Input of anterior and posterior semicircular canal interneurons encoding head-velocity to the dorsal Y group of the vestibular nuclei

Adaptive Balance in Posterior Cerebellum

Vestibular and optokinetic space is represented in three-dimensions in vermal lobules IX-X (uvula, nodulus) and hemisphere lobule X (flocculus) of the cerebellum. Vermal lobules IX-X encodes gravity and head movement using the utricular otolith and the two vertical semicircular canals. Hemispheric lobule X encodes self-motion using optokinetic feedback about the three axes of the semicircular canals. Vestibular and visual adaptation of this circuitry is needed to maintain balance during perturbations of self-induced motion. Vestibular and optokinetic (self-motion detection) stimulation is encoded by cerebellar climbing and mossy fibers. These two afferent pathways excite the discharge of Purkinje cells directly. Climbing fibers preferentially decrease the discharge of Purkinje cells by exciting stellate cell inhibitory interneurons. We describe instances adaptive balance at a behavioral level in which prolonged vestibular or optokinetic stimulation evokes reflexive eye movements that persist when the stimulation that initially evoked them stops. Adaptation to prolonged optokinetic stimulation also can be detected at cellular and subcellular levels. The transcription and expression of a neuropeptide, corticotropin releasing factor (CRF), is influenced by optokinetically-evoked olivary discharge and may contribute to optokinetic adaptation. The transcription and expression of microRNAs in floccular Purkinje cells evoked by long-term optokinetic stimulation may provide one of the subcellular mechanisms by which the membrane insertion of the GABAA receptors is regulated. The neurosteroids, estradiol (E2) and dihydrotestosterone (DHT), influence adaptation of vestibular nuclear neurons to electrically-induced potentiation and depression. In each section of this review, we discuss how adaptive changes in the vestibular and optokinetic subsystems of lobule X, inferior olivary nuclei and vestibular nuclei may contribute to the control of balance.

Histochemical Characterization of the Vestibular Y-Group in Monkey

The Y-group plays an important role in the generation of upward smooth pursuit eye movements and contributes to the adaptive properties of the vertical vestibulo-ocular reflex. Malfunction of this circuitry may cause eye movement disorders, such as downbeat nystagmus. To characterize the neuron populations in the Y-group, we performed immunostainings for cellular proteins related to firing characteristics and transmitters (calretinin, GABA-related proteins and ion channels) in brainstem sections of macaque monkeys that had received tracer injections into the oculomotor nucleus. Two histochemically different populations of premotor neurons were identified: The calretinin-positive population represents the excitatory projection to contralateral upgaze motoneurons, whereas the GABAergic population represents the inhibitory projection to ipsilateral downgaze motoneurons. Both populations receive a strong supply by GABAergic nerve endings most likely originating from floccular Purkinje cells. All premotor neurons express nonphosphorylated neurofilaments and are ensheathed by strong perineuronal nets. In addition, they contain the voltage-gated potassium channels Kv1.1 and Kv3.1b which suggests biophysical similarities to high-activity premotor neurons of vestibular and oculomotor systems. The premotor neurons of Y-group form a homogenous population with histochemical characteristics compatible with fast-firing projection neurons that can also undergo plasticity and contribute to motor learning as found for the adaptation of the vestibulo-ocular reflex in response to visual-vestibular mismatch stimulation. The histochemical characterization of premotor neurons in the Y-group allows the identification of the homologue cell groups in human, including their transmitter inputs and will serve as basis for correlated anatomical-neuropathological studies of clinical cases with downbeat nystagmus.

Simultaneous and opposing horizontal VOR adaptation in humans suggests functionally independent neural circuits

The healthy vestibulo-ocular reflex (VOR) ensures that images remain on the fovea of the retina during head rotation to maintain stable vision. VOR behavior can be measured as a summation of linear and nonlinear properties although it is unknown whether asymmetric VOR adaptation can be performed synchronously in humans. The purpose of the present study is twofold. First, examine whether the right and left VOR gains can be synchronously adapted in opposing directions. Second, to investigate whether the adaptation context transfers between both sides. Three separate VOR adaptation sessions were randomized such that the VOR was adapted Up-bilaterally, Down-bilaterally, or Mixed (one side up, opposite side down). Ten healthy subjects completed the study. Subjects were tested while seated upright, 1 meter in front of a wall in complete dark. Each subject made active (self-generated) head impulse rotations for 15 min while viewing a gradually increasing amount of retinal slip. VOR training demand changed by 10% every 90 s. The VOR changed significantly for all training conditions. No significant differences in the magnitude of VOR gain changes between training conditions were found. The human VOR can be simultaneously driven in opposite directions. The similar magnitude of VOR gain changes across training conditions suggests functionally independent VOR circuits for each side of head rotation that mediate simultaneous and opposing VOR adaptations.

NEW & NOTEWORTHY Our results indicate that humans have the adaptive capacity for concurrent and opposing directions of vestibulo-ocular reflex (VOR) motor learning. Context specificity of VOR adaptation is dependent on the error signal being unilateral or bilateral, which we illustrate via a lack of VOR gain transfer using unique adaptive demands.

