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PubMed1995

A case with symptoms of vestibular neuronitis caused by an intramedullary lesion.

We report on a patient with an intramedullary lesion who showed only symptoms of so-called vestibular neuronitis. From the neurological aspects and MRI findings, the lesion was thought to be localized to the vestibular nerve in the pons. Cerebrospinal fluid testing and MRI findings suggested a diagnosis of multiple sclerosis (MS), although the clinical features were not compatible with MS. This case may shed some light on the aetiology of vestibular neuronitis.

Adolescent↗
PubMed1994

Pattern of trkB protein-like immunoreactivity in vivo and the in vitro effects of brain-derived neurotrophic factor (BDNF) on developing cochlear and vestibular neurons.

The cochleo-vestibular ganglion (CVG) contains the neurons connecting the sensory epithelia of the inner ear to the cochlear and vestibular nuclei in the medulla. Expression of trkB protein-like immunoreactivity was studied in the developing CVG, using both Western blot and immunocytochemistry on tissue sections. Specific immunoreactivity was observed in the CVG from the 12th gestation day (gd) to the first postnatal week, reflecting the presence of high-affinity receptors for brain-derived neurotrophic factor (BDNF), a member of the NGF family of neurotrophins. Whole explants and dissociated cell cultures of cochlear (CG) and vestibular ganglion (VG) from mouse embryos and postnatal specimens were grown in neurotrophin-free medium to assay changes in neurite outgrowth and neuronal survival in response to the addition of physiological concentrations (0-5 ng/ml) of BDNF. Exogenous BDNF (2 ng/ml) promoted neurite outgrowth and neuronal survival in explants of both CG and VG, and the effects were stage-dependent. The onset of the response to BDNF occurred at gd 11-12. The response then reached a maximum between 14 and 18 gd and subsequently decreased, although it remained significantly present during the first postnatal week. BDNF-induced response was no longer observed in the mature cochlear and vestibular ganglion (after 30 postnatal days). The effects of BDNF on neuronal differentiation and survival were dose-dependent, starting at 0.5 ng/ml, with saturation at 2 ng/ml and half-maximal effect occurring between 1 and 1.5 ng/ml. On the basis of our results, we propose that BDNF may be physiologically involved in the control of both neuronal differentiation, and central and peripheral target-dependent neuronal death, in the CVG of embryos and early postnatal mice. BDNF may act alone or in cooperation with other neurotrophins to establish the afferent innervation of the inner ear sensory epithelium.

Animals↗
PubMed1976

Transfer characteristics of first and second order lateral canal vestibular neurons in gerbil.

Discharge patterns of first and second order vestibular neurons responding to angular acceleration in the plane of the lateral canals were studied in gerbil. The resting discharge activity of each cell was used to characterize the neuron by measureing the coefficient of variation and coefficient of skewness of the interspike interval distributions. Sinusoidal angular oscillations ranging in frequency from 0.0125 to 5.0 Hz were delivered by a velocity controlled rate-table. A PDP-12 minicomputer system was used on-line to display period and post-stimulus histograms of discriminated single unit activity. Off-line Fourier analysis of the period histograms was used to determine the phase of cell response to sinusoidal accelerations, while the average level and amplitude were determined by a least squares fitting algorithm applied over the fraction of the stimulus period where the cell discharged. First order neurons were found to have high discharge rates (average = 61.7 imp./sec) and bidirectional responses to rotation, and were of two groups called regular and irregular according to their resting discharge patterns. Second order neurons, located mainly in the medial and lateral vestibular nuclei, had low or even zero resting discharge rates (average = 17.8) resulting in more uni-directional responses and were of a single population. For frequencies less than 10 Hz, the Bode plots of the regular first order neurons are similar to that of a first order system with a time constant of about 2 sec as predicted by the torsion pendulum theory for cupula movement. The irregular first order neurons show an increasing gain above 0.5 Hz and a large phase lead relative to angular velocity above 1.0 Hz suggestive of a fractional power transfer function. The second order neurons show the phase and gain characteristics of the regular first order neurons being in phase with angular velocity above 1.0 Hz.

Anesthesia↗
PubMed1991

Responses and anatomical connections of peripheral vestibular neurons innervating otolith organs.

