In 1988 we introduced impulsive testing of semicircular canal 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 semicircular canal 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 6 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 7 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 semicircular canal stimulation, has become one of the giants of the vestibular world

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The Video Head Impulse Test.

The pathophysiology of spontaneous upbeat (UBN) and downbeat (DBN) nystagmus is reviewed in the light of several instructive clinical findings and experimental data. UBN due to pontine lesions could result from damage to the ventral tegmental tract (VTT), originating in the superior vestibular nucleus (SVN), coursing through the ventral pons and transmitting excitatory upward vestibular signals to the third nerve nucleus. A VTT lesion probably leads to relative hypoactivity of the drive to the motoneurons of the elevator muscles with, consequently, an imbalance between the downward and upward systems, resulting in a downward slow phase. The results observed in internuclear ophthalmoplegia suggest that the medial longitudinal fasciculus (MLF) is involved in the transmission of both upward and downward vestibular signals. Since no clinical cases of DBN due to focal brainstem damage have been reported, it may be assumed that the transmission of downward vestibular signals depends only upon the MLF, whereas that of upward vestibular signals involves both the MLF and the VTT. The main focal lesions resulting in DBN affect the cerebellar flocculus and/or paraflocculus. Apparently, this structure tonically inhibits the SVN and its excitatory efferent tract (i.e. the VTT) but not the downward vestibular system. Therefore, a floccular lesion could result in a disinhibition of the SVN-VTT pathway with, consequently, relative hyperactivity of the drive to the motoneurons of the elevator muscles, resulting in an upward slow phase. UBN also results from lesions affecting the caudal medulla. An area in this region could form part of a feedback loop involved in upward gaze-holding, originating in a collateral branch of the VTT and comprising the caudal medulla, the flocculus and the SVN, successively. Therefore, it is suggested that the main types of spontaneous vertical nystagmus due to focal central lesions result from a primary dysfunction of the SVN-VTT pathway, which becomes hypoactive after pontine or caudal medullary lesions, thereby eliciting UBN, and hyperactive after floccular lesions, thereby eliciting DBN. Lastly, since gravity influences UBN and DBN and may facilitate the downward vestibular system and restrain the upward vestibular system, it is hypothesized that the excitatory SVN-VTT pathway, along with its specific floccular inhibition, has developed to counteract the gravity pull. This anatomical hyperdevelopment is apparently associated with a physiological upward velocity bias, since the gain of all upward slow eye movements is greater than that of downward slow eye movements in normal human subjects and in monkeys.

Vertical nystagmus: clinical facts and hypotheses

An intact cerebellum is a prerequisite for optimal ocular motor performance. The cerebellum fine-tunes each of the subtypes of eye movements so they work together to bring and maintain images of objects of interest on the fovea. Here we review the major aspects of the contribution of the cerebellum to ocular motor control. The approach will be based on structural–functional correlation, combining the effects of lesions and the results from physiologic studies, with the emphasis on the cerebellar regions known to be most closely related to ocular motor function: (1) the flocculus/paraflocculus for high-frequency (brief) vestibular responses, sustained pursuit eye movements, and gaze holding, (2) the nodulus/ventral uvula for low-frequency (sustained) vestibular responses, and (3) the dorsal oculomotor vermis and its target in the posterior portion of the fastigial nucleus (the fastigial oculomotor region) for saccades and pursuit initiation.

/pdf/cerebellum-and-ocular-motor-control-33z9a7e16z.pdf

Cerebellum and ocular motor control.

Kobayashi, Yasushi, Kenji Kawano, Aya Takemura, Yuka Inoue, Toshihiro Kitama, Hiroaki Gomi, and Mitsuo Kawato. Temporal firing patterns of Purkinje cells in cerebellar ventral paraflocculus during ...

/pdf/temporal-firing-patterns-of-purkinje-cells-in-the-cerebellar-gyal1zumu9.pdf

Temporal firing patterns of Purkinje cells in the cerebellar ventral paraflocculus during ocular following responses in monkeys II. Complex spikes.