Climbing fibers mediate vestibular modulation of both "complex" and "simple spikes" in Purkinje cells

Climbing and mossy fibers comprise two distinct afferent paths to the cerebellum. Climbing fibers directly evoke a large multispiked action potential in Purkinje cells termed a "complex spike" (CS). By logical exclusion, the other class of Purkinje cell action potential, termed "simple spike" (SS), has often been attributed to activity conveyed by mossy fibers and relayed to Purkinje cells through granule cells. Here, we investigate the relative importance of climbing and mossy fiber pathways in modulating neuronal activity by recording extracellularly from Purkinje cells, as well as from mossy fiber terminals and interneurons in folia 8-10. Sinusoidal roll-tilt vestibular stimulation vigorously modulates the discharge of climbing and mossy fiber afferents, Purkinje cells, and interneurons in folia 9-10 in anesthetized mice. Roll-tilt onto the side ipsilateral to the recording site increases the discharge of both climbing fibers (CSs) and mossy fibers. However, the discharges of SSs decrease during ipsilateral roll-tilt. Unilateral microlesions of the beta nucleus (β-nucleus) of the inferior olive blocks vestibular modulation of both CSs and SSs in contralateral Purkinje cells. The blockage of SSs occurs even though primary and secondary vestibular mossy fibers remain intact. When mossy fiber afferents are damaged by a unilateral labyrinthectomy (UL), vestibular modulation of SSs in Purkinje cells ipsilateral to the UL remains intact. Two inhibitory interneurons, Golgi and stellate cells, could potentially contribute to climbing fiber-induced modulation of SSs. However, during sinusoidal roll-tilt, only stellate cells discharge appropriately out of phase with the discharge of SSs. Golgi cells discharge in phase with SSs. When the vestibularly modulated discharge is blocked by a microlesion of the inferior olive, the modulated discharge of CSs and SSs is also blocked. When the vestibular mossy fiber pathway is destroyed, vestibular modulation of ipsilateral CSs and SSs persists. We conclude that climbing fibers are primarily responsible for the vestibularly modulated discharge of both CSs and SSs. Modulation of the discharge of SSs is likely caused by climbing fiber-evoked stellate cell inhibition.

Calretinin as a Marker for Premotor Neurons Involved in Upgaze in Human Brainstem

Eye movements are generated by different premotor pathways. Damage to them can cause specific deficits of eye movements, such as saccades. For correlative clinico-anatomical post-mortem studies of cases with eye movement disorders it is essential to identify the functional cell groups of the oculomotor system in the human brain by marker proteins. Based on monkey studies, the premotor neurons of the saccadic system can be identified by the histochemical markers parvalbumin (PAV) and perineuronal nets in humans. These areas involve the interstitial nucleus of Cajal (INC) and the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF), which both contain premotor neurons for upgaze and downgaze. Recent monkey and human studies revealed a selective excitatory calretinin (CR)-positive input to the motoneurons mediating upgaze, but not to those for downgaze. Three premotor regions were identified as sources of CR input in monkey: y-group, INC and RIMLF. These findings suggest that the expression pattern of parvalbumin and CR may help to identify premotor neurons involved in up- or downgaze. In a post-mortem study of five human cases without neurological diseases we investigated the y-group, INC and RIMLF for the presence of parvalbumin and CR positive neurons including their co-expression. Adjacent thin paraffin sections were stained for the aggrecan (ACAN) component of perineuronal nets, parvalbumin or CR and glutamate decarboxylase. The comparative analysis of scanned thin sections of INC and RIMLF revealed medium-sized parvalbumin positive neurons with and without CR coexpression, which were intermingled. The parvalbumin/CR positive neurons in both nuclei are considered as excitatory premotor upgaze neurons. Accordingly, the parvalbumin-positive neurons lacking CR are considered as premotor downgaze neurons in RIMLF, but may in addition include inhibitory premotor upgaze neurons in the INC as indicated by co-expression of glutamate decarboxylase in a subpopulation. CR-positive neurons ensheathed by perineuronal nets in the human y-group are considered as the homolog premotor neurons described in monkey, projecting to superior rectus (SR) and inferior oblique (IO) motoneurons. In conclusion, combined immunostaining for parvalbumin, perineuronal nets and CR may well be suited for the specific identification and subsequent analysis of premotor upgaze pathways in clinical cases of isolated up- or downgaze deficits.

