Vestibular Function and Anatomy

The vestibular system, defined as the peripheral vestibular motion detectors and related central nervous system structures, senses motion in space and converts that motion into information that the remainder of the central nervous system can use to generate appropriate motor reflexes or facilitate complex processes such as the coordination of head, eye, and trunk movements or updating one's perception of his or her orientation in the world. The vestibular system, like the auditory system, converts physical stimuli into neural signals, but the vestibular system detects angular and linear acceleration, rather than sound. The vestibular system is present in all vertebrates and many invertebrates. Yet, despite the importance of spatial orientation in all mobile animals, the vestibular system is largely underappreciated until a malfunction occurs, at which point patients present to a physician, often an otolaryngologist, for treatment and education. This chapter discusses the anatomy of the peripheral vestibular system, the biophysics of sensory transduction, vestibular hair cell types and physiology, vestibular afferent types and physiology, and the organization of sensory inputs to the central nervous system, but it is only an introduction to this important, complex system.
The complexity of challenges presented to the vestibular system on a daily basis is elucidated with a simple example. Figure 130.1 follows an office worker through the simple processes of looking for a book on a shelf. She pushes her chair back, stands, and turns left to face the shelf. She then tilts her head (right ear down) to scan the book titles. One way to describe the motion the worker's head makes between the two positions is to decompose the movement into linear (straight-line) motion and angular (rotational) motion, by considering a path made by the center of her head. In this example, the linear motion can be described as backing away from the desk 2 m, with a leftward motion of 3 m and an upward motion of 1 m (Fig. 130.1B). In terms of rotary motion, the woman pitches her head upward 40 degrees from looking down at the paper to looking horizontal. She turns 90 degrees to the left around a vertical axis to face the bookshelf. She then tilts her head 75 degrees toward the right-ear-down position to read the book titles (Fig. 130.1A). Thus, the motion the worker uses to complete the movement between the starting and ending positions can be characterized by three linear movements and three angular movements.
The vestibular system must be able to detect both linear and angular motion in order for the brain to estimate the orientation of the body in space. An important additional piece of information needed for orientation is the direction of gravitational pull or the gravity vector. Among other things, the knowledge of the orientation of gravity allows humans to maintain a vertical stance. Even in the previous simple example, there is not only a complex interaction between the motions of the head, eyes, and body relative to one another, all of which use information generated by the vestibular system, but there is a visual, somatosensory, and vestibular system interaction that allows the worker to know her orientation relative to her surroundings

Gross Anatomy of Thevestibular System
Vertebrates have what amounts to an inertial guidance system made up of multiple sensors of linear acceleration and multiple sensors of angular acceleration in each inner ear.This guidance system, the vestibular labyrinth, is housed in a portion of the otic capsule in the petrous portion of the temporal bone. The bony labyrinth is the thick bone of the otic capsule that houses the membranous labyrinth suspended in perilymph. The membranous labyrinth holds endolymphatic fluid and the neuroepithelial structures of sensory transduction. The perilymphatic and endolymphatic spaces of the labyrinth are continuous with those of the cochlea; therefore, the composition and homeostatic mechanisms of the perilymph and endolymph discussed in the cochlea chapter (Chapter 129) apply to the vestibular system as well. As in the cochlea, proper function of the vestibular system depends on the unique composition of these fluids.
The vestibular labyrinth is a paired structure, with the right and left labyrinth mirroring one another. Subdivisions of the vestibular labyrinth include the three semicircular canals: the lateral or horizontal canal, the posterior canal, and the anterior or superior canal, all of which detect angular accelerations. The geometric layout of the canals is shown in Figure 130.2. The horizontal canals lie parallel to the line between the external auditory canal and the outer canthus of the eye, which is inclined 30 degrees above the horizontal axial plane. The vertical canals are roughly at right angles to the horizontal canals and to each other. When looking down at the top of the head, the anterior canal is oriented at approximately 45 degrees off midsagittal and 45 degrees anterior to the intraaural line. The posterior canal is aligned roughly 45 degrees behind the intraaural line; thus, the anterior canal on the left is roughly parallel to the posterior canal on the right, and the left posterior canal and the right anterior canal are similarly aligned.