The response patterns of the Scarpa's neurons and lateral vestibular nucleus (LVN) neurons that respond to lateral and antero-posterior tilting were investigated using cats. The following observations were made: 1) The Scarpa's neurons that respond to lateral tilting have directional sensitivity physiologically and may innervate the utricular macula; 2) The LVN neurons that respond to lateral tilting are found mostly in the ventral part of the LVN and also have directional sensitivity. They project toward the spinal cord into the lateral vestibulo-spinal tract.

Acoustic Maculae↗
PubMed2002

Convergence of the anterior semicircular canal and otolith afferents on cat single vestibular neurons.

The convergence between the anterior semicircular canal (AC) and utricular (UT) inputs, as well as the convergence between the AC and saccular (SAC) inputs in single vestibular neurons of decerebrated cats were investigated. Postsynaptic potentials were recorded intracellularly after selective stimulation of each pair of vestibular nerves AC/UT or AC/SAC. Neurons were recorded from the central parts of the vestibular nuclei, where the otolith afferents mainly terminate. Of a total of 105 neurons that were activated after stimulation of the AC and UT nerves, 42 received convergent inputs. Thirty-eight of these neurons received excitatory inputs from both afferents. Convergent neurons were further classified into vestibulospinal (n=28) and vestibulooculospinal (n=6) neurons by antidromic activation from the border between the C1 and C2 spinal cord and the oculomotor or trochlear nucleus. Eight neurons that were not antidromically activated from either site were classified as vestibular neurons. Forty three percent of the convergent vestibulospinal neurons and most of the convergent vestibulooculospinal neurons projected to the spinal cord through the medial vestibulospinal tract. The remaining vestibulospinal and vestibulooculospinal neurons descended through the ipsilateral lateral vestibulospinal tract. Of a total of 118 neurons that were activated after stimulation of the AC and/or SAC nerves, 51 received convergent inputs (27 vestibulospinal, 4 vestibulooculospinal, 5 vestibuloocular and 15 vestibular neurons). Forty-two of the convergent neurons received excitatory inputs from both afferents. Thirty seven percent of the convergent vestibulospinal neurons and all of the convergent vestibulooculospinal neurons projected to the spinal cord through the medial vestibulospinal tract. The remaining vestibulospinal and vestibulooculospinal neurons descended through the ipsilateral lateral vestibulospinal tract.

Afferent Pathways↗
PubMed1987

Pharmacological aspects of excitatory synaptic transmission to second-order vestibular neurons in the frog.

Synaptic excitation of second-order vestibular neurons is mediated by two principal afferents: vestibular afferents projecting into the brain via the VIIIth cranial nerve and commissural afferents from the contralateral vestibular nuclear complex. The shape of the excitatory postsynaptic potentials (EPSPs) generated by selectively activating these two inputs differs qualitatively, such that ipsilateral VIIIth nerve afferents generate a faster-rising EPSP than do the commissural afferents. We have investigated the synaptic pharmacology of these two inputs in the isolated, intact medulla of the frog in order to determine the nature of the transmitter substances released by the afferents and the nature of the subsynaptic receptors with which these transmitters interact. Electrical stimulation of the ipsilateral VIIIth cranial nerve evokes in the region of the vestibular nuclear complex a field potential that exhibits a presynaptic (afferent volley) and a postsynaptic (slow negativity) component. Bath application of glutamate receptor antagonists, such as kynurenic acid (KENYA), blocks the postsynaptic component of this field potential in a dose-dependent manner, without affecting the presynaptic volley, suggesting that the VIIIth nerve afferent releases glutamate and/or similar substances as its neurotransmitter. A comparison of the actions of various glutamate receptor antagonists to block this postsynaptic negativity gives a rank order of effectiveness such that KENYA greater than gamma-D-glutamylglycine (gamma DGG) = gamma-D-glutamylaminomethylsulfonic acid (GAMS) greater than gamma-D-glutamyltaurine (gamma DGT) much greater than gamma-D-glutamylaminomethylphosphonic acid (GAMP) greater than D-2-amino-5-phosphonovaleric acid (D-APV) greater than D,L-APV greater than D-2-amino-7-phosphonoheptanoic acid (APH). This rank order of effectiveness suggests that the VIIIth nerve transmitter activates second-order neurons through kainate (KA)/quisqualate (QUIS) synaptic receptors. Intracellular studies support these conclusions. Chemically mediated EPSPs evoked from ipsilateral VIIIth nerve stimulation are completely blocked by high concentrations of KENYA (greater than or equal to 1 mM). Occasionally an extremely short-latency, probably electrically mediated, component to these EPSPs persists in the presence of KENYA. The slower-rising EPSPs evoked from contralateral VIIIth nerve or contralateral vestibular nuclear complex stimulation are also completely blocked by KENYA, suggesting that the transmitter released by the commissural afferents is also glutamate and/or related compounds.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2004

Asymmetric responses to rotation at high frequencies in central vestibular neurons of the alert cat.