Transmitter inputs to different motoneuron subgroups in the oculomotor and trochlear nucleus in monkey

In all vertebrates the eyes are moved by six pairs of extraocular muscles enabling horizontal, vertical and rotatory movements. Recent work showed that each extraocular muscle is controlled by two motoneuronal groups: (1) Motoneurons of singly-innervated muscle fibers (SIF) that lie within the boundaries of motonuclei mediating a fast muscle contraction; and (2) motoneurons of multiply-innervated muscle fibers (MIF) in the periphery of motonuclei mediating a tonic muscle contraction. Currently only limited data about the transmitter inputs to the SIF and MIF motoneurons are available. Here we performed a quantitative study on the transmitter inputs to SIF and MIF motoneurons of individual muscles in the oculomotor and trochlear nucleus in monkey. Pre-labeled motoneurons were immunostained for GABA, glutamate decarboxylase, GABA-A receptor, glycine transporter 2, glycine receptor 1, and vesicular glutamate transporters 1 and 2. The main findings were: (1) the inhibitory control of SIF motoneurons for horizontal and vertical eye movements differs. Unlike in previous primate studies a considerable GABAergic input was found to all SIF motoneuronal groups, whereas a glycinergic input was confined to motoneurons of the medial rectus (MR) muscle mediating horizontal eye movements and to those of the levator palpebrae (LP) muscle elevating the upper eyelid. Whereas SIF and MIF motoneurons of individual eye muscles do not differ numerically in their GABAergic, glycinergic and vGlut2 input, vGlut1 containing terminals densely covered the supraoculomotor area (SOA) targeting MR MIF motoneurons. It is reasonable to assume that the vGlut1 input affects the near response system in the SOA, which houses the preganglionic neurons mediating pupillary constriction and accommodation and the MR MIF motoneurones involved in vergence.

A bi-hemispheric neuronal network model of the cerebellum with spontaneous climbing fiber firing produces asymmetrical motor learning during robot control

To acquire and maintain precise movement controls over a lifespan, changes in the physical and physiological characteristics of muscles must be compensated for adaptively. The cerebellum plays a crucial role in such adaptation. Changes in muscle characteristics are not always symmetrical. For example, it is unlikely that muscles that bend and straighten a joint will change to the same degree. Thus, different (i.e., asymmetrical) adaptation is required for bending and straightening motions. To date, little is known about the role of the cerebellum in asymmetrical adaptation. Here, we investigate the cerebellar mechanisms required for asymmetrical adaptation using a bi-hemispheric cerebellar neuronal network model (biCNN). The bi-hemispheric structure is inspired by the observation that lesioning one hemisphere reduces motor performance asymmetrically. The biCNN model was constructed to run in real-time and used to control an unstable two-wheeled balancing robot. The load of the robot and its environment were modified to create asymmetrical perturbations. Plasticity at parallel fiber-Purkinje cell synapses in the biCNN model was driven by error signal in the climbing fiber (cf) input. This cf input was configured to increase and decrease its firing rate from its spontaneous firing rate (approximately 1 Hz) with sensory errors in the preferred and non-preferred direction of each hemisphere, as demonstrated in the monkey cerebellum. Our results showed that asymmetrical conditions were successfully handled by the biCNN model, in contrast to a single hemisphere model or a classical non-adaptive proportional and derivative controller. Further, the spontaneous activity of the cf, while relatively small, was critical for balancing the contribution of each cerebellar hemisphere to the overall motor command sent to the robot. Eliminating the spontaneous activity compromised the asymmetrical learning capabilities of the biCNN model. Thus, we conclude that a bi-hemispheric structure and adequate spontaneous activity of cf inputs are critical for cerebellar asymmetrical motor learning.

Collateral projections from vestibular nuclear and inferior olivary neurons to lobules I/II and IX/X of the rat cerebellar vermis: a double retrograde labeling study

Axon collateral projections to various lobules of the cerebellar cortex are thought to contribute to the coordination of neuronal activities among different parts of the cerebellum. Even though lobules I/II and IX/X of the cerebellar vermis are located at the opposite poles in the anterior-posterior axis, they have been shown to receive dense vestibular mossy fiber projections. For climbing fibers, there is also a mirror-image-like organisation in their axonal collaterals between the anterior and posterior cerebellar cortex. However, the detailed organisation of mossy and climbing fiber collateral afferents to lobules I/II and IX/X is still unclear. Here, we carried out a double-labeling study with two retrograde tracers (FluoroGold and MicroRuby) in lobules I/II and IX/X. We examined labeled cells in the vestibular nuclei and inferior olive. We found a low percentage of double-labeled neurons in the vestibular nuclei (2.1 ± 0.9% of tracer-labeled neurons in this brain region), and a higher percentage of double-labeled neurons in the inferior olive (6.5 ± 1.9%), especially in its four small nuclei (18.5 ± 8.0%; including the β nucleus, dorsal cap of Kooy, ventrolateral outgrowth, and dorsomedial cell column), which are relevant for vestibular function. These results provide strong anatomical evidence for coordinated information processing in lobules I/II and IX/X for vestibular control.

