Semicircular Canal Primary Afferents Research Articles

Symmetries of a generic utricular projection: neural connectivity and the distribution of utricular information.

Sensory contribution to perception and action depends on both sensory receptors and the organization of pathways (or projections) reaching the central nervous system. Unlike the semicircular canals that are divided into three discrete sensitivity directions, the utricle has a relatively complicated anatomical structure, including sensitivity directions over essentially 360° of a curved, two-dimensional disk. The utricle is not flat, and we do not assume it to be. Directional sensitivity of individual utricular afferents decreases in a cosine-like fashion from peak excitation for movement in one direction to a null or near null response for a movement in an orthogonal direction. Directional sensitivity varies slowly between neighboring cells except within the striolar region that separates the medial from the lateral zone, where the directional selectivity abruptly reverses along the reversal line. Utricular primary afferent pathways reach the vestibular nuclei and cerebellum and, in many cases, converge on target cells with semicircular canal primary afferents and afference from other sources. Mathematically, some canal pathways are known to be characterized by symmetry groups related to physical space. These groups structure rotational information and movement. They divide the target neural center into distinct populations according to the innervation patterns they receive. Like canal pathways, utricular pathways combine symmetries from the utricle with those from target neural centers. This study presents a generic set of transformations drawn from the known structure of the utricle and therefore likely to be found in utricular pathways, but not exhaustive of utricular pathway symmetries. This generic set of transformations forms a 32-element group that is a semi-direct product of two simple abelian groups. Subgroups of the group include order-four elements corresponding to discrete rotations. Evaluation of subgroups allows us to functionally identify the spatial implications of otolith and canal symmetries regarding action and perception. Our results are discussed in relation to observed utricular pathways, including those convergent with canal pathways. Oculomotor and other sensorimotor systems are organized according to canal planes. However, the utricle is evolutionarily prior to the canals and may provide a more fundamental spatial framework for canal pathways as well as for movement. The fullest purely otolithic pathway is likely that which reaches the lumbar spine via Deiters' cells in the lateral vestibular nucleus. It will be of great interest to see whether symmetries predicted from the utricle are identified within this pathway.

Testing Human Otolith Function Using Bone‐Conducted Vibration

Bone-conducted vibration of the forehead, in the midline at the hairline (Fz) causes linear acceleration stimulation of both mastoids and results in an ocular vestibular-evoked myogenic potential (oVEMP), recorded by surface electromyogram (EMG) electrodes just beneath the eyes. The early n10 component of the oVEMP is symmetrical in healthy subjects, absent in patients with bilateral vestibular loss, and in patients after unilateral vestibular loss (uVL) n10 is small or absent on the side contralateral to the uVL, but of normal amplitude on the side contralateral to the healthy ear. The n10 component probably reflects mainly otolithic function, since in the guinea pig, primary otolith irregular neurons are selectively activated by bone-conducted vibration (BCV) at low intensities (0.1 g), whereas semicircular canal primary afferents are not activated even at high intensities (10 g).

Spatial symmetry groups as sensorimotor guidelines

While some aspects of neuroanatomical organization are related to packing and access rather than to function, other aspects of anatomical/physiological organization are directly related to function. The mathematics of symmetry groups can be used to determine logical structure in projections and to relate it to function. This paper reviews two studies of the symmetry groups of vestibular projections that are related to the spatial functions of the vestibular complex, including gaze, posture, and movement. These logical structures have been determined by finding symmetry groups of two vestibular projections directly from physiological and anatomical data. Logical structures in vestibular projections are distinct from mapping properties such as the ability to maintain two- and three-dimensional coordinate systems; rather, they provide anatomical/physiological foundations for these mapping properties. The symmetry group of the direct projection from the semicircular canal primary afferents to neck motor neurons is that of the cube (O, the octahedral group), which can serve as a discrete skeleton for coordinate systems in three-dimensional space. The symmetry group of the canal projection from the secondary vestibular afferents to the inferior olive and thence to the cerebellar uvula-nodulus is that of the square (D8), which can support coordinates for the horizontal plane. While the mathematical relationship between these symmetry groups and functions of the vestibular complex are clear, these studies open a larger question: what is the causal logic by which neural centers and their intrinsic organization affect each other and behavior? The relationship of vestibular projection symmetry groups to spatial function make them ideal projections for investigating this causal logic. The symmetry group results are discussed in relationship to possible ways they communicate spatial structure to other neural centers and format spatial functions such as body movements. These two projection symmetry groups suggest that all vestibular projections may have symmetry groups significantly related to function, perhaps all to spatial function.

