Contralateral Posterior Canal Research Articles

A novel maneuver for diagnosis and treatment of torsional-vertical down beating positioning nystagmus: anterior canal and apogeotropic posterior canal BPPV

IntroductionIn patients with benign paroxysmal positional vertigo, BPPV; a torsional-vertical down beating positioning nystagmus can be elicited in the supine straight head-hanging position test or in the Dix-Hallpike test to either side. This type of nystagmus can be explained by either an anterior canal BPPV or by an apogeotropic variant of the contralateral posterior canal BPPV Until now all the therapeutic maneuvers that have been proposed address only one possibility, and without first performing a clear differential diagnosis between them. ObjectiveTo propose a new maneuver for torsional-vertical down beating positioning nystagmus with a clear lateralization that takes into account both possible diagnoses (anterior canal-BPPV and posterior canal-BPPV). MethodsA prospective cohort study was conducted on 157 consecutive patients with BPPV. The new maneuver was performed only in those with torsional-vertical down beating positioning nystagmus with clear lateralization. ResultsTwenty patients (12.7%) were diagnosed with a torsional-vertical down beating positioning nystagmus. The maneuver was performed in 10 (6.35%) patients, in whom the affected side was clearly determined. Seven (4.45%) patients were diagnosed with an anterior canal-BPPV and successfully treated. Two (1.25%) patients were diagnosed with a posterior canal-BPPV and successfully treated with an Epley maneuver after its conversion into a geotropic posterior BPPV. ConclusionThis new maneuver was found to be effective in resolving all the cases of torsional-vertical down beating positioning nystagmus-BPPV caused by an anterior canal-BPPV, and in shifting in a controlled way the posterior canal-BPPV cases of the contralateral side into a geotropic-posterior-BPPV successfully treated during the followup visit. Moreover, this new maneuver helped in the differential diagnosis between anterior canal-BPPV and a contralateral posterior canal-BPPV.

Editors' note: Impaired vestibular responses in internuclear ophthalmoplegia: Association and dissociation

In “Impaired vestibular responses in internuclear ophthalmoplegia: Association and dissociation,” Choi et al. determined that the medial longitudinal fasciculus (MLF) serves as the main passage for the high-acceleration vestibulo-ocular reflex (VOR) from the contralateral posterior canal. They also concluded that the associations and dissociations of the vestibular dysfunction indicate variable combinations of damage to the vestibular fibers ascending or descending in the MLF in strokes causing isolated unilateral internuclear ophthalmoplegia (INO). Dr. Johnston raises concerns regarding these conclusions. Based on her experience, horizontal VOR latencies are normal in patients with INO, implying integrity of direct VOR. She explained that undamaged fibers in MLF or extrafascicular pathways can still initiate the VOR without delay. Dr. Kim, senior author of the study, points out, in Dr. Johnston's study, that the VOR was recorded only in the eye on the lesion side by adopting lower velocity and acceleration of head motion and did not specify the lesion extent or INO as an isolated finding. He also adds that her suggestion does not explain the preservation of the head impulse VOR for the ipsilesional anterior canal in the presence of impairments for other canals, and impaired VOR gains in the eye on the contralesional side in patients with strictly isolated unilateral INO. These findings support separate brainstem pathways for posterior and anterior canal projections pertaining to high acceleration stimuli. In “Impaired vestibular responses in internuclear ophthalmoplegia: Association and dissociation,” Choi et al. determined that the medial longitudinal fasciculus (MLF) serves as the main passage for the high-acceleration vestibulo-ocular reflex (VOR) from the contralateral posterior canal. They also concluded that the associations and dissociations of the vestibular dysfunction indicate variable combinations of damage to the vestibular fibers ascending or descending in the MLF in strokes causing isolated unilateral internuclear ophthalmoplegia (INO).

Impaired vestibular responses in internuclear ophthalmoplegia: Association and dissociation.