Calretinin inputs are confined to motoneurons for upward eye movements in monkey

Motoneurons of extraocular muscles are controlled by different premotor pathways, whose selective damage may cause directionally selective eye movement disorders. The fact that clinical disorders can affect only one direction, e.g., isolated up-/downgaze palsy or up-/downbeat nystagmus, indicates that up- and downgaze pathways are organized separately. Recent work in monkey revealed that a subpopulation of premotor neurons of the vertical eye movement system contains the calcium-binding protein calretinin (CR). With combined tract-tracing and immunofluorescence, the motoneurons of vertically pulling eye muscles in monkey were investigated for the presence of CR-positive afferent terminals. In the oculomotor nucleus, CR was specifically found in punctate profiles contacting superior rectus and inferior oblique motoneurons, as well as levator palpebrae motoneurons, all of which participate in upward eye movements. Double-immunofluorescence labeling revealed that CR-positive terminals lacked the γ-aminobutyric acid (GABA)-synthesizing enzyme glutamate decarboxylase, which is present in inhibitory afferents to all motoneurons mediating vertical eye movements. Therefore, CR-containing afferents are considered to be excitatory. In conclusion, a strong CR input is confined to motoneurons mediating upgaze, which derive from premotor pathways mediating saccades and smooth pursuit, but not from secondary vestibulo-ocular neurons in the magnocellular part of the medial vestibular nucleus. The functional significance of CR in these connections is unclear, but it may serve as a useful marker to locate upgaze pathways in the human brain.

Diversity of vestibular nuclei neurons targeted by cerebellar nodulus inhibition

A functional role of the cerebellar nodulus and ventral uvula (lobules X and IXc,d of the vermis) for vestibular processing has been strongly suggested by direct reciprocal connections with the vestibular nuclei, as well as direct vestibular afferent inputs as mossy fibres. Here we have explored the types of neurons in the macaque vestibular nuclei targeted by nodulus/ventral uvula inhibition using orthodromic identification from the caudal vermis. We found that all nodulus-target neurons are tuned to vestibular stimuli, and most are insensitive to eye movements. Such non-eye-movement neurons are thought to project to vestibulo-spinal and/or thalamo-cortical pathways. Less than 20% of nodulus-target neurons were sensitive to eye movements, suggesting that the caudal vermis can also directly influence vestibulo-ocular pathways. In general, response properties of nodulus-target neurons were diverse, spanning the whole continuum previously described in the vestibular nuclei. Most nodulus-target cells responded to both rotation and translation stimuli and only a few were selectively tuned to translation motion only. Other neurons were sensitive to net linear acceleration, similar to otolith afferents. These results demonstrate that, unlike the flocculus and ventral paraflocculus which target a particular cell group, nodulus/ventral uvula inhibition targets a large diversity of cell types in the vestibular nuclei, consistent with a broad functional significance contributing to vestibulo-ocular, vestibulo-thalamic and vestibulo-spinal pathways.

Modulated discharge of Purkinje and stellate cells persists after unilateral loss of vestibular primary afferent mossy fibers in mice

Cerebellar Purkinje cells are excited by two afferent pathways: climbing and mossy fibers. Climbing fibers evoke large "complex spikes" (CSs) that discharge at low frequencies. Mossy fibers synapse on granule cells whose parallel fibers excite Purkinje cells and may contribute to the genesis of "simple spikes" (SSs). Both afferent systems convey vestibular information to folia 9c-10. After making a unilateral labyrinthectomy (UL) in mice, we tested how the discharge of CSs and SSs was changed by the loss of primary vestibular afferent mossy fibers during sinusoidal roll tilt. We recorded from cells identified by juxtacellular neurobiotin labeling. The UL preferentially reduced vestibular modulation of CSs and SSs in folia 8-10 contralateral to the UL. The effects of a UL on Purkinje cell discharge were similar in folia 9c-10, to which vestibular primary afferents project, and in folia 8-9a, to which they do not project, suggesting that vestibular primary afferent mossy fibers were not responsible for the UL-induced alteration of SS discharge. UL also induced reduced vestibular modulation of stellate cell discharge contralateral to the UL. We attribute the decreased modulation to reduced vestibular modulation of climbing fibers. In summary, climbing fibers modulate CSs directly and SSs indirectly through activation of stellate cells. Whereas vestibular primary afferent mossy fibers cannot account for the modulated discharge of SSs or stellate cells, the nonspecific excitation of Purkinje cells by parallel fibers may set an operating point about which the discharges of SSs are sculpted by climbing fibers.

Sources of calretinin inputs to motoneurons of extraocular muscles involved in upgaze

Recent monkey studies showed that motoneurons of the oculomotor nucleus involved in upward eye movements receive a selective input from afferents containing calretinin (CR). Here, we investigated the sources of these CR-positive afferents. After injections of tract-tracers into the oculomotor nucleus (nIII) of two monkeys, the retrograde labeling was combined with CR-immunofluorescence in frozen brainstem sections. Three sources of CR inputs to nIII were found: the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF), the interstitial nucleus of Cajal, and the y-group. CR is not present in all premotor upward-moving pathways. The excitatory secondary vestibulo-ocular neurons in the magnocellular part of the medial vestibular nuclei contained nonphosphorylated neurofilaments, but no CR, and they received a strong supply of large CR-positive boutons. In conclusion, the present study presents evidence that only specific premotor pathways for upward eye movements--excitatory upgaze pathways--contain CR, but not the up vestibulo-ocular reflex pathways. This property may help to differentiate between premotor up- and downgaze pathways in correlative clinico-anatomical studies in humans.