Housed in the vestibule are the otolith organs—the utricle and the saccule—which detect linear acceleration. Neither organ is perfectly planar, but the utricle is primarily aligned parallel to the earth and is roughly aligned with the ipsilateral horizontal canal (Fig. 130.3). At rest, the saccule is perpendicular and at right angles to the utricle. The sensitivity of detection of translational acceleration is greatest in the plane of the macula. Thus, the utricular macula is sensitive in the horizontal plane, and the saccular macula is sensitive in the sagittal plane

Basic Physics of Mechanotransduction
Both the linear (utricle and saccule) and the angular (semicircular canal) acceleration sensors for the inner ear use a three-step process to convert accelerations of the head into useful information for the nervous system (Fig. 130.4). The elements used in these three steps are inertial mass, one or more sensory hair cells, and the nerve fibers connected to the hair cells through synaptic junctions. Figure 130.4A shows the system at rest (no acceleration). Figure 130.4B shows the response when the system is accelerated to the left. The resulting rightward movement of the mass (M) relative to the sensory hair cell deflects the sensory hairs, depolarizes the cell body, and increases the discharge rate of the attached nerve fiber.

Neuroepithelium
The sensory hair cells in the membranous labyrinth are similar to those in the cochlea in that both detect small deflections and transmit the information provided by the displacement to the central nervous system (Fig. 130.5). However, significant differences are found between the cochlea and the vestibular neuroepithelia. The vestibular hair cell body is surrounded by supporting and other haircells. Sensory bundles extend from the apical surface of the vestibular hair cell body and are usually in contact with a gelatinous membrane, the motion of which is affected by displacement of the mass element: either the cupula of the semicircular canal or the otolithic membrane for the utricle and saccule. These sensory hair bundles have two distinct ciliary types. The kinocilium is the tallest cilia and is near the edge of the top of the hair cell. Kinocilia are not found on cochlear hair cells. The position of the kinocilium determines the orientation of the hair cell. There are many stereocilia, arranged in columns and rows, and the closer they are to the kinocilium, the taller they are. This arrangement produces an orderly array of stereocilia and a means by which alignment of an individual hair cell can be determined by its so-called morphologic polarization vector, which is shown as an arrow in Figure 130.6. Experimental data show that a functional axis of alignment corresponds with the morphologic one. It is in this axis that a cell responds most vigorously to the displacement of the stereocilia. Displacement of the stereocilia perpendicular to the polarization axis causes no change in the resting potential of the hair cells. On any one sensory organ, neighboring hair cells tend to have polarization vectors that are aligned.
The electrical potential inside the body of hair cells differs from that of the fluids surrounding them because of active transport at the cell membrane. Bending the stereocilia on top of the cell toward the kinocilium opens potassium channels and temporarily increases the resting potential, depolarizing the cell. Deflection away from the kinocilium hyperpolarizes the cell. The channels responsible for transduction are located at the top of stereocilia in the utricle and are opened by relative motion of the stereocilia (1). The hair cells release a neurotransmitter (believed to be glutamate) that is excitatory to the hair cell afferents with which they connect. At rest, there is a baseline release of the neurotransmitter. This release is important because not only does deflection of the hair cell bundle toward the kinocilium (depolarizing the hair cell) increase transmitter release, but deflection of the hair cell bundle away from the kinocilium (hyperpolarizing the hair cell) reduces transmitter release. Thus, one hair cell detects both acceleration and deceleration along the axis of the morphologic polarization vector.
There are two morphologically and physiologically distinct types of hair cell bodies: type I or chalice hair cells and type II or cylindrical hair cells. The body of a type I hair cell is entirely engulfed by one afferent terminal. Efferent innervation is indirect, as the efferent nerve has its synapse on the afferent nerve ending. Type II hair cells can have one or more afferent nerve endings on the body of the cell. Type II hair cells can also be directly or indirectly innervated by vestibular efferent terminals. Type I and type II hair cells are not evenly distributed throughout the neuroepithelium of either the semicircular canal ampullae or the utricular maculae. As discussed later, they are innervated by different classes of vestibular afferents.
