Horizontal eye movement signals in second-order vestibular nuclei neurons in the cat

Signal processing in the vestibulo-ocular reflex

In this chapter, Robinson develops models to account for the neural control of the vestibulo-ocular reflex in response to horizontal and vertical head rotations. By combining knowledge of the discharge properties of the several subpopulations of neurons that contribute to vestibular eye movements with their known anatomical connections, these models seek to explain how specific signals are combined to enable the ocular motoneurons to program vestibular eye movements that compensate for head perturbations. Details such as the integration of raw vestibular signals, differences in the neuronal processing for vertical versus horizontal reflexes, and the role of individual pathways such as the medial longitudinal fasciculus are discussed.

Motion perception during selfmotion: The direct versus inferential controversy revisited

According to the traditional inferential theory of perception, percepts of object motion or stationarity stem from an evaluation of afferent retinal signals (which encode image motion) with the help of extraretinal signals (which encode eye movements). According to direct perception theory, on the other hand, the percepts derive from retinally conveyed information only. Neither view is compatible with a perceptual phenomenon that occurs during visually induced sensations of ego motion (vection). A modified version of inferential theory yields a model in which the concept of extraretinal signals is replaced by that of reference signals, which do not encode how the eyes move in their orbits but how they move in space. Hence reference signals are produced not only during eye movements but also during ego motion (i.e., in response to vestibular stimulation and to retinal image flow, which may induce vection). The present theory describes the interface between self-motion and object-motion percepts. An experimental paradigm that allows quantitative measurement of the magnitude and gain of reference signals and the size of the just noticeable difference (JND) between retinal and reference signals reveals that the distinction between direct and inferential theories largely depends on: (1) a mistaken belief that perceptual veridicality is evidence that extraretinal information is not involved, and (2) a failure to distinguish between (the perception of) absolute object motion in space and relative motion of objects with respect to each other. The model corrects these errors, and provides a new, unified framework for interpreting many phenomena in the field of motion perception.

The functions of the proprioceptors of the eye muscles

This article sets out to present a fairly comprehensive review of our knowledge about the functions of the receptors that have been found in the extraocular muscles--the six muscles that move each eye of vertebrates in its orbit--of all the animals in which they have been sought, including Man. Since their discovery at the beginning of the 20th century these receptors have, at various times, been credited with important roles in the control of eye movement and the construction of extrapersonal space and have also been denied any function whatsoever. Experiments intended to study the actions of eye muscle receptors and, even more so, opinions (and indeed polemic) derived from these observations have been influenced by the changing fashions and beliefs about the more general question of how limb position and movement is detected by the brain and which signals contribute to those aspects of this that are perceived (kinaesthesis). But the conclusions drawn from studies on the eye have also influenced beliefs about the mechanisms of kinaesthesis and, arguably, this influence has been even larger than that in the converse direction. Experimental evidence accumulated over rather more than a century is set out and discussed. It supports the view that, at the beginning of the 21st century, there are excellent grounds for believing that the receptors in the extraocular muscles are indeed proprioceptors, that is to say that the signals that they send into the brain are used to provide information about the position and movement of the eye in the orbit. It seems that this information is important in the control of eye movements of at least some types, and in the determination by the brain of the direction of gaze and the relationship of the organism to its environment. In addition, signals from these receptors in the eye muscles are seen to be necessary for the development of normal mechanisms of visual analysis in the mammalian visual cortex and for both the development and maintenance of normal visuomotor behaviour. Man is among those vertebrates to whose brains eye muscle proprioceptive signals provide information apparently used in normal sensorimotor functions; these include various aspects of perception, and of the control of eye movement. It is possible that abnormalities of the eye muscle proprioceptors and their signals may play a part in the genesis of some types of human squint (strabismus); conversely studies of patients with squint in the course of their surgical or pharmacological treatment have yielded much interesting evidence about the central actions of the proprioceptive signals from the extraocular muscles. The results of experiments on the eye have played a large part in the historical controversy, now in at least its third century, about the origin of signals that inform the brain about movement of parts of the body. Some of these results, and more of the interpretations of them, now need to be critically re-examined. The re-examination in the light of recent experiments that is presented here does not support many of the conclusions confidently drawn in the past and leads to both new insights and fresh questions about the roles of information from motor signals flowing out of the brain and that from signals from the peripheral receptors flowing into it. There remain many lacunae in our knowledge and filling some of these will, it is contended, be essential to advance our understanding further. It is argued that such understanding of eye muscle proprioception is a necessary part of the understanding of the physiology and pathophysiology of eye movement control and that it is also essential to an account of how organisms, including Man, build and maintain knowledge of their relationship to the external visual world. The eye would seem to provide a uniquely favourable system in which to study the way in which information derived within the brain about motor actions may interact with signals flowing in from peripheral receptors. The review is constructed in relatively independent sections that deal with particular topics. It ends with a fairly brief piece in which the author sets out some personal views about what has been achieved recently and what most immediately needs to be done. It also suggests some lines of study that appear to the author to be important for the future.

