Physiology of peripheral neurons innervating semicircular canals of the squirrel monkey. II. Response to sinusoidal stimulation and dynamics of peripheral vestibular system

Electronystagmography in the laboratory rat

A method is described for obtaining electronystagmograms from the awake laboratory rat. Threshold valves for rotation impulse and oscillatory acceleration were determined, as well as the time constant for the horizontal semicircular canal. The time constant appeared to be small. This might be attributed to the rapid natural head movements in this species. No visual suppression of vestibular nystagmus was found in the rat.

The response of horizontal semicircular canal afferents to sinusoidal rotation in the cat

Dynamic characteristics of primary vestibular afferents innervating the horizontal semicircular canal were studied in decerebrate, unanesthetized cats. Activities of individual afferent fibers were recorded intracranially by glass micropipettes. Frequency of sinusoidal rotation was varied from 0.014 Hz to 0.42 Hz, and phase and gain properties were examined. All of the fibers recorded fired spontaneously, and their firing rate ranged from 7 to 128 spikes/sec. Regularity of firing, phase lags, and gains were calculated in individual fibers. There was a tendency that the units with high spontaneous firing rates showed regular firing, larger phase lags, and lower gains that the units with low spontaneous firing rates. The transfer function of the system (firing rate of the primary afferent per angular acceleration of the head) was (formula: see text). A high frequency phase lead component was needed to account for the data obtained, indicating a slight deviation from the relationship predicted by the torsion pendulum model. The present phase properties were compared with those of vestibular nucleus neurons reported previously. It was suggested that a group of vestibular nucleus neurons transmits fairly faithfully the phase properties of primary afferents, and that another group of vestibular nucleus neurons receive additional influences from central structures, exhibiting larger phase lags than primary afferents.

Frequency response of the lateral-line organ of Xenopus laevis

The stimulus response relation of the epidermal lateral-line organ of Xenopus laevis was studied by recording activity of single afferent nerve fibres in isolated preparations. Linear frequency response analysis over a frequency range of 0.1--100Hz was performed under steady-state conditions, using small amplitude, sinusoidal water displacements produced by a glass sphere at a short distance from the skin. Period histograms of afferent nerve activity were computed, and amplitude, phase and mean activity of the response were determined by means of Fourier analysis. A standardization procedure at the start of each experiment made scaling of the frequency responses of different preparations unnecessary. The results show that for small stimulus amplitudes the response of the lateral-line organ over the whole range of frequencies studied can adequately be described as a modulation of the spontaneous activity. The amplitude of the response is proportional to the stimulus amplitude, and the phase of the response is independent of stimulus amplitude. The lateral-line organ of Xenopus laevis can thus be regarded as a linear system for stimuli which produce modulation of the spontaneous activity. The frequency response demonstrates unequivocally that the lateral-line organ of Xenopus laevis functions as a water velocity detector. For frequencies of stimulation from 0.1--20Hz the gain increases with a slope of 7.5 dB/oct, and up to 5Hz the response is almost in phase with the water velocity. The extent to which the different transmission steps between stimulus and response will contribute to the frequency response is discussed.

Responses of squirrel monkey vestibular neurons to audio-frequency sound and head vibration

A study was made of the response of peripheral vestibular neurons in the squirrel monkey to head vibration and air-borne sound in the frequency range from 50-4 00 Hz. Responses were measured in terms of the phase locking of discharge and changes in firing rate. The lowest phase-locking thresholds for vibration were -70 to -80 dB re 1 g, and median values in the most sensitive frequency range (200-400 Hz) were -20 to -40 dB re 1 g; the minimum and median thresholds for sound were 76 and 120-130 dB SPL, respectively. Rate-change thresholds were 10-30 dB above phase-locking thresholds. The squirrel monkey sacculus has no special sensitivity to vibration in comparison with the other vestibular end-organs; the median phase-locking threshold to sound of saccular neurons exceeded 100 dB SPL. Irregularly discharging neurons are more sensitive than regularly discharging units. Evidence is presented that the response to intense sound involves a hair-cell mechanism.

