Rhythmic activities of secondary vestibular efferent fibers recorded within the abducens nucleus during vestibular nystagmus

The Video Head Impulse Test

In 1988, we introduced impulsive testing of semicircular canal (SCC) function measured with scleral search coils and showed that it could accurately and reliably detect impaired function even of a single lateral canal. Later we showed that it was also possible to test individual vertical canal function in peripheral and also in central vestibular disorders and proposed a physiological mechanism for why this might be so. For the next 20 years, between 1988 and 2008, impulsive testing of individual SCC function could only be accurately done by a few aficionados with the time and money to support scleral search-coil systems—an expensive, complicated and cumbersome, semi-invasive technique that never made the transition from the research lab to the dizzy clinic. Then, in 2009 and 2013, we introduced a video method of testing function of each of the six canals individually. Since 2009, the method has been taken up by most dizzy clinics around the world, with now close to 100 refereed articles in PubMed. In many dizzy clinics around the world, video Head Impulse Testing has supplanted caloric testing as the initial and in some cases the final test of choice in patients with suspected vestibular disorders. Here, we consider seven current, interesting, and controversial aspects of video Head Impulse Testing: (1) introduction to the test; (2) the progress from the head impulse protocol (HIMPs) to the new variant—suppression head impulse protocol (SHIMPs); (3) the physiological basis for head impulse testing; (4) practical aspects and potential pitfalls of video head impulse testing; (5) problems of vestibulo-ocular reflex gain calculations; (6) head impulse testing in central vestibular disorders; and (7) to stay right up-to-date—new clinical disease patterns emerging from video head impulse testing. With thanks and appreciation we dedicate this article to our friend, colleague, and mentor, Dr Bernard Cohen of Mount Sinai Medical School, New York, who since his first article 55 years ago on compensatory eye movements induced by vertical SCC stimulation has become one of the giants of the vestibular world.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

Commissural mirror-symmetric excitation and reciprocal inhibition between the two superior colliculi and their roles in vertical and horizontal eye movements

The functional roles of commissural excitation and inhibition between the two superior colliculi (SCs) are not yet well understood. We previously showed the existence of strong excitatory commissural connections between the rostral SCs, although commissural connections had been considered to be mainly inhibitory. In this study, by recording intracellular potentials, we examined the topographical distribution of commissural monosynaptic excitation and inhibition from the contralateral medial and lateral SC to tectoreticular neurons (TRNs) in the medial or lateral SC of anesthetized cats. About 85% of TRNs examined projected to both the ipsilateral Forel's field H and the contralateral inhibitory burst neuron region where the respective premotor neurons for vertical and horizontal saccades reside. Medial TRNs received strong commissural excitation from the medial part of the opposite SC, whereas lateral TRNs received excitation mainly from its lateral part. Injection of wheat germ agglutinin-horseradish peroxidase into the lateral or medial SC retrogradely labeled many larger neurons in the lateral or medial part of the contralateral SC, respectively. These results indicated that excitatory commissural connections exist between the medial and medial parts and between the lateral and lateral parts of the rostral SCs. These may play an important role in reinforcing the conjugacy of upward and downward saccades, respectively. In contrast, medial SC projections to lateral SC TRNs and lateral SC projections to medial TRNs mainly produce strong inhibition. This shows that regions representing upward saccades inhibit contralateral regions representing downward saccades and vice versa.

Synaptic inputs and their pathways from fixation and saccade zones of the superior colliculus to inhibitory burst neurons and pause neurons

The caudal part of the superior colliculus (SC) plays an important role in the generation of saccades, whereas the rostral part of the SC is considered to be involved in visual fixation. The present study was performed to determine neural connections from the rostral and caudal parts of the SC to inhibitory burst neurons (IBNs) and pause neurons (PNs) in the nucleus raphe interpositus in the anesthetized cat, and to reveal the functional role of the rostral SC on eye movements. The intracellular potentials from IBNs and PNs were recorded, and the effects of stimulation of the SC on these neurons were analyzed. The results show that IBNs receive monosynaptic excitation from the contralateral caudal SC, and disynaptic inhibition from the ipsilateral caudal SC via contralateral IBNs. In addition, IBNs receive disynaptic inhibition from the rostral part of the SC on either side via inhibitory interneurons other than IBNs. Intracellular recording from PNs revealed that they receive convergent excitation from the rostral parts of the bilateral superior colliculi and that the rostral SC inhibits IBNs on both sides via PNs. The neural connections determined in this study support the functional independence of the rostral SC and are consistent with the notion that the "fixation zone" is localized in the rostral SC. These results show that the fixation zone in the rostral SC may suppress the initiation of bilateral saccades via pause neurons.