The inferior olivary nuclei clearly play a role in creating oculopalatal tremor, but the exact mechanism is unknown Oculopalatal tremor develops some time after a lesion in the brain that interrupts inhibition of the inferior olive by the deep cerebellar nuclei Over time the inferior olive gradually becomes hypertrophic and its neurons enlarge developing abnormal soma-somatic gap junctions However, results from several experimental studies have confounded the issue because they seem inconsistent with a role for the inferior olive in oculopalatal tremor, or because they ascribe the tremor to other brain areas Here we look at 3D binocular eye movements in 15 oculopalatal tremor patients and compare their behaviour to the output of our recent mathematical model of oculopalatal tremor This model has two mechanisms that interact to create oculopalatal tremor: an oscillator in the inferior olive and a modulator in the cerebellum Here we show that this dual mechanism model can reproduce the basic features of oculopalatal tremor and plausibly refute the confounding experimental results Oscillations in all patients and simulations were aperiodic, with a complicated frequency spectrum showing dominant components from 1 to 3 Hz The model's synchronized inferior olive output was too small to induce noticeable ocular oscillations, requiring amplification by the cerebellar cortex Simulations show that reducing the influence of the cerebellar cortex on the oculomotor pathway reduces the amplitude of ocular tremor, makes it more periodic and pulse-like, but leaves its frequency unchanged Reducing the coupling among cells in the inferior olive decreases the oscillation's amplitude until they stop (at approximately 20% of full coupling strength), but does not change their frequency The dual-mechanism model accounts for many of the properties of oculopalatal tremor Simulations suggest that drug therapies designed to reduce electrotonic coupling within the inferior olive or reduce the disinhibition of the cerebellar cortex on the deep cerebellar nuclei could treat oculopalatal tremor We conclude that oculopalatal tremor oscillations originate in the hypertrophic inferior olive and are amplified by learning in the cerebellum

/pdf/oculopalatal-tremor-explained-by-a-model-of-inferior-olivary-1pe5tx4fvk.pdf

Oculopalatal tremor explained by a model of inferior olivary hypertrophy and cerebellar plasticity.

1. Using single-unit recording and microstimulation methods, a group of flocculus target neurons (FTNs) were identified in the superior vestibular nucleus (SVN) and were studied using visual-vestibular interaction paradigms in alert squirrel monkeys. The response properties of these FTNs were characterized and compared with those of flocculus projecting neurons (FPNs). 2. FTNs were monosynaptically inhibited by single-pulse flocculus stimulation. The mean inhibition latency was 1.0 +/- 0.57 (SD) ms (n = 40) and the mean inhibition period was 6.7 +/- 2.69 ms. FTNs were also monosynaptically activated by VIIIth nerve stimulation. The mean response latency was 1.10 +/- 0.25 ms (n = 12). This is about the same as that of the FPNs (1.14 +/- 0.16 ms, n = 17). 3. The most characteristic response property of the FTNs is their firing rate modulation during visual following eye movements induced by sinusoidal rotation of an optokinetic drum at 0.5 Hz. This modulation was mainly related to eye velocity and was therefore termed a visual following eye velocity signal. The average eye velocity gain for all FTNs is 0.79 spikes.s-1.deg-1.s-1. In contrast, the responses of FPNs were not modulated under the same conditions. 4. Even though FTNs are inhibited by the flocculus, they have a relatively higher mean firing rate (124 +/- 23 spikes/s, n = 45) than FPNs (66 +/- 28 spikes/s, n = 42). The underlying mechanism may be related to commissural facilitation of FTNs and commissural inhibition of FPNs. 5. Thirty FTNs were identified as upward eye velocity FTNs because their firing rate increased for upward eye velocity during a visual following eye movement. The mean eye velocity sensitivity was 1.09 spikes.s-1.deg-1.s-1. Most of these cells also modulated during vestibuloocular reflex (VOR) in the dark, with firing rate increasing for downward head velocity. During VOR suppression the firing rate either did not modulate or modulated in phase with head or drum velocity with a smaller amplitude in comparison with the response during visual following. For all cells (with 1 exception) the response during a visual following eye movement can be approximately predicted by a linear vectorial subtraction of the response during VOR suppression and the response during VOR in the dark [modulation response vector of FTNs during visual following of the optokinetic stimulus (OKR) approximately modulation response vector of FTNs during VOR suppression-modulation response vector of FTNs during VOR in the dark].(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. I. General properties in comparison with flocculus projecting neurons.