To determine the role of the medial longitudinal fasciculus (MLF) in conveying vestibular signals. In 10 patients with isolated acute unilateral internuclear ophthalmoplegia (INO) due to an acute stroke, we performed comprehensive vestibular evaluation using video-oculography, head impulse tests with a magnetic search coil technique, bithermal caloric tests, tests for the ocular tilt reaction, and measurements of subjective visual vertical and cervical and ocular vestibular evoked myogenic potentials (VEMPs). The head impulse gain of the vestibulo-ocular reflex (VOR) was decreased invariably for the contralesional posterior canal (PC) (n = 9; 90%) and usually for the ipsilesional horizontal canal (n = 5; 50%). At least one component of contraversive ocular tilt reaction (n = 9) or contraversive tilt of the subjective visual vertical (n = 7) were common along with ipsitorsional nystagmus (n = 5). Cervical or ocular VEMPs were abnormal in 5 patients. The MLF serves as the main passage for the high-acceleration VOR from the contralateral PC. The associations and dissociations of the vestibular dysfunction in our patients indicate variable combinations of damage to the vestibular fibers ascending or descending in the MLF even in strokes causing isolated unilateral INO.

Qualitative analysis of the Dix-Hallpike maneuver in multi-canal BPPV using a biomechanical model: Introduction of an expanded Dix-Hallpike maneuver for enhanced diagnosis of multi-canal BPPV.

Introduction/ObjectiveMultiple canal BPPV can be a diagnostic challenge to the clinician. This is due in part to the complex anatomy of the labyrinth but also to complex and often simultaneous ocular responses that result from stimulation of multiple canals during traditional diagnostic testing. Our objective was to analyze the Dix-Hallpike maneuver used in the diagnosis of BPPV to look for patterns of simultaneous canal response and to develop a diagnostic maneuver that will allow separation of canal responses in multiple canal BPPV.MethodsA previously created and published 3D biomechanical model of the human labyrinths for the study of BPPV was used to analyze and compare the position and movement of otoliths in the Dix-Hallpike maneuver as well as in a proposed expanded version of the traditional Dix-Hallpike maneuver.ResultsThe traditional Dix-Hallpike maneuver with the head hanging may promote movement of otoliths in 5 of the six semicircular canals. The Dix-Hallpike maneuver with the head lowered only to the horizontal position allows for otoconia in only the lowermost posterior canal to fall to the most gravity dependent position. This position allows for minimal or no movement of otoconia in the contralateral posterior canal, or in either superior canal. Turning the head ninety degrees to the opposite side while still in the horizontal position will provoke otolith movement in only the contralateral posterior canal. The superior canals can then be examined for free otolith debris by extending the neck to a head-hanging position. These positions may be assumed directly from one to the next in the lying position. There seems to be no advantage to sitting up between positions.ConclusionThe Dix–Hallpike maneuver may cause simultaneous movement of otoliths present in multiple canals and create an obstacle to accurate diagnosis in multi-canal BPPV. An expanded Dix-Hallpike maneuver is described which adds intermediate steps with the head positioned to the right and left in the horizontal position before head-hanging. This expanded maneuver has helped to isolate affected semi-circular canals for individual assessment in multiple canal BPPV.

Semicircular Canal Function Before and After Surgery for Superior Canal Dehiscence

To characterize semicircular canal function before and after surgery for superior semicircular canal dehiscence (SCD) syndrome. Prospective unblinded study of physiologic effect of intervention. Tertiary referral center. Patients with SCD syndrome documented by history, sound- or pressure-evoked eye movements, vestibular-evoked myogenic potential testing, and high-resolution multiplanar computed tomographic scans. Nineteen subjects with SCD had quantitative measurements of their angular vestibulo-ocular reflexes (AVOR) in response to rapid rotary head thrusts measured by magnetic search coil technique before and after middle fossa approach and repair of the dehiscence. In 18 subjects, the dehiscence was plugged; and in 1, it was resurfaced. The AVOR gains (eye velocity/head velocity) for excitation of each of the semicircular canals. Vertigo resulting from pressure or loud sounds resolved in each case. Before surgery, mean AVOR gains were normal for the ipsilateral horizontal (0.94 +/- 0.07) and posterior (0.84 +/- 0.09) canals. For the superior canal to be operated on, AVOR gain was 0.75 +/- 0.13; but this was not significantly lower than the gain for the contralateral superior canal (0.82 +/- 0.11, p = 0.08). Mean AVOR gain decreased by 44% for the operated superior canals (to 0.42 +/- 0.11, p < 0.0001). There was a 13% decrease in gain for the ipsilateral posterior canal (p = 0.02), perhaps because plugging affected the common crus in some cases. There was a 10% decrease in gain for excitation of the contralateral posterior canal (p < 0.0001), which likely reflects the loss of the inhibitory contribution of the plugged superior canal during head thrusts exciting the contralateral posterior canal. Mean AVOR gain did not change for any of the other canals, but two subjects did develop hypofunction of all three ipsilateral canals postoperatively. Middle fossa craniotomy and repair of SCD reduce the function of the operated superior canal but typically preserve the function of the other ipsilateral semicircular canals.

