Neurodynamic response analysis of anterior semicircular canal afferents in the pigeon

Infrasonic hearing in birds: a review of audiometry and hypothesized structure-function relationships

The perception of airborne infrasound (sounds below 20 Hz, inaudible to humans except at very high levels) has been documented in a handful of mammals and birds. While animals that produce vocalizations with infrasonic components (e.g. elephants) present conspicuous examples of potential use of infrasound in the context of communication, the extent to which airborne infrasound perception exists among terrestrial animals is unclear. Given that most infrasound in the environment arises from geophysical sources, many of which could be ecologically relevant, communication might not be the only use of infrasound by animals. Therefore, infrasound perception could be more common than currently realized. At least three bird species, each of which do not communicate using infrasound, are capable of detecting infrasound, but the associated auditory mechanisms are not well understood. Here we combine an evaluation of hearing measurements with anatomical observations to propose and evaluate hypotheses supporting avian infrasound detection. Environmental infrasound is mixed with non-acoustic pressure fluctuations that also occur at infrasonic frequencies. The ear can detect such non-acoustic pressure perturbations and therefore, distinguishing responses to infrasound from responses to non-acoustic perturbations presents a great challenge. Our review shows that infrasound could stimulate the ear through the middle ear (tympanic) route and by extratympanic routes bypassing the middle ear. While vibration velocities of the middle ear decline towards infrasonic frequencies, whole-body vibrations - which are normally much lower amplitude than that those of the middle ear in the 'audible' range (i.e. >20 Hz) - do not exhibit a similar decline and therefore may reach vibration magnitudes comparable to the middle ear at infrasonic frequencies. Low stiffness in the middle and inner ear is expected to aid infrasound transmission. In the middle ear, this could be achieved by large air cavities in the skull connected to the middle ear and low stiffness of middle ear structures; in the inner ear, the stiffness of round windows and cochlear partitions are key factors. Within the inner ear, the sizes of the helicotrema and cochlear aqueduct are expected to play important roles in shunting low-frequency vibrations away from low-frequency hair-cell sensors in the cochlea. The basilar papilla, the auditory organ in birds, responds to infrasound in some species, and in pigeons, infrasonic-sensitive neurons were traced back to the apical, abneural end of the basilar papilla. Vestibular organs and the paratympanic organ, a hair cell organ outside of the inner ear, are additional untested candidates for infrasound detection in birds. In summary, this review brings together evidence to create a hypothetical framework for infrasonic hearing mechanisms in birds and other animals.

Models of vestibular semicircular canal afferent neuron firing activity

Semicircular canal afferent neurons transmit information about head rotation to the brain. Mathematical models of how they do this have coevolved with concepts of how brains perceive the world. A 19th-century "camera" metaphor, in which sensory neurons project an image of the world captured by sense organs into the brain, gave way to a 20th-century view of sensory nerves as communication channels providing inputs to dynamical control systems. Now, in the 21st century, brains are being modeled as Bayesian observers who infer what is happening in the world given noisy, incomplete, and distorted sense data. The semicircular canals of the vestibular apparatus provide an experimentally accessible, low-dimensional system for developing and testing dynamical Bayesian generative models of sense data. In this review, we summarize advances in mathematical modeling of information transmission by semicircular canal afferent sensory neurons since the first such model was proposed nearly a century ago. Models of information transmission by vestibular afferent neurons may provide a foundation for developing realistic models of how brains perceive the world by inferring the causes of sense data.

Influence of Magnitude and Duration of Altered Gravity and Readaptation to 1 g on the Structure and Function of the Utricle in Toadfish, Opsanus tau

