Pathways mediating optokinetic responses of vestibular nucleus neurons in the rat

1. The effects of various brain lesions on the responses of vestibular nuclear neurons (Vn) of the horizontal semicircular canal system to optokinetic stimulations were studied to elucidate the optokinetic path from the retina to the vestibular nuclei. A previous study performed in intact rats served as a control [2]. 2. It was shown that the pretectal region including the n. of the optic tract is the first central relay in the optokinetic path; it receives its functionally effective input from the contralateral eye. Unilateral lesions of this area rendered all Vn responses unidirectional when tested with binocular stimulation. Lesions of other visual centers such as the superior colliculi or visual cortices had no influence on the optokinetic response properties of Vn. 3. The area of the n. reticularis tegmenti pontis (NRTP) proved to be an important link between pretectum and vestibular nuclei: Unilateral lesions produced effects similar to those described for pretectal lesions. Pretectal axons to NRTP descend lateral to the MLF and tectospinal tract. 4. It was demonstrated that the vestibular commissure plays the crucial role in mediating the mirror image optokinetic effects to Vn on the opposite side and assures the bidirectionality of the responses to binocular stimulation. 5. Cerebellectomy did not significantly affect the Vn responses to the optokinetic stimuli presented in this study. 6. Electrical stimulation of the pretectum excited type II and inhibited type I Vn ipsilaterally and had the opposite effect on Vn located on the opposite side. NRTP stimulation excited type II and inhibited type I ipsilaterally; latency analysis of these effects suggested that the pretectal stimuli excited opsilateral NRTP neurons which, in turn, excited ipsilateral type II Vn. Ipsilateral type I inhibition as well as the concurrent contralateral type II inhibition and type I excitation are produced by the inhibitory action of type II on type I and the commisural system. 7. Systemic application of picrotoxin abolished all optokinetic responses of Vn except the type II activation. This finding further supports the hypothesis described above. 8. Unilateral pretectal or NRTP lesions abolished OKN to surround motion in the direction of the lesion.     

Firing characteristics of neurons mediating optokinetic responses to rat's vestibular neurons

1. The responses of single units in the pretectum (Pt) and in the n. reticularis tegmenti pontis (NRTP) to constant velocity horizontal rotation (0.25–60 deg/s) of a large-field visual pattern were studied in immobilized, non-anesthetized DA-HAN rats. In addition, responses of Pt and NRTP neurons to pure vestibular stimuli (rotation in the dark) were studied.
2. Pt neurons showed seven response types to optokinetic stimulation (Table 1). The most frequent response (48%) consisted of a very rapid increase in firing to steady state on temporonasal motion stimulation of the contralateral eye; nasotemporal stimuli yielded no change in resting rate as did stimulation of the ipsilateral eye. The response maximum occurred at a retinal slip velocity of 1 deg/s. None of the Pt units tested responded to pure vestibular stimuli.
3. NRTP neurons — as Pt units — most frequently (43%) increased their discharge rate on temporonasal stimulation of the contralateral eye and maintained a constant resting rate during nasotemporal motion. Peak response amplitudes also occurred with retinal slip velocities of 1 deg/s. Contrary to the fast time-to-peak of the responses of Pt neurons NRTP units showed a slow rise in frequency of firing to peak response levels.
4. NRTP neurons responded to pure vestibular stimuli (horizontal angular acceleration in the dark). The vestibular responses were synergistic with those evoked in the same neurons by optokinetic stimuli. Thus, the most frequently encountered type of optokinetic response (s. above) showed a type II vestibular response.
5. Comparison of OKN and Vn optokinetic responses with those of Pt and NRTP suggests that the unidirectional-selective Pt and NRTP neurons are important links in the central optokinetic path. In addition, the NRTP may represent the site at which the retinal slip signal and the eye velocity signal converge. This convergence has been postulated in models of the system [12].

