Optokinetic nystagmus and the vestibular nuclei

Galvanic stimulation of the vestibular periphery in guinea pigs during passive whole body rotation and self-generated head movement

Irregular vestibular afferents exhibit significant phase leads with respect to angular velocity of the head in space. This characteristic and their connectivity with vestibulospinal neurons suggest a functionally important role for these afferents in producing the vestibulo-collic reflex (VCR). A goal of these experiments was to test this hypothesis with the use of weak galvanic stimulation of the vestibular periphery (GVS) to selectively activate or suppress irregular afferents during passive whole body rotation of guinea pigs that could freely move their heads. Both inhibitory and excitatory GVS had significant effects on compensatory head movements during sinusoidal and transient whole body rotations. Unexpectedly, GVS also strongly affected the vestibulo-ocular reflex (VOR) during passive whole body rotation. The effect of GVS on the VOR was comparable in light and darkness and whether the head was restrained or unrestrained. Significantly, there was no effect of GVS on compensatory eye and head movements during volitional head motion, a confirmation of our previous study that demonstrated the extravestibular nature of anticipatory eye movements that compensate for voluntary head movements.

Optokinetic and vestibular responsiveness in the macaque rostral vestibular and fastigial nuclei

We recorded from rostral vestibular (VN) and rostral fastigial nuclei (FN) neurons that did not respond to eye movements during three-dimensional (3D) vestibular and optokinetic stimulation (OKS). The majority of neurons in both areas (76 and 69% in VN and FN, respectively) responded during both rotational and translational motion. Preferred directions scattered throughout 3D space for translation but showed some preference for pitch/roll over yaw for rotation. VN/FN neurons were also tested during OKS while monkeys suppressed their optokinetic nystagmus by fixating a head-fixed target. Only a handful of cells (VN: 17%, FN: 6%) modulated during 0.5-Hz OKS suppression, but the number of responsive cells increased (VN: 40%, FN: 48%) during 0.02-Hz OKS. Preferred directions for rotation and OKS were not matched on individual neurons, and OKS gains were smaller than the respective gains during rotation. These results were generally similar for VN and FN neurons. We conclude that optokinetic-vestibular convergence might not be as prevalent as earlier studies have suggested.

Pretectofugal fibers from the nucleus of the optic tract in monkeys

The nucleus of the optic tract (NOT) is the visuo-motor relay between the retina and preoculomotor structures in the pathway mediating optokinetic nystagmus (OKN). NOT lesions in monkeys produce no OKN toward the lesioned side. Then, efferent fibers from the NOT course through the brainstem and may reach the vestibular nucleus, which is proposed to be the final nucleus to the motor nucleus. In the present study, the tracer was injected through a micropipette in the NOT in four monkeys. Labeled terminals were observed ipsilaterally in the parabigeminal nucleus, superficial layers of the superior colliculus, dorsal and lateral terminal nuclei of the accessory optic system and pretectal nuclei and contralaterally in the NOT and superficial layers of the superior colliculus. Descending fibers from the NOT consisted of two major pathways: (1) fibers descended medially from the injection site through the reticularis pontis oralis to reach the lateral part of the ipsilateral nucleus reticularis tegmenti pontis; (2) fibers projecting into the dorsal cap of inferior olive, by far the greatest number of labeled fibers, descended ventrally along the lateral border of the reticularis pontis oralis and reached the medial lemniscus where they descended further and branched into the dorsolateral pontine nucleus, the lateral part of the nucleus reticularis tegmenti pontis, the peduncular pontine nucleus, the lateral pontine nucleus, the nucleus prepositus hypoglossi, the medial vestibular nucleus and finally the dorsal cap of the inferior olive. Consistent with the physiological data, the direct terminals to the medial vestibular nucleus could serve to drive the storage mechanisms and to produce OKN in the monkey.