Functional recovery of anterior semicircular canal afferents following hair cell regeneration in birds.

Streptomycin sulfate (1.2 g/kg i.m.) was administered for 5 consecutive days to 5-7-day-old white Leghorn chicks; this causes damage to semicircular canal hair cells that ultimately regenerate to reform the sensory epithelium. During the recovery period, electrophysiological recordings were taken sequentially from anterior semicircular canal primary afferents using an indentation stimulus of the canal that has been shown to mimic rotational stimulation. Chicks were assigned to an early (14-18 days; n = 8), intermediate (28-34 days; n = 5), and late (38-58 days; n = 4) period based on days after treatment. Seven untreated chicks, 15-67 days old, provided control data. An absence of background and indent-induced discharge was the prominent feature of afferents in the early period: only "silent" afferents were encountered in 5/8 experiments. In several of these chicks, fascicles of afferent fibers were seen extending up to the epithelium that was void of hair cells, and intra- and extracellular biocytin labeling revealed afferent processes penetrating into the supporting cell layer of the crista. In 3/8 chicks 74 afferents could be characterized, and they significantly differed from controls (n = 130) by having a lower discharge rate and a negligible response to canal stimulation. In the intermediate period there was considerable variability in discharge properties of 121 afferents, but as a whole the number of "silent" fibers in the canal nerve diminished, the background rate increased, and a response to canal stimulation detected. Individually biocytin-labeled afferents had normal-appearing terminal specializations in the sensory epithelium by 28 days poststreptomycin. In the late period, afferents (n = 58) remained significantly different from controls in background discharge properties and response gain. The evidence suggests that a considerable amount of variability exists between chicks in the return of vestibular afferent function following ototoxic injury and that the secretory function of regenerating hair cells might become functional before their transducer function.

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity.