     

Effects of kainic acid lesions of the nucleus reticularis tegmenti pontis on fast and slow phases of vestibulo-ocular and optokinetic reflexes in the pigmented rat

Summary The nucleus reticularis tegmenti pontis (NRTP) and adjacent pontine reticular formation were lesioned chemically using the neurotoxic agent kainic acid, and the effects of these lesions on horizontal ocular optokinetic and vestibular nystagmus were examined. Eye position was measured in the alert, NRTP-lesioned animals with the electromagnetic search coil technique. Optokinetic and vestibular stimuli consisted of steps of rotations or sinusoidal oscillations of a fullfield visual pattern surrounding the animal or of the animal in total darkness, respectively. In a first group of animals, small unilateral NRTP lesions were produced by placing a single kainic acid injection in the area of the left NRTP. In one third of the animals, ipsilateral quick phases of optokinetic and vestibular nystagmus were abolished. In the remaining animals, quick phases were deficient to various degrees or not affected at all. There were no changes in the characteristics of optokinetic step responses to ipsilateral pattern rotations which activate predominantly optokinetic pathways on the side of the brainstem lesion. In animals with ipsiversive quick phase deficits, contralateral pattern rotations elicited tonic eye deviations. In a second group of animals, large uni- or bilateral lesions were produced by injecting kainic acid into three separate rostral, middle and caudal levels of the right NRTP. These animals had uni- or bilateral quick phase deficits during optokinetic and vestibular nystagmus. Optokinetic nystagmus in response to velocity steps of pattern rotation towards the lesion side was strongly reduced in gain even in those animals that had no apparent deficits in the fast contraversive reset phases. In four out of six animals, responses to sinusoidal optokinetic pattern oscillations were reduced in gain and showed increased phase lags compared to controls. Vestibulo-ocular responses to velocity steps of head rotations were of normal gain but reduced in duration (measured from onset of stimulation to reversal of nystagmus). Sinusoidal vestibulo-ocular responses evoked by head oscillations exhibited reduced gain values and strongly increased phase leads in the frequency range below 0.5 Hz. The vestibular time constant was found to be around 4.5 s in animals with NRTP lesions compared to about 7.5 s in control animals. The present results show that large kainic acid lesions of the NRTP (and adjacent area) do not abolish optokinetic eye movements in the rat, in contrast to what has been reported after electrolytic lesions. The data suggest, however, that there is a failure of slow build-up of OKN slow phase velocity as well as a shortening of the vestibular time constant which correlates with the kainic acid lesions extending into rostromedial and caudal parts of the NRTP. The implications of these findings with respect to an involvement of these structures in velocity storage are discussed.Abbreviations CN cochlear nucleus - DpSC decussation, peduncle, superior, cerebellar - ip interpeduncular nucleus - MLF medial longitudinal fasciculus - NOT nucleus of optic tract - NRTPc nucleus reticularis tegmenti pontis, central subdivision - NRTPp nucleus reticularis tegmenti pontis, pericentral subdivision - p pontine nuclei - ph praepositus hypoglossi nucleus - pMC peduncle, middle cerebellar - pSC peduncle, superior cerebellar - Pyr pyramidal tract - Rcs raphe central superior - Rm raphe magnus - rpc reticular nucleus, pontine, caudal - rpo reticular nucleus, pontine, oral - TB trapezoid body - tM trapezoid nucleus, medial - tGd tegmental nucleus of von Gudden, dorsal - tGv tegmental nucleus of von Gudden, ventral - 5 trigeminal tract or trigeminal nerve - 5m mesencephalic trigeminal nucleus - 5mt motor trigeminal nucleus - 6n abducens nucleus - 7 facial nerve Prof. W. Precht died on March 12, 1985     