The horizontal rotatory vestibulo-ocular reflex (VOR) stabilizes gaze by moving the eyes at an angular velocity proportional to head velocity, and can accomplish this for a broad range of frequencies and amplitudes of head motion. Rotation at 5 Hz and above may be processed differently than lower frequencies by the VOR network. We recorded discharges and calculated spike densities of a small sample of vestibular neurons in alert cats during low-velocity rotation at frequencies up to 8 Hz. At high frequencies, we found both vestibular-only (V-only) and eye-movement-sensitive (EM) cells that generated asymmetric output signals. Asymmetry was primarily of the cutoff type, i.e., changes in spike density were smallest for rotation in the inhibitory direction. Most cells were identified as secondary neurons. The mean spike density was 23 sp/s, which was lower than previously reported in vestibular neurons of monkeys. A few neurons had very high sensitivities, associated with phase-locking, to rotation at high frequencies. In general, vestibular neurons carried a high-pass-filtered version of rotational signals. When synaptic inputs from the vestibular commissure were quantified, we found that the immediate change in probability of firing due to commissural vestibular input was inversely correlated with the degree of high-pass filtering. At high frequencies, increased asymmetry and phase-locking occurred in some neurons. A small number of neurons responded with increased probability of firing to both directions of rotation. Together, these observations suggest that high frequencies of rotation may be encoded differently than low frequencies by central vestibular neurons in alert animals.

Action Potentials↗
PubMed1985

The efferent vestibular neurons in the toad (Bufo bufo L.): their location and morphology. A horseradish peroxidase study.

Retrogradely labeled neurons are observed in the central nervous system of the toad after peripheral application of peroxidase to the anterior and posterior stumps of the VIIIth nerve. These efferent vestibular neurons are localized in the brainstem only ipsilaterally to the treated nerve; they are restricted within a region close to the motor nucleus of the VIIth nerve, outside the vestibular nuclear complex, and are predominantly localized to the borders between the gray and white matter in an arrangement that seems to surround the motor nucleus of the VIIth nerve. No evidence was found for the existence of labeled Purkinje cells in the cerebellar cortex and therefore of a cerebellolabyrinthine pathway. The efferent vestibular neurons are medium-sized cells with two prominent dendrites, often oriented in a mediolateral direction. A comparative analysis between these neurons and the contiguous motoneurons of the VIIth nerve permitted differentiation of the two groups of neurons as to: distribution in a mediolateral direction, distribution in depth from the brainstem surface, longitudinal extension in the caudorostral direction and morphologic characteristics and dendritic arrangement of the neurons.

Animals↗
PubMed1998

The relationship between rhombomeres and vestibular neuron populations as assessed in quail-chicken chimeras.

The aim of this study was to evaluate the role segmentation plays in the determination of neuronal identity in the hindbrain. We focused on two specific sets of hindbrain neurons, namely, the vestibulospinal and vestibulo-ocular neurons, which comprise distinct groups that can be identified and distinguished by virtue of their axonal projection pathways. The relationship between rhombomeres and the vestibular neuron groups was assessed by a combination of quail-chicken chimeric grafting and selective retrograde axonal tracing. Individual quail hemirhombomeres were transplanted homotopically and isochronically into a chicken embryo host. Subsequently, vestibulospinal and vestibulo-ocular neurons with specific axon trajectories were labeled retrogradely with biotin-conjugated dextran-amines. The relationship between the spatial domains of the vestibular neuron groups and rhombomere-derived domains had the following features: (1) some groups were derived from single rhombomeres; (2) some groups were derived from multiple contiguous rhombomeres; (3) two groups occupied domains that could not be defined in terms of whole rhombomere lengths; (4) some groups spanning multiple rhombomeres exhibited an internal cytoarchitectonic organization that related to individual rhombomeres; and (5) some groups exhibited limited boundary violation. These results support the notion that positional information within defined domains of the neural tube provides a groundplan for the regional determination of neuronal identity and axon pathfinding, and that hindbrain segmentation contributes to this process. But they also indicate that segmentation is not the only mechanism that defines the rostrocaudal domains of neuron types. Moreover, they emphasize that the relationship between rhombomeres and neuronal determination cannot be couched simply in terms of segmental iteration or of bimeric (paired rule) specification.