Post-synaptic potentials recorded from afferent nerve fibres of the posterior semicircular canal in the frog

Glass microelectrode recordings were made from single fibres of the posterior ampullary nerve in the isolated labyrinth of the frog (Rana esculenta). Potentials were recorded both at rest and during rotatory stimulation of the canal. At rest, the tracings revealed an intense background of small, largely summated potentials (0.5-10 mV amplitude, 3-6 msec duration), which underlay the discharge of spikes in all the impaled units. The frequency of the subthreshold events was related to the frequency of the propagated spikes, the latter ranging from 0 to 40/sec. Stimulation modulated the frequency of both spikes and subthreshold potentials, whose summation during excitation led to a positive shift of the fibre membrane potential. The small potentials proved to be dependent on Ca2+ and Mg2+ levels in the bath. Antidromic-stimulation of the posterior ampullary nerve indicated that the observed events do not represent an artifact due to extracellular field interference related to spike activity in the neighbouring fibres. Tetrodotoxin (10(-7)-10(-6) g/ml) applied externally to the preparation or previously perfused through the frog vessels abolishes the propagated spikes but left unaffected the small potentials which, even under drug treatment, were normally modulated by the stimulus. The subthreshold potentials thus appear to be EPSPs generated at the cyto-neural junction between the hair cells and the endings of the ampullary nerve fibres.

Vestibular and somatosensory interaction in the cat vestibular nuclei

The vestibular nuclei of cats were explored extracellulary with micropipettes to locate units with a resting discharge rate which responded to rotation in the horizontal plane. These units were examined for somatosensory input from neck and limbs. Fewer than half responded to somatosensory stimulation. The neck region was the body area most effective in influencing unitary activity. The response pattern most often noted was an increase and decrease in discharge frequency when the body was moved towards and away from the recording electrode respectively. Change in discharge rate was observed to be primarily dependant upon neck velocity and not upon absolute neck position. Half of the somatosensory units received input from either the forelimbs or the hindlimbs, while the remaining half responded to both.

Ewald's second law re-evaluated

Patients and experimental animals (cats) with one functioning horizontal semicircular canal were tested with precise rotatory stimuli. Nystagmus responses were quantified with EOG and a laboratory digital computer. After large-magnitude stimuli there was a statistically significant difference between the maximum slow component velocity of nystagmus induced by ampullopetal endolymph flow and that induced by ampullofugal endolymph flow in all patients and cats.

Adaptive distortions in the generator potential of semicircular canal sensory afferents

The generator potential in sensory afferents of frog crista ampullaris was extracellularly recorded from the cut end of the posterior ampullary nerve by means of suction electrodes. A servocontrolled turntable allowed suitable rotatary stimulations. The analysis of the recorded generator potential revealed a different time course from that predicted on the basis of the pendulum model. Adaptation and undershoots in the responses to velocity ramps, steps and sinusoids, were mainly responsible for the deviations, which became very evident only when fairly high acceleration rates were applied. Both adaptation and undershoots were produced presumably by the activation of an electrogenic pump, probably located in nerv terminals contacting the hair cells. In fact, the time course of the generator potential became much more consistent with the predictions from the pendulum model under treatments capable of hindering the ion pump activity.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. I. Motor output during sinusoidal linear accelerations

Decerebrate cats were subjected to sinusoidal linear accelerations along the animal's horizontal and vertical axes, while recording the EMG activity of both triceps brachii muscles. This activity was found to be sinusoidally modulated in response to the accelerations and thus phase and gain relations between motor output and input acceleration could be obtained. They were found to be the same for accelerations along each of the three axes. In particular the gain dropped by 14-20 dB over a frequency range from 0.2 to 1.0 Hz and the phase of the motor output showed a lag of 40-60 degrees at 1.0 Hz. Thus, it was concluded that (1) the dynamic behavior of utricular and saccular receptors is the same, (2) the changes in motor activity observed during accelerations along the vertical axis are mostly due to the activation of saccular afferents, and (3) the motor output cannot simply result from vestibular afferent activities being relayed directly to the spinal motoneurons via the vestibulo-spinal tracts.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. II. motor output during rotations in the horizontal plane

