Responses of Purkinje-cells of the cerebellar anterior vermis to stimulation of vestibular and somatosensory receptors

Effects of a short period of postural training on postural stability and vestibulospinal reflexes

The effects of postural training on postural stability and vestibulospinal reflexes (VSRs) were investigated in normal subjects. A period (23 minutes) of repeated episodes (n = 10, 50 seconds) of unipedal stance elicited a progressive reduction of the area covered by centre of pressure (CoP) displacement, of average CoP displacement along the X and Y axes and of CoP velocity observed in this challenging postural task. All these changes were correlated to each other with the only exception of those in X and Y CoP displacement. Moreover, they were larger in the subjects showing higher initial instability in unipedal stance, suggesting that they were triggered by the modulation of sensory afferents signalling body sway. No changes in bipedal stance occurred soon and 1 hour after this period of postural training, while a reduction of CoP displacement was apparent after 24 hours, possibly due to a beneficial effect of overnight sleep on postural learning. The same period of postural training also reduced the CoP displacement elicited by electrical vestibular stimulation (EVS) along the X axis up to 24 hours following the training end. No significant changes in postural parameters of bipedal stance and VSRs could be observed in control experiments where subjects were tested at identical time points without performing the postural training. Therefore, postural training led to a stricter control of CoP displacement, possibly acting through the cerebellum by enhancing feedforward mechanisms of postural stability and by depressing the VSR, the most important reflex mechanism involved in balance maintenance under challenging conditions.

Body Sway Increases After Functional Inactivation of the Cerebellar Vermis by cTBS

Balance stability correlates with cerebellar vermis volume. Furthermore, the cerebellum is involved in precise timing of motor processes by fine-tuning the sensorimotor integration. We tested the hypothesis that any cerebellar action in stance control and in timing of visuomotor integration for balance is impaired by continuous theta-burst stimulation (cTBS) of the vermis. Ten subjects stood quietly and underwent six sequences of 10-min acquisition of center of foot pressure (CoP) data after cTBS, sham stimulation, and no stimulation. Visual shifts from eyes closed (EC) to eyes open (EO) and vice versa were presented via electronic goggles. Mean anteroposterior and mediolateral CoP position and oscillation, and the time delay at which body sway changed after visual shift were calculated. CoP position under both EC and EO condition was not modified after cTBS. Sway path length was greater with EC than EO and increased in both visual conditions after cTBS. CoP oscillation was also larger with EC and increased under both visual conditions after cTBS. The delay at which body oscillation changed after visual shift was longer after EC to EO than EO to EC, but unaffected by cTBS. The time constant of decrease or increase of oscillation was longer in EC to EO shifts, but unaffected by cTBS. Functional inactivation of the cerebellar vermis is associated with increased sway. Despite this, cTBS does not detectably modify onset and time course of the sensorimotor integration process of adaptation to visual shifts. Cerebellar vermis normally controls oscillation, but not timing of adaptation to abrupt changes in stabilizing information.

Vestibular nucleus neurons respond to hindlimb movement in the decerebrate cat

The vestibular nuclei integrate information from vestibular and proprioceptive afferents, which presumably facilitates the maintenance of stable balance and posture. However, little is currently known about the processing of sensory signals from the limbs by vestibular nucleus neurons. This study tested the hypothesis that limb movement is encoded by vestibular nucleus neurons and described the changes in activity of these neurons elicited by limb extension and flexion. In decerebrate cats, we recorded the activity of 70 vestibular nucleus neurons whose activity was modulated by limb movements. Most of these neurons (57/70, 81.4%) encoded information about the direction of hindlimb movement, while the remaining neurons (13/70, 18.6%) encoded the presence of hindlimb movement without signaling the direction of movement. The activity of many vestibular nucleus neurons that responded to limb movement was also modulated by rotating the animal's body in vertical planes, suggesting that the neurons integrated hindlimb and labyrinthine inputs. Neurons whose firing rate increased during ipsilateral ear-down roll rotations tended to be excited by hindlimb flexion, whereas neurons whose firing rate increased during contralateral ear-down tilts were excited by hindlimb extension. These observations suggest that there is a purposeful mapping of hindlimb inputs onto vestibular nucleus neurons, such that integration of hindlimb and labyrinthine inputs to the neurons is functionally relevant.