Anterior semicircular canal benign paroxysmal positional vertigo and positional downbeating nystagmus

Purpose The aim of this study was to describe the clinical features and video-oculographic findings in patients with anterior semicircular canal benign paroxysmal positional vertigo (BPPV). Materials and methods Study Design. This is a prospective case series. Setting. The study was set at an outpatient clinic in a general hospital. Patients. Fourteen individuals with symptoms of BPPV and positional downbeating nystagmus (pDBN) were included in the study. The diagnosis was based on a history of brief episodes of vertigo and the presence of pDBN confirmed in the video-oculographic examination during Dix-Hallpike test (DH) or head-hanging maneuver. Intervention. Patients were treated by particle repositioning maneuver and the effectiveness was evaluated at 7, 30, and 180 days posttreatment. The treatment was repeated up to 4 times if pDBN was persistent. Main Outcome Measures. The main outcome measure is the number of patients without pDBN at 30 and 180 days. Results Video-oculography showed a predominant pDBN in response to DH. Of the 14 patients, 7 had arterial hypertension, and 5 of 14 cases presented abnormalities on the caloric test. Horizontal spontaneous nystagmus was found in 3 of 14 individuals. Positional nystagmus at different positional test was observed in 5 of 14 individuals, suggesting the involvement of several canals. Of the 14 patients, 10 (71%) did not present vertigo, and the positional tests were negative at 30 days. However, 3 cases presented a positive DH with persistence of BPPV episodes and pDBN at 30 days, and another developed a contralateral posterior canal affectation. One of the patients maintained a persistent pDBN at 180 days despite the repeated maneuvers. Conclusions Video-oculography demonstrates that anterior canal BPPV is characterized by a predominant downbeating nystagmus in response to DH. These individuals may show alterations in the vestibular caloric, and they can have multicanal affectation.

Vertical eye movement-related type II neurons with downward on-directions in the vestibular nucleus in alert cats

The vestibular nuclei and the interstitial nucleus of Cajal (INC) have been regarded as key elements of the velocity-to-position integrator for vertical eye movements. This paper reports a class of type II vestibular neurons that receives input from the INC and carries vertical eye movement signals that appear to represent an intermediate stage of the integration process. Extracellular recordings were made from neurons in and near the vestibular nuclei in alert cats. We encountered 39 neurons that exhibited an intense burst of spikes for downward saccades and a position-related tonic activity during intersaccadic intervals (d-type II neurons). They had a very high saccadic sensitivity (4.3+/-2.7 spikes/deg, mean +/- SD) as well as a high position sensitivity (3.2+/-1.6 (spikes/sec)/deg). Unlike the bursts of motoneurons, the bursts of these neurons declined gradually with an exponential-like time course and lasted well beyond the end of saccades. The mean time constant of the burst decay was 139+/-43 ms. The d-type II neurons were excited with disynaptic or trisynaptic latencies following stimulation of the contralateral vestibular nerve. The responses to vertical head rotations suggested inputs from the contralateral posterior canal. The d-type II neurons were excited with short latencies following stimulation of the ipsilateral INC, suggesting that they receive a direct excitatory input from vertical eye movement-related INC neurons with downward on-directions. The d-type II neurons were located in the rostral portion of the vestibular nuclei and the underlying reticular formation. These results suggest that d-type II neurons may be interposed between the burst-tonic neurons in the INC and pure tonic neurons in the vestibular nuclei and contribute to the oculomotor velocity-to-position integration.

Principles of linear and angular vestibuloocular reflex organization in the frog.