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity. J. Neurophysiol. 80: 3297-3311, 1998. The electrophysiological activity of horizontal semicircular canal primary afferents (HSCPA) was investigated in vivo in the barbiturate-anesthetized pigeon by means of extracellular single-fiber vestibular nerve action potential recordings. The spontaneous and driven discharges to pulse (step/trapezoid waveform, peak velocity = 120 degrees/s) and sum-of-sines (0.03, 0.09, 0.21, 0.39, 0.93, 1.83 Hz, peak velocity = 30 degrees/s for each frequency) rotations were measured both in normal control animals and a group of animals at 30, 40, 50, 60, 71, and 150 days postinjection sequence (PIS) of streptomycin sulfate. Prior to 30 days PIS, the activity in the nerve was not appropriately modulated during and after rotation. At 30 days PIS and thereafter, the responses resembled those observed in control animals but with systematic changes in parameters of fitted pulse responses and fitted Bode plots as days PIS increased. The return of parameters characterizing the neural dynamics of the semicircular canals were monotonic and could be best described by either linear or exponential functions. After 30 days PIS, the mechanical cupula-endolymph system, the function of which can be inferred from the cupula long time constant (tauL) following step velocity, did not change systematically (tauL = 6.92 +/- 3.96, 8.64 +/- 5.52, 8.35 +/- 4.21, 10.00 +/- 2.79, 9.05 +/- 3.67, 7.05 +/- 2.72; means +/- SD). However, the mean gain (G) of the HSCPA response to pulse rotation nearly doubled between 30 and 150 days PIS (from 1.31 +/- 0. 39 to 2.40 +/- 1.04) and returned linearly to control values (G = 2. 39 +/- 0.77) over this time period [G = 1.33 + 0.009(PIS-30), R2 = 0. 92, P < 0.05]. Meanwhile, neural adaptation as quantitated using a fractional operator, k, decayed exponentially (single exponential) to an asymptote. The time constant of this exponential was approximately 55 days [k = 0.034 + 0.33e-(PIS-30)/55.4, R2 = 0.99, P < 0.01]. Features of the spontaneous discharge previously shown to be correlated with k changed appropriately. That is, the coefficient of variation (CV) and frequency of firing (FF) decayed and grew asymptotically, respectively. These parameters also exhibited an exponential time course of return to control values from 30 to 150 days PIS [CV = 0.44 + 0.65e-(PIS-30)/21.5, R2 = 0.96, P < 0.01, and FF = 39.97 + 101.42(1 - e-(PIS-30)/32.6), R2 = 0.97, P < 0.01]. The trends of recovery for G, k, and tauL derived from analysis of the pulse response were confirmed by strong positive correlations with best fitted parameters obtained from analysis of the sum-of-sines frequency domain response of HSCPAs. There were statistically significant correlations (r = 0.90, P < 0.05 and r = 0.93, P < 0.05) between parameters (G, k) derived from pulse responses and those (G', k') from sum-of-sines responses, respectively. The cupula time constant based on sum-of-sines' data (tau'L) showed no statistically significant change between 30 and 150 days PIS (P > 0.05, analysis of variance). Thus the results in present study indicate that both the spontaneous discharge and the driven response to rotation of pigeon HSCPAs recovered their normal physiological status between 30 and 150 days PIS after hair cell death due to aminoglycoside ototoxicity. The recovery was systematic for the parameters chosen to be tested with the exception of the cupula long time constant, tauL. The mechanisms (changes in ciliary dynamics, changes in hair cell ionic currents, changes in bouton terminals, etc.) underlying these changes await further morphophysiological studies.

Adaptation of primate vestibuloocular reflex to altered peripheral vestibular inputs. I. Frequency-specific recovery of horizontal VOR after inactivation of the lateral semicircular canals.

1. The adaptive plasticity of the vestibuloocular reflex (VOR) following a selective lesion of the peripheral vestibular organs was investigated in rhesus monkeys whose lateral semicircular canals were inactivated by plugging of the canal lumen in both ears. Gain and phase of horizontal, vertical, and torsional slow-phase eye velocity were determined from three-dimensional eye movement recordings obtained acutely after the plugging operation, as well as in regular intervals up to 10 mo later. 2. Acutely after plugging, horizontal VOR was minimal during yaw rotation with gains of < 0.1 at all frequencies. Horizontal VOR gain gradually increased over time, reaching gains of 0.4-0.5 for yaw oscillations at 1.1 Hz approximately 5 mo after lateral canal inactivation. This response recovery was strongly frequency dependent: horizontal VOR gains were largest at the highest frequency tested and progressively decreased for lower frequencies. Below approximately 0.1 Hz, no consistent horizontal VOR could be elicited even 10 mo after plugging. 3. The frequency-dependent changes in gain paralleled changes in horizontal VOR phase. Below approximately 0.1-0.05 Hz large phase leads were present, similarly as in semicircular canal primary afferents. Smaller phase leads were also present at higher frequencies, particularly at 1.1 Hz (the highest frequency tested). 4. Consistent with the afferent-like dynamics of the adapted horizontal VOR, per- and postrotatory horizontal responses to constant-velocity yaw rotations were short lasting. Time constants of the slow-phase eye velocity envelope of the horizontal postrotatory nystagmus were approximately 2 s. Nonetheless, a consistent horizontal optokinetic afternystagmus was evoked in plugged animals. 5. A torsional component that was absent in intact animals was consistently present during yaw rotation acutely after lateral canal inactivation and remained approximately constant thereafter. The frequency response characteristics of this torsional component resembled those of the adapted horizontal slow-phase responses: gain decreased and large phase leads were introduced at frequencies below approximately 0.05-0.1 Hz. Torsional responses elicited by roll oscillations in supine position, on the other hand, were indistinguishable in their dynamics from intact animals. No consistent vertical nystagmus was elicited during yaw rotation. 6. Our results show that there is a slow, frequency-specific recovery of horizontal VOR after selective inactivation of the lateral semicircular canals. Both the spatial organization and the dynamic properties of the adapted VOR responses are distinctly different from responses in intact animals, suggesting complex changes in the underlying vestibuloocular circuitry.