On the pathway mediating optokinetic responses in vestibular nuclear neurons

Most vestibular nucleus neurons of the horizontal semicircular canal system respond to rotation of a large-field visual pattern in a direction-selective fashion. As shown previously, these optokinetic responses also occurred in the absence of the cerebellum. In the present study the pathways mediating the optokinetic responses of vestibular neurons were further investigated by placing lesions in various parts of the central nervous system. Optokinetic responses of vestibular neurons were studied a few days postoperatively in the fully alert cat. In addition, prior to the single unit recording, eye movements evoked by vestibular, optokinetic and combined vestibular-optokinetic stimuli were studied.Bilateral pretectal lesions most severely affected both the horizontal optokinetic responses of vestibular neurons and the horizontal optokinetic nystagmus. Unilateral pretectal lesions were less effective but led to alterations of responses in both ipsi- and contralateral vestibular nuclei whereas optokinetic nystagmus was primarily affected upon optokinetic stimulation to the side of the lesion. Bilateral lesions in the region of the nucleus reticularis tegmenti pontis or also laterorostral to it yielded results very similar to those described for bilateral prectectal lesions, except that the vestibulo-ocular reflex still showed an improvement in phase and gain when tested at low frequency in the light. Lesions of the central tegmental tracts, medial longitudinal fascicles, superior colliculus and inferior olive had no, or only slight, effects on optokinetic responses.The results suggest that the responses of vestibular neurons to horizontal optokinetic stimuli are crucially dependent upon the integrity of the pretectum. One major optokinetic pretectofugal pathway reaches the vestibular nuclei through the area of the nucleus reticularis tegmenti pontis. Neurons in this nucleus may be in monosynaptic and/or oligosynaptic contact with vestibular neurons. Indirect routes through a cerebellar loop involving the vermis and deep cerebellar nuclei or inferior olive play a minor role.     

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.     

A comparison of the horizontal and vertical optokinetic reflexes of the rabbit

Summary The horizontal and vertical monocular optokinetic reflexes of the rabbit were measured under closed-loop and open-loop conditions. A random noise, optokinetic stimulus subtending 70×70 deg was presented to the left eye of rabbits placed in front of a rear projection tangent screen. The position of the right eye (nonstimulated) was measured using an infrared light projection technique. During open-loop optokinetic stimulation the eye position signal was fed back to sum with a time-integrated velocity command signal driving the optokinetic stimulus. The dynamics of eye movements evoked by horizontal and vertical optokinetic stimulation were different. Horizontally evoked eye movements never exceeded a deviation of 15 deg before being interrupted by resetting saccades, which returned the eye past the primary position. By contrast, vertical eye deviations greater than 20 deg were often maintained for intervals exceeding 10 s without resetting. The closed-loop gain of optokinetically evoked horizontal eye movements was higher for monocular posterior-anterior optokinetic stimulation than for anterior-posterior stimulation. The vertical optokinetic gain for up-down stimulation was slightly greater than the gain for down-up stimulation. The vertical up-down, open-loop optokinetic gain was greater than the down-up gain over a range of retinal slip velocities of 0.5–5.0 deg/s. Measurement of the horizontal vestibulo-ocular reflex during simultaneous horizontal optokinetic stimulation demonstrated that visual and vestibular information combine linearly to produce reflex eye movements. These data suggest that the higher gain of the horizontal optokinetic reflex may compensate in part for the reduced gain of the horizontal vestibulo-ocular reflex at lower angular accelerations of the head. An equivalent vertical optokinetic gain would be obviated by the contribution of the utricular otoliths to the vertical vestibulo-ocular reflex at low frequencies of head movement.This research was supported by the National Institutes of Health Grant EY00848 and the Oregon Lions Sight and Hearing Foundation     

Visual-vestibular interaction in the flocculus of the alert monkey

Summary The activity of Purkinje cells (P-cells) was recorded in the flocculus of alert Rhesus monkeys under different conditions of visual-vestibular stimulation. Stimulus conditions were vestibular, optokinetic, combined and conflicting. About 10–20% of all P-cells were activated in their simple spike activity during conflicting stimulation to the recording side (type I) and gave no response or much less during vestibular stimulation. About half of these P-cells were also activated during optokinetic stimulation to the recording side at velocities above 40–60 deg/s. Simple and complex spike activity behaved in a reciprocal way with overlapping but not identical working ranges. Simple spike modulation was unidirectional, complex spike activity always bidirectional. Modulation of simple spike activity cannot be related to one single parameter of the sensory input or the oculomotor output. The hypothesis is put forward that the vestibular nuclei and the flocculus behave in a complementary fashion in processing visual-vestibular information, the flocculus being specialized for high velocity optokinetic nystagmus and suppression of vestibular nystagmus.Supported by a grant from the Swiss National Foundation for Scientific Research 3.343-2.78     

Vestibular nuclei activity in the alert monkey during suppression of vestibular and optokinetic nystagmus