Directional preponderance of vertical eye movements induced by cross-axis adaptation of the vestibulo-ocular reflex in the cat

OBJECTIVE

We examined the emerging vertical component after cross-axis vestibulo-ocular reflex (VOR) adaptation.

METHODS

For the adaptive conditioning, the whole body of a subject was rotated horizontally while a visual pattern was rotated vertically for 1h.

RESULTS

During the conditioning period, the vertical component in response to the vertical visual stimulation was an up-down asymmetric, and its amplitude gradually increased as the stimulus progressed. After conditioning, the direction of VOR was changed via an emerging vertical component during horizontal head rotation in total darkness. Even though the retinal slip-velocity was greater for downward than for upward during the conditioning period, the emerging upward component was significantly larger than the emerging downward component.

CONCLUSION

These results may be helpful for developing an optimal vestibular-compensation system by visual-vestibular mismatch/optokinetic training.

Saccade and vestibular ocular motor adaptation

PURPOSE

This paper focuses on motor learning within the saccadic and vestibulo-ocular reflex (VOR) oculomotor systems, vital for our understanding how the brain keeps these subsystems calibrated in the presence of disease, trauma, and the changes that invariably accompany normal development and aging. We will concentrate on new information related to multiple time scales of saccade motor learning, adaptation of the VOR during high-velocity impulses, and the role of saccades in VOR adaptation. The role of the cerebellum in both systems is considered.

METHODS

Review of data involving saccade and VOR motor learning.

RESULTS

Data supports learning within the saccadic and VOR oculomotor systems is influenced by 1). Multiple time scales, with different rates of both learning and forgetting (seconds, minutes, hours, days, and months). In the case of forgetting, relearning on a similar task may be faster. 2). Pattern of training, learning and forgetting are not similarly achieved. Different contexts require different motor behaviors and rest periods between training sessions can be important for memory consolidation.

CONCLUSIONS

The central nervous system has the difficult task of determining where blame resides when motor performance is impaired (the credit assignment problem). Saccade and VOR motor learning takes place at multiple levels within the nervous system, from alterations in ion channel and membrane properties on single neurons, to more complex changes in neural circuit behavior and higher-level cognitive processes including prediction.

Neuronal Substrates of Motor Learning in the Velocity Storage Generated During Optokinetic Stimulation in the Squirrel Monkey

Chronic motor learning in the vestibuloocular reflex (VOR) results in changes in the gain of this reflex and in other eye movements intimately associated with VOR behavior, e.g., the velocity storage generated by optokinetic stimulation (OKN velocity storage). The aim of the present study was to identify the plastic sites responsible for the change in OKN velocity storage after chronic VOR motor learning. We studied the neuronal responses of vertical eye movement flocculus target neurons (FTNs) during the optokinetic afternystagmus (OKAN) phase of the optokinetic response (OKR) before and after VOR motor learning. Our findings can be summarized as follows. 1) Chronic VOR motor learning changes the horizontal OKN velocity storage in parallel with changes in VOR gain, whereas the vertical OKN velocity storage is more complex, increasing with VOR gain increases, but not changing following VOR gain decreases. 2) FTNs contain an OKAN signal having opposite directional preferences after chronic high versus low gain learning, suggesting a change in the OKN velocity storage representation of FTNs. 3) Changes in the eye-velocity sensitivity of FTNs during OKAN are correlated with changes in the brain stem head-velocity sensitivity of the same neurons. And 4) these changes in eye-velocity sensitivity of FTNs during OKAN support the new behavior after high gain but not low gain learning. Thus we hypothesize that the changes observed in the OKN velocity storage behavior after chronic learning result from changes in brain stem pathways carrying head velocity and OKN velocity storage information, and that a parallel pathway to vertical FTNs changes its OKN velocity storage representation following low, but not high, gain VOR motor learning.

The origin of downbeat nystagmus: an asymmetry in the distribution of on-directions of vertical gaze-velocity Purkinje cells

Various hypotheses on the origin of cerebellar downbeat nystagmus (DBN) have been presented; the exact pathomechanism, however, is still not known. Based on previous anatomical and electrophysiological studies, we propose that an asymmetry in the distribution of on-directions of vertical gaze-velocity Purkinje cells leads to spontaneous upward ocular drift in cerebellar disease, and therefore, to DBN. Our hypothesis is supported by a computational model for vertical eye movements.

