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To maintain a clear and stable view of the environment, we use reflexes to rotate our eyes in a direction that is opposite to any head motion that occurs. The reflexes are driven by both vestibular and visual inputs. When the head rotates to the right, the angular vestibulo-ocular reflex causes the eyes to rotate to the left, such that gaze remains stable in space. If the head translates to the right, the translational vestibulo-ocular reflex can rotate the eyes to the left to compensate for the potential motion of the retinal images of near targets. These vestibulo-ocular reflexes operate in synergy with visuo-ocular reflexes, the best understood being optokinetic nystagmus. This chapter summarizes the basic properties of these reflexes, with an emphasis on behavioral observations.

Neuronal systems must learn how to adapt to changes in their intrinsic structure or in the environment. The vestibular system is no exception. Several features have made it an attractive system in which to study the neurobiology of learning. These include the relatively simple circuitry of its reflex pathways and their close relation to the cerebellum, a major site of motor learning. This chapter discusses two examples of adaptive plasticity: motor learning involving the vestibulo-ocular reflex (VOR) and visual feedback; and compensation for the imbalance introduced by removal of the vestibular labyrinth or section of the vestibular nerve on one side.

Some of the functional properties of different otolith-ocular reflexes are discussed in this chapter, such as the linear vestibulo-ocular reflexes, which are activated by linear acceleration. The latter part of the chapter focuses on the interaction of otolith and canal signals during rotatory head movements. Stabilization of the line of sight and maintenance of spatial orientation requires appropriate transformation of sensory inputs of different modalities to oculomotor output. Rapid information about head movements and orientation in space is provided by the vestibular organs. The utricular and saccular otolith organs detect linear accelerations of the head, and hence convey information about translatory head movements as well as about head orientation relative to gravity, while the semicircular canals, activated by head angular accelerations, provide afferent information. Otolith signals interact with semicircular canal signals in at least two important ways involving the velocity storage system. First, otolith signals are processed to complement the canal-ocular reflexes at frequencies of head rotations below 0.1 Hz. Second, static otolith input interacts through the velocity storage integrator with head velocity signals to provide a spatial reference about head angular velocity. The otolith organs provide complementary signals that are used to detect the direction of the head rotation in space, i.e. relative to gravity, as well as eccentricity of rotation during fast head movements. Dynamic otolith signals can initiate appropriate transformations of canal-driven head velocity signals to account for eccentricity of ocular rotation and target distance.

This chapter reviews the prevalence of vestibular disorders and their impact on daily living. It discusses the physiological principles that underlie manifestations of vestibular disorders. It describes tests of vestibular function with an emphasis on their relationship to basic mechanisms. Research into the mechanisms of compensation for unilateral vestibular injury, based on animal studies of the vestibulo-ocular reflex, is also discussed. The chapter describes strategies for future research designed to ameliorate the deficits caused by vestibular dysfunction. It deals almost exclusively with peripheral disorders, focusing on underlying scientific principles.

Model systems have been critical to developing our understanding of cerebellar function. The vestibulo-ocular reflex stabilizes the eyes during head movement and depends on the cerebellum to maintain accurate function. Classical conditioning of the eye blink reflex is an example of predictive motor learning where the role of the cerebellum is to appropriately time the conditioned response. Voluntary goal-directed behaviour, such as target-directed eye movements, harnesses the cerebellar circuitry to maintain accuracy and compensates for self-induced perturbations that occur during the movement such as an eye blink. In the general context of everyday movement, the role of the cerebellum in the actions and reactions that underlie animal athleticism is likely to be pervasive, but also inextricably intertwined with the wider motor control networks.

The vestibulo-ocular reflexes help to stabilize the visual image on the retina, and the vestibulocollic (vestibular neck) reflexes play a role in restoring the head position in space during head movements. The vestibular nucleus neurons play a crucial role in both reflex pathways. It was observed that individual vestibular relay neurons receive inputs from two to three semicircular canal pairs, or from canals and otolith organs, when natural stimuli were applied in alert cats. However, according to observations in anesthetized cats, it has been considered that the primary afferent fibers from each semicircular canal have their own target neurons in the vestibular nuclei.