Summary Single neurons were recorded in the vestibular nuclei of monkeys trained to suppress nystagmus by visual fixation during vestibular or optokinetic stimulation. During optokinetic nystagmus vestibular nuclei neurons exhibit frequency changes. With the suppression of optokinetic nystagmus this neuronal activity on average is attenuated by 40% at stimulus velocities of 40 °/s. At a stimulus velocity of 5 °/s responses are, under both conditions, close to threshold. For steps in velocity, suppression of vestibular nystagmus shortens the time constants of the decay of neuronal activity from 15–35 s to 5–9 s, while the amplitude of the response remains unchanged. The results are discussed in relation to current models of visual-vestibular interaction. These models use a feedback mechanism which normally operates during vestibular and optokinetic nystagmus. Nystagmus suppression interrupts this feedback loop.Supported by the Swiss National Foundation for Scientific Research (SNF 3.233.77) and the Deutsche Forschungsgemeinschaft (U.W. Buettner, Bue 379/2)     

Vestibular signals in the parasolitary nucleus

Vestibular primary afferents project to secondary vestibular neurons located in the vestibular complex. Vestibular primary afferents also project to the uvula-nodulus of the cerebellum where they terminate on granule cells. In this report we describe the physiological properties of neurons in a "new" vestibular nucleus, the parasolitary nucleus (Psol). This nucleus consists of 2,300 GABAergic neurons that project onto the ipsilateral inferior olive (beta-nucleus and dorsomedial cell column) as well as the nucleus reticularis gigantocellularis. These olivary neurons are the exclusive source of vestibularly modulated climbing fiber inputs to the cerebellum. We recorded the activity of Psol neurons during natural vestibular stimulation in anesthetized rabbits. The rabbits were placed in a three-axis rate table at the center of a large sphere, permitting vestibular and optokinetic stimulation. We recorded from 74 neurons in the Psol and from 23 neurons in the regions bordering Psol. The activity of 72/74 Psol neurons and 4/23 non-Psol neurons was modulated by vestibular stimulation in either the pitch or roll planes but not the horizontal plane. Psol neurons responded in phase with ipsilateral side-down head position or velocity during sinusoidal stimulation. Approximately 80% of the recorded Psol neurons responded to static roll-tilt. The optimal response planes of evoked vestibular responses were inferred from measurement of null planes. Optimal response planes usually were aligned with the anatomical orientation of one of the two ipsilateral vertical semicircular canals. The frequency dependence of null plane measurements indicated a convergence of vestibular information from otoliths and semicircular canals. None of the recorded neurons evinced optokinetic sensitivity. These results are consistent with the view that Psol neurons provide the vestibular signals to the inferior olive that eventually reached the cerebellum in the form of modulated climbing fiber discharges. These signals provide information about spatial orientation about the longitudinal axis.     

Activity of vestibular nuclei neurons during vestibular and optokinetic stimulation in the alert mouse

As a result of the availability of genetic mutant strains and development of noninvasive eye movements recording techniques, the mouse stands as a very interesting model for bridging the gap among behavioral responses, neuronal response dynamics studied in vivo, and cellular mechanisms investigated in vitro. Here we characterized the responses of individual neurons in the mouse vestibular nuclei during vestibular (horizontal whole body rotations) and full field visual stimulation. The majority of neurons ( approximately 2/3) were sensitive to vestibular stimulation but not to eye movements. During the vestibular-ocular reflex (VOR), these neurons discharged in a manner comparable to the "vestibular only" (VO) neurons that have been previously described in primates. The remaining neurons [eye-movement-sensitive (ES) neurons] encoded both head-velocity and eye-position information during the VOR. When vestibular and visual stimulation were applied so that there was sensory conflict, the behavioral gain of the VOR was reduced. In turn, the modulation of sensitivity of VO neurons remained unaffected, whereas that of ES neurons was reduced. ES neurons were also modulated in response to full field visual stimulation that evoked the optokinetic reflex (OKR). Mouse VO neurons, however, unlike their primate counterpart, were not modulated during OKR. Taken together, our results show that the integration of visual and vestibular information in the mouse vestibular nucleus is limited to a subpopulation of neurons which likely supports gaze stabilization for both VOR and OKR.     

