order premotor neurons were found in the pons, midbrain, and cerebellum, including dorsolateral anterior interpositus nucleus and rostral fastigial nucleus. At 5 d, labeling revealed higher-order premotor structures. Labeled fourth-order Purkinje cells were found in ipsilateral cerebellar cortex in cerebellar lobule HVI and in lobule I. The former has been implicated in eyeblink conditioning and the latter in vestibular control. Labeled neurons in anterior interpositus were studied, using neurotransmitter immunoreactivity to classify individual cell types and delineate their interconnectivity. Labeled third-order premotor neurons were immunoreactive for glutamate and corresponded to large excitatory projection neurons. Labeled fourth-order premotor interneurons were immunoreactive for GABA (30%), glycine (18%), or both GABA and glycine (52%) and form a functional network within anterior interpositus involved in modulation of motor commands. These results identify a complete eyeblink premotor pathway, deep cerebellar interconnectivity, and specific neurons responsible for the generation of eyeblink responses.

Direction discrimination thresholds of vestibular and cerebellar nuclei neurons.

To understand the roles of the vestibular system in perceptual detection and discrimination of self-motion, it is critical to account for response variability in computing the sensitivity of vestibular neurons. Here we study responses of neurons with no eye movement sensitivity in the vestibular (VN) and rostral fastigial nuclei (FN) using high-frequency (2 Hz) oscillatory translational motion stimuli. The axis of translation (i.e., heading) varied slowly (1 degrees /s) in the horizontal plane as the animal was translated back and forth. Signal detection theory was used to compute the threshold sensitivity of VN/FN neurons for discriminating small variations in heading around all possible directions of translation. Across the population, minimum heading discrimination thresholds averaged 16.6 degrees +/- 1 degrees SE for FN neurons and 15.3 degrees +/- 2.2 degrees SE for VN neurons, severalfold larger than perceptual thresholds for heading discrimination. In line with previous studies and theoretical predictions, maximum discriminability was observed for directions where firing rate changed steeply as a function of heading, which occurs at headings approximately perpendicular to the maximum response direction. Forward/backward heading thresholds tended to be lower than lateral motion thresholds, and the ratio of lateral over forward heading thresholds averaged 2.2 +/- 6.1 (geometric mean +/- SD) for FN neurons and 1.1 +/- 4.4 for VN neurons. Our findings suggest that substantial pooling and/or selective decoding of vestibular signals from the vestibular and deep cerebellar nuclei may be important components of further processing. Such a characterization of neural sensitivity is critical for understanding how early stages of vestibular processing limit behavioral performance.

Multimodal Integration in Rostral Fastigial Nucleus Provides an Estimate of Body Movement

The ability to accurately control posture and perceive self-motion and spatial orientation requires knowledge of the motion of both the head and body. However, whereas the vestibular sensors and nuclei directly encode head motion, no sensors directly encode body motion. Instead, the convergence of vestibular and neck proprioceptive inputs during self-motion is generally believed to underlie the ability to compute body motion. Here, we provide evidence that the brain explicitly computes an internal estimate of body motion at the level of single cerebellar neurons. Neuronal responses were recorded from the rostral fastigial nucleus, the most medial of the deep cerebellar nuclei, during whole-body, body-under-head, and head-on-body rotations. We found that approximately half of the neurons encoded the motion of the body in space, whereas the other half encoded the motion of the head in space in a manner similar to neurons in the vestibular nuclei. Notably, neurons encoding body motion responded to both vestibular and proprioceptive stimulation (accordingly termed bimodal neurons). In contrast, neurons encoding head motion were sensitive only to vestibular inputs (accordingly termed unimodal neurons). Comparison of the proprioceptive and vestibular responses of bimodal neurons further revealed similar tuning in response to changes in head-on-body position. We propose that the similarity in nonlinear processing of vestibular and proprioceptive signals underlies the accurate computation of body motion. Furthermore, the same neurons that encode body motion (i.e., bimodal neurons) most likely encode vestibular signals in a body-referenced coordinate frame, since the integration of proprioceptive and vestibular information is required for both computations.

Optokinetic and Vestibular Responsiveness in the Macaque Rostral Vestibular and Fastigial Nuclei

We recorded from rostral vestibular (VN) and rostral fastigial nuclei (FN) neurons that did not respond to eye movements during three-dimensional (3D) vestibular and optokinetic stimulation (OKS). The majority of neurons in both areas (76 and 69% in VN and FN, respectively) responded during both rotational and translational motion. Preferred directions scattered throughout 3D space for translation but showed some preference for pitch/roll over yaw for rotation. VN/FN neurons were also tested during OKS while monkeys suppressed their optokinetic nystagmus by fixating a head-fixed target. Only a handful of cells (VN: 17%, FN: 6%) modulated during 0.5-Hz OKS suppression, but the number of responsive cells increased (VN: 40%, FN: 48%) during 0.02-Hz OKS. Preferred directions for rotation and OKS were not matched on individual neurons, and OKS gains were smaller than the respective gains during rotation. These results were generally similar for VN and FN neurons. We conclude that optokinetic-vestibular convergence might not be as prevalent as earlier studies have suggested.