Animals↗
PubMed1987

Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflexes of the squirrel monkey.

The morphology of 35 vestibular neurons whose firing rate was related to vertical eye movements was studied by injection of horseradish peroxidase intracellularly into physiologically identified vestibular axons in alert squirrel monkeys. The intracellularly injected cells were readily classified into four main groups. One group of cells, down position-vestibular-pause neurons (down PVPs; N = 12), increased their firing rate during downward eye positions, paused during saccades, and were located in the medial vestibular nucleus (MV) and the adjacent ventrolateral vestibular nucleus (VLV). They had axons that crossed the midline and ascended in the medial longitudinal fasciculus (MLF) to terminate in the trochlear nucleus, the lateral aspect of the caudal oculomotor nucleus, and the dorsal aspect of the rostral oculomotor nucleus. A second group of cells (N = 15) were also located in the MV and VLV, but increased their firing rate during upward eye positions, and paused during saccades. These cells had axons that crossed the midline and ascended in the contralateral MLF to terminate in the medial aspect of the oculomotor nucleus. A third group of cells (N = 4) were located in the superior vestibular nucleus, generated bursts of spikes during upward saccades, and increased their tonic firing rate during upward eye positions. These cells had axons that ascended laterally to the ipsilateral MLF to terminate in regions of the trochlear and oculomotor nuclei similar to those in which down PVPs terminated. A fourth group of cells (N = 4), located in the VLV, had axons that projected to the spinal cord, although they had firing rates that were significantly correlated with vertical eye position. Electrical stimulation of the vestibular nerve evoked spikes at monosynaptic latencies in each of the above classes of cells, six of which were injected with horseradish peroxidase. Each group of cells had collateral projections to other areas of the brainstem. Some of the neurons that projected to the contralateral trochlear and oculomotor nuclei had collaterals that crossed the midline to terminate in the oculomotor nucleus ipsilateral to the soma, and some gave rise to small collaterals that terminated in the abducens nucleus. Other areas of the brainstem that received collateral inputs from neurons projecting to oculomotor and trochlear nuclei included the interstitial nucleus of Cajal, the caudal part of the dorsal raphe nucleus, the nucleus raphe obscurus, Roller's nucleus, the intermediate and caudal interstitial nuclei of the MLF, and the nucleus prepositus.

Animals↗
PubMed1986

Morphology of vertical canal related second order vestibular neurons in the cat.

The morphology of vertical canal related second order vestibular neurons in the cat was studied with the intracellular horseradish peroxidase method. Neurons were identified by their monosynaptic potentials following electrical stimulation via bipolar electrodes implanted into individual semicircular canal ampullae. Anterior and posterior canal neurons projected primarily to contralateral or ipsilateral motoneuron pools (excitatory and inhibitory pathways, respectively). The axons of contralaterally projecting neurons crossed the midline at the level of the abducens nucleus and bifurcated into an ascending and a descending main branch which travelled in the medial longitudinal fasciculus (MLF). Two types of anterior canal neurons were observed, one with unilateral and one with bilateral oculomotor projection sites. For both neuron classes, the major termination sites were in the contralateral superior rectus and inferior oblique subdivisions of the oculomotor nucleus. In neurons which terminated bilaterally, major collaterals recrossed the midline within the oculomotor nucleus to reach the ipsilateral superior rectus motoneuron pool. Other, less extensive, termination sites of both neuron classes were in the contralateral vestibular nuclear complex, the facial nucleus, the medullary and pontine reticular formation, midline areas within and neighboring the raphé nuclei, and the trochlear nucleus. The ascending main axons continued further rostrally to reach the interstitial nucleus of Cajal and areas around the fasciculus retroflexus. The descending branches proceeded further caudal in the medial vestibulo-spinal tract but were not followed to their spinal target areas. In addition to two previously described posterior canal related neuron types (Graf et al. 1983), we found neurons with bilateral oculomotor terminals and a spinal collateral. Typical for posterior canal neurons, the major termination sites were in the trochlear nucleus (superior oblique motoneurons) and in the inferior rectus subdivision of the oculomotor nucleus. Axon collaterals recrossed the midline to reach ipsilateral inferior rectus motoneurons. The axons of ipsilaterally projecting neurons ascended through the reticular formation to join the MLF caudal to the trochlear nucleus. The main target sites of anterior canal related neurons were in the trochlear nucleus and the inferior rectus subdivision of the oculomotor nucleus. Minor collaterals reached the pontine reticular formation and areas in between the fiber bundles of the ipsilateral MLF.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗
PubMed2005

The involvement of Cav3.2/alpha1H T-type calcium channels in excitability of mouse embryonic primary vestibular neurones.