Visual sensory substitution in vestibular compensation: neuronal substrates in the alert cat

The purpose of this study was to investigate adaptive changes in the activity of vestibular nuclei neurons unilaterally deprived of their primary afferent inputs when influenced by visual motion cues. These neuronal changes might account for the established role that vision plays in the compensation for posturo-kinetic deficits after the loss of vestibular inputs. Neuronal recordings were made in alert, non-paralysed cats that had undergone unilateral vestibular nerve sections. The unit responses collected in both Deiters' nuclei were compared to those previously recorded in intact cats. We analysed the extracellular activity of Deiters' nucleus neurons, as well as the optokinetic reflex (OKR) evoked during sinusoidal translation of a whole-field optokinetic stimulus in the vertical plane. In intact cats, we found the unit firing rate closely correlated with the visual surround translation velocity, and the relationship between the discharge rate and the motion frequency was tuned around an optimal frequency. The maximum firing rate modulation was generally below the 0.25 Hz stimulus frequency; unit responses were weak or even absent above 0.25 Hz. From the 4th day to the end of the 3rd week after ipsilateral deafferentation, a majority of cells was found to display maximum discharge modulation during vertical visual stimulation at 0.50 Hz, and even at 0.75 Hz, indicating that the frequency bandwidth of the visually induced responses of deafferented vestibular nuclei neurons had been extended. Consequently, the frequency-dependent attenuation in the sensitivity of vestibular neurons to visual inputs was much less pronounced. After the first 3 weeks postlesion, the unit response characteristics were very similar to those observed prior to the deafferentation. On the nucleus contralateral to the neurectomy, the maximum modulation of most cells was tuned to the low frequencies of optokinetic stimulation, as also seen prior to the lesion. We found, however, a subgroup of cells displaying well-developed responses above 0.50 Hz. Under all experimental conditions, the neuronal response phase still remained closely correlated with the motion velocity of the vertical sinusoidal visual pattern. We hypothesize that Deiters' neurons deprived of their primary afferents may transiently acquire the ability to code fast head movements on the basis of visual messages, thus compensating, at least partially, for the loss of dynamic vestibular inputs during the early stages of the recovery process. Since the overall vertical OKR gain was not significantly altered within the 0.0125 Hz–1 Hz range of stimulation after the unilateral neurectomy, it can be postulated that the increased sensitivity of deafferented vestibular neurons to visual motion cues was accounted for by plasticity mechanisms operating within the deafferented Deiters' nucleus. The neuroplasticity mechanisms underlying this rapid and temporary increase in neuronal sensitivity are discussed.     

Optokinetic responses of vestibular nucleus neurons in the rat

1. Vestibular nucleus neurons of the brown rat (DA-HAN) responding to horizontal angular acceleration in the dark (type I and II neurons) have been studied during horizontal optokinetic stimulation in the time and frequency domain. For recording animals were nonanesthetized and paralyzed.
2. All type I and type II neurons studied responded in a direction-selective fashion to rotation of large-field visual pattern. With both eyes open, type I (type II) neurons increased (decreased) their discharge on optokinetic stimuli directed away from the recording side and decreased (increased) firing on rotation towards the recording side. Covering one eye, abolished the inhibition of type II and excitation of type I on the ipsilateral side and removed type II excitation and type I inhibition on the opposite side.
3. The missing responses of vestibular units to nasotemporal stimulation in monocular condition were paralleled by the absence of OKN on stimulation in the same direction.
4. Response maxima (± Δf) of vestibular units occurred at stimulus velocities of 1 deg/s (here equal retinal slip velocity). Below and above this velocity a sharp response decline occurred. The mean firing increases were larger than the decreases. There were no significant differences in mean ± Δf values between monocular and binocular conditions.
5. Frequency domain data show that response phase was in phase with surround velocity only at very low frequencies. With higher frequencies a progressive phase lag was noted; similarly the sensitivity decreased with increasing frequency.