Otolith-semicircular canal interaction during postrotatory nystagmus in humans.

The otolith-semicircular canal interaction during postrotatory nystagmus was studied in ten normal human subjects by applying fast, short-lasting, passive head and body tilts (15, 30, 45, or 90 degrees in the roll or pitch plane) 2 s after sudden stop from a constant-velocity rotation (100 degrees/s) about the earth-vertical axis in yaw. Eye movements were measured with three-dimensional magnetic search coils. Following the head tilt, activity in the semicircular canal primary afferents continues to reflect the postrotatory angular velocity vector in head-centered coordinates, whereas otolith primary afferents signal a different orientation of the head relative to gravity. Despite the change in head orientation relative to gravity, postrotatory eye velocity decayed closely along the axis of semicircular canal stimulation (horizontal in head coordinates) for large head tilts (90 degrees) and also for small head tilts (15-45 degrees) for reorientations in the pitch plane. Only for small head tilts (15-45 degrees) in the roll plane was there a reorientation of the eye rotation axis toward the gravitational vector. This reorientation was approximately compensatory for 15 degrees head tilts. For 30 degrees and 45 degrees head tilts the eye rotation axis tilted toward the gravitational vector by about the same amount as for 15 degrees head tilts. These results suggest that, with the exception of small head tilts in the roll plane, there was no compelling data showing a relationship between the eye rotation axis and head tilt and that postrotatory nystagmus is largely organized in head-centered rather than gravity-centered coordinates in humans. This indicates a rudimentary, nonlinear, and direction-specific interaction of semicircular canal and otolith signals in the central vestibular system in humans.

Three-dimensional orientation of the eye rotation axis during the Purkinje sensation

The otolith-semicircular canal interaction during postrotatory nystagmus was studied in six normal human subjects by applying fast, short-lasting, passive head and body tilts (90° in the roll of pitch plane) 2 s after sudden stop from a constant velocity rotation (100°/s) about the earth-vertical axis in yaw. Eye movements were measured with 3-D magnetic search coils. Following the head tilt, activity in the semicircular canal primary afferents continues to reflect the postrotatory angular velocity vector in head-centered coordinates, whereas otolith primary afferents signal a different orientation of the head relative to gravity. Pitch (roll) tilts away from upright during postrotatory nystagmus after yaw rotation elicited a transient vertical (torsional) VOR. Despite the change in head orientation relative to gravity, postrotatory eye velocity decayed closely along the axis of semicircular canal stimulation (horizontal in head coordinates). These results suggest that postrotary nystagmus is largely organized in head-centered rather than gravity-centered coordinates in humans as suggested by the Purkinje-sensation.