Gravity has remained constant during animal evolution and the neural sensory systems detecting acceleration forces have remained remarkably conserved among vertebrates. The utricular organ senses the sum of inertial force due to head translation and head tilt relative to gravitational vertical. Change in gravitational force would be expected to have profound effects on how an organism maintains equilibrium. We characterize the physiology of utricular afferents to applied accelerations in the oyster toadfish, Opsanus tau, in normal 1 g to establish benchmarks, after 1–32-day exposures to 2.24 g (resultant) via centrifugation (hypergravity, HG), after 4- and 16-day exposures to 1.12 g (resultant), and following 1–8 days recovery to HG exposures to study re-adaptation to 1 g. Afferents were also examined during activation of efferent vestibular pathway. Centrifugation at 2.24 g included 228°/s constant angular velocity component, and thus horizontal canal afferent responses to yaw rotation were recorded as an internal control in each fish. Afferents studied after 228°/s rotation for 4 and 16 days without centripetal acceleration, called On-Center-Control, were indistinguishable from their control counterparts. Principal response to HG was an adjustment of afferent sensitivity as a function of magnitude and duration of exposure: an initial robust increase at 3–4 days followed by a significant decrease from 16 to 32 days. Initial increase observed after 4 days of HG took >4 days in 1 g to recover, and the decrease observed after 16 days of HG took >2 days to readapt to 1 g. Hair cells in striola and medial extrastriola macula regions were serially reconstructed in 3D from thin sections using transmission electron microscopy in control fish and fish exposed to 4 and 16 days of HG. Despite the highly significant differences in afferent physiology, synaptic body counts quantified in the same fish were equivalent in their inter-animal variability and averages. No clear role of the efferent pathway as a feedback mechanism regulating afferent behavior to HG was found. Transfer from 1 g to HG imparts profound effects on gravitational sensitivity of utricular afferents and the accompanying transfer from the HG back to the 1 g resembles in part (as an analog) the transfer from 1 g to the micrograms.

Afferent innervation patterns of the pigeon horizontal crista ampullaris

The vestibular semicircular canals are responsible for detection of rotational head motion although the precise mechanisms underlying the transduction and encoding of movement information are still under study. In the present investigation, we utilized neural tracers and immunohistochemistry to quantitatively examine the topology and afferent innervation patterns of the horizontal semicircular canal crista (HCC) in pigeons (Columba livia). Two hundred and eighty-six afferents from five horizontal canal organs were identified of which 92 units were sufficiently labeled and isolated to perform anatomical reconstructions. In addition, a three-dimensional contour map of the crista was constructed. Bouton afferents were located only in the peripheral regions of the receptor epithelium. Bouton afferents had the most complex innervation patterns with significantly longer and more numerous branches as well as a higher branch order than any other fiber type. Bouton fibers also contained significantly more bouton terminals than did dimorph afferents. Calyx afferents were located only in the apex and central planar regions. Calyx fibers had the largest axonal diameters yet the smallest fiber lengths and innervation areas, the fewest number of branches, the lowest branch order, and the fewest total number of terminals of all fiber types. Dimorph afferents were located throughout the central crista with afferent terminations that were larger and more complex than calyx fibers but less so than bouton fibers. Overall, the pigeon HCC morphology and innervation shares many common features with those of other animal classes.

Planar relationships of the semicircular canals in two strains of mice

The mouse is increasingly important as a subject of vestibular research. Although many studies have focused on the vestibular responses of mice to angular rotation, the geometry of their semicircular canals has not been described. High-voltage X-ray computed tomography was used to measure the anatomy of the semicircular canals of two strains of mice, C57Bl/6J and CBA/CaJ. The horizontal plane of a stereotaxic coordinate system was defined by the midpoints of the external auditory meati and the point where the incisors emerge from the maxilla. The centroids of the lumens of the bony canals were calculated, and planes that describe the canals were fit using a least-squares regression analysis to the resulting points. Vectors normal to each regressed plane were used to represent the corresponding canal's axis of rotation, and angles of these vectors relative to skull landmarks as well as to each other were calculated. The horizontal canal of the mouse was found to be angled anteriorly upward 17.8 degrees for CBA/CaJ and 32.6 degrees for C57Bl/6J from the reference horizontal plane. Angles between ipsilateral canals deviated up to 12.3 degrees from orthogonal, and angles between contralateral synergistic canals (left anterior-right posterior, right anterior-left posterior, and horizontal-horizontal) deviated from parallel by up to 14.8 degrees. The orientations of the canals within the head as well as the orientations of the canals relative to each other were significantly different between the two strains, suggesting that care must be taken in the design and interpretation of developmental and physiologic studies involving different mouse strains.