     

Visual and oculomotor signals in nucleus reticularis tegmenti pontis in alert monkey

A small region in the dorsal midline portion of the nucleus reticularis tegmenti pontis (NRTP) in monkeys contains neurons that respond to focal visual stimuli or during saccadic eye movements or both. None of these cells or any others in this region respond to the motion of large visual fields (optokinetic stimulation), although such responses were specifically sought. Thus, this group of NRTP neurons forms a completely different set of cells than those previously described in more rostral but closely adjacent portions of the pontine nuclei which respond well to optokinetic stimulation. The most frequently encountered cell type in this region of NRTP (153 neurons) produced a high-frequency burst of discharges during saccadic eye movements. Neural discharge (burst intensity or duration) was not related to saccade metrics. Instead, peak burst frequency and/or the number of spikes in a unit's burst reached a maximum when the saccade moved the eyes to a circumscribed region (movement field) of the animal's visual field. There were two subtypes of these burst neurons. In one type (44%) the movement fields were smaller and entirely contained within the oculomotor range. In the other type (56%) the movement fields consisted of a whole sector (some as wide as 180 degrees) of the entire oculomotor range. All the neurons in this sample that we were able to test in total darkness continued to produce bursts of discharges of similar profile during spontaneous saccades into their movement field. All the movement fields were retinotopically organized, although a few cells (22%) showed a marked variation of burst metrics with initial eye position. Another small group of cells in NRTP (8 neurons) responded to small spots of light turned on within a circumscribed region of the visual field while the animal maintained fixation on a separate spot of light. These visual neurons produced no saccade-related discharge. A larger group of neurons (24 out of 52 tested cells) produced both a visual response and a saccadic burst. The visual field of this type of cell was always smaller and was contained within the movement field of the cell. The response of both types of NRTP visual neurons was enhanced when the visual stimulus was to be the target for a saccadic eye movement. On double-saccade trials the visual stimulus was never present in the hemifield containing the cell's visual field.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of natural neck afferent stimulation on vestibulo-spinal neurons in the decerebrate cat

Summary The response patterns of antidromically identified vestibulo-spinal Deiters' neurons and other vestibular nuclear neurons to stimulation of neck receptors and horizontal semicircular canal receptors were investigated in precollicular decerebrate cats.The neck receptors were stimulated by sinusoidal oscillation (frequency 0.2 Hz and 10 ° amplitude) and trapezoidal movements of the cervical axis vertebrae in the horizontal plane, while the head was stereotaxically fixed in a prone position. The influence of horizontal semicircular canal receptors was examined by sinusoidal turntable oscillations (0.2 Hz, 10 °).Out of 151 antidromically driven Deiters' neurons, 16 units responded to neck rotation and 11 cells out of 148 Deiters' neurons were influenced by the horizontal semicircular canal receptors. Only 3 of these neurons were affected by convergent cervical and vestibular influences and revealed antagonistic response patterns. Neck-responsive units were found to be scattered throughout Deiters' nucleus, whereas semicircular-canal-influenced neurons were predominantly encountered in its rostro-ventral part.Altogether, 68 neck-responsive neurons in the vestibular nuclear complex were tested. These units showed a periodic, direction-specific modulation of the discharge rate in response to sinusoidal oscillation. Almost all responses were related to input angular velocity at 0.2 Hz with a mean gain of 1.15 imp/s per deg/s. During trapezoidal neck displacement, responsive units also exhibited tonical components in their discharge rates.These results suggest kinetic-static response characteristics of neck receptors. The cervico-vestibulo-spinal pathway may contribute to tonic neck reflexes; vestibular and neck reflexes may partially interact in Deiters' nucleus.Supported by Sonderforschungsbereich Hirnforschung und Sinnesphysiologie (SFB 70) der Deutschen Forschungsgemeinschaft     

Response of central vestibular neurons to horizontal linear acceleration in the rat

Responses of central vestibular neurons to horizontal sinusoidal translation (F:0.25 Hz) were recorded in albino rat. 57.5% of vestibular neurons were responding to this stimulation by a modulation of their firing rate, the mean phase angle of the response, averaged from the whole population being 22±79 deg. lag, relative to the peak of contralateral acceleration. Dynamic characteristics of phase and gain were studied and appeared to be different from previous reports on primary afferents: the gain decreased or was flat with increasing acceleration at one frequency, and the phase lag which was flat in the same conditions increased with increasing frequency. A phase lead of some units has been observed at low frequency (0.1 Hz).Regarding the convergence between otolith and canal inputs on nuclear vestibular neurons, it was shown that the major pattern of convergence is between canal and otolith inputs of same polarity.Supported by the Deutsche Forschungsgemeinschaft This project was supported by the CNRS (ATP N^o 3626)     