Commissural excitation and inhibition by the superior colliculus in tectoreticular neurons projecting to omnipause neuron and inhibitory burst neuron regions

Previous electrophysiological studies have shown that the commissural connections between the two superior colliculi are mainly inhibitory with fewer excitatory connections. However, the functional roles of the commissural connections are not well understood, so we sought to clarify the physiology of tectal commissural excitation and inhibition of tectoreticular neurons (TRNs) in the "fixation " and "saccade " zones of the superior colliculus (SC). By recording intracellular potentials, we identified TRNs by their antidromic responses to stimulation of the omnipause neuron (OPN) and inhibitory burst neuron (IBN) regions and analyzed the effects of stimulation of the contralateral SC on these TRNs in anesthetized cats. TRNs in the caudal SC (saccade neurons) projected to the IBN region, and received mono- or disynaptic inhibition from the entire rostrocaudal extent of the contralateral SC. In contrast, TRNs in the rostral SC projected to the OPN or IBN region and received monosynaptic excitation from the most rostral level of the contralateral SC, and mono- or disynaptic inhibition from its entire rostrocaudal extent. Among the rostral TRNs with commissural excitation, IBN-projecting TRNs also projected to Forel's field H (vertical gaze center), suggesting that they were most likely saccade neurons related to vertical saccades. In contrast, TRNs projecting only to the OPN region were most likely fixation neurons. Most putative inhibitory neurons in the rostral SC had multiple axon branches throughout the rostrocaudal extent of the contralateral SC, whereas excitatory commissural neurons, most of which were rostral TRNs, distributed terminals to a discrete region in the rostral SC.

Physiological Characterization of Synaptic Inputs to Inhibitory Burst Neurons From the Rostral and Caudal Superior Colliculus

The caudal superior colliculus (SC) contains movement neurons that fire during saccades and the rostral SC contains fixation neurons that fire during visual fixation, suggesting potentially different functions for these 2 regions. To study whether these areas might have different projections, we characterized synaptic inputs from the rostral and caudal SC to inhibitory burst neurons (IBNs) in anesthetized cats. We recorded intracellular potentials from neurons in the IBN region and identified them as IBNs based on their antidromic activation from the contralateral abducens nucleus and short-latency excitation from the contralateral caudal SC and/or single-cell morphology. IBNs received disynaptic inhibition from the ipsilateral caudal SC and disynaptic inhibition from the rostral SC on both sides. Stimulation of the contralateral IBN region evoked monosynaptic inhibition in IBNs, which was enhanced by preconditioning stimulation of the ipsilateral caudal SC. A midline section between the IBN regions eliminated inhibition from the ipsilateral caudal SC, but inhibition from the rostral SC remained unaffected, indicating that the latter inhibition was mediated by inhibitory interneurons other than IBNs. A transverse section of the brain stem rostral to the pause neuron (PN) region eliminated inhibition from the rostral SC, suggesting that this inhibition is mediated by PNs. These results indicate that the most rostral SC inhibits bilateral IBNs, most likely via PNs, and the more caudal SC exerts monosynaptic excitation on contralateral IBNs and antagonistic inhibition on ipsilateral IBNs via contralateral IBNs. The most rostral SC may play roles in maintaining fixation by inhibition of burst neurons and facilitating saccadic initiation by releasing their inhibition.