Semicircular canal geometry, afferent sensitivity, and animal behavior

The geometry of the semicircular canals has been used in evolutionary studies to predict the behaviors of extinct animals. These predictions have relied on an assumption that the responses of the canals can be determined from their dimensions, and that an organism's behavior can be determined from these responses. However, the relationship between a canal's sensitivity and its size is not well known. An intraspecies comparison among canal responses in each of three species (cat, squirrel monkey, and pigeon) was undertaken to evaluate various models of canal function and determine how their dimensions may be related to afferent physiology. All models predicted the responses of the cat afferents, but the models performed less well for squirrel monkey and pigeon. Possible causes for this discrepancy include incorrectly assuming that afferent responses accurately represent canal function or errors in current biophysical models of the canals. These findings leave open the question as to how reliably canal anatomy can be used to estimate afferent responses and how closely afferent responses are related to behavior. Other labyrinthine features, such as orientation of the horizontal canal, which is reliably held near earth-horizontal across many species, may be better to use when extrapolating the posture and related behavior of extinct animals from labyrinthine morphology.

Determinants of spatial and temporal coding by semicircular canal afferents

The vestibular semicircular canals are internal sensors that signal the magnitude, direction, and temporal properties of angular head motion. Fluid mechanics within the 3-canal labyrinth code the direction of movement and integrate angular acceleration stimuli over time. Directional coding is accomplished by decomposition of complex angular accelerations into 3 biomechanical components-one component exciting each of the 3 ampullary organs and associated afferent nerve bundles separately. For low-frequency angular motion stimuli, fluid displacement within each canal is proportional to angular acceleration. At higher frequencies, above the lower corner frequency, real-time integration is accomplished by viscous forces arising from the movement of fluid within the slender lumen of each canal. This results in angular velocity sensitive fluid displacements. Reflecting this, a subset of afferent fibers indeed report angular acceleration to the brain for low frequencies of head movement and report angular velocity for higher frequencies. However, a substantial number of afferent fibers also report angular acceleration, or a signal between acceleration and velocity, even at frequencies where the endolymph displacement is known to follow angular head velocity. These non-velocity-sensitive afferent signals cannot be attributed to canal biomechanics alone. The responses of non-velocity-sensitive cells include a mathematical differentiation (first-order or fractional) imparted by hair-cell and/or afferent complexes. This mathematical differentiation from velocity to acceleration cannot be attributed to hair cell ionic currents, but occurs as a result of the dynamics of synaptic transmission between hair cells and their primary afferent fibers. The evidence for this conclusion is reviewed below.

Responses of irregularly discharging chinchilla semicircular canal vestibular-nerve afferents during high-frequency head rotations

Mammalian vestibular-nerve afferents innervating the semicircular canals have been divided into groups according to their discharge regularity, gain at 2-Hz rotational stimulation, and morphology. Low-gain irregular afferents terminate in calyx endings in the central crista, high-gain irregular afferents synapse more peripherally in dimorphic (bouton and calyx) endings, and regular afferents terminate in the peripheral zone as bouton-only and dimorphic endings. The response dynamics of these three groups have been described only up to 4 Hz in previous studies. Reported here are responses of chinchilla semicircular canal vestibular-nerve afferents to rotational stimuli at frequencies up to 16 Hz. The sensitivity of all afferents increased with increasing frequency with the sensitivity of low-gain irregular afferents increasing the most and matching the high-gain irregular afferents at 16 Hz. All afferents increased their phase lead with respect to stimulus velocity at higher frequencies with the highest leads in low-gain irregular afferents and the lowest in regular afferents. No attenuation of sensitivity or shift in phase consistent with the presence of a high-frequency pole over the range tested was noted. Responses were best fit with a torsion-pendulum model combined with a lead operator (tau(HF1)s + 1)(tau(HF2)s + 1). The discharge regularity of individual afferents was correlated to the value of each afferent's lead operator time constants. These findings suggest that low-gain irregular afferents are well suited for encoding the onset of rapid head movements, a property that would be advantageous for initiation of reflexes with short latency such as the vestibulo-ocular reflex.