We compared the spatial organization patterns of linear and angular vestibuloocular reflexes in frogs by recording the multiunit spike activity from cranial nerve branches innervating the lateral rectus, the inferior rectus, or the inferior obliquus eye muscles. Responses were evoked by linear horizontal and/or vertical accelerations on a sled or by angular accelerations about an earth-vertical axis on a turntable. Before each sinusoidal oscillation test in darkness, the static head position was systematically altered to determine those directions of horizontal linear acceleration and those planes of angular head oscillation that were associated with minimal response amplitudes. Inhibitory response components during angular accelerations were clearly present, whereas inhibitory response components during linear accelerations were absent. Likewise was no contribution from the vertical otolith organs (i.e., lagena and saccule) observed during vertical linear acceleration. Horizontal linear acceleration evoked responses that originated from eye muscle-specific sectors on the contralateral utricular macula. The sectors of the inferior obliquus and lateral rectus muscles on the utricle had an opening angle of 45 and 60 degrees, respectively and overlapped to a large extent in the laterorostral part of the utricle. Both sectors were coplanar with the horizontal semicircular canals. The sector of the inferior rectus muscle was narrow (opening 5 degrees), laterocaudally oriented, and slightly pitched up by 6 degrees. Angular acceleration evoked maximal responses in the inferior obliquus muscle nerve that originated from the ipsilateral horizontal and the contralateral anterior vertical canals in a ratio of 50:50. Lateral rectus excitation originated from the contralateral horizontal and anterior vertical semicircular canals in a ratio of 80:20. The excitatory responses of the inferior rectus muscle nerve originated exclusively from the contralateral posterior vertical canal. Measured data and known semicircular canal plane vectors were used to calculate the spatial orientation of maximum sensitivity vectors for the investigated eye muscle nerves in semicircular canal coordinates. Comparison of the directions of maximal sensitivity vectors of responses evoked by linear or angular accelerations in a given eye muscle nerve showed that the two vector directions were oriented about orthogonally with respect to each other. With this arrangement the linear and the angular vestibuloocular reflex can support each other dynamically whenever they are co-activated without a change in the spatial response characteristics. The mutual adaptation of angular and linear vestibuloocular reflexes as well as the differences in their organization described here for frogs may represent a basic feature common for vertebrates in general.

Eye movement-related vestibular neurons that receive input from the contralateral posterior canal

Eye movement-related vestibular neurons that receive input from the contralateral posterior canal

1524 Eye movement-related vestibular neurons that receive input from the contralateral posterior canal

In olfactory bulb slices from young rats, the field potential evoked in the glomerular layer by stimulation in the olfactory nerve layer consisted of two negative components: an early component (N1) which was blocked by bath application of the kainate/amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μM), and a late, prolonged component (N2; duration ≥350 msec) which was unaffected by CNQX, was enhanced by reduction of Mg2+ in the medium, and was blocked by the N-methyl-d-aspartate receptor antagonist dl-2-amino-5-phosphonovalerate (50 μM). A comparison of the glomerular field potentials before and after knife cuts that isolated the glomerular layer from the deeper layers of the olfactory bulb indicated that both N1 and N2 were produced by currents generated, for the most part, within the glomeruli. A laminar analysis of the field potential profiles evoked by olfactory nerve stimulation in standard medium, or in the presence of CNQX, showed that N1 and N2 reversed polarity in the external plexiform and mitral cell layers, suggesting that both components reflected synaptic responses in the distal, apical dendrites of mitral/tufted cells. Simultaneous field potential recordings in the glomerular layer and intracellular recordings in the mitral cell layer showed that: (i) N1 is associated with a brief, short-latency spiking activity of mitral cells, and (ii) N2 is associated with prolonged mitral cell spiking, since N2 and the late cell firing had similar time-courses, and both were blocked by bath applied dl-2-amino-5-phosphonovalerate.Application of the GABAA receptor antagonist bicuculline methiodide (10 μM) to standard medium selectively enhanced N2. The enhanced N2 was significantly reduced by dl-2-amino-5-phosphonovalerate. Strychnine, an antagonist of glycine receptors, had similar effects to those of bicuculline, but only at high concentrations that have been previously shown to block GABAA receptors; at low concentrations strychnine had no effect. The effects of all drugs tested were reversible.In the rat olfactory bulb, activation of the olfactory nerve evokes a kainate/AMPA receptor-mediated response in the distal, apical dendrites of mitral/tufted cells, followed by a slow N-methyl-d-aspartate receptor-mediated response which triggers prolonged discharge of mitral cells. GABAA receptor-mediated inhibition appears to suppress, preferentially, this N-methyl-d-aspartate receptor-mediated component. The presence of prolonged N-methyl-d-aspartate receptor-mediated postsynaptic activity at the primary synapses of the olfactory system may play a key role in olfactory processing by facilitating synaptic integration and plasticity.