The neuroepithelium contains other cell types as well. Supporting cells have the nuclei located at the basal end of the sensory epithelial, above the basement membrane. These cells are believed to make and secrete the extracellular macromolecules of the cupula and otolith membrane. Dark cells can be found at the margins of the transitional epithelium surrounding the neuroepithelium. These dark cells are located directly above pigmented cells and are thought to produce the ionic composition of the endolymph.

Microanatomy and Biophysics of the Semicircular Canals
The semicircular canal is a membranous structure shaped like a torus or hollow doughnut (Fig. 130.7). Maximum sensitivity is to rotation in the plane of the torus. At one end of the torus, there is an enlargement, the ampulla. A gelatinous flap, the cupula, completely seals one side of the ampulla from the other. Because the cupula is elastic, any pressure difference causes it to deflect. The interior of the torus is filled with endolymph, a liquid with the density and viscosity of water. The membranous portion of the canal is attached to the temporal bone. When the head is turned, the membranous labyrinth moves with it, but the endolymph inside has an inertial mass that tends to oppose the turning motion. This oppositional force causes pressure buildup across the cupula, deflecting the cupula from its equilibrium position. Within the physiologic range of motion, this deflection most nearly resembles the motion of the head of a drum or clamped diaphragm when pressure is uniformly applied to one side.
Cilia are embedded in the gelatinous cupula. As the cupula is deflected, the stereocilia bend either toward or away from the kinocilium, producing an increase or decrease, respectively, in the firing rate of the vestibular nerve. The kinocilia are parallel to the long axis of the canal. In the lateral semicircular canal, hair cells are arranged such that the kinocilia are closest to the vestibule; maximal excitation occurs with ampullopetal flow of endolymph. In the posterior and superior semicircular canals, this arrangement is reversed—the kinocilium are farthest from the vestibule. Thus, ampullofugal flow is excitatory.
The crista ampullaris has been divided into central, intermediate, and peripheral zones. In humans and other mammals, type I hair cells are relatively more common in the central zone than in the intermediate and peripheral zones (2). In contrast, type II cells are relatively less common in the central zone than in other zones.
More than half a century ago, Steinhausen constructed a biophysical model of the semicircular canal known as the torsion pendulum model The behavior of this model is determined by the mass of the endolymph, the viscous damping properties of the endolymph, and the springlike restoring force of the cupula. Estimates of these physical properties and knowledge of the geometric properties of the semicircular canal allow observers to relate deflection of the cupula to angular acceleration of the head. With this model, it can be predicted that cupular deflection is proportional to head velocity over a frequency bandwidth of approximately 0.1 to 10 Hz. Above and below this frequency bandwidth, the cupular deflection is not as great, and the sensitivity of the semicircular canal to velocity decreases. At 0 Hz, which corresponds to constant-velocity rotation, the torsion-pendulum model predicts there will be no response at all. The prediction made with this model agrees with the perception of a person who is turned at a constant velocity around a vertical axis. Subjects sense initial acceleration, but in the absence of other cues such as vision, subjects feel they are no longer rotating after 30 to 60 seconds. Rotating subjects who are suddenly brought to a full stop feel a sensation of turning in the opposite direction. This sensation is due to the inertia of the endolymph, which continues to rotate, deflecting the cupula in the direction opposite to that experienced with the initial rotation. Reflexive eye movements measured during these steps of velocity mirror the sensation felt by the subjects and are the basis of cupulometry used in the Bárány test and other tests of the vestibuloocular reflex.