Afferent innervation of the utricular macula in pigeons

Biotinylated dextran amine (BDA) was used to retrogradely label afferents innervating the utricular macula in adult pigeons. The pigeon utriclar macula consists of a large rectangular-shaped neuroepithelium with a dorsally curved anterior edge and an extended medioposterior tail. The macula could be demarcated into several regions based on cytoarchitectural differences. The striola occupied 30% of the macula and contained a large density of type I hair cells with fewer type II hair cells. Medial and lateral extrastriola zones were located outside the striola and contained only type II hair cells. A six- to eight-cell-wide band of type II hair cells existed near the center of the striola. The reversal line marked by the morphological polarization of hair cells coursed throughout the epithelium, near the peripheral margin, and through the center of the type II band. Calyx afferents innervated type I hair cells with calyceal terminals that contained between 2 and 15 receptor cells. Calyx afferents were located only in the striola region, exclusive of the type II band, had small total fiber innervation areas and low innervation densities. Dimorph afferents innervated both type I and type II hair cells with calyceal and bouton terminals and were primarily located in the striola region. Dimorph afferents had smaller calyceal terminals with few type I hair cells, extended fiber branches with bouton terminals and larger innervation areas. Bouton afferents innervated only type II hair cells in the extrastriola and type II band regions. Bouton afferents innervating the type II band had smaller terminal fields with fewer bouton terminals and smaller innervation areas than fibers located in the extrastriolar zones. Bouton afferents had the most bouton terminals on the longest fibers, the largest innervation areas with the highest innervation densities of all afferents. Among all afferents, smaller terminal innervation fields were observed in the striola and large fields were located in the extrastriola. The cellular organization and innervation patterns of the utricular maculae in birds appear to represent an organ in adaptive evolution, different from that observed for amphibians or mammals.

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.

Dynamics and kinematics of the angular vestibulo-ocular reflex in monkey: effects of canal plugging

Dynamics and kinematics of the angular vestibulo-ocular reflex in monkey: effects of canal plugging. J. Neurophysiol. 80: 3077-3099, 1998. Horizontal and roll components of the angular vestibulo-ocular reflex (aVOR) were elicited by sinusoidal rotation at frequencies from 0.2 Hz (60 degrees/s) to 4.0 Hz ( approximately 6 degrees/s) in cynomolgus monkeys. Animals had both lateral canals plugged (VC, vertical canals intact), both lateral canals and one pair of the vertical canals plugged (RALP, right anterior and left posterior canals intact; LARP, left anterior and right posterior canal intact), or all six semicircular canal plugged (NC, no canals). In normal animals, horizontal and roll eye velocity was in phase with head velocity and peak horizontal and roll gains were approximately 0.8 and 0.6 in upright and 90 degrees pitch, respectively. NC animals had small aVOR gains at 0.2 Hz, and the temporal phases were shifted approximately 90 degrees toward acceleration. As the frequency increased to 4 Hz, aVOR temporal gains and phases tended to normalize. Findings were similar for the LARP, RALP, and VC animals when they were rotated in the planes of the plugged canals. That is, they tended to normalize at higher frequencies. A model was developed incorporating the geometric organization of the canals and first order canal-endolymph dynamics. Canal plugging was modeled as an alteration in the low frequency 3-db roll-off and corresponding dominant time constant. The shift in the low-frequency 3-dB roll-off was seen in the temporal responses as a phase lead of the aVOR toward acceleration at higher frequencies. The phase shifted toward stimulus velocity as the frequency increased toward 4.0 Hz. By incorporating a dynamic model of the canals into the three-dimensional canal system, the spatial responses were predicted at all frequencies. Animals were also stimulated with steps of velocity in planes parallel to the plugged lateral canals. This induced a response with a short time constant and low peak velocity in each monkey. Gains were normalized for step rotation with respect to time constant as (steady state eye velocity)/(stimulus acceleration x time constant). Using this procedure, the gains were the same in canal plugged as in normal animals and corresponded to gains obtained in the frequency analysis. The study suggests that canal plugging does not block the afferent response to rotation, it merely shifts the dynamic response to higher frequencies.

