Spatiotemporal response properties of cerebellar Purkinje cells to animal displacement: a population analysis

Clinical Heterogeneity of Essential Tremor: Understanding Neural Substrates of Action Tremor Subtypes

Essential tremor (ET) is a common movement disorder affecting millions of people. Studies of ET patients and perturbations in animal models have provided a foundation for the neural networks involved in its pathophysiology. However, ET encompasses a wide variability of phenotypic expression, and this may be the consequence of dysfunction in distinct subcircuits in the brain. The cerebello-thalamo-cortical circuit is a common substrate for the multiple subtypes of action tremor. Within the cerebellum, three sets of cerebellar cortex-deep cerebellar nuclei connections are important for tremor. The lateral hemispheres and dentate nuclei may be involved in intention, postural and isometric tremor. The intermediate zone and interposed nuclei could be involved in intention tremor. The vermis and fastigial nuclei could be involved in head and proximal upper extremity tremor. Studying distinct cerebellar circuitry will provide important framework for understanding the clinical heterogeneity of ET.

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

Adaptation of spatio-temporal convergent properties in central vestibular neurons in monkeys

The spatio-temporal convergent (STC) response occurs in central vestibular cells when dynamic and static inputs are activated. The functional significance of STC behavior is not fully understood. Whether STC is a property of some specific central vestibular neurons, or whether it is a response that can be induced in any neuron at some frequencies is unknown. It is also unknown how the change in orientation of otolith polarization vector (orientation adaptation) affects STC behavior. A new complex model, that includes inputs with regular and irregular discharges from both canal and otolith afferents, was applied to experimental data to determine how many convergent inputs are sufficient to explain the STC behavior as a function of frequency and orientation adaptation. The canal-otolith and otolith-only neurons were recorded in the vestibular nuclei of three monkeys. About 42% (11/26 canal-otolith and 3/7 otolith-only) neurons showed typical STC responses at least at one frequency before orientation adaptation. After orientation adaptation in side-down head position for 2 h, some canal-otolith and otolith-only neurons altered their STC responses. Thus, STC is a property of weights of the regular and irregular vestibular afferent inputs to central vestibular neurons which appear and/or disappear based on stimulus frequency and orientation adaptation. This indicates that STC properties are more common for central vestibular neurons than previously assumed. While gravity-dependent adaptation is also critically dependent on stimulus frequency and orientation adaptation, we propose that STC behavior is also linked to the neural network responsible for localized contextual learning during gravity-dependent adaptation.

Abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions in cervical dystonia

The cerebellum can influence the responsiveness of the primary motor cortex (M1) to undergo spike timing-dependent plastic changes through a complex mechanism involving multiple relays in the cerebello-thalamo-cortical pathway. Previous TMS studies showed that cerebellar cortex excitation can block the increase in M1 excitability induced by a paired-associative stimulation (PAS), while cerebellar cortex inhibition would enhance it. Since cerebellum is known to be affected in many types of dystonia, this bidirectional modulation was assessed in 22 patients with cervical dystonia and 23 healthy controls. Exactly opposite effects were found in patients: cerebellar inhibition suppressed the effects of PAS, while cerebellar excitation enhanced them. Another experiment comparing healthy subjects maintaining the head straight with subjects maintaining the head turned as the patients found that turning the head is enough to invert the cerebellar modulation of M1 plasticity. A third control experiment in healthy subjects showed that proprioceptive perturbation of the sterno-cleido-mastoid muscle had the same effects as turning the head. We discuss these finding in the light of the recent model of a mesencephalic head integrator. We also suggest that abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions of the integrator in cervical dystonia.

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.

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 (theta(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.

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.