The vestibulo-ocular reflex (VOR) is reduced or cancelled by a smooth pursuit eye movement programmed to match the velocity of the moving target whenever there is an attempt made to follow a moving visual target by combining a smooth pursuit eye movement with a head movement generated in the direction of target movement. The objective of this study is to determine if there is a second mechanism that can cancel the VOR. The experiments in this study involved four squirrel monkeys that were prepared for chronic recording of eye movements. The monkeys were seated on a vestibular turntable with restrained heads. Their eye movements were recorded using magnetic search coil technique. Results showed that there is a cancellation of the vestibulo-ocular reflex generated by the sudden turntable jerks. There is also a cancellation of the vestibulo-ocular reflex produced by the unpredictable head movements.

In recent years, the vestibular system has been implicated not only in the reflex movements produced by labyrinthine inputs but also in the control of active motor behaviors, especially eye and head movements during gaze. This chapter discusses the experimental proof pertaining to the functional organization of vestibulo-ocular and vestibulo-oculo-collic pathways, with focus on recent researches of the signals carried by secondary vestibular neurons on the vertical canal systems. The synaptic organization of the vestibulo-ocular reflex (VOR) and vestibulocollic reflex (VCR) pathways stemming from the semicircular canals have been extensively studied and the fundamental patterns of excitatory and inhibitory connections with specific canal-muscles relationships have been well established. Although excitatory connections from the anterior involve the pathways through the deep reticular formations in the cat, the major part of the excitatory pathways ascend in the contralateral medial longitudinal fasciculus (MLF), and the inhibitory pathways in the ipsilateral MLF.

This chapter examines the mechanisms underlying sensorimotor integration by discussing three cases where anatomical and physiological studies of circuits are co-evolving with computer models of the circuit: the first focuses on the dorsal bending reflex in the leech, the second deals with the vestibulo-ocular reflex (VOR) in mammals, and the third is concerned with rhythmic behaviors generated by the spinal cord. The chapter explains each case in detail, beginning with the computational model for the local bending reflex in the leech and proceeding with a discussion of a model network incorporating known pathways, connections, and physiology of the VOR. It also considers how time is represented in nervous systems and describes the segmental swimming oscillator before concluding with an overview of modeling of neurons.

The primary role of the vestibulo-ocular reflex (VOR) is to stabilize the eyes for clear vision throughout angular eye movements. Another alternative approach to VOR testing at higher frequencies is to use active head oscillations in which a rotational sensor attached to a head strap would monitor head movement. This method needs the use of a computer and other technology in its implementation. In this chapter, the use of active head oscillations for VOR testing is reviewed in laboratories. Previous research on active head movements are also presented in the chapter. These previous studies employed active head movements at frequencies both below and above 2 Hz with an earth-fixed visual target. Certain physiologic considerations in using active head movements in place of passive chair rotations are also clarified in this chapter. The methods, test protocol, and data acquisition and analysis in VOR testing are sequentially explained. Meniere's disease is also briefly discussed.

This chapter covers locomotion on foot, and questions such as how far ahead humans look when placing their feet on uncertain terrain, and what sources of information are used to guide locomotion. It also examines the roles of eye, head, and body movements during turns, and of the reflexes that coordinate them. In short, it addresses the role of vision in moving through the environment and avoiding hazards, as well as how the eyes, head, and body work together to get high acuity foveae to where they are needed to best serve human behaviour. It specifically deals with two common situations that are encountered in everyday life: crossing busy roads and avoiding colliding with other pedestrians. When moving through the environment, eyes, head, and body often share a common alignment. However, in many situations they do not, and flexible co-ordination is obtained by two reflexes: the vestibulo-ocular reflex, which allows the eyes to maintain direction independent of the head, and the vestibulo-collic reflex, which maintains head direction independent of the body.