Microanatomy and Biophysics of the Otolith Organs
All the sensors in the vestibular system combine a mass element connected to sensory hair cells (Fig. 130.4). In the case of the otolith organs (utricle and saccule), the mass is composed of calcium carbonate crystals known as otoconia that are embedded in a gelatinous supporting substrate. Displacement of this structure due to linear acceleration or change in orientation with respect to gravity affects a number of sensory hair cells. Each cell has a polarization vector oriented in a slightly different direction, making each one maximally sensitive to acceleration in that particular direction.
The calcium carbonate crystals are suspended in the otolith membrane. Under this membrane are a number of sensory hair cells, each of which can have one or more afferent connections to the vestibular nerve. The drawing in Figure 130.8 is an exploded view. In reality, the stereocilia from the hair cells are in direct contact with the otolith membrane. In Figure 130.8, each of the sensory hair cells has a polarization vector with a small arrow indicating its direction of maximal excitation in the plane. The large arrow at the top of the otolithic mass represents linear acceleration that deflects the otolith mass in the direction of the arrow. Hair cells that have polarization vectors aligned with the arrow and in the same direction are excited maximally, whereas hair cells that have polarization vectors perpendicular to the acceleration are not stimulated.
In the maculae of the utricle and saccule, type I hair cells are relatively more prevalent close to the striola than in the peripheral zone areas. The striola is a zone that runs the length of the macula, is about 100 microns wide, and divides the macula into the medial and lateral extra striola zones. The orientation of the hair cells on either side of the striola is roughly 180 degrees out of phase. Because the striola is C-shaped in the utricle, orientation vectors of hair cells in the utricle are aligned in all directions of the plane of the utricular macula.
It is from an array of these hair cells that the brain can estimate the magnitude and direction of linear acceleration. If all polarization vectors were identically aligned, it would be impossible to determine the magnitude and direction of an acceleration vector in the plane of the otolithic macula. At least two different orientations are needed to resolve the vector in two dimensions. At least three separate orientations are needed to resolve the magnitude and direction of an acceleration vector in three dimensions.
As in the simple example described earlier, each otolith organ has sensory hair cells arranged in a wide variety of orientations of its polarization vectors (Fig. 130.9). Because of this architecture, the asymmetries inherent in the sensitivity of a single hair cell can be canceled out within one otolith organ itself. The orientation of the polarization vectors is toward the striola in the utricular macula and away from the striola in the saccular macula. The right and left otolith organs, like semicircular canals, have mirror symmetry around the sagittal plane. The exact neural connections of the otolith organs have not been as extensively studied as those of the pairs of semicircular canals. Thus, the exact circuitry for resolving linear acceleration in three-dimensional space has not been determined.
A simplified model of the response of the otolith organ to linear acceleration and changes in orientation with respect to gravity can be made with a mass, a spring, and a damper (Fig. 130.4). In this case, the mass is the otoconia macula minus the buoyant force placed on it by the surrounding endolymph. The spring and the damping factors come from the viscoelastic properties of the gelatinous structure in which the otoconia are embedded.
The response characteristics of the otolith organs can be predicted with the above model. Although the specific gravity of the otoconia has been found to be 2.7 times that of the endolymph, the damping forces of the otolithic membrane are more difficult to measure. These damping forces prevent oscillation of the otolith membrane in response to a given linear acceleration. However, when direct recordings are taken from otolith afferents, physiologic performance deviates from that predicted in the model. There are two types of neuronal otolith afferent populations that can be defined that are similar to those in the semicircular canals. The first population appears to respond to head position, and its responses closely follow those predicted with the model for sinusoidal stimulation at frequencies up to 0.1 Hz. A second population of neurons encodes information on linear acceleration. These neurons display increasing gain in proportion to higher-frequency stimuli.
theoretically, how two otolith organs operating in the same plane react to head tilt or to acceleration of the head in the plane of the otolithic macula. If there is no acceleration in that plane, the nerve firing rate of each otolith organ is constant and equal. When the head is tilted to the left, the firing rate of the nerve innervating the left otolith organ increases, while the firing rate of the nerve innervating the right otolith organ decreases. Maximum sensitivity is obtained by means of subtracting the firing rate of the right nerve from that of the left. Acceleration of the head to the right causes deflection of both otoconia to the left in a manner similar to a head tilt to the left. This acceleration produces an increase of the firing rate of the left nerve and decrease in the firing rate of the right nerve. This model (Fig. 130.10) shows that the asymmetry present in one hair cell innervating an otolith organ can be canceled by combining it with a signal from a hair cell that has the same polarization factor in the other side. It also shows that the otolith organs are influenced by both tilt orientation with respect to gravity and linear acceleration.