The cerebellum and sensorimotor coupling: Looking at the problem from the perspective of vestibular reflexes

Cerebellar modules process afferent information and deliver outputs relevant for both reflex and voluntary movements. The response of cerebellar modules to a given input depends on the whole array of signals impinging on them. Studies on vestibular reflexes indicate that the response of the cerebellar circuits to the vestibular input is modified by the integration of multiple visual, vestibular and somatosensory afferent signals. In this way the cerebellum slowly adapts these reflexes when they are not adequate to the behavioural condition and allows their fast modifications when the relative position of the body segments and that of the body in space are changed. Studies on voluntary movements indicate that the cerebellum is responsible for motor learning that consists of the development of new input-output associations. Several theoretical, anatomical and clinical studies are consistent with the hypothesis that the cerebellum allows the delivery of motor commands which vary according to the condition of the motor apparatus. Finally, the cerebellum could change the relation between visual information and aimed reaching movements according to the position of the eyes in the orbit and of the neck over the body. We propose that, due to the large expansion of its cortex, an important function of the cerebellum could be that of expanding the range of sensorimotor associations according to all the factors characterizing the behavioural condition. Indeed, following cerebellar lesion, learning is often lost, the movement results impaired and requires an increased attention. In the light of the recently discovered connections of the cerebellum with the rostral regions of the frontal lobe, it can be suggested that the ability of cerebellar circuits to modify the rules of input-output coupling according to a general context is a fundamental property allowing the cerebellum to control not only motor but also cognitive functions.

Responses of Purkinje-cells of the cerebellar anterior vermis to stimulation of vestibular and somatosensory receptors

In decerebrate cats, sinusoidal rotation of the forepaw around the wrist modifies the activity of the ipsilateral forelimb extensor triceps brachii (TB) and leads to plastic changes of adaptive nature in the gain of vestibulospinal (VS) reflexes (VSRs). Both effects are depressed by functional inactivation of the cerebellar anterior vermis, which also decreases the gain of VSRs. In order to better understand the mechanisms of these phenomena, the simple spike activity of Purkinje (P-) cells was recorded from the vermal cortex of the cerebellar anterior lobe during individual and/or combined stimulation of somatosensory wrist, neck and vestibular receptors. About one third of the recorded units were affected by sinusoidal rotation of the ipsilateral forepaw around the wrist axis (0.16 Hz, +/-10 degrees ). Most of these neurons ( approximately 60%) increased their activity during ventral flexion of the wrist and decreased it during the oppositely directed movement, with an average phase lag of -141 degrees with respect to the position of maximal dorsiflexion. The remaining cells ( approximately 40%) were excited during dorsiflexion of the wrist, with an average phase lead of 59 degrees with respect to the extreme dorsal flexion. Both populations showed comparable response gains, with an average value of 0.42+/-0.52, S.D., imp/s/deg. About half of the recorded units were also tested during sinusoidal roll tilt of the animal around the longitudinal axis (0.16 Hz, +/-10 degrees ), leading to stimulation of labyrinthine receptors. When both stimuli were applied simultaneously, the responses to combined stimulation usually corresponded to the sum of individual responses. While the phase distribution of somatosensory responses was clearly bimodal, vestibular responses showed phase angle values uniformly scattered between +/-180 degrees and 0 degrees , so that, during combined stimulation, each neuron could be maximally activated by coupling the two stimuli with a particular phase relation. Finally, a proportion of the recorded neurons was also tested during sinusoidal rotation of the body around its longitudinal axis, with the head fixed in space, leading to stimulation of neck receptors. The proportion of neurons affected by individual stimulation of vestibular or neck receptors (81% and 72%, respectively) was larger than that of wrist-driven neurons. Convergence of signals from vestibular, somatosensory wrist and neck receptors was found in 18% of the neurons analyzed. In conclusion, the results of this study show that somatosensory signals from the forelimb: i) modulate the activity of a sizeable proportion of neurons located within the cerebellar anterior vermis and ii) interact widely with labyrinthine and neck signals at this level. Moreover, iii) this corticocerebellar region is largely dominated by vestibular and neck signals that may be utilized to build up a neuronal representation of the position of body in space. These findings suggest that: 1) the modulation of TB activity induced by rotation of the ipsilateral wrist may at least partially depend upon the simultaneous changes in P-cell activity and 2) the interaction of vestibular and somatosensory wrist signals at P-cell level may represent the substrate of the plastic changes that affect the VSR when animal tilt and wrist rotation are driven together. A preliminary report of these data has been presented [ Responses of cerebellar Purkinje cells to forepaw rotation in decerebrate cat. Pflügers Arch 440:R31].