Determinants of semicircular canal afferent response dynamics in fish

Present results separate the relative contributions of semicircular canal biomechanics from hair cell/afferent biophysics in determining the amplitude and phase of afferent responses to sinusoidal motion of the head. Separation was achieved by combining electrical polarization of the endolymph with mechanical indentation of the canal limb to modulate the instantaneous firing rate of horizontal semicircular canal afferents. The electrical stimulus drives hair cell transduction currents via modulation of the Nernst-Planck potential, whereas the mechanical stimulus mimics head rotation and modulates the open probability of the transduction channels. Responses for electrical polarization therefore reflect post-transduction-current (PTC) mechanisms, and responses for mechanical stimulation include the additional influence of canal mechanics. Linear transfer functions defining individual afferent response dynamics were obtained for low levels of each stimuli and are reported in Bode form providing gain (spikes/s per micron or mV) and phase (deg re: peak stim) over the frequency range from 0.02 to 40 Hz. Combined results for electrical and mechanical stimuli distinguish the component of sensory signal processing carried out by canal mechanics from that carried out by the hair cell/afferent complexes. Individual afferents were categorized according to their response to the mechanical stimuli as low-gain velocity (LG), high-gain velocity (HG) or acceleration (A) sensitive, groups as originally defined by Boyle and Highstein to describe interafferent diversity present within the population. In contrast to the results for mechanical stimuli, all afferent groups exhibit nearly equal increases in gain and phase for increasing frequencies of electrical stimulation. Comparison of individual afferent responses for the two stimuli leads to the conclusion that the LG, HG, and A groups are distinguished primarily by diversity in the mechanical activation of associated hair cells and not by PTC mechanisms. Even though PTC processing does not contribute significantly to determining these groups, it is the primary determinant underlying high-frequency gain and phase enhancements observed in the population average. Comparison of mechanical and electrical responses also reveals the mechanical lower-corner responsible for phase enhancements and gain decreases in all afferents at low frequencies of mechanical stimulation (< 0.05 Hz). Results imply that LG afferents encode angular head velocity by canceling a phase lag and gain attenuation due to the mechanics with a phase lead and gain enhancement due to PTC mechanisms above approximately 0.2 Hz. In contrast, A group afferents encode angular head acceleration by combining high-frequency phase leads and gain enhancements present in both the mechanics and PTC mechanisms across the physiological frequency spectrum. HG afferents fall between these two extremes, and, other than the influence of the mechanical lower-corner, their response primarily reflects PTC processing.

A method for controlled mechanical stimulation of single semicircular canals

In the present technical report, we describe a method of mechanical stimulation for single semicircular canals that is reproducible, sensitive, and discrete. The mechanical stimulator is capable of delivering reliable controlled stimuli of different waveforms over a wide frequency range that produce endolymph movements and consequent cupula deflections without motion being imparted to the animal. The results of electrophysiological experiments where the responses from pigeon (Columba livia) single horizontal semicircular canal afferent fibers produced by mechanical stimulation across a broad frequency bandwidth are reported. Comparisons between afferent fiber responses elicited by natural yaw rotation and mechanical stimulation were conducted, with the results indicating that the two stimulation methods produced responses from the same afferent unit that could be equated in magnitude. In addition, results are described from several control experiments that were conducted in order to determine the efficacy of the mechanical stimulation technique.

Some hydrodynamic properties of the posterior canal in the frog labyrinth related to neuronal responses

Measurements of the posterior-canal radius of curvature (R), the semicircular-duct radius (r) and the cupula radius (rC) were performed in the frog labyrinth. The Steinhausen-van Egmond equation led to an estimate of the canal endolymph flow, cupula deflection and sensory hair bending during constant angular accelerations (0.2-64 degrees/s2) of opposite directions and increasing duration (1.2-12 s). This analysis suggests that the non-linear canal afferent discharge behaviour exhibited by some units may not arise at the presynaptic level but rather postsynaptically, at the encoder site.