Ca2+ influx through voltage-gated calcium channels probably influences neuronal ontogenesis. Many developing neurones transiently express T-type/Cav3 calcium channels that contribute to their electrical activity and potentially to their morphological differentiation. Here we have characterized the electrophysiological properties and the functional role of a large T-type calcium current that is present in mouse developing primary vestibular neurones at embryonic day E17. This T-type current showed fast activation and inactivation, as well as slow deactivation kinetics. The overlap of activation and inactivation parameters produced a window current between -65 and -45 mV. Recovery from short-term inactivation was slow suggesting the presence of the Cav3.2 subunit. This T-type current was blocked by micromolar concentrations of Ni2+ and was inhibited by fast perfusion velocities in a similar fashion to recombinant Cav3.2 T-type channels expressed in HEK-293 cells. More importantly, current clamp experiments have revealed that the T-current could elicit afterdepolarization potentials during the repolarization phase of action potentials, and occasionally generate calcium spikes. Taken together, we demonstrate that the Cav3.2 subunit is likely to be the main T-type calcium channel subunit expressed in embryonic vestibular neurones and should play a key role in the excitability of these neurones during the ontogenesis of vestibular afferentation.

Action Potentials↗
PubMed1993

Clinical observation of 32 cases of vestibular neuronitis.

Clinical observations were made on 32 patients with vestibular neuronitis in the last 5 years: 14 male and 18 female, ranging from 15-76 years, the average being 46.3. Within the age distribution of the cases, two peaks were observed: a younger and an older group. Five patients (15%) had recently contracted infection of the upper respiratory tract before the onset of the disease, and two developed benign paroxysmal positional vertigo during the recovery period. Spontaneous nystagmus was observed in 19 cases (59%) in the first medical examination. There were several patients in whom there was the possibility of central lesions based upon the findings of equilibrium examinations. No relation was found between the degree of canal paresis and the time required for recuperation; however, the older patients took much longer to recover from the disease. Our results showed that there were considerable individual variations regarding the clinical findings, which may suggest that the pathological entity that creates this disease is not simple.

Adolescent↗
PubMed2003

Commissural inputs to secondary vestibular neurons in alert cats after canal plugs.

Gaze is stabilized during head movements primarily by the vestibuloocular reflex (VOR). After a unilateral canal plug, the VOR's response is reduced. Recovery of the VOR may be brought about by changes in the efficacy of brain stem synapses or by other mechanisms. We measured the responses of horizontal secondary vestibular neurons (HSNs) to stimulation of the contralateral labyrinth. HSN responses in normal alert cats were compared with those in cats that had recovered from unilateral horizontal semicircular canal (HSCC) plugs. After recovery, excitatory commissural inputs to HSNs on the plugged side elicited significantly smaller responses than in normal cats with no change in mean discharge rates. However, mean discharge rates tended to be higher after recovery for cells receiving inhibitory commissural inputs. The change in resting rate invalidates any direct comparison of inhibitory inputs. These results are interpreted in terms of possible mechanisms for recovery from unilateral vestibular loss by the VOR neural network. We conclude that after unilateral HSCC plugs, changes in brain stem excitatory synapses and/or excitability of secondary vestibular neurons may participate in the restoration of normal vestibular reflexes.

Action Potentials↗
PubMed1993

Responses of vestibular neurons to stimulation of cortical sensorimotor areas in the cat.

Selected areas of sensorimotor cortex were stimulated with short trains adequate to evoke focal movements. Neurons belonging to the vestibular nuclear complex, including the lateral vestibular nucleus (LVN), were mainly affected by the stimulation of wrist-elbow, shoulder and cortical areas. The units responded to cortical stimulation either with a constant latency lower than 10 msec (short-latency units), or with variable latencies higher than 10 msec (long-latency units). Particular attention was paid to the first group of responses showing the involvement of direct and/or oligosynaptic facilitatory inputs from the cortical cells. Moreover, many of these responsive vestibular nuclear neurons were identified as projecting to the spinal cord.