Asymmetric short-term adaptation of the vertical vestibulo-ocular reflex in humans

Anatomical and electrophysiological studies have demonstrated up-down asymmetries in vertical ocular motor pathways. We investigated whether these asymmetries extend to the capacity for short-term adaptation of the vertical vestibulo-ocular reflex (VVOR) in humans. Specifically, we asked whether smooth pursuit signals are sufficient to asymmetrically adapt the VVOR. Healthy human subjects (N=8), positioned 90 degrees left-ear-down and fixating with their eyes upon a small laser dot (diameter: 0.1 degrees) projected on a sphere (distance: 1.4 m) were trained toward low VVOR gain for 30 min with symmetric and asymmetric visual VVOR cancellation paradigms, while being oscillated (0.2 Hz, +/-20 degrees) on a motorized turntable about the interaural earth-vertical axis. During asymmetric VVOR cancellation, the target was head-fixed in either the pitch-up or pitch-down half-cycles of oscillation (= trained direction) and space-fixed during the other half-cycles (= untrained direction). During symmetric VVOR cancellation, the target was head-fixed throughout the oscillations. Before and after adaptation, the pitch-up and pitch-down VOR gains were assessed during turntable oscillation in complete darkness. Before adaptation, average gains of pitch-up (0.75+/-0.15 SD) and pitch-down (0.79+/-0.19 SD) VOR were not significantly different (paired t test: P>0.05). On an average, relative gain reductions induced by selective pitch-up (pitch-up VOR: 32%; pitch-down VOR: 21%) and pitch-down (pitch-up VOR: 18%; pitch-down VOR: 30%) VOR cancellation were significantly (P<0.05) larger in the trained than in the untrained direction. Symmetric visual VVOR cancellation led to a significantly (P<0.01) larger relative gain reduction of the pitch-down (41%) than the pitch-up (33%) VOR. None of the paradigms led to significant changes of phase or offset. We conclude that, in human subjects, the smooth pursuit system is capable to asymmetrically decrease the gain of the VVOR equally well in both the upward and downward direction. The unexpected asymmetric decrease of the VVOR gain after symmetric visual cancellation may be related to the directional preferences of vertical gaze-velocity sensitive Purkinje cells in the flocculus for the downward direction.

The extraocular motor nuclei: organization and functional neuroanatomy

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

Inferior olive and oculomotor system

Three subnuclei within the inferior olive are implicated in the control of eye movement; the dorsal cap (DC), the beta-nucleus and the dorsomedial cell column (DMCC). Each of these subnuclei can be further divided into clusters of cells that encode specific parameters of optokinetic and vestibular stimulation. DC neurons respond to optokinetic stimulation in one of three planes, corresponding to the anatomical planes of the semicircular canals. Neurons in the beta-nucleus and DMCC respond to vestibular stimulation in the planes of the vertical semicircular canals and otoliths. Each these olivary nuclei receives excitatory and inhibitory signals from pre-olivary structures. The DC receives excitatory signals from the ipsilateral nucleus of the optic tract (NOT) and inhibitory signals from the contralateral nucleus prepositus hypoglossi (NPH). The beta-nucleus and DMCC receive inhibitory signals from the ipsilateral nucleus parasolitarius (Psol) and excitatory signals from the contralateral dorsal Y group. Consequently, the olivary projection to the cerebellum, although totally crossed, still represents bilateral sensory stimulation. Inputs to the inferior olive from the NOT, NPH, Psol or Y-group discharge at frequencies of 10-100 imp/s. CFRs discharge at 1-5 imp/s; a frequency reduction of an order of magnitude. Inferior olivary projections to the contralateral cerebellum are sagittally arrayed onto multiple cerebellar folia. These arrays establish coordinate systems in the flocculus and nodulus, representing head-body movement. These climbing fiber-defined spatial coordinate systems align Purkinje cell discharge onto subjacent cerebellar and vestibular nuclei. In the oculomotor system, olivo-cerebellar circuitry enhances and modifies eye movements based on movement of the head-body in space.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Chronic changes in inputs to dorsal Y neurons accompany VOR motor learning

Gain changes in the vestibuloocular reflex (VOR) during visual-vestibular mismatch stimulation serve as a model system for motor learning. The cerebellar flocculus and its target neurons in the brain stem (FTN) are candidates for the storage of these novel VOR gains. We have recently studied the changes in vertical flocculus Purkinje cells after chronic VOR motor learning. Recently we recorded Y neurons (a vertical type of FTNs) after chronic VOR motor learning and compared these records with vertical floccular Purkinje cells to document any changes in inputs to FTNs and understand how Y neurons and the vertical Purkinje cells fit into a general model for the vertical VOR. Analysis illustrates that the changes observed in Purkinje cells are not transferred to Y neurons, suggesting that the gain of their synaptic interconnection was modified. We quantified changes in both populations and employed simulations to study changes in parallel pathways to FTNs and to extract the role of the flocculus in VOR adaptation. Low-gain adaptation results in more drastic changes than its high-gain counterpart, causing increases in head velocity sensitivity in parallel pathways. Simulations suggest that cerebellar and brain stem plasticity both participate in novel VOR gain storage and that results obtained following floccular lesion are the product of different mechanisms than those operating in the intact animal.