Asymmetries of the vertical optokinetic reflex (VOKR) and the vertical vestibulo-ocular reflex (VVOR) have been observed in many species. Gains of upward OKR slow phases were higher than those of downward OKR slow phases. An upward preponderance was also observed in the VVOR of cats. This difference between upward and downward eye responses has been shown to be largely affected by gravity. Gravity also affects the VVOR response because the animals should be pitched around the interaural axis to a 90-degree incline to reveal such vertical vestibular asymmetry. In this condition, the vertical canals were stimulated without activating the otolithic receptors since the direction of gravity's action on the maculae was constant.

Gaze is defined as the coordinated sum of eye and head movements and gaze position is the position of the eye in space, while eye position is the position of the eye in the orbit. In this chapter, the authors discuss how several causal factors (protocol conditions including desired gaze amplitude and pathologic situations including reduced inclination to move the head in hemianopia) influence the selection of a gaze mode despite statistical selection. The results of the methods employed in this study show that there is a strong connection between the gaze type as defined by relative latency of eye and head movement and the VOR gain and gaze amplitude. It is also evident that some motor brain mechanism organizes a relative latency between head and eye movement.

The horizontal cervico-ocular reflex (HCOR) comprises compensatory eye movements that are evoked by rotation of the body about the fixed head. This head goes together with the horizontal vestibulo-ocular reflex (HVOR) in the sense that a counterclockwise rotation of the body about the fixed head evokes a counter clockwise compensatory eye movement. This counterclockwise eye movement is evoked if the head and body are rotated in a clockwise direction or if the head is rotated clockwise about the body. The gain of the HCOR is maximal for relatively low-frequency stimuli. In addition to the compensatory slow-phase eye movement, the HCOR is depicted by a fast-phase eye movement that is compensatory as well.

This chapter presents the results of the experiments that indicate that there is little evidence to support the existence of a central cancellation mechanism of the VOR in normal human subjects, as proposed by Robinson. The changes in gaze velocity with the frequency composition of the stimulus are discussed in this chapter. The enhancement of the highest frequency component is presented. The role of the vestibulo-ocular reflex during head-free pursuit is analyzed and investigated here. It is concluded that although there is little evidence to support the central cancellation hypothesis, other nonvisual mechanisms may have a role in vestibulo-ocular reflex suppression.

In this chapter a series of experiments are described where large-amplitude gaze shifts were recorded in the head-free condition, and eye saccades in the head-fixed condition. The amplitude of head-fixed saccades was chosen in order to correspond to the amplitude of the eye movements in the orbit in head-free condition. By comparing head-free and head-fixed eye saccades of equivalent size, vestibulo-ocular reflex (VOR) gain changes during large gaze shifts involving both the eye and the head can be indirectly measured. Quantification of the VOR gain variations during large gaze shifts has been done by a systematic comparison of saccade dynamics in head-free and head-fixed human subjects, under certain conditions, such as identical eye saccade amplitude and starting position. This analysis is only possible when the head velocity attains a sufficient level, and is thus reserved for the terminal part of the gaze shift. In addition, the validity of the estimation of the VOR gain and of the hypotheses proposed to explain the observations has been tested by model simulations.

The gain of eye velocity during head-fixed pursuit is significantly modified by the frequency composition of a mixed-frequency, pseudorandom stimulus. The model of the visual and nonvisual control of smooth eye movement that has been developed is discussed here, as well as the results of a series of experiments related to head-eye coordination. In order to simulate the changes in gain and phase of eye velocity that occur during a number of oculomotor tasks, a model of oculomotor control is presented here. The results of the experiment indicate that there are two separate mechanisms involved in the visual control of eye movement. Evidence suggests that optimal VOR suppression can be achieved only through visual feedback.

Two primary issues are being studied in relation to the control of coordinated eye and head movements. The first one is the possible interaction of the vestibulo-ocular reflex (VOR) during the saccadic component of a gaze shift and the second one is the possible coordination of eye and head trajectors through shared access to a common premotor drive. This chapter presents an alternate view of central eye-head coordination, using global gaze error as a precursor of motoneural drives. The model of eye-head coupling is also conceptually justified in this chapter. Some simulation examples are presented, including the gaze shifts within and beyond the oculomotor range and coupling of eye and head trajectories.