“Velocity leakage” in the pigeon vestibulo-ocular reflex

The transfer characteristics of the vestibulo-ocular reflex (VOR), and of the semicircular canal primary afferents (SCPAs) that drive it, have been studied in several species. In monkeys and cats, the dominant time constant describing horizontal VOR dynamics (tau hv) is longer than that (tau c) of horizontal SCPAs. This lengthening of the time constant has been attributed to a "velocity storage" mechanism that has been modeled as a positive feedback loop in the VOR pathways. We have studied the transfer characteristics of horizontal and vertical VOR and SCPAs in unanesthetized pigeons. In this species the dominant time constants of both the horizontal and vertical VOR (tau hv and tau vv) are shorter that tau c. This finding indicates that time constants characterizing the lower frequency response of the VOR can be lengthened or shortened depending on the species. We propose that in the pigeon the "velocity leakage" mechanism can be modeled by substituting negative feedback for positive feedback in the model of the VOR pathways. Negative feedback can also account for the further shortening of tau hv and tau vv as VOR gain increases with arousal. Additionally, making the negative feedback loop nonlinear can model the dependency of lower frequency VOR phase on amplitude, and skew in VOR waveforms. Pigeon VOR and SCPA dynamics also differ in their adaptive properties and higher frequency behavior. A predominance of input from highly adaptive SCPAs is proposed to account for the increased adaptation of the vertical VOR as compared with SCPAs overall. A pure time-delay associated with VOR operation can explain the phase lag of the VOR relative to SCPAs at higher frequencies.

Morphological correlates of response dynamics and efferent stimulation in horizontal semicircular canal afferents of the toadfish, Opsanus tau

1. We used the intraaxonal labeling technique to study correlations between the terminal dendritic morphology of horizontal semicircular canal primary afferents and their response dynamics to sinusoidal head rotation and combined electrical stimulation of central efferent vestibular neurons. Thirty-eight canal afferents were identified by their sensitivity and phase of response to rotation between 0.1 and 1.0 Hz (+/- 10 degrees/s) and were subsequently labeled with horseradish peroxidase or biocytin. The afferent's dendritic field and synaptic specializations in the neuroepithelium of the crista were examined under light microscopy. 2. Rate and regularity of background discharge of the afferent were not correlated with its axon diameter or relative location of its dendritic field in the crista. 3. Response sensitivity of the afferent to rotation was correlated both with the relative location of its dendritic field in the crista and with the number of terminal endings it possesses. Afferents having low sensitivities, slow dynamics, and few terminal endings supply the peripheral portions of the crista; afferents with higher sensitivities, faster dynamics, and greater number of terminal endings supply the more central portions. It is suggested that the differences in sensitivity among the afferents reflect principally the variations in both the cupular dynamics along the crista and the number of possible hair cell contact sites in the neuroepithelium. 4. Response phase of the afferent was correlated only with the extent of its dendritic processes along the transverse axis of the crista. Afferents having transversely oriented dendritic fields had less phase lags relative to acceleration than did those having a more longitudinally oriented dendritic field. 5. Efferent stimulation produced a change in both the afferent's discharge rate and its response sensitivity to rotation. Afferents discharge rate and its response sensitivity to rotation. Afferents having a centrally located dendritic field and acceleration afferents, defined by their response to rotation, were the most affected by efferent stimulation. These results suggest that efferent innervation is either directed toward, or most efficacious in, the central regions of the crista and that it may select specific hair cell-afferent complexes.

A frequency and time domain study of the horizontal and vertical vestibuloocular reflex in the pigeon.