Discharge characteristics of vestibular and saccade neurons in the rostral midbrain of alert cats

1. The interstitial nucleus of Cajal (INC) is involved in generation of vertical eye movements. To further explore the neural substrates of INC involvement in eye movements, we examined the discharge characteristics of neurons in the rostral midbrain that discharge for vestibular stimulation and saccades in alert cats. We analyzed 75 vestibular and saccade neurons (VSNs) that showed characteristic discharge during pitch rotation. Of these, 50 exhibited gradually increasing activity during upward slow phases and burst of action potentials during downward fast phases induced by downward pitch. The remaining 25 cells showed the opposite response pattern: gradually increasing activity during downward slow phases and a burst during upward fast phases induced by upward pitch. We classified these cells as downward VSNs and upward VSNs, respectively. The effects of electrical stimulation of the contralateral vestibular nerve were tested for 13 downward VSNs and 2 upward VSNs; all of them were activated at short latencies. 2. All vertical VSNs also showed burst activity associated with spontaneous saccades. The preferred direction for the burst was always the same for saccades and vestibular fast phases. The onset of burst activity preceded the onset of fast eye movement by an average of 35 +/- 14 ms (mean +/- SD). During fast eye movement in other directions, many of these cells also showed an inconsistent burst and the burst often began after the onset of saccades. Half of the upward VSNs examined showed a pause in activity during downward fast eye movement. 3. Burst parameters in the preferred directions were correlated with saccade parameters in half of the vertical VSNs examined, although the correlation coefficients were typically 0.5-0.6. 4. All vertical VSNs had irregular resting activity. Half of the downward VSNs examined showed eye-position sensitivity toward the direction opposite to the preferred direction for the burst activity but only if the period of analysis was restricted to discharge shortly before and after saccades. 5. All downward VSNs examined during sinusoidal rotation in several vertical planes received strong excitatory input from the contralateral anterior canal. Three of four upward VSNs examined received contralateral posterior canal excitation; the exception received ipsilateral posterior canal excitation.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensorimotor transformation in the cat's vestibuloocular reflex system. I. Neuronal signals coding spatial coordination of compensatory eye movements.

1. Coordinate transformation between vestibular and oculomotor reference frames is required to produce compensatory, vestibuloocular reflex (VOR) eye movements in view of the differing, although similar, peripheral three-dimensional geometry of the semicircular canals and the extraocular muscles. This transformation requires convergence of canal-specific signals at some level in the VOR arc. 2. To investigate the networks that underlie the transformation between oculomotor and vestibular reference frames for spatial coordination of compensatory eye movements, we recorded intracellularly in decerebrate cats from 93 identified second-order vestibulooculomotor and abducens internuclear neurons in the medial longitudinal fasciculus (MLF) between abducens and trochlear nuclei and characterized their spatial response properties. Neurons were classified according to whether they were activated from the labyrinth ipsi- or contralateral to the recording site and according to the canal from which they received their strongest input. Our data revealed neurons that carried single- and multiple-canal signals. 3. Twenty-two neurons received their primary input from the ipsi- or contralateral anterior canal (AC). Ten nonconvergent AC neurons responded maximally to rotation in a plane within 10 degrees of the AC plane, whereas 6 AC neurons received sufficient convergent input from the orthogonal vertical canals to shift their plane of maximum response > 10 degrees from the AC plane. Of 29 ipsi- and contralateral posterior canal (PC) neurons, 9 were non-convergent, whereas 13 received sufficient convergent input from the orthogonal vertical canals to shift their response plane > 10 degrees from the PC plane. The rotational responses of 6 AC and 12 PC neurons indicated that they received convergent input from the horizontal canals. However, when these responses were averaged, the horizontal responses tended to cancel, leaving little net population response to horizontal rotation. We also encountered 19 ascending axons of second-order neurons activated primarily from the horizontal canals. Nine of these received significant input from the vertical canals, but the mean population response was within 4 degrees of the horizontal canal plane. 4. Our results demonstrate single canal responses and specific canal-canal convergence within the population of vertical vestibulooculomotor neurons. This convergence is thought to participate in the coordinate transformation within the specific geometry of the VOR system. The data indicate response shifts of a population of AC neurons toward the plane of the inferior oblique muscle, and of a population of PC neurons toward the plane of the superior oblique muscle. Conspicuously absent are neurons that fall into the plane of either the superior rectus or the inferior rectus muscles. (ABSTRACT TRUNCATED AT 400 WORDS)