Einstein recognized that an ambiguity presented between linear acceleration and gravity, and in aviation, is a problem during the acceleration of takeoff, when pilots have trouble differentiating the acceleration of the airplane from the gravity vector. Because translational motion in one direction creates the same inertial force as gravity to tilt in the opposite direction (Fig. 130.10), this problem is known as a tilt-translational ambiguity. Recent research has demonstrated that the central nervous system uses semicircular canal information (activated during tilt but not during translation) in combination with the otolith input to distinguish, for example, tilting the head upward versus accelerating forward as in a car, sled, or airplane (3). This mechanism works poorly at low frequency rotations. In the circumstance where the rotational component of the motion is at low frequency (<0.1 Hz), the brain uses visual or tactile information to help interpret the otolith's signal. In the absence of non-otolith input, such as vision or rotation at frequencies above 0.1 Hz, the system defaults to interpreting linear acceleration as tilt (or gravity). Returning to the aviation example, fighter pilots taking off from an aircraft carrier deck at night will feel as if they are tilted backwards during forward acceleration. The natural correction for this feeling is to steer the plane downward, which could result in disaster.

Vestibular Afferents
Based on the anatomy of peripheral termination, there are three distinct vestibular afferent types: calyx, dimorphic, and bouton. Calyx afferents terminate exclusively on type I hair cells at the calyx endings. Calyx endings may terminate on one or several hair cells. Dimorphic afferents have both calyx endings on type I hair cells and bouton endings on type II hair cells. Dimorphic afferents are likely the most prevalent. Bouton afferents have only bouton endings and thus only terminate on type II hair cells. These three afferent types differ immunohistochemically. Calrentinin, a calcium binding protein, is seen only in calyx afferents; peripherin, which is an intermediate filament protein, is seen in bouton afferents; and neither of these markers is seen in dimorphic afferents.
There are other anatomic distinctions between these afferent types. Calyx afferents have characteristically thick axons, whereas bouton afferents are thinner. Dimorphic afferents, however, can be thick or thin. The processes to the calyx endings are thicker than the processes to the bouton endings. The distribution of the three fiber types is also characteristic. Calyx afferent endings are found in the central zone of the crista ampullaris, whereas dimorphic afferents terminate in the central, intermediate, and peripheral zones, and bouton fibers terminate in the peripheral zone. Similarly, utricular calyx afferents terminate in the striola region, whereas dimorphic afferent terminals are seen throughout the macula and bouton afferents and generally terminate peripherally.
Afferents also differ in their discharge regularity, conduction velocity, and sensitivity to vestibular and galvanic stimulation. Although response gains, conduction velocity, and discharge regularity over a population of neurons all fall along a continuum, based on these characteristics, vestibular afferents fall into three general groups that correspond well with the three groups (calyx, dimorphic, and bouton) determined by peripheral morphology. Calyx afferents innervating the central ampulla or striola are large fibers that are irregularly firing, sensitive to galvanic stimulation, and have a low sensitivity to angular motion. Dimorphic afferents may have thick or thin axons. Those terminating more centrally tend to have thicker axons and are irregularly firing, galvanically sensitive, and sensitive to (rotational or linear) stimulation. Dimorphic afferents terminating peripherally (either in the macula or crista) and bouton afferents tend to be thinner fibers with lower galvanic and natural stimulation thresholds and are regularly firing with slower conduction velocities (4). These different afferent types may be of more interest than just physiologic curiosity. The high sensitivity irregular afferents are sensitive to small perturbations but have nonlinear dynamics because they readily silence when the head moves in the inhibitory direction. These afferents may be best suited for quick, nonlinear reflexes such as vestibulospinal responses to inhibit a fall. In contrast, the linear characteristics of the thinner, more regular afferents are appropriate for linear vestibular reflexes, like the vestibuloocular reflex, that must work over a wide range of frequencies and peak velocities (5).