Dynamic properties of the action potential encoder in an insect mechanosensory neuron

A variety of sensory receptors show adaptation to dynamic stimuli that can be well characterized as fractional differentiation of the input signal. The cause of this behavior is unknown, but because it can be represented by linear systems theory, it has been assumed to arise during early linear processes of transduction or adaptation, rather than during the nonlinear process of action potential encoding. I measured the action potential encoding properties of an insect mechanoreceptor by direct electrical stimulation of the sensory cell axon and found a dynamic response that is identical to the response given by mechanical stimulation. This indicates that the fractional differentiation is a property of the encoder rather than the transducer.

Dimensional analysis and dynamic response characterization of mammalian peripheral vestibular structures

Extensive morphometric measurements were made on the vestibular system of the rabbit ( Oryctulagus cuniculus), the gerbil (Meriones unguiculatus), the chinchilla (Chinchilla laniger ), and the squirrel monkey (Saimiri sciureus) from serial sections of temporal bones. Additionally, a more limited set of measurements were also completed on the owl monkey (Aotus trivirgatus), the Capuchin monkey (Cebus sp.), the harp seal ( Pagophilus groenlandicus Erxleben , 1777), and the two-toed sloth ( Choloepus sp.). The following measurements were made: 1) radius of curvature (R) of each membranous semicircular canal (herein called semicircular duct-see nomenclature in Nomina Anatomica (1968) ), 2) cross-sectional diameter of the ducts and the osseous semicircular canals, and 3) some pertinent morphometrics of the cristae ampullares and the utricle. In all species studied 1) the radii of curvature of the three semicircular ducts are dissimilar, with that of the lateral duct being as small as, or smaller than, those of the anterior and posterior ducts; 2) R for the anterior duct is largest in the harp seal and the rabbit; 3) the canal and duct dimensions are largest in the Capuchin and squirrel monkeys, the two-toed sloth, and the harp seal, and smallest in the gerbil; 4) the proportion of otic fluid "space" that is occupied by endolymph shows a ranking of gerbil greater than rabbit greater than two-toed sloth greater than chinchilla = owl monkey greater than squirrel monkey greater than Capuchin monkey greater than harp seal; and 5) the gross ampullary and utricular dimensions are largest in the harp seal and smallest in the gerbil. These measurements were used for determining the time constants describing semicircular-canal dynamics in the Steinhausen (1931, 1933) and Oman -Marcus (1980) equations.

The response of primary horizontal semicircular canal neurons in the rat and guinea pig to angular acceleration

In rats and guinea pigs, primary afferent neurons from the horizontal semicircular canal were divided into two categories, regular and irregular, on the basis of the regularity of their resting activity. Regular neurons tend to have higher average resting rates than irregular neurons and in response to a constant angular acceleration stimulus of 16.7 deg/s2 regular neurons tended to have lower sensitivity and longer time constants than irregular cells. Some irregular neurons are more sensitive to incremental accelerations than to decremental accelerations of the same magnitude, whereas regular neurons tend to show symmetrical sensitivity. In response to sinusoidal angular acceleration stimuli (fixed frequencies) in the range 0.01-1.5 Hz, cells which fired regularly at rest tended to have smaller gain and longer phase lag re acceleration at most frequencies than irregular cells. Transfer functions were obtained for averaged data for regular and irregular neurons separately in both species. In both species there is evidence of systematic variation between neurons within each category, and this systematic variation is obscured by averaging across neurons.

Sensitivity of the vestibular system to acoustic stimuli

Cochlea-deprived pigeons were placed on a rotating platform and stimulated with sound after fenestration of the lateral canal. The whole nerve action potentials evoked by the sound stimulus were suppressed by the rotatory stimulus. The time course of this suppression makes the lateral crista the most acceptable site of generation of the action potential. In another type of experiment, single unit responses were recorded from the lateral ampullary nerve during stimulation with sound. Phase-lock of the response to the stimulus was best for frequencies fom 0.5 to 1 kHz.