Decerebrate cats with the spinal cord sectioned at low thoracic levels were submitted to rotations in the horizontal plane. The position of the animal with respect to the axis of rotation was such that horizontal canal afferents were activated either alone or in combination with macular afferents. The EMG activity from the triceps brachii muscles of both forelimbs was recorded. The main findings were as follows. (1) The motor output to each forelimb extensor is increased by an increase in the activity of the horizontal canal afferents from the contralateral labyrinth. The phase of the motor output with respect to that of the vestibular afferents shows a lag which increases with frequency, reaching about 85 degrees at 1.0 Hz. (2) The macular and horizontal canal inputs are independently processed in the central nervous system and the motor output in response to both inputs applied simultaneously is a linear summation of the outputs expected for each of the inputs.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. III. Motor output during rotations in the vertical plane

Decerebrate cats with the spinal cord sectioned at low thoracic levels were subjected to sinusoidal rotations in the vertical plane, while recording the EMG activity of both triceps brachii muscles. The applied stimulus activated utricular and saccular receptors as well as the vertical semicircular canals. Using results presented in preceding papers 2,3, namely the dynamic response characteristics of the motor output to macular inputs and the fact that the responses to otolith and canal stimulation summate linearly 3, it is deduced that the dynamic characteristics of the motor output in response to stimulation of the vertical canals are the same as those to horizontal canal stimulation. The implication of these findings vis-à-vis the problem of central processing of vestibular inputs and the problem of postural stability is discussed.

Nystagmic modulation of neuronal activity in rabbit cerebellar flocculus

1. The responses of neuronal elements in the flocculus of the awake, restrained rabbit were recorded during horizontal vestibular nystagmus in the dark. 2. Purkinje cells showed both vestibular (Types I and II) and eye movement modulation of simple spike activity. Type I Purkinje cells most commonly were inhibited in association with the ipsilaterally directed fast phase of nystagmus and excited during contralaterally directed fast phases. Type II Purkinje cells had a similar modulation but in the opposite direction. Variations on this pattern included an increase in firing during fast phases in both directions. 3. Presumed mossy fibers and granule cells also exhibited both vestibular and nystagmic modulation in various combinations. The nystagmic modulation often began during the fast phase and continued into the early part of the slow phase. Mossy fibers showing both vestibular and nystagmic modulation probably arise from the vestibular nuclei and/or the perihypoglossal nuclei. 4. Floccular control of brain stem nuclei utilizes not only vestibular but also eye movement signals and probably all sensory and internal signals involved in the regulation of gaze.

Transfer characteristics of first and second order lateral canal vestibular neurons in gerbil

Discharge patterns of first and second order vestibular neurons responding to angular acceleration in the plane of the lateral canals were studied in gerbil. The resting discharge activity of each cell was used to characterize the neuron by measureing the coefficient of variation and coefficient of skewness of the interspike interval distributions. Sinusoidal angular oscillations ranging in frequency from 0.0125 to 5.0 Hz were delivered by a velocity controlled rate-table. A PDP-12 minicomputer system was used on-line to display period and post-stimulus histograms of discriminated single unit activity. Off-line Fourier analysis of the period histograms was used to determine the phase of cell response to sinusoidal accelerations, while the average level and amplitude were determined by a least squares fitting algorithm applied over the fraction of the stimulus period where the cell discharged. First order neurons were found to have high discharge rates (average = 61.7 imp./sec) and bidirectional responses to rotation, and were of two groups called regular and irregular according to their resting discharge patterns. Second order neurons, located mainly in the medial and lateral vestibular nuclei, had low or even zero resting discharge rates (average = 17.8) resulting in more uni-directional responses and were of a single population. For frequencies less than 10 Hz, the Bode plots of the regular first order neurons are similar to that of a first order system with a time constant of about 2 sec as predicted by the torsion pendulum theory for cupula movement. The irregular first order neurons show an increasing gain above 0.5 Hz and a large phase lead relative to angular velocity above 1.0 Hz suggestive of a fractional power transfer function. The second order neurons show the phase and gain characteristics of the regular first order neurons being in phase with angular velocity above 1.0 Hz.

