Spatial and temporal characteristics of vestibular convergence

Adaptation of spatio-temporal convergent properties in central vestibular neurons in monkeys

The spatio-temporal convergent (STC) response occurs in central vestibular cells when dynamic and static inputs are activated. The functional significance of STC behavior is not fully understood. Whether STC is a property of some specific central vestibular neurons, or whether it is a response that can be induced in any neuron at some frequencies is unknown. It is also unknown how the change in orientation of otolith polarization vector (orientation adaptation) affects STC behavior. A new complex model, that includes inputs with regular and irregular discharges from both canal and otolith afferents, was applied to experimental data to determine how many convergent inputs are sufficient to explain the STC behavior as a function of frequency and orientation adaptation. The canal-otolith and otolith-only neurons were recorded in the vestibular nuclei of three monkeys. About 42% (11/26 canal-otolith and 3/7 otolith-only) neurons showed typical STC responses at least at one frequency before orientation adaptation. After orientation adaptation in side-down head position for 2 h, some canal-otolith and otolith-only neurons altered their STC responses. Thus, STC is a property of weights of the regular and irregular vestibular afferent inputs to central vestibular neurons which appear and/or disappear based on stimulus frequency and orientation adaptation. This indicates that STC properties are more common for central vestibular neurons than previously assumed. While gravity-dependent adaptation is also critically dependent on stimulus frequency and orientation adaptation, we propose that STC behavior is also linked to the neural network responsible for localized contextual learning during gravity-dependent adaptation.

Convergence of linear acceleration and yaw rotation signals on non-eye movement neurons in the vestibular nucleus of macaques

Roughly half of all vestibular nucleus neurons without eye movement sensitivity respond to both angular rotation and linear acceleration. Linear acceleration signals arise from otolith organs, and rotation signals arise from semicircular canals. In the vestibular nerve, these signals are carried by different afferents. Vestibular nucleus neurons represent the first point of convergence for these distinct sensory signals. This study systematically evaluated how rotational and translational signals interact in single neurons in the vestibular nuclei: multisensory integration at the first opportunity for convergence between these two independent vestibular sensory signals. Single-unit recordings were made from the vestibular nuclei of awake macaques during yaw rotation, translation in the horizontal plane, and combinations of rotation and translation at different frequencies. The overall response magnitude of the combined translation and rotation was generally less than the sum of the magnitudes in responses to the stimuli applied independently. However, we found that under conditions in which the peaks of the rotational and translational responses were coincident these signals were approximately additive. With presentation of rotation and translation at different frequencies, rotation was attenuated more than translation, regardless of which was at a higher frequency. These data suggest a nonlinear interaction between these two sensory modalities in the vestibular nuclei, in which coincident peak responses are proportionally stronger than other, off-peak interactions. These results are similar to those reported for other forms of multisensory integration, such as audio-visual integration in the superior colliculus. NEW & NOTEWORTHY This is the first study to systematically explore the interaction of rotational and translational signals in the vestibular nuclei through independent manipulation. The results of this study demonstrate nonlinear integration leading to maximum response amplitude when the timing and direction of peak rotational and translational responses are coincident.

The integration of neural information by a passive kinetic stimulus and galvanic vestibular stimulation in the lateral vestibular nucleus

Despite an easy control and the direct effects on vestibular neurons, the clinical applications of galvanic vestibular stimulation (GVS) have been restricted because of its unclear activities as input. On the other hand, some critical conclusions have been made in the peripheral and the central processing of neural information by kinetic stimuli with different motion frequencies. Nevertheless, it is still elusive how the neural responses to simultaneous GVS and kinetic stimulus are modified during transmission and integration at the central vestibular area. To understand how the neural information was transmitted and integrated, we examined the neuronal responses to GVS, kinetic stimulus, and their combined stimulus in the vestibular nucleus. The neuronal response to each stimulus was recorded, and its responding features (amplitude and baseline) were extracted by applying the curve fitting based on a sinusoidal function. Twenty-five (96.2%) comparisons of the amplitudes showed that the amplitudes decreased during the combined stimulus (p < 0.001). However, the relations in the amplitudes (slope = 0.712) and the baselines (slope = 0.747) were linear. The neuronal effects by the different stimuli were separately estimated; the changes of the amplitudes were mainly caused by the kinetic stimulus and those of the baselines were largely influenced by GVS. Therefore, the slopes in the comparisons implied the neural sensitivity to the applied stimuli. Using the slopes, we found that the reduced amounts of the neural information were transmitted. Overall, the comparisons of the responding features demonstrated the linearity and the subadditivity in the neural transmission.