The cerebellum may implement the appropriate coupling of sensory inputs and motor responses: evidence from vestibular physiology

Starting from a survey of current ideas on the role of the cerebellum in sensorimotor transformations, the present review summarizes the results of recent experiments showing that (a) somatosensory signals modify the spatial organization of the postural reflexes, thus leading to body stability, and (b) otolith input changes the plane of reflex eye movements, by keeping it perpendicular to the gravito-inertial vector. Evidence will be given that both transformations require the integrity of specific cerebellar regions. These data indicates that the cerebellum allows an optimal input-output coupling in relation to the ultimate behavioural goal of the motor activity.

Three dimensional spatial-temporal convergence of otolith related signals in vestibular only neurons in squirrel monkeys

Spatio-temporal convergence (STC) properties of 19 vestibular only neurons' responses to translational head movements were examined in an alert, behaving squirrel monkey. In addition to standard tests that included 1.2 Hz yaw/pitch/roll rotations and inter-aural/naso-occipital/dorso-ventral translations, we also observed responses to translations along multiple directions in several orthogonal planes. Neural responses were fitted first by a model that permitted STC in all planes, characterized by a non-zero minimum and a gradual shift of phase. We then evaluated statistically whether models with fewer independent dynamic parameters yielded equally satisfactory results. The responses of 13 neurons were adequately fit by simple cosine models (1-D) as well as models that allowed complex STC behavior. Of the six neurons exhibiting STC, five could be modeled with two independent phase parameters (2-D) while the remaining neuron required a model with three independent phase parameters (3-D). The maximum translation sensitivity and phase, Smax and varphimax, and minimum translation sensitivity and phase, Smin and varphimin, were estimated from the reconstructed sensitivity and phase surfaces. The tuning ratio, Smin/Smax, in STC neurons was>0.40 while in 1-D neurons it was <0.25. Furthermore, the maximum response vectors of most 1-D neurons lay within 20 degrees of either the horizontal or sagittal plane while those of STC neurons lay >20 degrees from both planes. No difference in other response properties, such as varphimax or rotational responses, was found between neurons exhibiting STC and the simple cosine tuning. Our results suggest that the STC behavior observed in otolith-related vestibular neurons probably arises from summing inputs from afferents, with diverse response dynamics, innervating different otolith macula.

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.

Modelling transfer characteristics of vestibular neurons in the fastigial nucleus of the behaving monkey on the basis of canal-otolith interaction

The interaction of vestibular inputs with different dynamic and spatial behavior, i.e., canal-otolith interaction, leads to spatio-temporal convergence. Vestibular neurons in the fastigial nucleus often exhibit spatio-temporal convergence. The present report demonstrates that the discharge rates of most vestibular neurons in the primate fastigial nucleus can be simulated at different stimulus frequencies and orientations by a simple linear summation of the signals of the semicircular canals and the otoliths. In this way, a number of complex characteristics that depend on frequency, i.e. changing response-vector orientations, large phase changes, absence and presence of spatio-temporal convergence, can be easily explained.

Differential induction of brain-derived neurotrophic factor mRNA in rat inferior olive subregions following unilateral labyrinthectomy

Vestibular compensation, the neuronal process underlying behavioral recovery from vestibular dysfunction produced by unilateral labyrinthectomy, is attributed to functional reorganization of neuronal circuits in the brainstem and cerebellum. Climbing fibers originating from the inferior olive are suggested to play a crucial role in this compensatory process. To assess the possible contribution of brain-derived neurotrophic factor (BDNF) to the function of climbing fibers during vestibular compensation, we investigated the BDNF mRNA expression in the rat inferior olive after unilateral labyrinthectomy by quantitative in situ hybridization. We found several induction patterns depending on the subregions of the inferior olive 6 h after unilateral labyrinthectomy. First, in the dorsal cap and the ventrolateral outgrowth, the expression levels increased on the side contralateral to the lesion and decreased on the ipsilateral side. Second, in the beta nucleus, C subnucleus of the medial accessory olive, and A/B subnuclei of the medial accessory olive, we detected an increase in the mRNA expression level on the side contralateral to the lesion, but no changes in the expression level on the ipsilateral side. In the beta nucleus, where the strongest induction was observed, the mRNA expression level increased nearly five-fold. Third, in the dorsomedial cell column, the mRNA expression levels increased on both sides. Finally, we did not detect significant changes in mRNA expression levels in the other subregions of the inferior olive, the dorsal accessory olive, principal olive and rostral medial accessory olive. The changes in BDNF mRNA expression reverted to control levels by 72 h after the labyrinthectomy. The inferior olive subregions that showed changes in BDNF mRNA expression levels send climbing fibers to the cerebellar cortical regions that, in turn, project to the vestibular nuclei. Therefore, BDNF induced in these subregions could contribute to the functional reorganization of the olivo-cerebellar system for vestibular control.