Visual and vestibular projections to tongue motoneurons

The rabbit hypoglossal neurons, localized in the mediocaudal part of the XIIth nucleus, receive visual and vestibular inputs able to induce reflex responses functionally driven both to prepare the oral cavity for food reception and to ensure a correct postural arrangement of the tongue. The aim of the present research was to show a possible convergence of visual input on the hypoglossal neurons modulated by the vestibular system and, thus, demonstrate that visual input plays a part in the control of the tongue posture. It was found that 78% of tested neurons responded to both photic stimulation of the retinae, performed by a conventional strobe unit, and electrical stimulation of the labyrinth, with different patterns of response. Moreover, visual input significantly modified both the hypoglossal neuron response and the electromyographic genioglossal response to caloric stimulation of the labyrinth. Because a significant visual influence on the hypoglossal nucleus response and the genioglossal muscle response to labyrinthine stimulation was observed, it can be concluded that vision does integrate the spatial information of the labyrinth to modulate the postural tone of the tongue muscles.

Role of the nucleus of the optic tract in monkeys in relation to optokinetic nystagmus

Nine monkeys were used in order to clarify the role of the nucleus of the optic tract (NOT) in the generation of optokinetic nystagmus (OKN). In 3 monkeys whose NOTs were almost totally damaged, optokinetic stimulus toward the lesioned side failed to generate either eye deviation or OKN and revealed only voluntary saccades, whereas that toward the side contralateral to the lesion generated normal gain of OKN. The phenomenon was identical in either monocular or binocular stimulation. In two of 3 monkeys whose NOTs were partially destroyed, optokinetic stimulus toward the lesioned side produced OKN, but the gain of OKN, but the gain of OKN was at maximum less than 60% in both binocular and monocular stimulation. In the remaining one monkey whose NOT was injured, but superficially, OKN showed normal gain in both directions. In 3 other monkeys whose NOTs were spared, this OKN asymmetry was not observed. Pursuit and saccadic eye movements were normal in all NOT-lesioned monkeys. Visually induced eye movements in the vertical axis were likewise normal. The present experiment suggests that the NOT in monkeys may be the first relay station in the horizontal optokinetic path and that in primates as in non-primates both crossed and uncrossed fibers reach motor centers for OKN via the NOT.

Visual input to the hypoglossal nucleus

Photic stimulation of the retina elicits in the mediocaudal part of the XIIth nucleus the appearance of pronounced field potentials and responses of single hypoglossal units. The latency of the evoked potentials, characterized by a complex sequence of positive-negative waves, was 32.75 +/- 2.5 ms for the initial component p1, 42.25 +/- 3.30 ms for n1, 49.75 +/- 3.09 ms for p2, and 67 +/- 3.94 ms for n2. The hypoglossal neurons, antidromically identified by electrical stimulation of the XIIth nerve, responded with several combinations of excitation-inhibition at various latencies. Nineteen percent of the units responded to both retinae and 10% only to one retina, mainly to the contralateral one. As the mediocaudal part of the hypoglossal nucleus receives vestibular information, it can be concluded that the hypoglossal units adjust their firing in relation to head position and also by using visual information.

Analysis of signal content of Purkinje cell responses to optokinetic stimuli in the rabbit cerebellar flocculus by selective lesions of brainstem pathways

The heterogeneous signal content of floccular Purkinje cell responses to optokinetic stimuli was analyzed in alert rabbits by means of selective lesions to brainstem pathways. Extracellular spike activities of Purkinje cells were recorded from rostral areas of the flocculus where local electrical stimulation elicited abduction of the ipsilateral eye. Chronic unilateral destruction of the nucleus reticularis tegmenti pontis, interrupting the visual mossy fiber afferent pathway to the flocculus, reduced the gain of the optokinetic eye movement (OKR) to one-third of the control. Concomitantly, simple spike responses of Purkinje cells to optokinetic stimuli were reduced to less than one-third of the control values. Severance of the visual climbing fiber afferent pathway by rostral inferior olivary lesions reduced the OKR gain little, and decreased the simple spike responses of the Purkinje cells only slightly. Bilateral lesions of the rostral half of the medial vestibular nucleus and rostro-ventral part of the lateral vestibular nucleus, which reduced the eye velocity in the OKR to less than one-third of the control value, did not induce any appreciable change in the simple spike responses of the Purkinje cells. It is concluded that visual mossy fiber signals are the most dominant factor which determines Purkinje cell responses to optokinetic stimuli, while visual climbing fiber signals and eye velocity mossy fiber signals make only subsidiary contributions.