Otolith and canal integration on single vestibular neurons in cats

In this review, based primarily on work from our laboratory, but related to previous studies, we summarize what is known about the convergence of vestibular afferent inputs onto single vestibular neurons activated by selective stimulation of individual vestibular nerve branches. Horizontal semicircular canal (HC), anterior semicircular canal (AC), posterior semicircular canal (PC), utricular (UT), and saccular (SAC) nerves were selectively stimulated in decerebrate cats. All recorded neurons were classified as either projection neurons, which consisted of vestibulospinal (VS), vestibulo-oculospinal (VOS), vestibulo-ocular (VO) neurons, or non-projection neurons, which we simply term "vestibular'' (V) neurons. The first three types could be successfully activated antidromically from oculomotor/trochlear nuclei and/or spinal cord, and the last type could not be activated antidromically from either site. A total of 1228 neurons were activated by stimulation of various nerve pair combinations. Convergent neurons were located in the caudoventral part of the lateral, the rostral part of the descending, and the medial vestibular nuclei. Otolith-activated vestibular neurons in the superior vestibular nucleus were extremely rare. A high percentage of neurons received excitatory inputs from two nerve pairs, a small percentage received reciprocal convergent inputs and even fewer received inhibitory inputs from both nerves. More than 30% of vestibular neurons received convergent inputs from vertical semicircular canal/otolith nerve pairs. In contrast, only half as many received convergent inputs from HC/otolith-nerve pairs, implying that convergent input from vertical semicircular canal and otolith-nerve pairs may play a more important role than that played by inputs from horizontal semicircular canal and otolith-nerve pairs. Convergent VS neurons projected through the ipsilateral lateral vestibulospinal tract (i-LVST) and the medial vestibulospinal tract (MVST). Almost all the VOS neurons projected through the MVST. Convergent neurons projecting to the oculomotor/trochlear nuclei were much fewer in number than those projecting to the spinal cord. Some of the convergent neurons that receive both canal and otolith input may contribute to the short-latency pathway of the vestibulocollic reflex. The functional significance of these convergences is discussed.

Cerebellar climbing fibers modulate simple spikes in Purkinje cells

Purkinje cells have two action potentials: Climbing fiber responses (CFRs) and simple spikes (SSs). CFRs reflect the discharge of a single climbing fiber at multiple synaptic sites on the proximal dendrite of the Purkinje cell. SSs reflect the summed action of a subset of parallel fiber synapses on Purkinje cell dendritic spines. Because mossy fiber afferents terminate on granule cells, the ascending axons of which bifurcate, giving rise to parallel fibers, the modulation of SSs has been attributed to mossy fiber afferent signals. This inference has never been tested. Conversely, the low discharge frequency of CFRs has led many to conclude that they have a unique and intermittent role in cerebellar signal processing. We examine the relative potency of vestibularly modulated mossy fiber and climbing fiber signals in evoking CFRs and SSs in Purkinje cells of the uvula-nodulus in chloralose-urethane-anesthetized rabbits. Vestibular primary afferents were blocked by unilateral labyrinthectomy (UL). A UL destroys the vestibular primary afferent signal to the ipsilateral uvula-nodulus, while leaving intact the vestibular climbing fiber signal from the contralateral inferior olive. After UL, vestibular stimulation modulated CFRs and SSs in ipsilateral uvula-nodular Purkinje cells, demonstrating that the primary vestibular afferent mossy fiber input to the ipsilateral uvula-nodulus was not necessary for SS modulation. Unilateral microlesions of the caudal half of the beta-nucleus of the inferior olive reduced a modulated climbing fiber signal to the contralateral uvula-nodulus, causing loss of both vestibularly modulated CFRs and SSs in contralateral Purkinje cells. Vestibular climbing fibers not only evoke low-frequency CFRs, but also indirectly modulate higher-frequency SSs. This modulation must be attributed to cerebellar interneurons. Golgi cell inhibition of granule cells may provide the interneuronal mechanism for CFR-induced SS modulation.

Central vestibular system: vestibular nuclei and posterior cerebellum

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between "active" and "passive" movements. The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and beta-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

Response properties of Y group neurons to crossed otolith inputs in the cat

The response properties of extracellularly recorded Y group neurons on the lesioned side were examined in decerebrate cats after acute hemilabyrinthectomy, with the use of constant velocity off-vertical axis rotations (OVAR) to stimulate the remaining intact otolith receptors. During rotation in the clockwise or counterclockwise direction, Y group neurons displayed a spectrum of position-dependent bidirectional response sensitivities, ranging from one- to two-dimensional. Some two-dimensional neurons even exhibited unidirectional responses with change in OVAR velocity. These findings indicate that Y group neurons have the capacity to code spatiotemporal signals arising from the contralateral otolith. The best response orientations of one-dimensional and two-dimensional neurons were found predominantly along the antero-posterior direction, thus providing a spatial framework for the otolithic reflexes.