Neurotoxic effect of lead at low concentrations

The effects of lead exposure at low concentrations were evaluated by studying the post-rotatory nystagmus (PRN) in two groups of rats exposed for 3 months to 50 parts per million (ppm) of sodium acetate and 50 ppm of lead acetate, respectively, in the drinking water. Only animals treated with lead acetate showed changes of the PRN parameters which were significantly related to the concentration of lead in the blood and in brain structures. The patterns of PRN responses were characterized and classified into four types: progressively inhibitory (40%), prematurely inhibitory (25%), late inhibitory (25%), and excitatory-inhibitory (10%). No alterations of the PRN parameters were observed in the animals treated with sodium acetate. The results show that exposure to lead, even at low concentrations, impairs both sensory and motor functions. The findings also point out that the vestibular system and brain stem structures which generate and control the PRN represent targets of the action of this heavy metal. Finally, the results indicate that the evaluation of the vestibulo-ocular-reflex can provide a test suited for the screening of the neurotoxic effects of lead even in the absence of clinical signs typical of lead intoxication.

Behavior of neurons in the abducens nucleus of the alert cat--I. Motoneurons

The activity of 53 antidromically identified abducens motoneurons was analyzed in alert cats during spontaneous and vestibular induced eye movements. Conduction velocities ranged from 13 to 70 m/s and all motoneurons increased their discharge rates with successive eye positions in the abducting direction. Motoneurons were recruited from -19 degrees to +7 degrees. Within the oculomotor range frequency saturation was never observed for any cell. The slope of rate-position (k) relationships ranged from 2 to 17.7 spikes/s/deg (n = 40, mean 8.7 +/- 2.5). Regression analysis showed that the rate-position plots could be fit by straight lines but in most cases exponential curves produced slightly better statistical fits. Steeper slopes suggest that successively larger increases in k are required for the lateral rectus muscle to maintain more eccentric fixations in the on direction. Interspike intervals for a constant eye position exhibited low variability (less than 3.5%) for fixations shorter than 1 s. Over longer periods, variability increased in proportion to the duration of the fixation in exponential-like fashion up to 14%. Abducens motoneurons showed considerable variability in frequency during repeated fixations of the same eye position. Discharge rates were found to depend upon both the direction of the previous eye movement and, more importantly, the animal's level of alertness. The rate-position regression lines for fixation periods after saccades in the on direction significantly differed in slopes (100%) and thresholds (20%) from those in the off direction. The observed static hysteresis in abducens motoneuron behavior was in opposite direction to that previously described for the mechanical properties of the lateral rectus. This suggests both neural and mechanical factors are significantly involved in determining final eye position. The animal's level of alertness was evaluated in this study by counting the number of saccadic movements/s occurring in "alert" (1 +/- 0.2 saccades/s), and "drowsy" (0.5 +/- 0.2 saccades/s) circumstances. Comparison of the rate-position regression lines between the two conditions showed a significant decrease in slopes (100%) and elevation of thresholds (70%). Discharge rate of abducens motoneurons increased abruptly 8.9 +/- 2.8 ms prior to saccades in the horizontal on direction, and decreased 14.8 +/- 4.05 m before saccades in the off direction. During purely vertical saccades the firing frequency of abducens motoneurons did not change. Burst frequency did not saturate during saccades, but increased with saccadic velocity in a linear fashion.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional organization of premotor neurons in the cat medial vestibular nucleus related to slow and fast phases of nystagmus

Extracellular spikes were recorded from secondary vestibular neurons in the cat medial vestibular nucleus (MVN) and were identified as type I or II neurons by horizontal rotation. Type I neurons were further classified as excitatory or inhibitory premotor neurons on the basis of their axonal termination in the contralateral or ipsilateral abducens nucleus, demonstrated by spike-triggered averaging of abducens nerve discharges, or by antidromic activation using systematic microstimulation within the abducens nucleus. Both excitatory and inhibitory premotor type I MVN neurons exhibited a rhythmic modulation of their firing rate in association with nystagmus elicited by rotation or electrical stimulation of the vestibular nerve. Their tonic activity during the slow phase was suppressed at the quick phase directed to the ipsilateral side. Excitatory type I MVN neurons terminating in the contralateral abducens nucleus sent collateral axons to the contralateral MVN. These commissural neurons also showed a nystagmus-related discharge pattern. Type II MVN neurons activated at short latency by stimulation of the contralateral vestibular nerve exhibited burst discharges when the activity of ipsilateral type I neurons was suppressed at the quick phase. These type II neurons made monosynaptic inhibitory connection with type I neurons as shown by the post-spike average of the membrane potential of secondary MVN neurons triggered from spikes of single type II neurons. Thus, the inhibitory action originating from burst activity of type II MVN neurons contributes to suppression of type I premotor MVN neurons during fast eye movements.