Convergence of directional vestibular and neck signals on cerebellar Purkinje cells

 Convergence of spatially oriented vestibular and neck signals within the cerebellar anterior vermis in decerebrate cats was studied by recording the simple spike discharge of Purkinje (P) cells during wobble either of the whole animal (vestibular input) or of the body under a fixed head (neck input) at 0.156 Hz, 5° and 2.5°, respectively. Both clockwise (CW) and counterclockwise (CCW) rotations were performed. Units that had equal response amplitudes to CW and CCW rotations (narrowly tuned neurons) were described by a single vector (S _max), characterized by a gain, a direction and a temporal phase. Units with different response amplitudes to CW and CCW rotation (broadly tuned neurons) were described by two vectors (S _max and S _min). In addition to these bidirectional units, there were also unidirectional units which responded either to CW or CCW rotation; in these cases the gain of S _max equals that of S _min. On the whole, 77% and 63% of the P cells responding to vestibular and neck stimulation, respectively, showed a bidirectional broadly tuned or unidirectional behavior. These response patterns were attributed to the convergence of signals with different spatial and temporal properties. About 50% of the P cells from which recordings were made responded to stimulation of both sensory systems. However, the gains of the S _max vectors of the neck responses were much greater than those of the vestibular responses, at least for small amplitudes of rotation, and were positively correlated with them. Usually the differences in the orientation components of the neck and vestibular S _max vectors were larger, while the differences in temporal phases were smaller than 90°. These findings suggest that periodic changes in the phase difference and gain ratio of the neck to the vestibular response may occur during dynamic displacement of the head over the body, depending on the stimulus direction. As a result of these data, the P cells of the cerebellar vermis are expected to show prominent responses to head rotation, which could affect the spatially organized postural responses by utilizing vestibular and reticular targets. Received: 10 June 1997 / Received after revision: 23 October 1997 / Accepted: 25 November 1997     

Regional blood flow in genetically obese (ob/ob) mice

1. Experiments have been undertaken on 11 decerebrate cats to investigate the effects of natural vestibular stimulation on the activity of cerebellar fastigial neurons. 2. From recordings in the rostral portion of the nucleus during sinusoidal lateral (roll) and horizontal (yaw) rotation, distinctive patterns of response were observed. 3. The majority of neurons sensitive to vestibular stimulation showed responses to a single modality of vestibular activation. During lateral tilt some neurones showed positional sensititivy, others gave responses related tothe velocity of movement. Other neurones responded in phase with the velocity of movement in the horizontal plane. 4. Aside from these neuronal responses, others provided indications of a convergence of inputs from different sets of vestibular receptors. In particular, several neurons showed a pattern of response that indicated tht they received inputs from otolith receptors and ampullar receptors of the vertical canal. At low velocities of movement their response was positional but with inreasing velocity the magnitude of the response increased and there was a marked phase shift of the discharge towards head velocity. 5. Neurons responding to horizontal rotation often showed positional responses during lateral tilt. There were also indications of a convergence of ampullar inputs from both vertical and horizontal canals. 6. The neural pathways mediating these resonses are discussed in consideration of previous neuroanatomical and neurophysiological data. We consider it likely that several pathways may act to evoke the patterns of response observed, and a role of the cerebellar cortex is indicated.     

Response of vestibular neurons to head rotations in vertical planes. II. Response to neck stimulation and vestibular-neck interaction

