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Unilateral cerebellar lobectomy was carried out in young and mature macaques. There is no significant abnormality in saccadic eye movement or in the vestibulo-ocular reflex. There is a constant drift of the eyes in the contralateral direction, and the abilities to maintain eccentric gaze and to make smooth pursuit movements are jointly affected. The syndrome is transient in young monkeys, and even bilateral cerebellar lobectomy carried out serially leaves some ability to hold eccentric gaze and pursue smoothly. The findings permit the conclusion that the primate oculomotor organization depends on the active maintainence of straight ahead gaze. Saccades move the eyes in a coordinated way, and are also associated with correlated innervation to hold the gaze in the new position. Since cerebellar interventions which cause centripetal drift also always produce defects in pursuit movements, the abilities to maintain eccentric gaze and to pursue smoothly must have a common origin. [ABSTRACT FROM AUTHOR]

Since during active eye-head turning the eyes move first and with higher velocity than the head, the lines of sight reach the target while the head is still moving. Then for the remaining duration of the head movement the eyes maintain their fixation by performing a movement which is counter to that of the head and perfectly compensates for it. It is agreed that this compensatory eye movement is critically influenced by visual, vestibular and neck afferents and that it is not initiated centrally. We have investigated a) the relative contribution of the vestibular and neck afferents to the compensatory eye movement made during active and passive head turning in monkeys and b) the mechanisms underlying the recovery of compensatory eye movements following either the removal of the vestibular or neck loop or both. Our results have shown that 1. normal monkeys display perfect ocular stability in darkness, 2. at least 95% of ocular stability is due to the vestibular loop, and 3. the contribution of the neck loop is negligible. Following bilateral vestibulectomy the recovery of compensatory eye movements occurs gradually and reaches 90% within seven weeks but only during active head movements. We have shown that there are at least three mechanisms underlying this recovery: 1. an increase in gain of the neck loop. 2. the occurrence of a centrally programmed compensatory eye movement, and 3. a recalibration of the saccadic and head motor system. [ABSTRACT FROM AUTHOR]

Intracellular and extracellular responses were recorded with glass micro-electrodes from motoneurons in the IIIrd and IVth cranial nuclei of anesthesized rabbits. Five subgroups of neurons innervating the superior rectus (SR), inferior oblique (IO), inferior rectus (IR), medial rectus (MR), and superior oblique (IVth) extraocular muscles were identified by their antidromic activation from the branches of the IIIrd and IVth cranial nerves. The relative positions of the subgroups thus determined were consistent with the histological data on the rabbit. In the SR, IO, IR, and IVth subgroups the effects of ipsilateral VIIIth nerve stimulation were inhibitory, producing disynaptic IPSPs, while the effects of contralateral VIIIth nerve stimulation were excitatory, producing disynaptic EPSPs. In the MR subgroup, however, a mixture of EPSPs and IPSPs was produced by VIIIth nerve stimulation: this was particularly clear on the ipsilateral side. Sites relaying these VIIIth nerve effects to each of the five subgroups were explored by direct stimulation of various brain stem sites. Stimulation of the superior vestibular nucleus (SV) produced IPSPs monosynaptically in all five subgroups on the ipsilateral side as well as in the contralateral MR subgroup. Stimulation of the medial vestibular nucleus (MV) produced EPSPs monosynaptically in all of the five subgroups on the contralateral side as well as in the ipsilateral MR subgroup. Stimulation of the brachium conjunctivum (BC) also produced EPSPs monosynaptically in the contralateral SR, IO, and IR subgroups. Further, while the recording electrode was placed within each of the five subgroups to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla and cerebellum were systematically tracked with a monopolar stimulating electrode. It was thus confirmed that the SV is the sole inhibitory relay site, while excitation is relayed by both the MV and the BC. The origin of the BC pathway was traced to the Y-Group for the IO, to the lateral nucleus of the cerebellum (LN) for the IR, and to both the Y-Group and the LN for the SR subgroup. [ABSTRACT FROM AUTHOR]

Intra- and extra-cellular responses were recorded with glass microelectrodes from motoneurons in the VIth cranial nuclei of anesthesized rabbits. VIth nucleus motoneurons were identified by their antidromic activation from the VIth nerve. In these motoneurons stimulation of the ipsilateral VIIIth nerve produced IPSPs with disynaptic latencies (mean and S.D., 1.08 ± 0.1 msec) while stimulation of the contralateral VIIIth nerve produced EPSPs with disynaptic latencies (mean and S.D., 1.20 ± 0.18 msec). Correspondingly, direct stimulation of the ipsilateral medial vestibular nucleus (MV), produced IPSPs with monosynaptic latencies (mean and S.D., 0.61±0.15 msec) while direct stimulation of the contralateral MV produced EPSPs with monosynaptic latencies (mean and S.D., 0.61±0.09 msec). Further, with the recording electrode placed within the VIth nucleus to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla was systematically tracked with a monopolar stimulating electrode. It was demonstrated that the inhibitory relay cells could be effectively stimulated in the rostral half of the ipsilateral MV and the excitatory relay cells in the rostral half of the contralateral MV. Pharmacological investigation suggested that the inhibitory transmitter involved in the vestibular inhibition is gamma amino-butyric acid or a related substance. Electric stimulation of the flocculus produced a prominant depression in the inhibitory vestibulo-ocular reflex pathway to the VIth nucleus, while the excitatory pathway was free of any similar flocculus inhibition. [ABSTRACT FROM AUTHOR]

Field and intracellular potentials evoked in the trochlear nucleus (TN) of the cat following stimulation of the ipsi and contralateral vestibular nerves (V, V) and the vestibular nuclei (VN) were recorded with microelectrodes. Single shock stimulation of either V or V evokes in the TN the presynaptic potentials, n and n, which are generated by the action currents of repetitively firing axons of vestibular neurons reaching the TN via the medial longitudinal fascicle (MLF). In the case of V stimulation a slow negative potential (n) follows the presynaptic components of the field complex while a slow positive potential (p-wave) is evoked by V stimuli. The n wave is composed of the excitatory synaptic and action currents generated in trochlear motoneurons (TMns) while the p-wave is produced by the inhibitory synaptic current. Disynaptic EPSPs and IPSPs are recorded intracellularly in TMns following V and V stimulation, respectively. Each synaptic potential shows a biphasic rising phase due to the synchronous n and n presynaptic barrage. On stimulation of the ipsilateral superior and contralateral medial vestibular nuclei, the latencies of the IPSPs and EPSPs, respectively, are reduced to the monosynaptic range. Thus, it has been directly demonstrated that the VN are the mediating links for both the short latency excitatory and inhibitory vestibuloocular reflexes. The above data suggest that IPSPs are for the most part generated at or near the soma of the motoneurons. As for the site of generation of the EPSPs, a predominantly dendritic origin is suggested. The organization of the neuronal circuitry is discussed in relation to the vestibular induced eye movements. [ABSTRACT FROM AUTHOR]

Examines the rotary-vestibular reactions of infants. Deviation of the eyes towards the direction of rotation; Direction of the face towards straight forward and the eyes towards slow horizontal excursions; Manifestation of axial head rotation.