Changes in dorsal neck muscle activity related to imposed eye movement in the decerebrate pigeon

Movements of the head and eyes are known to be intimately related. Eye position has also been shown to be closely related to the electromyographic activity of dorsal neck muscles; however, extraocular muscle proprioception has not generally been considered to play a part in the control of such movements. We have previously shown that, in the pigeon, imposed movements of one eye modify the vestibular responses of several dorsal neck muscles in ways that are dependent on stimulus parameters such as the amplitude and velocity of imposed eye movement. The present study examines more closely the interactions between imposed eye movements and different muscle pairs. The three neck muscle pairs studied each responded to afferent signals from the extraocular muscles in discrete and specific ways which appeared to be correlated with their different actions. Complementary effects of imposed eye movements in the horizontal plane were seen for both the complexus and splenius muscle pairs, with imposed eye movements in one direction producing the largest inhibition of the ipsilateral muscle's vestibular response and imposed eye movements in the opposite direction the largest inhibition of the contralateral muscle's vestibular response. During roll tilt oscillation (ear-up/ear-down) in the frontal plane, similar complementary effects of imposed eye movement were seen in the complexus muscle pair, but the splenius muscle pair showed little tuning, with similar inhibition for imposed eye movement directed either upwards or downwards. In contrast to these complementary effects, the biventer cervicis muscle pair showed no vestibular modulation during vestibular stimulation in the horizontal plane and their spontaneous activity was not altered by imposed eye movement. During roll-tilt oscillation (ear-up/ear-down) in the frontal plane imposed eye movement directed vertically upwards increased both muscles' vestibular responses and imposed eye movement directed vertically downwards inhibited both muscles' vestibular responses. Section of the ophthalmic branch of the trigeminal nerve (deafferenting the eye muscles) abolished the effects of imposed eye movement on the neck muscle pairs. In conjunction with further control experiments these results provide compelling evidence that proprioceptive signals from the extraocular muscles reach the neck muscles and provide them with a functionally significant signal. We have previously shown that signals from the extraocular muscles appear to be involved in the control of the vestibulo-ocular reflex. It follows from the experiments reported here that proprioceptive signals from the extraocular muscles are also likely to be involved in the control of gaze.

Neuronal adaptation accompanying metamorphosis in the flatfish

Flatfish provide a natural paradigm to investigate adaptive changes in the central nervous system of vertebrates. During their metamorphosis, the animals undergo a 90 degrees tilt to one side or the other to become the bottom-adapted adult flatfish. The eye on the down side is pushed over to the up side. Thus, vestibular and oculomotor coordinate systems rotate 90 degrees relative to each other. As a result, during swimming movements different types of compensatory eye movements are produced before and after metamorphosis by the same vestibular stimulation. Intracellular staining of central neurons with horseradish peroxidase revealed that in postmetamorphic flatfish second-order horizontal canal neurons contact vertical eye muscle motoneuron pools on both sides of the brain via pathways that are absent in all other vertebrates studied. These unique connections provide the necessary and sufficient connectivity to adapt the flatfish's eye movement system to the animals' postmetamorphic existence. Although the adult fish has a bilaterally asymmetric appearance, the central nervous connectivity reestablishes symmetry in the vestibulo-oculomotor system.