1. The horizontal and vertical vestibuloocular reflex (HVOR and VVOR, respectively) was studied in four chronically instrumented pigeons. Eye movements were measured using the magnetic search-coil technique and were produced by rotation in the dark. During the rotation paradigms, the pigeons were either pharmacologically aroused (using amphetamine) or drug free (normal). The pigeon HVOR and VVOR were tested using step and sinusoidal rotational stimulation. The range of frequencies (0.03-6.0 Hz) and the magnitude of the sinusoidal stimuli were chosen to match those used in a previous study of the responses of semicircular canal primary afferents (SCPAs) in unanesthetized (i.e., normal) pigeons. 2. The gain of the HVOR and VVOR in both normal and aroused pigeons was independent of stimulus magnitude (6-30 degrees/s) over the frequency range tested. In aroused pigeons, the frequency independent gain (G) of the HVOR (G = 0.6) and VVOR (G = 0.9) was roughly twice that for normal pigeons. Pigeon VOR phase under all combinations of orientation and arousal level was independent of stimulus magnitude except at the lowest frequency tested (0.03 Hz). At this frequency, phase lead decreased as stimulus magnitude increased for the HVOR and VVOR in both normal and aroused pigeons. 3. The step and sinusoidal gains were greater for the VVOR than for the HVOR under the same level of arousal. Neither the gain nor the dominant time constant of the VOR (tau vor) differed for rotation direction (clockwise or counterclockwise) for the HVOR or VVOR in normal or aroused pigeons. 4. The mean value of tau vor was 4.0 +/- 0.5 (SE) s as estimated from frequency response data and 4.3 +/- 0.4 s as estimated from step response data for the HVOR and VVOR in both normal and aroused pigeons. In comparison, the mean value of the dominant or cupular time constant (tau c) of normal pigeon SCPAs was approximately 10 s as estimated from frequency response data. These results indicate that tau vor is shorter than tau c in the pigeon. 5. At higher frequencies, the pigeon HVOR and VVOR exhibit an increasing phase lag unaccompanied by a gain change--characteristics produced by a pure time delay. The value of this time delay was about 7 ms for both the HVOR and VVOR in both normal and aroused pigeons. The HVOR and VVOR in normal and aroused pigeons lacked the higher frequency lead characteristics present in the frequency responses of most pigeon SCPAs.

Spontaneous and driven responses of semicircular canal primary afferents in the unanesthetized pigeon.

Spontaneous activity from 120 vestibular semicircular canal (SC) primary afferents and driven responses from 20 SC primary afferents to 2.3-decade bandwidth (0.029-6.152 Hz) sum of sinusoidal angular rotations were characterized in 10 unanesthetized pigeons. The results were compared with those previously gathered from barbiturate anesthetized pigeons. The average spontaneous mean firing rate (MFR) of SC primary afferents in unanesthetized pigeons was found to be 168 impulses per second (I/s), n = 120, and is approximately 80% higher than that for SC primary afferents in anesthetized pigeons [93 I/s, n = 149 (Ref. 9) and 92 I/s, n = 124 (Ref. 22)]. The spontaneous discharge of SC primary afferents from unanesthetized pigeons was classified according to the coefficient of variation (CV) of 512-1,024 interspike intervals (ISIs) into regular (CV less than 0.1), intermediate (0.1 less than or equal to CV less 0.4), and irregular (0.4 less than or equal to CV) categories. The percentages of SC primary afferents falling within each CV category were similar for unanesthetized and anesthetized pigeons (22). The relation between the spontaneous mean ISI and its standard deviation (SD) for SC primary afferents in the unanesthetized pigeon is best described by a power function model, which provides a significantly better fit than does a linear model. The parameters of this power function model are similar for SC primary afferents in unanesthetized and anesthetized pigeons (10). The spontaneous ISIs of individual SC primary afferents in the unanesthetized pigeon were found to be distributed either normally, log-normally, or according to the first passage time of the Wiener-Levy (WL) process. The gain of anterior SC primary afferents at 0.25 Hz is similar for anesthetized (2.93 I X s-1 X deg-1 X s-1, n = 14) (11) and for unanesthetized (3.01 I X s-1 X deg-1 X s-1, n = 14) pigeons. Bode plots constructed from the responses to sum sinusoidal angular accelerations reveal that SC primary afferents in unanesthetized pigeons display a phase lead and gain enhancement at frequencies between 0.6 and 6 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Differences between cats in response properties of horizontal semicircular canal primary afferents.