Anesthesia↗
PubMed1993

Encoding of head acceleration in vestibular neurons. I. Spatiotemporal response properties to linear acceleration.

1. Extracellular recordings were made in and around the medial vestibular nuclei in decerebrated rats. Neurons were functionally identified according to their semicircular canal input on the basis of their responses to angular head rotations around the yaw, pitch, and roll head axes. Those cells responding to angular acceleration were classified as either horizontal semicircular canal-related (HC) or vertical semicircular canal-related (VC) neurons. The HC neurons were further characterized as either type I or type II, depending on the direction of rotation producing excitation. Cells that lacked a response to angular head acceleration, but exhibited sensitivity to a change in head position, were classified as purely otolith organ-related (OTO) neurons. All vestibular neurons were then tested for their response to sinusoidal linear translation in the horizontal head plane. 2. Convergence of macular and canal inputs onto central vestibular nuclei neurons occurred in 73% of the type I HC, 79% of the type II HC, and 86% of the VC neurons. Out of the 223 neurons identified as receiving macular input, 94 neurons were further studied, and their spatiotemporal response properties to sinusoidal stimulation with pure linear acceleration were quantified. Data were obtained from 33 type I HC, 22 type II HC, 22 VC, and 17 OTO neurons. 3. For each neuron the angle of the translational stimulus vector was varied by 15, 30, or 45 degrees increments in the horizontal head plane. In all tested neurons, a direction of maximum sensitivity was identified. An interesting difference among neurons was their response to translation along the direction perpendicular to that that produced the maximum response ("null" direction). For the majority of neurons tested, it was possible to evoke a nonzero response during stimulation along the null direction always had response phases that varied as a function of stimulus direction. 4. These spatiotemporal response properties were quantified in two independent ways. First, the data were evaluated on the basis of the traditional one-dimensional principle governed by the "cosine gain rule" and constant response phase at different stimulus orientations. Second, the response gain and phase values that were empirically determined for each orientation of the applied linear stimulus vector were fitted on the basis of a newly developed formalism that treats neuronal responses as exhibiting two-dimensional spatial sensitivity. Thus two response vectors were determined for each neuron on the basis of its response gain and phase at different stimulus directions in the horizontal head plane.(ABSTRACT TRUNCATED AT 400 WORDS)

Acceleration↗
PubMed2000

[The role of the thalamic ventrolateral nucleus in the cortical effect on the activity of vestibular neurons].

Electrocoagulation of lateral vestibular nucleus (NVL) reduces inhibitory effect of the motor and somatosensory areas and enhances the inhibitory effect of limbic, vestibular, and orbital cortical areas. Facilitating effect was enhanced by electrostimulation of the motor area and reduced by the stimulation of other cortical areas. Following the coagulation of the NVL, the ascending afferent flow to the cortex seems to be reduced. This results in diminishing of the cortical neurones tone and readjusts the descending influences upon the NVL neurones activity.

Animals↗
PubMed1998

Timing of secondary vestibular neuron responses to a range of rotational head movements.

Secondary vestibular neurons exhibit a wide variety of responses to a head movement, with the response of each secondary neuron depending upon the particular primary afferents converging onto it. A single head movement is thereby registered in a distributed manner. This paper focuses on implications of afferent convergence to the relative timing of secondary neuron response modulation during rotational movements about a combination of horizontal axes. In particular, the neurons of interest are those that receive input from afferents innervating the vertical semicircular canals, and the movements of interest are those that have a sinusoidal component about one vertical canal axis and a sinusoidal component about another, approximately orthogonal, vertical canal axis. Under these conditions, the present research shows that it is possible for two or more secondary neurons to have a different relative timing of response (i.e., different relative phase of the periodic modulation in firing rate) for different head movements, and for the neurons to switch their order of response for different movements. For particular head movements, those same neurons will respond in phase. From the point of view of the nervous system, the relative timing of neuron responses may tell which movement is taking place, but with certain restrictions as discussed in the present paper. Shown here is that, among those head movements for which the two components of rotation may be at any phase relative to one another and have any relative amplitude, an in-phase response of just two neurons cannot identify a single motion. Two neurons that respond in phase for one motion must respond in phase for an entire range of motions; all motions in that range are thus response-equivalent, in the sense that the pair of neurons cannot distinguish between the two motions. On the other hand, an in-phase response of three neurons can identify a single motion, for certain patterns of primary afferent convergence.

Afferent Pathways↗