Effects of ocular fixation on perrotatory nystagmus in damped pendular rotation test

Effects of ocular fixation on pendular rotation nystagmus were investigated in 65 patients. There were 25 with peripheral vestibular or vestibulo-cochlear disorders, 17 with central vestibular disorders, five with congenital nystagmus, 16 patients over 60 years old with vertigo in whom peripheral vestibular disorders were ruled out, however, the causes were unknown. Damped pendular rotation test (DPRT) was performed both under darkness and employing mental arithmetic and under ocular fixation. These findings were related to those of caloric vestibular suppression test (VST) by Takemori and those of optokinetic pattern test (OKP), eye tracking test (ETT), and spontaneous nystagmus. Thirteen of 17 patients with central vestibular disorders and five with congenital nystagmus showed loss of visual suppression during ocular fixation in DPRT, whereas in cases of peripheral lesions, visual suppression was observed. Loss of visual suppression during ocular fixation in DPRT was often seen in cases of brainstem and cerebellar lesions. In brainstem lesions, perrotatory nystagmus was evoked during ocular fixation, whereas no nystagmus was seen in darkness with eyes open. In cerebellar lesions, perrotatory nystagmus was partly suppressed or decreased during ocular fixation. Relationships between the direction of the visual suppression during ocular fixation in DPRT and the side of the lesion were not apparent. Ocular fixation test in DPRT has a diagnostic value not only for central lesions, but for differentiating brainstem lesion from cerebellar lesion with the findings in DPRT under darkness. The findings under ocular fixation in DPRT are closely related to those of VST in cases of caloric nystagmus.

Eye movement related activity and morphology of second order vestibular neurons terminating in the cat abducens nucleus

Intracellular records were obtained from axons of second order vestibular neurons in, and around, the left abducens nucleus in alert cats implanted with stimulating electrodes on both vestibular nerves and the left VIth nerve. Twelve secondary vestibular neurons were identified by their increase in firing rate with horizontal head rotation to the left and/or increasing eye position to the right. Following HRP injection, somatic location, axonal trajectory and termination sites were determined. Each of the above cells collateralized extensively in the abducens nucleus in a fashion consistent with their being either inhibitory (n = 7; left) or excitatory (n = 6; right) vestibular neurons in the disynaptic horizontal vestibulo-ocular reflex pathway. These vestibular neurons also arborized extensively in other posterior brainstem eye-movement related areas as well as sending an axon to the spinal cord.

Postsynaptic inhibition underlying spike suppression of secondary vestibular neurons during quick phases of vestibular nystagmus

Synaptic mechanisms of spike suppression of vestibular neurons during quick phases of vestibular nystagmus were investigated by intracellular recording in the rostrolateral part of the cat medial vestibular nucleus. When repetitive spike discharges of vestibular neurons were abruptly suppressed at the quick phase, the membrane potential shifted steeply in the hyperpolarizing direction. After the commissural IPSP was inverted into depolarization by intracellular injection of Cl-ions, the hyperpolarizing deflection of the membrane potential at the quick phase was also inverted into a depolarizing potential. The results indicate that an abrupt generation of IPSPs in vestibular neurons underlies the quick phase suppression of spike activity in these neurons.