1. Seven upward eye velocity flocculus target neurons (FTNs) and two flocculus projecting neurons (FPNs) were studied before and after ipsilateral flocculus inactivation by injection of muscimol in the alert squirrel monkey. An additional seven FTNs and seven FPNs recorded from the corresponding FTN and FPN areas were recorded after injection. Response properties of FTNs and FPNs were characterized by visual-vestibular interaction paradigms and were compared before and after flocculus inactivation. 2. In FTNs the mean firing rate increased within 2-5 min after muscimol injection in the flocculus and reached a plateau level in approximately 10-20 min. The average mean firing rate for seven FTNs increased from 117 to 174 spikes/s, a net increase of 57 spikes/s (49%). Accompanying the large increase of the mean firing rate, a spontaneous nystagmus in the darkness developed with the slow phase directed upward and contralateral. 3. The firing rate modulation during visual following of a sinusoidal optokinetic drum (0.5 Hz) decreased within 2-5 min after muscimol injection in the flocculus and reached a level of 0 in approximately 10-20 min for all FTNs. After that, some cells remained unmodulated for the period of recording; other cells gradually reversed their phase and developed a modulation out of phase with drum velocity. The depletion of the visual following eye velocity signal on superior vestibular nucleus (SVN) FTNs accompanied a small but consistent decrease of visual following eye velocity amplitude. The average maximum decrease of eye velocity was 26 +/- 9% (mean +/- SD). 4. After flocculus inactivation, even though the modulation response at 0.5 Hz during visual following was abolished, a slow-component eye velocity signal with the same on direction was revealed by a constant-velocity optokinetic stimulus. It is concluded that there are at least two kinds of eye velocity signals during the optokinetic response. These signals are combined at the FTNs and are subsequently relayed to the oculomotor neurons. The source of the fast component is the flocculus, and the source of the slow component is another, as yet unidentified brain structure. 5. The effect of flocculus inactivation on the modulation amplitude during the vestibuloocular reflex (VOR) in darkness was variable: two cells did not change, two cells decreased, and three cells increased their amplitude. The response phase tended to move toward a phase lead, but the change was small. The effect on VOR suppression was more prominent.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. II. Signal components revealed by reversible flocculus inactivation

1. Properties of superior vestibular nucleus (SVN) neurons and their projection to the cerebellar flocculus were studied in alert squirrel monkeys by using chronic unit and eye movement recording and microstimulation techniques. Twenty-three cells were antidromically activated from the ipsilateral flocculus, and seventeen of these were also orthodromically activated from the ipsilateral VIIth nerve at monosynaptic latencies. Only 1 of these 23 units was also inhibited by flocculus stimulation. According to their response properties, 9 of the cells were pure vestibular, 2 were vestibular-pause, and 12 were position-vestibular cells. The mean eye position sensitivity of these position-vestibular cells was significantly lower than that of cells projecting to the oculomotor nucleus (OMN). No eye movement-only neurons were antidromically activated from the flocculus. No cells could be antidromically activated from both the oculomotor nucleus and the flocculus.

Properties of superior vestibular nucleus neurons projecting to the cerebellar flocculus in the squirrel monkey

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Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. I. General properties in comparison with flocculus projecting neurons.

Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. II. Signal components revealed by reversible flocculus inactivation

Properties of superior vestibular nucleus neurons projecting to the cerebellar flocculus in the squirrel monkey