Responses of rostral fastigial nucleus neurons of conscious cats to rotations in vertical planes

The rostral fastigial nucleus (RFN) of the cerebellum is thought to play an important role in postural control, and recent studies in conscious nonhuman primates suggest that this region also participates in the sensory processing required to compute body motion in space. The goal of the present study was to examine the dynamic and spatial responses to sinusoidal rotations in vertical planes of RFN neurons in conscious cats, and determine if they are similar to responses reported for monkeys. Approximately half of the RFN neurons examined were classified as graviceptive, since their firing was synchronized with stimulus position and the gain of their responses was relatively unaffected by the frequency of the tilts. The large majority (80%) of graviceptive RFN neurons were activated by pitch rotations. Most of the remaining RFN units exhibited responses to vertical oscillations that encoded stimulus velocity, and approximately 50% of these velocity units had a response vector orientation aligned near the plane of a single vertical semicircular canal. Unlike in primates, few feline RFN neurons had responses to vertical rotations that suggested integration of graviceptive (otolith) and velocity (vertical semicircular canal) signals. These data indicate that the physiological role of the RFN may differ between primates and lower mammals. The RFN in rats and cats in known to be involved in adjusting blood pressure and breathing during postural alterations in the transverse (pitch) plane. The relatively simple responses of many RFN neurons in cats are appropriate for triggering such compensatory autonomic responses.

Hyperventilation evoked by activation of the vicinity of the caudal inferior olivary nucleus depends on the fastigial nucleus in anesthetized rats

Several studies have demonstrated that cerebellar deep nuclei, particularly the rostral fastigial nucleus (FNr), are involved in respiratory modulation. These nuclei receive inputs from the contralateral caudal inferior olivary nuclei of the medulla. The objectives of this study were to determine whether electrical and chemical activation of the vicinity of the caudal inferior olivary nuclei (vIOc) affected respiration and, if true, whether the FNr was involved in the vIOc stimulation-evoked ventilatory responses. Experiments were conducted in 30 anesthetized and spontaneously breathing rats. Our results showed that 1) electrical (25 or 100 microA at 10 or 20 Hz for 10 s) and chemical (1 or 100 mM, 25-50 nl N-methyl-D-aspartate) stimulation of the vIOc augmented ventilation predominantly via increasing tidal volume; 2) the responses to the electrical stimulation were almost eliminated by lesion of the contralateral FNr via microinjection of ibotenic acid; and 3) the respiratory responses to electrical stimulation in the vicinity of the rostral IO were 65-70% smaller compared with that evoked by vIOc stimulation. These findings strongly suggest that vIOc neurons play a significant role in modulation of respiratory activity, largely depending on their projections to the FNr.

Properties of Cerebellar Fastigial Neurons During Translation, Rotation, and Eye Movements

The most medial of the deep cerebellar nuclei, the fastigial nucleus (FN), receives sensory vestibular information and direct inhibition from the cerebellar vermis. We investigated the signal processing in the primate FN by recording single-unit activities during translational motion, rotational motion, and eye movements. Firing rate modulation during horizontal plane translation in the absence of eye movements was observed in all non-eye-movement-sensitive cells and 26% of the pursuit eye-movement-sensitive neurons in the caudal FN. Many non-eye-movement-sensitive cells recorded in the rostral FN of three fascicularis monkeys exhibited convergence of signals from both the otolith organs and the semicircular canals. At low frequencies of translation, the majority of these rostral FN cells changed their firing rates in phase with head velocity rather than linear acceleration. As frequency increased, FN vestibular neurons exhibited a wide range of response dynamics with most cells being characterized by increasing phase leads as a function of frequency. Unlike cells in the vestibular nuclei, none of the rostral FN cells responded to rotational motion alone, without simultaneously exhibiting sensitivity to translational motion. Modulation during earth-horizontal axis rotation was observed in more than half (77%) of the neurons, although with smaller gains than during translation. In contrast, only 47% of the cells changed their firing rates during earth-vertical axis rotations in the absence of a dynamic linear acceleration stimulus. These response properties suggest that the rostral FN represents a main processing center of otolith-driven information for inertial motion detection and spatial orientation.

Multiple reference frames for motion in the primate cerebellum.