Low threshold calcium spikes in medial vestibular nuclei neurones in vitro: a role in the generation of the vestibular nystagmus quick phase in vivo?

Intracellular recordings were obtained from medial vestibular nuclei neurones in guinea-pig brainstem slices. A subpopulation of neurones in this nucleus was found to have burst firing properties. Using ionic channel blockers the underlying mechanism was shown to be a low threshold calcium spike. It is speculated that this property could be implicated in the generation of the quick phase of the vestibular nystagmus in the behaving guinea-pig.

Neural correlates of horizontal vestibulo-ocular reflex cancellation during rapid eye movements in the cat

1. The aim of the present study is to describe the behaviour of identified second-order vestibular neurones in the alert cat during eye saccades. A selection of neurones which are involved in horizontal eye movements has been made. The activity has been compared with a selected sample of abducens motoneurones recorded in the same animals. 2. Alert head-fixed cats were used for this study. Eye movements were recorded by the scleral search coil technique. Abducens motoneurones were identified by antidromic stimulation from the VIth nerve with chronically implanted electrodes. They were recorded extracellularly. 3. Second-order vestibular neurones were identified by orthodromic stimulation from the vestibular organs. They were recorded intra-axonally and injected with horseradish peroxidase after recording of their physiological characteristics. Their morphology was reconstructed from frozen sections. 4. All the recorded vestibular neurones showed various amounts of eye position sensitivity. The firing rate (F) - horizontal eye position (H) characteristics are compared for abducens and vestibular neurones. The population average values are F = 33 + 4 H for motoneurones and F = 51 + 2.4 H for vestibular neurones. 5. All recorded vestibular neurones showed an increase of discharge rate during contralateral horizontal saccades and a strong decrease or pause during ipsilateral saccades. Firing rate - horizontal eye velocity sensitivity has been calculated. 6. Results suggest a strong inhibitory input on vestibular neurones from the saccadic generator. This mechanism underlies the suppression of the vestibulo-ocular reflex during saccades. Our results suggest that in the cat, for saccades of amplitude smaller than 20 deg, there is a variable degree of suppression which is provided by a projection of excitatory bursters (EBNs) on second-order vestibular neurones through inhibitory type II neurones. 7. We also conclude from this study that the eye position sensitivity of vestibular second-order neurones is in fact a motor signal indicating a motor error, i.e. the amount of head or eye movement which remains to be done in order to align gaze on target with the eyes centred in the orbit.

Functional coupling of the stabilizing gaze reflexes during vertical linear motion in the alert cat