1. We have studied the responses of neurons in the lateral and descending vestibular nuclei of decerebrate cats to stimulation of neck receptors, produced by rotating the body in vertical planes with the head stationary. The responses to such neck stimulation were compared with the responses to vestibular stimulation produced by whole-body tilt, described in the preceding paper. 2. After determining the optimal vertical plane of neck rotation (response vector orientation), the dynamics of the neck response were studied over a frequency range of 0.02-1 Hz. The majority of the neurons were excited by neck rotations that brought the chin toward the ipsilateral side; most neurons responded better to roll than to pitch rotations. The typical neck response showed a low-frequency phase lead of 30 degrees, increasing to 60 degrees at higher frequencies, and a gain that increased about threefold per decade. 3. Neck input was found in about one-half of the vestibular-responsive neurons tested with vertical rotations. The presence of a neck response was correlated with the predominant vestibular input to these neurons; neck input was most prevalent on neurons with vestibular vector orientations near roll and receiving convergent vestibular input, either input from both ipsilateral vertical semicircular canals, or from canals plus the otolith organs. 4. Neurons with both vestibular and neck responses tend to have the respective orientation vectors pointing in opposite directions, i.e., a head tilt that produces an excitatory vestibular response would produce an inhibitory neck response. In addition, the gain components of these responses were similar. These results suggest that during head movements on a stationary body, these opposing neck and vestibular inputs will cancel each other. 5. Cancellation was observed in 12 out of 27 neurons tested with head rotation in the mid-frequency range. For most of the remaining neurons, the response to such a combined stimulus was greatly attenuated: the vestibular and neck interaction was largely antagonistic. 6. Neck response dynamics were similar to those of the vestibular input in many neurons, permitting cancellation to take place over a wide range of stimulus frequencies. Another pattern of interaction, observed in some neurons with canal input, produced responses to head rotation that had a relatively constant gain and remained in phase with position over the entire frequency range; such neurons possibly code head position in space.     

Discharge characteristics of medial rectus and abducens motoneurons in the goldfish.

1. The discharge of antidromically identified medial rectus and abducens motoneurons was recorded in restrained unanesthesized goldfish during spontaneous eye movements and in response to vestibular and optokinetic stimulation. 2. All medial rectus and abducens motoneurons exhibited a similar discharge pattern. A burst of spikes accompanied spontaneous saccades and fast phases during vestibular and optokinetic nystagmus in the ON-direction. Firing rate decreased for the same eye movements in the OFF-direction. All units showed a steady firing rate proportional to eye position beyond their recruitment threshold. 3. Motoneuronal position (ks) and velocity (rs) sensitivity for spontaneous eye movements were calculated from the slope of the rate-position and rate-velocity linear regression lines, respectively. The averaged ks and rs values of medial rectus motoneurons were higher than those of abducens motoneurons. The differences in motoneuronal sensitivity coupled with structural variations in the lateral versus the medial rectus muscle suggest that symmetric nasal and temporal eye movements are preserved by different motor unit composition. Although the abducens nucleus consists of distinct rostral and caudal subgroups, mean ks and rs values were not significantly different between the two populations. 4. Every abducens and medial rectus motoneuron fired an intense burst of spikes during its corresponding temporal or nasal activation phase of the "eye blink." This eye movement consisted of a sequential, rather than a synergic, contraction of both vertical and horizontal extraocular muscles. The eye blink could act neither as a protective reflex nor as a goal-directed eye movement because it could not be evoked in response to sensory stimuli. We propose a role for the blink in recentering eye position. 5. Motoneuronal firing rate after ON-directed saccades decreased exponentially before reaching the sustained discharge proportional to the new eye position. Time constants of the exponential decay ranged from 50 to 300 ms. Longer time constants after the saccade were associated with backward drifts of eye position and shorter time constants with onward drifts. These postsaccadic slide signals are suggested to encode the transition of eye position to the new steady level. 6. Motoneurons modulated sinusoidally in response to sinusoidal head rotation in the dark, but for a part of the cycle they went into cutoff, dependent on their eye position recruitment threshold. Eye position (kv) and velocity (rv) sensitivity during vestibular stimulation were measured at frequencies between 1/16 and 2 Hz. Motoneuronal time constants (tau v = rv/kv) decreased on the average by 25% with the frequency of vestibular stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vestibular nuclei activity during optokinetic after-nystagmus (OKAN) in the alert monkey

Summary Neurons which receive an input from the horizontal semicircular canals were recorded from the vestibular nuclei in chronically prepared monkeys (Macaca mulatta) during optokinetic after-nystagmus (OKAN). In complete darkness the vestibular neurons showed activity changes which closely paralleled the strength of nystagmus. The activity of vestibular units returned to baseline levels of spontaneous discharge only when all after-nystagmus had ceased, or when it was inhibited by stationary visual stimuli. The possible role of vestibular neurons in the generation of OKAN and its significance in vestibulo-visual interaction is discussed.Supported in part by Swiss National Foundation for Scientific Research 3.672-0.77 and Emil Barell-Foundation of Hoffman-La Roche, Basel, Switzerland