Knowledge of body motion through space is necessary for spatial orientation, self-motion perception, and postural control. Yet, sensory afferent signals may not directly provide such information to the brain. Because motion detected by the vestibular end organs is encoded in a head-fixed frame of reference, a coordinate transformation is thus required to encode body motion. In this study, we investigated whether cerebellar motion-sensitive neurons encode the translation of the body through space. We systematically changed both the direction of motion relative to the body and the static orientation of the head relative to the trunk. The activities of motion-sensitive neurons in the most medial of the deep cerebellar nuclei, the rostral fastigial nucleus, were compared with those in the brainstem vestibular nuclei. We found a distributed representation of reference frames for motion in the rostral fastigial nucleus, in contrast to cells in the vestibular nuclei, which primarily encoded motion in a head-fixed reference frame. This differential representation of motion-related information implies potential differences in the functional roles of these areas.

Trunk position influences vestibular responses of fastigial nucleus neurons in the alert monkey.

Vestibulospinal reflexes play an important role for body stabilization during locomotion and for postural control. For an appropriate distribution of vestibular signals to spinal motoneurons, the orientation of the body relative to the head needs to be taken into account. For different trunk positions, identical vestibular stimuli must activate different sets of muscles to ensure body stabilization. Because the cerebellar vermis and the underlying fastigial nucleus (FN) might be involved in this task, vestibular neurons in the rostral FN of alert rhesus monkeys were recorded during sinusoidal vestibular stimulation (0.1-1.0 Hz) in the roll and pitch planes at different trunk-re-head positions (center and +/-45 degrees ). From the sensitivity and phase values measured in these planes, the response properties in the intermediate planes and the stimulus orientation eliciting the optimal response [response vector orientation (RVO)] were calculated. In most neurons, the RVOs rotated systematically with respect to the head, when trunk-re-head position was altered, so that they tended to maintain their orientation with respect to the trunk. Sensitivity and phase at the RVO were not affected. This pattern was the same for neurons in the right and left FN and independent of stimulus frequency. The average sensitivity of this partially compensatory RVO shift in response to trunk-re-head displacements, evaluated by linear regression analyses, was 0.59 degrees / degrees (n = 73 neurons). These data show that FN neurons may encode vestibular information in a coordinate system that is closer to a trunk-centered than to a head-centered reference frame. They indicate an important role of this nucleus in motor programs related to posture and gait control.

Multimodal signal integration in vestibular neurons of the primate fastigial nucleus.

The rostral fastigial nucleus contains vestibular neurons, which presumably are involved in spinal mechanisms (neck, gait, posture) and which are not modulated with individual eye movements. Single-unit recordings in the alert behaving monkey during natural stimulus conditions reveal that virtually all neurons demonstrate integration of several sensory inputs. This applies not only for canal-canal and canal-otolith interaction, but also for otolith-otolith interaction. There is also some evidence that most neurons receive not only an utriculus but also a sacculus input. Furthermore, most neurons also respond to large-field optokinetic stimulation, reflecting visual-vestibular interaction. Neurons are also affected by the head on trunk position, which would allow these neurons to operate in a body-centered rather than a head-centered reference frame. These complex, multisensory features could permit fastigial nucleus neurons to rather specifically affect spinal motor functions.

Microinjection of acetazolamide into the fastigial nucleus augments respiratory output in the rat.

The rostral fastigial nucleus (FNr) of the cerebellum facilitates the respiratory response to hypercapnia. We hypothesized that some FNr sites are chemosensitive to focal tissue acidosis and contribute, at least partially, to respiratory modulation. Minute ventilation (VE) was recorded in 21 anesthetized and spontaneously breathing rats. Acetazolamide (AZ; 50 microM) was microinjected unilaterally into the FNr while an isocapnic condition was maintained throughout the experiment. AZ (1 or 20 nl) injection into the FNr significantly elevated VE (46.0 +/- 6.7%; P < 0.05), primarily via an increase in tidal volume (31.7 +/- 3.8%; P < 0.05), with little effect on arterial blood pressure. This augmented ventilatory response was initiated at 6.3 +/- 0.8 min and reached the peak at 19.7 +/- 4.1 min after AZ administration. The same dose of AZ delivered into the interposed and lateral cerebellar nuclei, or vehicle injection into the FNr, failed to elicit detectable cardiorespiratory responses. To determine whether the ventilatory response to AZ injection into the FNr resulted from an increase in respiratory central drive, the minute phrenic nerve activity (MPN) was recorded in seven paralyzed and ventilated rats. Similar to VE, MPN was increased by 38.9 +/- 8.9% (P < 0.05) after AZ administration. Our results suggest that elevation of CO2/H+ within the FNr facilitates respiratory output, supporting the presence of ventilatory chemoreception in rat FNr.

Fastigial nucleus-mediated respiratory responses depend on the medullary gigantocellular nucleus.