The response properties of cat horizontal canal afferents (N = 81) were characterized by three parameters: their long time constants (tau), low frequency gain constants (G1), and middle frequency gain constants (Gm). An average value of each of these parameters was calculated for each of eight animals and comparisons were made across animals. There were significant differences between individual animals in their average values of tau and Gm. There was also a significant negative correlation between tau's and Gm's. An animal with a larger average tau tended to have a smaller average Gm. We also used anatomic data on membranous canal duct diameter from the literature to independently estimate the potential effect of interanimal anatomic variability on the predicted range of tau and Gm values in a population. We then compared the data from our 81 afferents with the predictions from the anatomic data.

A nonlinear model of semicircular canal primary afferents in bullfrog.

A nonlinear model of semicircular canal primary afferents in bullfrog.B N Segal, and J S OuterbridgeB N Segal, and J S OuterbridgePublished Online:01 Apr 1982https://doi.org/10.1152/jn.1982.47.4.563MoreSectionsPDF (3 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited BySome hydrodynamic properties of the posterior canal in the frog labyrinth related to neuronal responsesNeuroscience Letters, Vol. 66, No. 3 More from this issue > Volume 47Issue 4April 1982Pages 563-78 https://doi.org/10.1152/jn.1982.47.4.563PubMed6978388History Published online 1 April 1982 Published in print 1 April 1982 Metrics

Tilt responses of semicircular canal primary afferents.

The purpose of our study was to determine the sensitivity of semicircular canal afferents to linear acceleration in intact animals prepared to preserve the physical integrity of the end organ and nerve. The responses of physiologically identified lateral and vertical canal afferents to static head tilts are described, and the implications of these findings, especially with regard to positional nystagmus, are discussed.

Long-term adaptive changes in primate vestibuloocular reflex. II. Electrophysiological observations on semicircular canal primary afferents.

I. Single-unit recordings were obtained from 551 fibers in the vestibular portion of the eighth nerve in awake rhesus monkeys. Slightly more than half of these recordings were from normal animals and the remainder from animals adapted to dove prism (leftright) reversing spectacles for 2 or more weeks. 2. Horizontal vestibuloocular reflex (VOR) gains were determined at intervals throughout the recording period (several weeks for each animal), using passive sinusoidal oscillations of the whole animal about the vertical axis at 0.2 Hz, 220”. Mean horizontal VOR gain for the normal animals was 0.94 (range, 0.91-0.98) and for the prism adapted, 0.15 (range, 0.05-0.2). The latter showed large phase lags relative to normal (mean, -93”; range, -15 to -160”). 3. On the basis of their responses to angular head oscillations and the dynamic and static phases of forward or backward head tilting, eighth nerve fibers were classified as otolith or semicircular canal afferents, and the latter assigned to one of the three canals. The proportions of the various fiber types were very similar in the normal and prism-adapted animals, and each included a small percentage of fibers that were unresponsive to any of the stimuli employed. 4. Canal fiber responses to sinusoidal angular oscillations were normalized to provide a best estimate of their sensitivity to head velocity (in spikes l Udeg l s-l) when the response plane of the canal in question coincided with the horizontal stimulus plane. 5. Nonparametric statistical tests applied to the population data for the canal afferents in normal and adapted animals failed to reveal any significant differences in their resting discharge rates or their responses to sinusoidal angular head oscillations (sensitivity or phase) with one minor exception: activity of horizontal canal units in the adapted animals was on average a few degrees more phase advanced than that in the normals. Since this difference was small and uncorrelated with the much larger phase lags seen in the vestibuloocular responses of the adapted animals, it is thought to result from biased sampling. Correlations between these measured parameters (resting discharge rate, sensitivity, phase) were also not significantly different in normal and adapted animals. 6. We conclude that the signals carried by the semicircular canal primary afferents in the monkey do not undergo any significant modification when the animal adapts to reversing prisms and the modifiable elements responsible for the observed changes in the vestibuloocular reflex must lie within the central nervous system.