Spatiotemporal processing of linear acceleration: primary afferent and central vestibular neuron responses

Spatiotemporal convergence and two-dimensional (2-D) neural tuning have been proposed as a major neural mechanism in the signal processing of linear acceleration. To examine this hypothesis, we studied the firing properties of primary otolith afferents and central otolith neurons that respond exclusively to horizontal linear accelerations of the head (0.16-10 Hz) in alert rhesus monkeys. Unlike primary afferents, the majority of central otolith neurons exhibited 2-D spatial tuning to linear acceleration. As a result, central otolith dynamics vary as a function of movement direction. During movement along the maximum sensitivity direction, the dynamics of all central otolith neurons differed significantly from those observed for the primary afferent population. Specifically at low frequencies (</=0.5 Hz), the firing rate of the majority of central otolith neurons peaked in phase with linear velocity, in contrast to primary afferents that peaked in phase with linear acceleration. At least three different groups of central response dynamics were described according to the properties observed for motion along the maximum sensitivity direction. "High-pass" neurons exhibited increasing gains and phase values as a function of frequency. "Flat" neurons were characterized by relatively flat gains and constant phase lags (approximately 20-55 degrees ). A few neurons ("low-pass") were characterized by decreasing gain and phase as a function of frequency. The response dynamics of central otolith neurons suggest that the approximately 90 degrees phase lags observed at low frequencies are not the result of a neural integration but rather the effect of nonminimum phase behavior, which could arise at least partly through spatiotemporal convergence. Neither afferent nor central otolith neurons discriminated between gravitational and inertial components of linear acceleration. Thus response sensitivity was indistinguishable during 0.5-Hz pitch oscillations and fore-aft movements. The fact that otolith-only central neurons with "high-pass" filter properties exhibit semicircular canal-like dynamics during head tilts might have important consequences for the conclusions of previous studies of sensory convergence and sensorimotor transformations in central vestibular neurons.

Linear spatio-temporal convergence in vestibular neurons of the primate nucleus fastigii

Vestibular responses in the primate fastigial nucleus (FN) do often not follow the simple cosine tuning observed in primary vestibular afferents. The present report demonstrates that these more complex patterns can mostly be attributed to simple linear summation of spatially and temporally diverse cosine-tuned input signals (linear spatio-temporal convergence, STC). Analyses following from this elementary finding, however, reveal frequency-dependent properties in many FN neurons, which are difficult to reconcile with existing concepts of possible functions of STC in central vestibular areas. The demonstration that STC linearity holds for FN responses is thus of both theoretical and practical relevance, allowing shortening of future experimental protocols and facilitating comparison of the observed spatio-temporal response dynamics with those at other stages of vestibular signal processing.

Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, delta, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in delta values (i.e. between 0/infinity and finite values) with velocity can be computed for each neuron. The shifts in delta values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.

Neck input modifies the reference frame for coding labyrinthine signals in the cerebellar vermis: a cellular analysis