Electrical stimulation of the rostral fastigial nucleus (FNr) alters respiration via activation of local neurons. We hypothesized that this FNr-mediated respiratory response was dependent on the integrity of the nucleus gigantocellularis of the medulla (NGC). Electrical stimulation of the FNr in 15 anesthetized and tracheotomized spontaneously breathing rats significantly altered ventilation by 35.2 +/- 11.0% (P < 0.01) with the major effect being excitatory (78%). This respiratory response did not significantly differ from control after lesions of the NGC via bilateral microinjection of kainic or ibotenic acid (4.5 +/- 1.9%; P > 0.05) but persisted in sham controls. Eight other rats, in which horseradish peroxidase (HRP) solution was previously microinjected into the left NGC, served as nonstimulation controls or were exposed to either 15-min repeated electrical stimulation of the right FNr or hypercapnia for 90 min. Histochemical and immunocytochemical data showed that the right FNr contained clustered HRP-labeled neurons, most of which were double labeled with c-Fos immunoreactivity in both electrically and CO(2)-stimulated rats. We conclude that the NGC receives monosynaptic FNr inputs and is required for fully expressing FNr-mediated respiratory responses.

Responses of rostral fastigial neurons to linear acceleration in an alert monkey.

Vestibular-only neuronal responses to angular acceleration have been systematically characterized in the rostral fastigial nucleus (FN) by several studies. However, responses of these neurons to linear acceleration have not been examined. In this study, we recorded single-unit activity of vestibular-only neurons in an alert monkey during pure sinusoidal linear acceleration along different directions in the horizontal plane. Spatiotemporal response properties were quantified by computing two-dimensional response ellipses in the horizontal plane. Based on this analysis, neurons were classified as narrowly or broadly tuned. About 29% (5/17) of neurons were broadly tuned. The other 71% (12/17) were narrowly tuned. Unlike vestibular nuclei neurons, all recorded FN neurons exhibited irregular resting discharge rates (CV*0.2). Based on studies of linear motion-sensitive neurons in the vestibular nuclei, the data suggest that irregular neurons in the rostral FN and the vestibular nuclei have similar responses to linear acceleration in behaving monkeys.

Canal-otolith interaction in the fastigial nucleus of the alert monkey.

To determine the contribution of the otoliths as well as the horizontal and vertical semicircular canals to the response of "vestibular only" neurons in the rostral fastigial nucleus of the alert monkey, we applied natural sinusoidal vestibular stimuli (0.6 Hz; +/-15 deg) around different axes. During the experiment the monkey sat erect in a primate chair with the head immobile. Semicircular canal responses were investigated during tilted yaw stimulation around an earth vertical axis. The tilt angle was varied by 30 deg and included the optimal plane for horizontal canal stimulation (15 deg nose down from the stereotactic plane). The otoliths and mainly the vertical canals made contributions during stimulation around an earth-fixed horizontal axis (vertical stimulation). Head orientation was also slowly altered (2-3 deg/s) over a range of 180 deg under both stimulus conditions (tilted yaw and vertical stimulation). Neuronal data for each paradigm were fitted by a least squares best-sine function. Computation of the hypothetical contributions made by all three pairs of semicircular canals and the otoliths to these responses showed that 74% of the 46 neurons investigated received an otolith input; in most instances it was combined with a canal input. Neurons most often received input from the horizontal and vertical canals as well as the otoliths. Only a minority of neurons received a purely otolith (13%), vertical canal (13%), or horizontal canal (4%) input. Conventional criteria (head position-related activity, spatiotemporal convergence, STC) failed to detect an otolith contribution in several such instances. Thus, canal-otolith convergence is the general rule at this central stage of vestibular information processing in the fastigial nucleus. The large variety of response types allows these neurons to participate in multiple tasks of vestibulospinal movement control.

Otolith processing in the deep cerebellar nuclei.

To investigate the otolith contribution to the responses of "vestibular only" neurons in the rostral fastigial nucleus (FN), single-unit activity was recorded in the alert monkey with the head fixed during static and dynamic stimulation (+/- 15 deg, 0.06-1.4 Hz) around an earth-fixed horizontal axis. Head orientation could be altered allowing for roll, pitch, and intermediate planes of orientation. For the vast majority of neurons a response vector orientation (RVO) with an optimal response and a null-response at a head orientation 90 deg apart could be determined. Presumably more than 30% of the vestibular only neurons had an otolith input, as indicated by responses to static tilt, head-position-related activity, large phase changes (> 100 deg) of neuronal activity between 0.06 and 1.4 Hz, changes of the RVO at different frequencies and complex responses (spatio-temporal convergence). Thus, neurons in FN reflecting an otolith or a combined canal-otolith input are much more common than up to now thought. Vestibular-only neurons are most likely involved in vestibulospinal mechanisms. Their precise functional role has yet to be determined.