Functional characterization of primary vestibular afferents in the frog

1. In order to more accurately identify the nature of the vestibular input to central neurons, the response properties of single semicircular canal and otolith units in the frog VIIth nerve were studied in curarized preparations. 2. An equation describing the response plane was calculated for each canal on the basis of null point measurements. These results show that the ipsilateral canal planes are orthogonal within 2-5 degrees, and the pairs of right-left synergists are essentially coplanar. A head position of 10-20 degrees maxilla nose up produces optimal horizontal canal and minimal vertical canal activation with horizontal rotation. 3. The frequency response of the horizontal canal was examined in the range 0.025-0.5 Hz. Comparatively shorter phase-lags and a 10 fold greater acceleration gain in this frequency range distinguish the frog from the mammalian species studied. 4. Otolithic responses were tonic, phasic-tonic, and phasic in nature. The preponderance of the latter two groups is stressed (94%). Tonic responses were proportional to the gravitational vector change. Phasic responses were proportional to velocity during transitions in head position and phase-led displacement (30-80%) with sinusoidal acceleration in roll and pitch. 5. Efferent vestibular neurons respond to rotation in the horizontal (usually Type III) as well as vertical planes. Responses in the vertical planes result from canal and/or otolithic input to these neurons indicating that the vestibular efferent system receives extensive multi-labyrinthine convergence.

Functional changes in the oculomotor system of the monkey at various stages of barbiturate anesthesia and alertness

1. Single units in the III. and VI. nerve nuclei were continuously recorded together with vestibular stimuli and eye movements in macaques before, during, and after administration of barbiturate. 2. The visual input was functionally detached from the oculomotor system during the deeper stages of anesthesia, whereas some kind of vestibulo-ocular response could always be elicited. 3. The finding of various phase values between the maximum impulse rate IRmax of oculomotor units and the maximum stimulus velocity vmax during 1 Hz sinusoidal vestibular stimulation ranging from about 65 deg phase lead to 65 deg phase lag is suggested as important for the explanation of the phase shifts between head rotation and eye movement during anesthesia. 4. The phase relationship between IRmax and vmax was found to be unchanged, whereas the characteristic of IRmax versus vmax was highly sensitive to arousal stimuli for some oculomotor neurons. This sensitivity was represented exclusively by activation rather than inhibition.

Voluntary, non-visual control of the human vestibulo-ocular reflex

Voluntary control of the human vestibulo-ocular reflex with and without visual targets was investigated. Subjects were rotated sinusoidally from 0.1 to 1.0 Hz using d.c. electro-oculography to record eye position. The ratio of eye to head movement, or gain, of the vestibulo-ocular reflex was measured. When subjects were rotated in the dark at 0.3 Hz whilst performing mental arithmetic the gain was 0.65. When subjects were asked to fixate imaginary targets in the dark that were stationary in space, the gain rose to 0.95. When they imagined targets rotating with them on the chair, the gain dropped to 0.35. Our results indicate that the ability to modulate the gain of the vestibulo-ocular reflex does not depend entirely on the smooth pursuit system. Higher centers must modulate eye velocity so that it is appropriate to the subject's choice of a frame of reference, whether or not vision is available.

The origin of slow potentials in semicircular canals of the frog

Slow potentials generated in the sensory organ of the ampulla were recorded in isolated semicircular canals of the frog by means of fluid electrodes. These potentials, which may be picked up from the intrampullar fluid and those from the ampullar nerve appear to be generated at different stages of the process taking place in crista ampullaris. Slow intra-ampullar potentials apparently reflect receptor potentials of hair cells. They are preserved after degeneration of the nerve fibre endings and are relatively insensitive to DNP poisoning; their amplitude is maximum at high K+ concentrations. Slow nerve potentials appear to be due to electronic spreading of post-synaptic excitatory potentials generated at cytoneural junctions. They disappear after degeneration of the nerve fibres, in low Ca++ and high Mg++ solutions and are extremely sensitive to DNP poisoning. An analysis of the time-course of the slow ampullar and nerve potentials referred to the discharge of impulses in afferent fibres was performed with a view to interpreting the transduction mechanism of semicircular canals.