The activity of 68 neurons, mainly Purkinje cells, was recorded from the cerebellar anterior vermis of decerebrate cats during wobble of the whole animal (at 0.156 Hz, 5 degrees), a mixture of tilt and rotation, leading to stimulation of labyrinth receptors. Most of the neurons (65/68) were affected by both clockwise and counterclockwise rotations. Twenty-four units showing responses of comparable amplitude to these stimuli (narrowly tuned cells) were represented by a single vector (Smax), whose preferred direction corresponded to the direction of stimulation giving rise to the maximal response. The remaining 41 units, however, showed different amplitude responses to these rotations (broadly tuned cells) and were characterized by two spatially and temporally orthogonal vectors (Smax and Smin), suggesting that labyrinthine signals with different spatial and temporal properties converged on these cells. All these units were tested while the body was aligned with the head (control position), as well as after static displacement of the body under a fixed head by 15 degrees and/or 30 degrees around a vertical axis passing through C1-C2, thus leading to stimulation of neck receptors. The orientation component of the response vector of the Purkinje cells to vestibular stimulation changed following body-to-head displacement. Moreover, the amplitude of vector rotation corresponded, on the average, to that of body rotation. Changes in temporal phase, gain and tuning ratio of the responses were also observed. We propose that information from neck receptors regulates the convergence of labyrinthine signals with different spatial and temporal properties on corticocerebellar units. Due to their strict relationship with the motor system, these units may give rise to appropriate responses in the limb musculature, by modifying the spatial organization of the vestibulospinal reflexes according to the requirements of body stability. The cerebellar vermis may thus represent an important structure, where frames of reference can be altered to account for changes in position of trunk, head and neck.

Fastigial nucleus activity during different frequencies and orientations of vertical vestibular stimulation in the monkey

Neurons in the rostral part of the fastigial nucleus (FN) respond to vestibular stimulation but are not related to eye movements. To understand the precise role of these vestibular-only neurons in the central processing of vestibular signals, unit activity in the FN of alert monkeys (Macaca mulatta) was recorded. To induce vestibular stimulation, the monkey was rotated sinusoidally around an earth-fixed horizontal axis at stimulus frequencies between 0.06 (+/-15 degrees) and 1.4 Hz (+/-7.5 degrees). During stimulation head orientation was changed continuously, allowing for roll, pitch, and intermediate planes of orientation. At a frequency of 0.6 Hz, 59% of the neurons had an optimal response orientation (ORO) and a null response (i.e., no modulation) 90 degrees apart. The phase of neuronal response was constant except for a steep shift of 180 degrees around the null response. This group I response is compatible with a semicircular canal input, canal convergence, or a single otolith input. Several other features indicated more complex responses, including spatiotemporal convergence (STC). 1) For 35% of the responses at 0.6 Hz, phase changes were gradual with different orientations. Fifteen percent of these had a null response (group II), and 20% showed only a minimal response but no null response (group III). The remaining responses (6%), classified as group IV, were characterized by a constant sensitivity at different orientations in most instances. 2) For the vast majority of neurons, the stimulus frequency determined the response group, i.e., an individual neuron could show a group I response at one frequency and a group II (III or IV) response at another frequency. 3) ORO changed with frequency by >45 degrees for 44% of the neurons. 4) Although phase changes at different frequencies were close to head velocity (+/-45 degrees ) or head position (+/-45 degrees ) for most neurons, they exceeded 90 degrees for 29% of the neurons between 0.1 and 1.0 Hz. In most cases, this was a phase advance. The change in sensitivity with change in frequency showed a similar pattern for all neurons; the average sensitivity increased from 1.24 imp. s-1. deg-1 at 0.1 Hz to 2.97 imp. s-1. deg-1 at 1.0 Hz. These data demonstrate that only an analysis based on measurements at different frequencies and orientations reveals a number of complex features. They moreover suggest that for the vast majority of neurons several sources of canal and otolith information interact at this central stage of vestibular information processing.

Control of spatial orientation of the angular vestibulo-ocular reflex by the nodulus and uvula of the vestibulocerebellum