Pontine metastasis with dissociated bilateral horizontal gaze paralysis

A clinicopathological case of pontine metastatic tumour is reported with an oculomotor syndrome including bilateral horizontal gaze paralysis affecting saccades and foveal pursuit. During full-field pursuit, oculocephalic movement, and after caloric stimulation, the right eye alone was able to move slowly only 30 degrees to the right of the midline. Convergence and vertical eye movements were unaffected in either eye. The lesion lay in the whole left pontine tegmentum and partly in the right pontine tegmentum which was also strongly compressed and displaced to the right. The bilateral horizontal gaze paralysis resulted from damage to both paramedian pontine reticular formations. The unusual combination of an absence of foveal pursuit with the persistence of a rightward full-field pursuit analysed in the light of recent experimental work, suggested a clear separation between the brainstem pathways of these two types of pursuit movement. Lastly, according to our data and other clinicopathological findings previously reported, it appeared also that the paramedian pontine reticular formation role in the triggering of voluntary vertical saccades is less significant in man than in the monkey.

The pontine reticular formation is part of the output pathway for amphetamine- and apomorphine-induced lateral head movements: evidence from experimental lesions in the rat

Electrolytic lesions in the caudal part of the medial pontine reticular formation (PRF) in the rat abolished apomorphine- or amphetamine-induced stereotypic lateral head movements and turning to the damaged side. Rats with unilateral PRF lesions turned and circled only to the intact side, and rats with bilateral lesions did not turn at all. PRF lesions were also effective in abolishing or reducing amphetamine-induced circling in rats with unilateral nigrostriatal bundle damage. Thus, head movements induced by dopamine agonists are added to the class of head movements mediated by the PRF. It is proposed that the decussations of the motor pathways for drug-induced turning are located between the midbrain and caudal pons.

Optokinetic suppression of aberrant vestibular reactions: an observation

It has long been known that optokinetic nystagmus may be modified by vestibular inputs. We have suppressed an aberrant vestibular response and associated gastrointestinal sensations by use of an optokinetic stimulus simultaneous to the positional stimulus in a patient with paroxysmal positional vertigo. This single subject study utilized several optokinetic conditions, and patterned its stimulus presentations to control for visual fixation and vestibular habituation. Objective recordings of eye movements (ENG) were made simultaneously with subjective evaluation of "dizziness" on a simple magnitude estimation scale.

Optokinetic test comprising both acceleration and constant velocity stimulation (ACV-OKN test)

Normal subjects were exposed to two kinds of optokinetic stimulation, viz. at speeds of 60 degrees/s and 90 degrees/s preceded by acceleration at 6 degrees/s2 and 4.5 degrees/s2, respectively. In most subjects the speed of the eye in the slow nystagmus phase equalled the speed of the rotating device during the acceleration at velocities up to about 60 degrees/s. The eye then lagged behind the speed of the optokinetic stimulus. The upper limit for linear increase in the eye speed of the slow nystagmus phase has been named the "optokinetic fatigue threshold". A new clinical test permitting quantitative assessment of the optokinetic response has been introduced. The maximum eye speed in the slow nystagmus phase has been found to be the most appropriate parameter. Presentation of the results in the form of special charts, named optokinograms, is recommended. Optokinetic disturbances of varying kinds were noted mostly in patients with CNS disorders, but also in patients with diseases of the inner ear.