Otolith-controlled responses from the first-order neurons of the labyrinth of the bullfrog (Rana catesbeiana) to changes in linear acceleration

The experiments reported here represent a study of otolith-controlled vestibular units showing an extreme degree of adaptation in stationary spatial positions. These units cannot, therefore, be characterized as static position sensors. They respond to tilting movements as such, showing an output related to the direction and velocity of the movements. Phase shifts in responses to oppositely directed interrupted and continuous sinu­soidal full-circle tilts about horizontally placed head axes indicate that, beside neuronal adaptation, mechanical factors may contribute to the observed response asymmetries. The possible structural basis for such asymmetries is discussed. A frequency analysis of the recorded data yields Bode plots of fre­quency-dependent phase and gain, and these relations are discussed against the background of various mathematical models suggested by a number of authors. The conclusion is reached that, in contrast to the case of the semicircular canals, it is difficult to fit the phase behaviour of otolith-controlled vestibular sense endings to model equations containing only one or two frequency constants. Evidently, the otolith organ is a mechanically complex system, and its receptor units are characterized by considerable nonlinearities in the transduction process apart from a wide range of adaptation in their responses to constant levels of acceleration. The results are discussed in comparison with recent vestibular re­search, especially on the mammalian labyrinth.

Responses of peripheral vestibular neurons to angular and linear accelerations in the squirrel monkey

Peripheral neurons innervating semicircular canals can respond to constant linear accelerations. Evidence is presented that, in our preparation, the response is artifactual and arises from thermal gradients introduced by the surgical exposure. Otolith neurons do not respond to even intense angular accelerations. Canal plugging abolishes the response of the corresponding afferents to angular acceleration, without obviously affecting resting activity. The procedure does not prevent the canals from responding to linear accelerations. The latter response, unlike that seen in intact canals, is not due to thermal gradients and may be related to the mechanisms underlying the persistent component of barbecue nystagmus.

On-line identification of sensory systems using pseudorandom binary noise perturbations

A technique of on-line identification of linear system characteristics from sensory systems with spike train or analog voltage outputs was developed and applied to the semicircular canal. A pseudorandom binary white noise input was cross-correlated with the system's output to produce estimates of linear system unit impulse responses (UIRs), which were then corrected for response errors of the input transducers. The effects of variability in the system response characteristics and sensitivity were studied by employing the technique with known linear analog circuits. First-order unit afferent responses from the guitarfish horizontal semicircular canal were cross-correlated with white noise rotational acceleration inputs to produce non-parametric UIR models. In addition, the UIRs were fitted by nonlinear regression to truncated exponential series to produce parametric models in the form of low-order linear system equations. The experimental responses to the white noise input were then compared with those predicted from the UIR models linear convolution, and the differences were expressed as a percent mean-square-error (%MSE). The average difference found from a population of 62 semicircular canal afferents was relatively low mean and standard deviation of 10.2 +/- 5.9 SD%MSE, respectively. This suggests that relatively accurate inferences can be made concerning the physiology of the semicircular canal from the linear characteristics of afferent responses.

The postural response of normal dogs to sinusoidal displacement

1. Normal dogs were trained to adopt a laterally symmetric stance on a horizontal platform. Sinusoidal oscillation of the platform in the cephalocaudal direction caused the dogs to adopt a strategy of response which would keep them from falling down during the period of imposed motion or perturbation.2. A Fourier analysis of the response variables provided a quantitative measure of the distortion in the induced movement at the various hind leg joints and in the motion of the body. Certain aspects of the distortion could be accounted for by recognized random events such as drift and panting. The remainder of the distortion was task related and therefore provided evidence that the dog's postural control system behaved essentially as a non-linear system.3. The motion of the body was less distorted than the motion at the joints. The frequency response of the body motion resembled that of a second order linear system, but the amplitude of the body motion did not vary in constant proportion with changes in the amplitude of the input. Further, the phase relation between body motion and platform motion was not constant when the amplitude of the input was changed. Therefore, it was concluded that the control of the body position was probably non-linearly related to the input disturbance but that the low pass filter characteristics tended to minimize the appearance of distortion in that system output.4. Control of the position of the hind limb is related to the control of the torque generated at the hind limb joints. To the extent that joint angle and change in joint angle are related to the torque at a joint, the distortion of the motion at the joints clearly demonstrates that control of the hind limb during this postural task was non-linearly related to the sinusoidal input.5. The uniformity of the response parameters, as assessed from the Fourier coefficients, indicated that all of the tested dogs adopted the same or nearly the same strategy for solving the problem of adjusting their postural control in response to the perturbation. Therefore, a reasonable hypothesis for future testing is that the central programme which generates this particular postural response will be structured similarly from one dog to the next.