Physiology of central pathways

The relative simplicity of the neural circuits that mediate vestibular reflexes is well suited for linking systems and cellular levels of analyses. Notably, a distinctive feature of the vestibular system is that neurons at the first central stage of sensory processing in the vestibular nuclei are premotor neurons; the same neurons that receive vestibular-nerve input also send direct projections to motor pathways. For example, the simplicity of the three-neuron pathway that mediates the vestibulo-ocular reflex leads to the generation of compensatory eye movements within ~5ms of a head movement. Similarly, relatively direct pathways between the labyrinth and spinal cord control vestibulospinal reflexes. A second distinctive feature of the vestibular system is that the first stage of central processing is strongly multimodal. This is because the vestibular nuclei receive inputs from a wide range of cortical, cerebellar, and other brainstem structures in addition to direct inputs from the vestibular nerve. Recent studies in alert animals have established how extravestibular signals shape these "simple" reflexes to meet the needs of current behavioral goal. Moreover, multimodal interactions at higher levels, such as the vestibular cerebellum, thalamus, and cortex, play a vital role in ensuring accurate self-motion and spatial orientation perception.

Integration of canal and otolith inputs by central vestibular neurons is subadditive for both active and passive self-motion: implication for perception

Traditionally, the neural encoding of vestibular information is studied by applying either passive rotations or translations in isolation. However, natural vestibular stimuli are typically more complex. During everyday life, our self-motion is generally not restricted to one dimension, but rather comprises both rotational and translational motion that will simultaneously stimulate receptors in the semicircular canals and otoliths. In addition, natural self-motion is the result of self-generated and externally generated movements. However, to date, it remains unknown how information about rotational and translational components of self-motion is integrated by vestibular pathways during active and/or passive motion. Accordingly, here, we compared the responses of neurons at the first central stage of vestibular processing to rotation, translation, and combined motion. Recordings were made in alert macaques from neurons in the vestibular nuclei involved in postural control and self-motion perception. In response to passive stimulation, neurons did not combine canal and otolith afferent information linearly. Instead, inputs were subadditively integrated with a weighting that was frequency dependent. Although canal inputs were more heavily weighted at low frequencies, the weighting of otolith input increased with frequency. In response to active stimulation, neuronal modulation was significantly attenuated (∼ 70%) relative to passive stimulation for rotations and translations and even more profoundly attenuated for combined motion due to subadditive input integration. Together, these findings provide insights into neural computations underlying the integration of semicircular canal and otolith inputs required for accurate posture and motor control, as well as perceptual stability, during everyday life.

Projection neurons of the vestibulo-sympathetic reflex pathway

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer Fluoro-Gold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and Fluoro-Gold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial, and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to Fluoro-Gold injection sites in both the RVLM and CVLM, and the remainder were contralateral. As a group, cells projecting to the RVLM were located slightly rostral to those with terminals in the CVLM. Individual activated projection neurons were multipolar, globular, or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

Responses of non-eye movement central vestibular neurons to sinusoidal horizontal translation in compensated macaques after unilateral labyrinthectomy

After vestibular labyrinth injury, behavioral deficits partially recover through the process of vestibular compensation. The present study was performed to improve our understanding of the physiology of the macaque vestibular system in the compensated state (>7 wk) after unilateral labyrinthectomy (UL). Three groups of vestibular nucleus neurons were included: pre-UL control neurons, neurons ipsilateral to the lesion, and neurons contralateral to the lesion. The firing responses of neurons sensitive to linear acceleration in the horizontal plane were recorded during sinusoidal horizontal translation directed along six different orientations (30° apart) at 0.5 Hz and 0.2 g peak acceleration (196 cm/s(2)). This data defined the vector of best response for each neuron in the horizontal plane, along which sensitivity, symmetry, detection threshold, and variability of firing were determined. Additionally, the responses of the same cells to translation over a series of frequencies (0.25-5.0 Hz) either in the interaural or naso-occipital orientation were obtained to define the frequency response characteristics in each group. We found a decrease in sensitivity, increase in threshold, and alteration in orientation of best responses in the vestibular nuclei after UL. Additionally, the phase relationship of the best neural response to translational stimulation changed with UL. The symmetry of individual neuron responses in the excitatory and inhibitory directions was unchanged by UL. Bilateral central utricular neurons still demonstrated two-dimension tuning after UL, consistent with spatio-temporal convergence from a single vestibular end-organ. These neuronal data correlate with known behavioral deficits after unilateral vestibular compromise.