Effects of leg-to-body position on the responses of rat cerebellar and vestibular nuclear neurons to labyrinthine stimulation

The spatial organization of vestibulospinal (VS) reflexes, elicited by labyrinthine signals and related to head motion, depends on the direction of body tilt, due to proprioceptive neck afferents acting through the cerebellar anterior vermis. The responses of Purkinje cells located within this region to labyrinthine stimulation are modulated by the head-to-body position. We investigated, in urethane-anesthetized rats, whether a 90° leg-to-trunk displacement modifies the responses of corticocerebellar and vestibular nuclear neurons to the labyrinthine input, which would indicate that VS reflexes are tuned by the leg-to-trunk position. With this aim, unit activity was recorded during "wobble" stimuli that allow evaluating the gain and spatiotemporal properties of neuronal responses. The response gain of corticocerebellar units showed a significant drop in the leg-rotated position with respect to the control one. Following a change in leg position, a proportion of the recorded neurons showed significant changes in the direction and phase of the response vector. In contrast, vestibular nuclear neurons did not show significant modifications in their response gain and direction. Thus, proprioceptive afferents signaling leg-to-trunk position seem to affect the processing of directional labyrinthine signals within the cerebellar cortex.

Adaptive tuning of perceptual timing to whole body motion

In a previous work we have shown that sinusoidal whole-body rotations producing continuous vestibular stimulation, affected the timing of motor responses as assessed with a paced finger tapping (PFT) task (Binetti et al. (2010). Neuropsychologia, 48(6), 1842-1852). Here, in two new psychophysical experiments, one purely perceptual and one with both sensory and motor components, we explored the relationship between body motion/vestibular stimulation and perceived timing of acoustic events. In experiment 1, participants were required to discriminate sequences of acoustic tones endowed with different degrees of acceleration or deceleration. In this experiment we found that a tone sequence presented during acceleratory whole-body rotations required a progressive increase in rate in order to be considered temporally regular, consistent with the idea of an increase in "clock" frequency and of an overestimation of time. In experiment 2 participants produced self-paced taps, which entailed an acoustic feedback. We found that tapping frequency in this task was affected by periodic motion by means of anticipatory and congruent (in-phase) fluctuations irrespective of the self-generated sensory feedback. On the other hand, synchronizing taps to an external rhythm determined a completely opposite modulation (delayed/counter-phase). Overall this study shows that body displacements "remap" our metric of time, affecting not only motor output but also sensory input.

Internal models and neural computation in the vestibular system

The vestibular system is vital for motor control and spatial self-motion perception. Afferents from the otolith organs and the semicircular canals converge with optokinetic, somatosensory and motor-related signals in the vestibular nuclei, which are reciprocally interconnected with the vestibulocerebellar cortex and deep cerebellar nuclei. Here, we review the properties of the many cell types in the vestibular nuclei, as well as some fundamental computations implemented within this brainstem-cerebellar circuitry. These include the sensorimotor transformations for reflex generation, the neural computations for inertial motion estimation, the distinction between active and passive head movements, as well as the integration of vestibular and proprioceptive information for body motion estimation. A common theme in the solution to such computational problems is the concept of internal models and their neural implementation. Recent studies have shed new insights into important organizational principles that closely resemble those proposed for other sensorimotor systems, where their neural basis has often been more difficult to identify. As such, the vestibular system provides an excellent model to explore common neural processing strategies relevant both for reflexive and for goal-directed, voluntary movement as well as perception.

The cerebellum and sensorimotor coupling: Looking at the problem from the perspective of vestibular reflexes

Cerebellar modules process afferent information and deliver outputs relevant for both reflex and voluntary movements. The response of cerebellar modules to a given input depends on the whole array of signals impinging on them. Studies on vestibular reflexes indicate that the response of the cerebellar circuits to the vestibular input is modified by the integration of multiple visual, vestibular and somatosensory afferent signals. In this way the cerebellum slowly adapts these reflexes when they are not adequate to the behavioural condition and allows their fast modifications when the relative position of the body segments and that of the body in space are changed. Studies on voluntary movements indicate that the cerebellum is responsible for motor learning that consists of the development of new input-output associations. Several theoretical, anatomical and clinical studies are consistent with the hypothesis that the cerebellum allows the delivery of motor commands which vary according to the condition of the motor apparatus. Finally, the cerebellum could change the relation between visual information and aimed reaching movements according to the position of the eyes in the orbit and of the neck over the body. We propose that, due to the large expansion of its cortex, an important function of the cerebellum could be that of expanding the range of sensorimotor associations according to all the factors characterizing the behavioural condition. Indeed, following cerebellar lesion, learning is often lost, the movement results impaired and requires an increased attention. In the light of the recently discovered connections of the cerebellum with the rostral regions of the frontal lobe, it can be suggested that the ability of cerebellar circuits to modify the rules of input-output coupling according to a general context is a fundamental property allowing the cerebellum to control not only motor but also cognitive functions.