Both semicircular canal afferents and otolith afferents are cosine tuned, which means they have one best characteristic response vector. For utricular afferents, these vectors can lie anywhere in the horizontal plane and are dependent on the orientation vector of the hair cells that they innervate. The response of the afferent is proportional to the cosine angle between the direction of stimulation and the orientation vector of the afferent. Similarly, the rotational vector of maximum stimulation for semicircular canal afferents is in the plane of rotation of the canal. The response of the fiber decreases as cosine of the angle between the plane of rotation and the canal plane. The cosine tuning of the afferent is consistent with the fact that transmitter release by hair cells is proportional to the cosine of the angle between the displacement of the hair cell bundle and the direction of stimulation (Fig. 130.6). Thus, both the hair cells and the afferents are cosine tuned. The coding of the vestibular system is such that the direction of stimulation is encoded by the afferent population stimulated, and the intensity of the movement is encoded by the intensity of the response of the stimulated afferent.
All of the vestibular epithelium are also innervated by vestibular efferent neurons. These neurons have cell bodies in the brainstem in areas around the genu of the facial nerve. Their fibers in humans run mixed with the vestibular afferent fibers and can terminate either presynaptically (on type II hair cells) or postsynaptically on calyx or bouton endings. The function of the vestibular efferent system in mammals is unknown.

Vestibular Brainstem
Vestibular afferents are bipolar neurons that have cell bodies in the inferior and superior Scarpa (vestibular) ganglion. The peripheral (dendritic) processes of these neurons exit the neuroepithelium and collect in the inferior and superior vestibular nerves. The inferior division includes neurons from the posterior canal and saccule, and the anterior division includes utricular, horizontal canal, and anterior canal afferents (Fig. 130.11). Axonal branches of primary afferent ramify in the vestibular nuclei. Afferent terminals from the different end organs primarily innervate the various divisions of the vestibular nuclei, although terminations are seen in the cerebellum and other brainstem nuclei as well. The precise terminations by end organ (semicircular canal or otolith) in the central nervous system are similar in many species (6). Not only does the brainstem region receive convergent output from different branches of the vestibular nerve, but individual neurons receive afferent input from one, two, or more end organs (canal ampullae or otolith maculae). Thus, the vestibular nuclei integrate information from multiple ipsilateral receptors.
There are four major vestibular nuclei in the brainstem: the lateral (Deiters), superior, medial, and inferior (spinal, descending) nuclei. In addition, there are several minor vestibular nuclei, including nucleus y, that are identified in various species by various investigators. The vestibular nuclei not only receive vestibular information but other information pertaining to spatial orientation as well. These inputs include optokinetic signals through the accessory optic system, neck proprioceptive signals, and Purkinje cell projections from the cerebellar cortex. From the vestibular nuclei, vestibular signals are passed throughout the central nervous system. The dominant output of the vestibular nuclei are to the ocular motor nuclei, via the medial longitudinal fasciculus and the ascending tract of Deiters; to the spinal cord, via the medial and lateral vestibulospinal tracts; to the cerebellum, via the cerebellar peduncles; and to the contralateral vestibular nuclei, via the vestibular commissural system. Other pathways connect the vestibular nuclei with the autonomic system, which has implications in motion sickness and blood pressure control, and with the thalamus.