Properties of vestibular neurones projecting to neck segments of the cat spinal cord

1. Vestibular neurones projecting to the upper cervical grey matter (vestibulocollic neurones) were identified by localized microstimulation in the C3 segment of the cat spinal cord.2. The neurones were found in the lateral (Deiters'), medial and descending nuclei bilaterally and projected to the spinal cord in the lateral and medial vestibulospinal tracts (LVST and MVST). Ipsilateral axons of Deiters' neurones were mostly in the LVST, axons of medial and descending neurones in the MVST; a few Deiters' neurones had axons in the MVST; some descending neurones had axons in the LVST. Most axons of contralateral neurones were in the MVST.3. The axons of 62% of ipsilateral vestibulocollic Deiters' neurones not only gave off a collateral to C3, but also extended as far as the cervical enlargement (;branching'); some of these neurones projected as far as the upper thoracic cord, almost none to the lumbar cord. Ipsilateral descending nucleus neurones branch in the same fashion, but there is no branching in the relatively small medial nucleus population.4. A large majority of vestibulocollic neurones receive monosynaptic excitation from the ipsilateral labyrinth and a number are inhibited by stimulation of the contralateral labyrinth (commissural inhibition). It is possible that commissural inhibition acts on a broad population of vestibular neurones involved in the control of eye, head and trunk movement.5. Vestibulocollic neurones do not make up a homogeneous population acting only on the neck. Instead it is likely that subpopulations, for example branching and non-branching neurones, have different functions.

Horizontal vestibular nystagmus. II. Activity patterns of medial vestibular neurones during nystagmus

1. Identified medial vestibular neurones were studied before, during and after nystagmogenic labyrinthine stimulation in the Encéphale isolé cat. Motor discharges were simultaneously recorded from the contralateral abducens nerve. 2. 87.9% of the recorded neurones showed changes in tonic firing frequency during repetitibe labyrinthine stimulation with no nystagmic modulation in behaviour. 3. The secondary vestibular neurones projecting monosynaptically to the contralateral abducens motoneurones were included in this non-rhythmic population. 4. Only 2% of the recorded population fired rhythmically both during nystagmogenic stimulation and poststimulation nystagmus. These neurones showed plasticity in behaviour regarding the phases of nystagmus recorded from the contralateral abducens nerve. 5. The remaining 10.1% of the medial vestibular neurones were excited or inhibited by repetitive stimulation of the labyrinth but showed burst firing pattersn correlated with poststimulation nystagmic discharges.

The damped pendular rotation test in central vestibular disorders

Nystagmic rhythm in the damped pendular rotation test (DPRT) was analysed electronystagmographically both in healthy subjects (control) and patients with peripheral and central vestibular lesions. The patients were tested in a semi-dark room with eyes open and, in addition, a serial subtraction task was performed to maintain the state of alertness throughout the rotation. In the controls 49 years of age and under dysrhythmia was found in 11%, while in persons 50 years of age and over the abnormal rate was 29%. In patients with acoustic tumors, who had undergone surgery in which the translabyrinthine and middle fossa approach had been utilized, dysrhthmia was present post-operatively in only a few cases. In patients 49 years of age and under, operated on through the suboccipital approach, dysrhythmia was found post-operatively, or was at least more remarkable than pre-operatively in most cases. Nystagmic rhythm was, on the other hand, regular with cerebellar degenerative process or atrophy. Pre-operative dysrhythmia in patients with cerebellar and cerebral tumors was in some cases post-operatively converted to regular rhythmic nystagmus in pendular stimulations. Therefore, in cerebellar and cerebral tumors, dysrhythmia was thought to be due to secondary effects in brainstem, as these tumors can cause intracranial hypertension and circulatory disturbances in surrounding brain tissues. In Menière's disease and sudden deafness, dysrhythmia and salvos in DPRT were found in the age groups, 49 years of age and under and 50 years and over, as frequently as in the healthy and control group. In one case with Menière's disease which showed dysrhythmia, labyrinthectomy was done. This surgical manipulation of the labyrinth did not eliminate dysrhythmia, though a temporary improvement was obtained. This result indicates that the dysrhythmia may be of central origin. These findings also suggest that nystagmic rhythm in DPRT with mental arithmetic is significant in assessing brainstem dysfunctions, when the age and state of alertness of the patient are taken into consideration, or when comparing the pre- and post-operative findings in intracranial lesions.