Responses of floccular Purkinje cells to sinusoidal vertical rotation and effects of muscimol infusion into the flocculus in alert cats

To understand how the flocculus is involved in the integration process in the vertical VOR, we examined the responses of floccular Purkinje cells during whole-body rotation in many vertical and horizontal planes in alert cats. The great majority of cells that responded to pitch rotation received excitation from the contralateral posterior canal, whereas the minority received excitation from the anterior canal, either on the ipsilateral or contralateral side; 35% of these vertical canal responding cells received convergent inputs, either from the ipsilateral or contralateral horizontal canal. After infusion of a GABA agonist (muscimol) into the area where many posterior canal responding Purkinje cells were recorded, the downward VOR induced by nose-up pitch was selectively impaired at low stimulus frequencies, together with an impairment of gaze holding after downward saccades. These results indicate a crucial role for posterior canal responding floccular Purkinje cells in generating downward eye position signals. Since the flocculus inhibits VOR interneurons related to the anterior and horizontal canals but not the posterior canal (Ito, 1984), our results suggest the possibility that posterior canal responding floccular Purkinje cells may form a transfloccular inhibitory pathway to anterior canal related VOR interneurons. This pathway may facilitate the positive feedback effect in the vestibular commissural inhibitory pathway between the co-planar vertical canals for the posterior canal signal, and may thereby be specifically involved in integration in the vertical VOR.

Influence of Vestibular and Visual Climbing Fiber Signals on Purkinje Cell Discharge in the Cerebellar Nodulus of the Rabbit

Extracellular recordings of the climbing fiber responses (CFRs) of single Purkinje cells were made in the cerebellar nodulus and ventral uvula of anesthetized pigmented rabbits. Natural vestibular and optokinetic stimuli were used to evoke responses from these Purkinje cells. Using a null response technique we sorted vestibularly-evoked CFRs into two canal-related categories. CFRs were related to either the ipsilateral anterior and contralateral posterior semicircular canal (iAC+cPC) or to the ipsilateral posterior and contralateral anterior semicircular canal (iPC+cAC). Both canal-related CFRs also responded to downward optokinetic stimulation.

Vertical nystagmus provoked by locally applied calorization at the vertical semicircular canals: a study in pigeons.

Calorization of the vertical semicircular canals in pigeons was performed by irrigation of water through a polyethylene cannula, that was attached at a selected site to the bony wall of the anterior or posterior canal of the labyrinth. The temperature differences between the stimulated site and the corresponding location inside the contralateral mastoid were detected by two thermistors. (a) Calorization of an anterior or posterior canal evoked a dissociated vertical nystagmus in both eyes. (b) The direction of the nystagmus and of the vertical slow phase velocity proved to be temperature-dependent. (c) The vertical eye movements disappeared when the stimulated canal was placed horizontally during calorization. (d) During parabolic flight the vertical and horizontal eye movements, elicited by sustained calorization, disappeared during short periods of microgravity and proved to be dependent to the g load. (e) All vertical canal calorizations evoked eye movements in the plane parallel to the stimulated canal. Left and right eye moved in opposite vertical directions, depending on the selected site of calorization on the vertical canal. (f) The same mode (warm or cold) of calorization of an anterior canal provoked similar vertical movements of both eyes as compared to the vertical eye movements that were induced by calorization of the contralateral posterior canal. The results are discussed in view of the 'convection current' and the 'expansion/contraction' theory and they are compared to previously reported results of vestibular nerve stimulation.