One important function of the vestibular commissural system is inhibition. Experimental data show that the discharge frequencies of neurons excited during ipsilateral angular acceleration are also excited due to a decrease of crossed inhibition, which is caused by a decrease in discharge rate from the contralateral paired semicircular canal. This reciprocal mechanism is the basis of the so-called push-pull connection that increases the sensitivity of the system through use of the difference in signals between the functionally paired semicircular canals (left horizontal–right horizontal, left anterior–right posterior, left posterior–right anterior) in either ear. In this way, the paired canals complement one another and tend to cancel out the asymmetries inherent in the hair cell transduction mechanisms and afferent firing patterns mentioned earlier. The neural signals from these pairs of canals converge in a synergistic way in the nervous system, allowing the system to function even in the presence of a complete unilateral lesion. However, responses in patients with unilateral lesions are more asymmetric than those among healthy persons given high enough angular acceleration to reveal these inherent asymmetries, which are apparent when the push-pull redundancy is not available.
The best studied vestibular reflex is the vestibuloocular reflex. Vestibuloocular reflexes are of two types: compensatory reflexes that stabilize gaze during motion and orienting reflexes that align the eye with the gravitational vector. One of the challenges for the nervous system is to translate signals from the semicircular canal planes into coordinates appropriate for effector action. Those who study the vestibular system use an external frame of reference, as shown in Figure 130.3. Linear acceleration or rotational acceleration occurs around three axes that are perpendicular to each other: the interaural or pitch axis, the nasal-occipital or roll axis, and the rostral-caudal or yaw axis. The vestibule-oculo-motor system, however, is thought to use a coordinate system based on the orientation of the three pairs of semicircular canals. Experiments have shown that stimulation of afferent branches of the eighth cranial nerve that come exclusively from one semicircular canal produces reflexive eye movements that tend to rotate around the axis of greatest sensitivity for that canal. The three agonist-antagonist pairs of eye muscles themselves do not produce eye movements that completely correspond to these axes of orientation of the semicircular canals. Thus, there is a distribution of signals from the semicircular canals to produce compensatory eye movement of the desired magnitude and direction.
According to a simplified analysis, the connection between the three pairs of semicircular canals and the three pairs of eye muscles can be described with a set of nine constant coefficients. First-order analysis indicates that the translation of incoming vestibular systems needed to produce compensatory eye movement is a relatively simple operation for the brain to perform. This operation is contrasted to the more complicated series of commands that must be given when signals from the vestibular system are used to stabilize the head on the neck or the body with leg muscles.
The nervous system can adapt its response by comparing vestibular input to other sensory input. When the head moves, the vestibuloocular reflex tends to stabilize the image of an object in space on the retina by producing an eye movement compensatory to the head movement. At any time, the functional anatomic connections needed to stabilize an object can be thought of as a set of nine constant coefficients that distribute the incoming vestibular systems to the ocular motor neurons to form the reflexive eye movement response. For example, the motion of the head 10 degrees to the right produces eye movement 10 degrees to the left.
Provisions have been made in the nervous system for this response to adapt when necessary, owing to factors such as disease or aging. One such example is people with myopia who wear eyeglasses. If the magnification of the lens is 1.2 times, rotation of the head 10 degrees to the right produces rotation of the world as viewed by the eye 12 degrees to the left and therefore demands a corresponding reflexive eye movement 12 degrees to the left. The nervous system makes this form of adaptive change to resolve a conflict between afferent inputs, in this case vestibular and visual inputs. In this example, the nervous system can correspondingly increase the amount of eye movement produced for a given head movement so that the error between the head motion input and eye motion response is reduced to nearly zero. This gain plasticity requires participation of the floccular lobe of the cerebellum.

Current Vestibular Issues
Like most fields in basic sciences, the vestibular system is actively studied in a number of excellent laboratories. Among the many actively investigated areas are the pharmacology of the vestibular periphery, interactions between active head movements and the passive vestibular reflexes, the role of vestibular signals in spatial orientation, the function of vestibular efferent system, physiologic and cellular mechanisms of adaptation and compensation after vestibular injury, and the adaptation of the vestibular system to microgravity. In addition, efforts are ongoing to develop prosthetic devices to aid patients with vestibular deficits. This research holds the promise of improving our understanding of this vital, well conserved, and underappreciated “sixth” sensory system.