Eye-head coordination is mainly achieved by means of stabilizing reflexes (VOR, VCR, OKR) and orienting movements (eye-neck surgery) underlying the close cooperation of the visual and vestibular systems in gaze stabilization. The functional coupling of these different sensorimotor subsystems has been principally analysed using rotatory stimulation of the whole body and/or of the visual surround. The aim of the present study was to investigate the dynamic properties of these stabilizing gaze reflexes and their coupling during linear motion in the vertical plane. These investigations were performed in the alert cat under open-loop conditions (head fixed). Otolith stimulation consisted of vertically translating the cat in total darkness using sinusoidal linear motion (0.025 Hz-1.39 Hz; 290 mm peak-to-peak amplitude). Optokinetic stimulation was provided by sinusoidally moving a pseudo-random visual pattern in front of the cat and in the vertical plane, with identical kinematic parameters. Normal visual-otolith interaction was performed by translating the cat in front of the stationary visual surround while conflicting interaction was provided by moving the animal and the visual pattern in phase and at the same velocity (visual stabilization). Results showed that the vertical otolith-neck reflex is very poorly developed or absent in the low frequency range of motion (0.025 Hz-0.25 Hz) while consistent EMG activity is found during pure optokinetic stimulation. EMG responses are in phase with the visual surround velocity in the upward direction and with the upward OKR velocity. A close correlation is observed between the EMG gain and the OKR gain, which both decrease in this low frequency range, indicating that gaze stabilization would be mainly ensured by the OKR and a functional oculo-collic coupling or eye-neck surgery in the vertical plane. On the contrary, gaze stabilization is principally achieved by way of the otolith-neck reflex in the higher frequency range of motion (above 0.25 Hz). EMG responses recorded during otolith stimulation exhibit a relatively constant gain and a phase lead with respect to motion velocity which progressively reduces as the stimulus frequency increases up to 1.39 Hz. When present, EMG responses evoked during the optokinetic stimulation show strong gain attenuation and phase lag. Normal visual-otolith interaction induces neck muscle activity which parallels the optokinetic and the otolith responses in the low and high frequency ranges, respectively. The motor responses are however improved in terms of gain and phase values in the whole frequency range when both sensory inputs are combined.(ABSTRACT TRUNCATED AT 400 WORDS)

Axonal trajectories of posterior canal-activated secondary vestibular neurons and their coactivation of extraocular and neck flexor motoneurons in the cat

Unit activities of 148 secondary vestibular neurons related to the posterior semicircular canal were recorded extracellularly in anesthetized cats. Axonal projections of these neurons were examined by their antidromic responses to stimulation of the excitatory target motoneurons of the contralateral (c-) inferior rectus muscle (IR) and bilateral (bi-) motoneuron pools of longus capitis muscles, neck flexors, in the C1 segment (C1LC). The neurons were classified into 4 groups according to their axonal projections. The first group of neurons, termed vestibulo-oculo-collic (VOC) neurons, sent axon collaterals both to the c-IR motoneuron pool and to the c-C1LC motoneuron pool. The majority of them (72%) were located in the descending nucleus. The second group of neurons were termed vestibuloocular (VO) neurons and sent their axons to the c-IR motoneuron pool but not to the cervical cord. Most of them (86%) were located in the medial nucleus. The third group of neurons, termed vestibulo-collic (contralateral) (VCc) neurons, sent axons to the c-C1LC motoneuron pool via the contralateral ventral funiculus but not to the oculomotor nuclei. They were mostly (75%) found in the descending nucleus. The last group of neurons were vestibulo-collic (ipsilateral) (VCi) neurons, which gave off axons to the ipsilateral (i-) C1LC motoneuron pool via the ipsilateral ventral funiculus but not to the oculomotor nuclei. One of them also sent an axon collateral to the c-C1LC motoneuron pool. The majority of them (74%) were located in the ventral part of the lateral nucleus. It was also observed in some of the VOC and VCi neurons that they produced unitary EPSPs in the c-C1LC and i-C1LC motoneurons, respectively. Their synaptic sites were estimated to be on the cell somata and/or proximal dendrites of the motoneurons.

Reticulo-spinal neurons participating in the control of synergic eye and head movements during orienting in the cat. II. Morphological properties as revealed by intra-axonal injections of horseradish peroxidase