Eye velocity produced by the angular vestibulo-ocular reflex (aVOR) tends to align with the summed vector of gravity and other linear accelerations [gravito-inertial acceleration (GIA)]. Defined as "spatial orientation of the aVOR," we propose that it is controlled by the nodulus and uvula of the vestibulocerebellum. Here, electrical stimulation, injections of the GABAA agonist, muscimol, and single-cell recordings were utilized to investigate this spatial orientation. Stimulation, injection, and recording sites in the nodulus were determined in vivo by MRI and verified in histological sections. MRI proved to be a sensitive, reliable way to localize electrode placements. Electrical stimulation at sites in the nodulus and sublobule d of the uvula produced nystagmus whose slow-phase eye-velocity vectors were either head centric or spatially invariant. When head centric, the eye velocity vector remained within +/- 45 degrees of the vector obtained with the animal upright, regardless of head position with respect to gravity. When spatially oriented, the vector remained relatively constant in space in one on-side position, with respect to the vector determined with the animal upright. A majority of induced movements from the nodulus were spatially oriented. Spatially oriented movements were generally followed by after-nystagmus, which had the characteristics of optokinetic after-nystagmus (OKAN), including orientation to the GIA. After muscimol injections, horizontal-to-vertical cross-coupling was lost or reduced during OKAN in tilted positions. This supports the hypothesis that the nodulus mediates yaw-to-vertical or roll cross-coupling. The injections also shortened the yaw-axis time constant and produced contralateral horizontal spontaneous nystagmus, whose velocity varied as a function of head position with regard to gravity. Nodulus units were tested with static head tilt, sinusoidal oscillation around a spatial horizontal axis with the head in different orientations relative to the pitching plane, and off-vertical axis rotation (OVAR). The direction of the response vectors of the otolith-recipient units in the nodulus, determined from static and/or dynamic head tilts, were confirmed by OVAR. These vector directions lay close to the planes of the vertical canals in 7/10 units; many units also had convergent input from the vertical canals. It is postulated that the orientation properties of the aVOR result from a transfer of otolith input regarding head tilt along canal planes to canal-related zones of the nodulus. In turn, Purkinje cells in these zones project to vestibular nuclei neurons to control eye velocity around axes normal to these same canal planes.

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.

Spatiotemporal response properties of cerebellar Purkinje cells to neck displacement

The activity of 184 Purkinje cells and 58 unidentified neurons located within the cerebellar anterior vermis was recorded in decerebrate cats during wobble of the body under a fixed head. This stimulus induced a neck displacement of constant amplitude (2.5 degrees) whose direction rotated at the constant velocity of 56.2 degrees/s on the horizontal plane, both in the clockwise and counterclockwise directions. It was then possible to evaluate the spatiotemporal characteristics of unit responses to neck displacement in the vertical planes; 131 of 184 Purkinje cells (71%) and 35 of 58 unidentified cells (60%) responded to clockwise and/or counterclockwise rotations. In particular, among the responsive units, 44% of the Purkinje cells and 37% of the unidentified cells showed an equal amplitude modulation during clockwise and counterclockwise rotations. These units are expected to show a maximal response sensitivity for neck displacement in a preferred direction, a null response for perpendicularly oriented stimuli and a constant temporal phase (narrowly tuned neurons). In 28% of the Purkinje cells and 40% of the unidentified cells, responses of different amplitudes were observed during clockwise and counterclockwise rotations. These neurons should display a preferred direction of response to neck displacement, lack of null response directions and a temporal phase changing with the stimulus direction (broadly tuned neurons). Finally, 27% of the Purkinje cells and 23% of the unidentified cells responded only to wobble in the clockwise or counterclockwise direction (unidirectional units). This behavior predicts equal sensitivities for all the directions of neck displacement and a response phase changing linearly with the direction of neck displacement. A maximal sensitivity vector (Smax), aligned with the preferred direction of the neuron, was evaluated for the bidirectional narrowly tuned and broadly tuned units. Its amplitude and temporal phase corresponded to the response characteristics expected for stimuli in the preferred direction of the cell. Smax directions were distributed over the horizontal plane. Most of them, however, were closer to the pitch than to the roll axis and pointed towards the animal's tail. Among pitch-related Purkinje cells, the temporal phase of Smax was small with a predominance of phase lags; phase leads of rather large amplitude were usually observed for roll-related Purkinje cells. The possibility that the recorded population of units coded the direction of neck displacement was tested by assuming that each cell gave a vectorial contribution related to its response properties and that the vectorial sum of such contributions represented the outcome of the population code. Dynamic body-to-head displacements in four different directions were simulated and for each direction 12 population vectors were evaluated at regular intervals of the stimulus cycle. The direction of the population vector was related to that of the stimulus, but the correspondence was close only for the pitch direction. Moreover, the amplitude of the population vector depended upon the direction of the stimulus, being larger for pitch than for roll displacements. Due to the efferent connections of the explored cerebellar region, the neuronal signals generated by the Purkinje cell population are probably transferred to the spinal cord, where they may differentially affect the amplitude and the spatial properties of the neck reflexes according to the direction of neck displacement.