A mechanism by which the hair cells of the inner ear transduce mechanical energy into a modulated train of action potentials

Physical models of the hair cells of the inner ear were built and analyzed. These models suggest that a straightforward physical process is capable of modulating the electrical resistance of the hair cell. Strong evidence in the literature indicates that such a change in resistance would modulate an otherwise steady electrical current which flows across the hair cell. This would cause the resting potential of the hair cell to change in a systematic fashion, eventually giving rise to the modulated train of action potentials in the neurons leading from the hair cell to the central nervous system.

Vestibulo-ocular and optokinetic reactions to rotation and their interaction in the rabbit

1. Compensatory eye movements due to sinusoidal yaw movements on a torsion swing were measured in alert rabbits. A range of combinations of frequencies (0.048-1.8 Hz) and amplitudes (1-25 degrees ) were used. Gain (cumulative slow phase eye movement amplitude/swing amplitude) and phase (eye position vs. swing position - 180 degrees ) were calculated from averaged records.2. Eyes were either closed (canal-ocular reactions only), open in earth-fixed visual surroundings (natural interaction of vestibular and optokinetic reactions), or looking at platform-fixed surroundings, which rotated with the animal (conflict situation). In some rabbits, the same stimulus programme was applied a month after bilateral destruction of the labyrinths (optokinetic reactions only).3. For canal-ocular reactions, no true threshold was found. Yet the system showed a small but systematic non-linearity which is tentatively explained by an acceleration-dependence of gain. For the higher frequencies (0.40-1.8 Hz) used, gain was 0.55-0.75, with a decrease at the lower frequencies, down to 0.16-0.33 at 0.048 Hz. The response showed a phase-lead of about 45 degrees at 0.048 Hz and was nearly in phase at 1-1.8 Hz. The long time constant of the cupula-endolymph system was estimated at about 3.3 sec.4. With earth-fixed visual surroundings a frequency-independent gain (range 0.55-0.82) with negligible phase error was found for the entire stimulus range tested. This natural combination of canal-ocular and optokinetic systems appears to function very efficiently, with mutual correction of the defects of the systems apart.5. With platform-fixed visual surroundings the canal-ocular system was severely inhibited and its non-linearities were markedly enhanced by the optokinetic system, especially when the torsion swing moved slowly.6. The general shape of input-output relations of optokinetic reactions after labyrinthectomy were similar to those found earlier in normal animals, but gain was subnormal for the entire stimulus range tested.

The projection of the vestibular nerve to the cerebral cortex in the cat

1. The cerebral cortex in the cat has been examined for responses to contra-lateral vestibular nerve shocks and to natural stimuli.2. Stimulation of the nerve produces short latency (3.5 msec) surface positive responses in very restricted areas. Within these areas, unitary discharges occur during the equivalent negative phase below the surface.3. It has not been possible to evoke unitary activity in these areas by natural stimulation during light anaesthesia with pentobarbitone, chloralose or urethane.4. The distribution and properties of the shock evoked responses have been studied with special attention to the problem of stimulus spread. They are shown to be overlapped by somato-sensory and auditory projections, but to be distinguishable from them. Evidence is presented that previously reported longer latency responses were probably due to stimulus spread.5. Reasons are discussed for the extreme sensitivity of the vestibulocortical pathway to anaesthesia.