Previously we described physiological properties of pontine reticulo-spinal neurons which generate bursts and decaying tonic discharges related to eye movements and neck muscle activity during ipsiversive gaze shifts (Grantyn and Berthoz 1987). Two of these "eye-neck reticulo-spinal neurons" (EN-RSN) were labeled by intra-axonal injections of HRP. The present report provides a detailed description of their morphology with an emphasis on the topography of axon collaterals, bouton numbers, and the structure of preterminal ramifications in different target areas. The cell bodies of labeled EN-RSNs were located rostro-ventrally to the abducens nucleus. Their descending axons issued 8 and 13 collaterals (left and right EN-RSN, respectively) at different rostro-caudal levels, between the abducens nucleus and the pyramidal decussation. On the basis of the size of their cell bodies, the isodendritic type of dendritic branching and their multiple collateralization, EN-RSNs correspond to the class of "generalized" reticular neurons, often referred to as The Scheibels' neurons. Collaterals of EN-RSNs terminated in the following structures: the abducens and facial nuclei, the medial and lateral vestibular nuclei, the nn. prepositus and intercalatus, and the bulbar reticular formation. As judged from bouton numbers, the strongest connection of both neurons was with the abducens nuclei. Terminations in the rostral part of the medial vestibular and prepositus nuclei indicate that EN-RSNs may also influence oculomotor output activity through these indirect routes. In the facial nucleus, a majority of terminations was found in its medial subdivision containing motoneurons of ear muscles. However, other subdivisions were also contacted by EN-RSNs. Most terminations in the rostral bulbar reticular formation are distributed to the dorsal, gigantocellular field. Within this field, there is a substantial contribution to the zone characterized by the highest density of reticulo-spinal neurons projecting directly to neck motoneurons. Other target areas which may participate in the modulation of spinal cord activity by EN-RSNs are the ventral reticular nucleus in the caudal medulla and the lateral vestibular nucleus. EN-RSNs also establish connections with precerebellar structures: the prepositus and the paramedian reticular nuclei. The numbers of boutons on collaterals issued within 6 mm of the injection site varied between 37 and 469. The occurrence of presumed axo-somatic contacts was low (0-8.2%) and not characteristic for any particular target area. Local accumulations of boutons in the form of small and large field clusters was a common observation.(ABSTRACT TRUNCATED AT 400 WORDS)

High thresholds for movement perception in schizophrenia may indicate abnormal extraneous noise levels of central vestibular activity

A theoretical argument proposes that thresholds for visual perception of movement should be abnormally high in schizophrenia. This may reflect a central vestibular dysfunction, consisting of abnormally high levels of extraneous noise within the neural activity of the central vestibulo-cerebellar complex. Two experiments are reported with results that support the hypothesis. To some extent, the disorder may explain the smooth pursuit eye movement dysfunction in schizophrenia. Relations to the dopamine hypothesis in schizophrenia are discussed.

Axon collaterals of anterior semicircular canal-activated vestibular neurons and their coactivation of extraocular and neck motoneurons in the cat

We studied the ascending and descending axonal trajectories of excitatory vestibular neurons related to the anterior semicircular canal, by means of local stimulation and spike-triggered signal averaging techniques in anesthetized cats. More than 200 vestibular neurons related to the ampullary nerve of the anterior semicircular canal (ACN) were identified as vestibulo-ocular neurons by antidromic stimulation of the contralateral inferior oblique (IO) muscle motoneuron pool. In the descending, medial and ventral lateral nuclei, about 60% of these vestibulo-ocular neurons were also activated antidromically by upper cervical spinal cord stimulation (vestibulo-ocular-collic (cervical) = VOC). These VOC neurons produced unitary EPSPs in the majority of neck extensor motoneurons located at the C1 segment. None of the VOC neurons had axons descending as far as the thoracic level. Most of these VOC neurons were activated monosynaptically following stimulation of the ACN. The conduction velocity of the descending axons of VOC neurons was approximately 63 m/s, which was significantly faster than that of the ascending axons. The remaining 40% of the vestibulo-ocular neurons were not activated antidromically following spinal cord stimulation at intensities of 1 mA or more (vestibulo-ocular = VO). Most of the VO neurons were activated polysynaptically by ACN stimulation. The superior vestibular nucleus contained VO neurons that were activated mono- and polysynaptically following ACN stimulation.