Vestibular responses and branching of interstitiospinal neurons.

1. Interstitiospinal neurons were activated by antidromic stimulation of the ventromedial funiculus of the spinal cord at C1 and C4 in cerebellectomized cats under chloralose anesthesia. 46% of these neurons responded only at C1 (N cells) and the remaining 54% responded at C4 also (D cells). There is no topographical difference in the location of N and D cells. Conduction velocities of N cells were significantly slower than those of D cells. 2. Stimulation of the contralateral whole vestibular nerve evoked firing of 31% of both N and D cells; some responded early enough to suggest disynaptic connections, many responded late. Stimulation of the ipsilateral whole vestibular nerve evoked firing of several cells, one spontaneously discharging D cell was inhibited. 3. Stimulation of the contralateral individual semicircular canal nerves evoked firing of 33% of N cells and 13% of D cells. Most of these responses were late. N cells responded not only to the vertical canals but also to the horizontal canal, whereas D cells responded to the horizontal canal, but seldom to the vertical ones. Most canal responding neurons received specific input, only two N cells received convergent input from both the anterior and horizontal canals. Stimulation of the ipsilateral canals did not evoke excitation of any cells tested; one D cell was inhibited by stimulation of the horizontal canal nerve. 4. Stimulation of the rostral medial vestibular nucleus evoked characteristic negative field potentials centered in the contralateral interstitial nucleus of Cajal (INC). Approximately 60% of both N and D cells received excitation from the contralateral vestibular nuclei. About 17% of these responding neurons received monosynaptic excitation, most frequently from the rostral medial nucleus. Stimulation of the ipsilateral vestibular nuclei evoked firing of 12% of both N and D cells. 5. Twenty-nine neurons were fired antidromically by weak stimuli applied to the ipsilateral vestibular nuclei. Twenty-seven of the 29 were activated only from C1 and were found in the INC (10 cells) and in the reticular formation dorsal to the INC (19 cells). Measurement of the spread of the effect of stimulus current and comparison of latencies to stimulation of the vestibular nuclei and C1 indicated that these neurons have axon collaterals going to the ipsilateral vestibular nuclei. Only one of them received excitation from the contralateral posterior canal, others did not respond to the labyrinth. Some were activated by stimulation of the vestibular nuclei.

Response characteristics of semicircular canal and otolith systems in cat. II. Responses of trochlear motoneurons

1. The electrical activity of single trochlear motoneurons (TMns) and axons of second order vestibular neurons presumably terminating on these motoneurons were studied during natural stimulation of semicircular canals and otolith organs in cats anesthetized with Ketamine. 2. Null point analysis showed that TMns received an excitatory canal input from the contralateral posterior canal, and labyrinthine lesion experiments suggested that the functionally synergistic, ipsilateral anterior canal provides an inhibitory input. A small number of motoneurons showed orthogonal canal convergence. 3. In addition to the canal projections most TMns received an otolithic input. Firing rate was proportional to lateral head tilt and was of the beta type. Most units also responded to pitch with an increase and decrease in firing rate on nose-up and nose-down positioning, respectively. Lesion experiments indicated that the otolith responses are the results of reciprocal innervation of TMns by contralateral (excitatory) and ipsilateral (inhibitory) otolith projections. 4. During sinusoidal rotation in yaw (canal only stimulation) the mean phase lag re acceleration of the response of TMns increased from 60 degrees at 0.025 Hz to 126 degrees at 1.0 Hz. In roll (canal plus otolith stimulation) the phase lag of TMn responses measured 180 degrees and 130 degrees at 0.025 and 1.0 Hz, respectively. Phase-lags measured in Vi and Vc axons were less by ca. 15 degrees. 5. The otolith contribution to TMn responses in roll was calculated by vectorial subtraction of the yaw from the roll responses: A phase lag of 10 (0.025 Hz) to 90 degrees (0.5 Hz) re. displacement was noted and gain was constant over the same range. Similar lag dynamics were revealed in TMns when studied during ramp displacement of the head. 6. The possible functional role of central canal-otolith convergence and the differences between the response of primary vestibular afferents and secondary vestibular neurons and TMns will be discussed.