Vestibularly evoked climbing-fiber responses modulate simple spikes in rabbit cerebellar Purkinje neurons

The nodulus receives a primary vestibular afferent input from the ipsilateral labyrinth and a vestibularly related climbing-fiber input originating from the contralateral labyrinth. Previously we demonstrated that increased discharge of vestibularly evoked climbing-fiber responses (CFRs) in nodular Purkinje cells was correlated with decreased discharge of simple spikes (SSs). This left unresolved the question of whether vestibularly evoked antiphasic behavior of CFRs and SSs reflects a common neural mechanism or the activation of two separate parallel pathways. We answered this question using natural vestibular stimulation to modulate the discharge of uvula-nodular Purkinje cells recorded extracellularly in unilaterally labyrinthectomized, chloralose urethane-anesthetized rabbits. In such animals, vestibular primary afferents projecting to the uvula-nodulus as mossy fibers remained intact on the side contralateral to the unilateral labyrinthectomy. The discharge of CFRs recorded in ipsilateral nodular Purkinje cells was increased by ipsilateral roll-tilt while the discharge of SSs was increased by contralateral roll-tilt. These polarities were reversed for Purkinje cells recorded in the contralateral uvula-nodulus. The polarity of SS discharge recorded from Purkinje cells on both sides of the nodulus was opposite to that of the vestibular primary mossy-fiber afferents. SSs continued to respond to contralateral roll-tilt even when the primary vestibular afferent mossy-fiber pathway was destroyed by the unilateral labyrinthectomy. Although the discharge of SSs recorded in the contralateral uvula-nodulus was increased by contralateral roll-tilt, this modulation was reduced relative to that observed in Purkinje cells recorded in the ipsilateral uvula-nodulus. We conclude that vestibularly evoked CFRs caused the modulation of SS discharge.

Capacity of vertical VOR adaptation in squirrel monkey

Squirrel monkeys were trained using newly developed visual-vestibular mismatch paradigms to test the asymmetrical simultaneous induction of vertical vestibuloocular reflex (VOR) gain changes in opposite directions (high and low) either in the upward and downward directions or in response to high- and low-frequency stimuli. The first paradigm consists of sinusoidal head movement [A sin(omegat)] and a full rectified sinusoidal optokinetic stimulus [+/-|A sin(omegat)|], whereas the second paradigm consists of the sum of two sinusoids with different frequencies [A sin(omega(1)t) + A sin(omega(2)t) for head motion and +/-[A sin(omega(1)t) - A sin(omega(2)t)] for the optokinetic stimulus, omega(1) = 0.1pi, omega(2) = 5pi]. The first paradigm induced a half rectified sinusoidal eye-velocity trace, i.e., suppression of the VOR during upward head motion and enhancement during downward head motion or vise versa, whereas the second paradigm induced suppression of the VOR at the low-frequency omega(1) and enhancement at the high-frequency omega(2) or vise versa. After 4 h of exposure to these paradigms, VOR gains of up and down or high and low frequency were modified in opposite directions. We conclude that the monkey vertical VOR system is capable of up-down directionally differential adaptation as well as high-low frequency differential adaptation. However, experiments also suggest that these gain controls are not completely independent because the magnitudes of the gain changes during simultaneous asymmetrical training were less than those achieved by symmetrical training or training in only one of the two components, indicating an influence of the gain controls on each other. These results confine the adaptive site(s) responsible for vertical VOR motor learning to those that can process up and downward or low- and high-frequency head signal separately but not completely independently.

Acute Adaptation of the Vestibuloocular Reflex: Signal Processing by Floccular and Ventral Parafloccular Purkinje Cells

The gain of the vertical vestibuloocular reflex (VVOR), defined as eye velocity/head velocity was adapted in squirrel monkeys by employing visual-vestibular mismatch stimuli. VVOR gain, measured in the dark, could be trained to values between 0.4 and 1.5. Single-unit activity of vertical zone Purkinje cells was recorded from the flocculus and ventral paraflocculus in alert squirrel monkeys before and during the gain change training. Our goal was to evaluate the site(s) of learning of the gain change. To aid in the evaluation, a model of the vertical optokinetic reflex (VOKR) and VVOR was constructed consisting of floccular and nonfloccular systems divided into subsystems based on the known anatomy and input and output parameters. Three kinds of input to floccular Purkinje cells via mossy fibers were explicitly described, namely vestibular, visual (retinal slip), and efference copy of eye movement. The characteristics of each subsystem (gain and phase) were identified at different VOR gains by reconstructing single-unit activity of Purkinje cells during VOKR and VVOR with multiple linear regression models consisting of sensory input and motor output signals. Model adequacy was checked by evaluating the residual following the regressions and by predicting Purkinje cells' activity during visual-vestibular mismatch paradigms. As a result, parallel changes in identified characteristics with VVOR adaptation were found in the prefloccular/floccular subsystem that conveys vestibular signals and in the nonfloccular subsystem that conveys vestibular signals, while no change was found in other subsystems, namely prefloccular/floccular subsystems conveying efference copy or visual signals, nonfloccular subsystem conveying visual signals, and postfloccular subsystem transforming Purkinje cell activity to eye movements. The result suggests multiple sites for VVOR motor learning including both flocculus and nonflocculus pathways. The gain change in the nonfloccular vestibular subsystem was in the correct direction to cause VOR gain adaptation while the change in the prefloccular/floccular vestibular subsystem was incorrect (anti-compensatory). This apparent incorrect directional change might serve to prevent instability of the VOR caused by positive feedback via the efference copy pathway.