Keisuke Kushiro

Differences between otolith- and semicircular canal-activated neural circuitry in the vestibular system.

In the last two decades, we have focused on establishing a reliable technique for focal stimulation of vestibular receptors to evaluate neural connectivity. Here, we summarize the vestibular-related neuronal circuits for the vestibulo-ocular reflex, vestibulocollic reflex, and vestibulospinal reflex arcs. The focal stimulating technique also uncovered some hidden neural mechanisms. In the otolith system, we identified two hidden neural mechanisms that enhance otolith receptor sensitivity. The first is commissural inhibition, which boosts sensitivity by incorporating inputs from bilateral otolith receptors, the existence of which was in contradiction to the classical understanding of the otolith system but was observed in the utricular system. The second mechanism, cross-striolar inhibition, intensifies the sensitivity of inputs from both sides of receptive cells across the striola in a single otolith sensor. This was an entirely novel finding and is typically observed in the saccular system. We discuss the possible functional meaning of commissural and cross-striolar inhibition. Finally, our focal stimulating technique was applied to elucidate the different constructions of axonal projections from each vestibular receptor to the spinal cord. We also discuss the possible function of the unique neural connectivity observed in each vestibular receptor system.

Effect of anxiety on antero-posterior postural stability in patients with dizziness

The purpose of this study was to investigate the effect of anxiety on the postural stability of a variety of dizzy patients during upright standing. To address this issue, 54 patients complaining of dizziness were enrolled in this study. The degree of anxiety in patients was evaluated on the basis of a routine vestibular examination together with their dizziness handicap inventory (DHI) scores as well as the hospital anxiety and depression scale (HADS). The patients were divided into 3 groups. If there was no vestibular dysfunction, they were defined as psychogenic (PSY) (N=16). The remaining subjects were further divided on the basis of their HADS score. If the score of A (anxiety) was less than 5, they are defined as organic (ORG) (N=25), and the rest were defined as a combination of psychogenic and organic (PSY+ORG) (N=13). Posturographic measurements were performed in a quiet and stable standing position on a force platform, as one of the vestibular examinations. The total length, the area of body sway, and the ratio of maximum perturbation of antero-posterior axis (A/P ratio) were registered. Spectrum analyses of the left-right axis and antero-posterior axis were also performed by using the fast Fourier transform (FFT) method of body sway. We found a significant correlation between anxiety and postural instability in the antero-posterior axis in all subjects as a group and in either group PSY or PSY+ORG. However, no significant correlation was found in group ORG. Using power spectrum analysis (FFT), we identified 3 frequency components of postural sway: group A (0.02-0.21Hz), group B (0.22-2.01Hz), and group C (2.01-10Hz). Statistical significance of the data was examined by ANOVA. Group C reflected somatosensory inputs, and group A reflected vestibular inputs. The power of group C decreased in the high anxiety group, whereas the power of group A increased in the high anxiety group. These phenomena disappeared in the eyes-closed condition. Our study shows that the effect of visual input on vestibular and somatosensory input is affected by anxiety. In conclusion, our results indicate that anxiety affects the postural perturbation in the antero-posterior axis and that anxiety possibly affects the interactions of visual inputs with vestibular and somatosensory inputs in the maintenance of postural balance in patients complaining of dizziness.

Effect of masticating chewing gum on postural stability during upright standing.

The purpose of this study was to investigate the effect of masticating chewing gum on postural stability during upright standing. To address this issue, 12 healthy subjects performed quiet standing on a force platform for the posturography study. The subjects were instructed to stand as stable as possible on the force platform in order to record the trajectory of the center-of-pressure (COP). After measuring the postural sway in the initial condition (pre-condition), the subjects were asked to stand while masticating chewing gum (gum-condition). Following the gum-condition, quiet standing without mastication was evaluated (post-condition) to ensure the effect of masticating chewing gum on postural stability. The trajectory and velocity of the COP were analyzed for each condition. We found that the postural stability tended to enhance during mastication of chewing gum. The rectangle area of the COP trajectory significantly diminished in the gum-condition and significantly enlarged in the post-condition. A similar effect was observed in the maximum velocity and standard deviation (SD) of the fore-aft amplitude of the COP trajectory. The values were significantly smaller in the gum-condition compared to those in the post-condition. These findings suggest that mastication of chewing gum affects the postural control by enhancing the postural stability during upright standing.

Direction-dependent differences in temporal kinematics for vertical prehension movements

Abstract In our daily lives, we can appropriately perform movements on the earth, suggesting that the central nervous system takes into account gravitational forces that act on our bodies during the movements. Recently, gravitational forces have been observed to generate the direction-dependent differences in the spatial properties of the kinematics of prehension movements. However, little is known about how gravitational forces affect the temporal properties of the kinematics of these movements. In this study, we tried to elucidate the gravitational effects on the temporal properties of the kinematics of movements by comparing upward (against gravity) and downward (with gravity) movements. As a result, we found the direction-dependent differences in temporal kinematics in both the reaching and grasping components of movements. For the reaching component, a shorter acceleration time was observed for the upward movements compared to the downward movements. For the grasping component, participants opened their hands earlier and faster for the upward movements than for the downward movements. These direction-dependent differences in the temporal kinematics suggested that the central nervous system takes into account and takes advantage of gravitational effects in the motor plans and controls of vertical prehension movements.

Dynamic characteristics of otolith ocular response during counter rotation about dual yaw axes in mice

The central vestibular system plays an important role in higher neural functions such as self-motion perception and spatial orientation. Its ability to store head angular velocity is called velocity storage mechanism (VSM), which has been thoroughly investigated across a wide range of species. However, little is known about the mouse VSM, because the mouse lacks typical ocular responses such as optokinetic after nystagmus or a dominant time constant of vestibulo-ocular reflex for which the VSM is critical. Experiments were conducted to examine the otolith-driven eye movements related to the VSM and verify its characteristics in mice. We used a novel approach to generate a similar rotating vector as a traditional off-vertical axis rotation (OVAR) but with a larger resultant gravito-inertial force (>1g) by using counter rotation centrifugation. Similar to results previously described in other animals during OVAR, two components of eye movements were induced, i.e. a sinusoidal modulatory eye movement (modulation component) on which a unidirectional nystagmus (bias component) was superimposed. Each response is considered to derive from different mechanisms; modulations arise predominantly through linear vestibulo-ocular reflex, whereas for the bias, the VSM is responsible. Data indicate that the mouse also has a well-developed vestibular system through otoliths inputs, showing its highly conserved nature across mammalian species. On the other hand, to reach a plateau state of bias, a higher frequency rotation or a larger gravito-inertial force was considered to be necessary than other larger animals. Compared with modulation, the bias had a more variable profile, suggesting an inherent complexity of higher-order neural processes in the brain. Our data provide the basis for further study of the central vestibular system in mice, however, the underlying individual variability should be taken into consideration.

Effect of chewing gum on static posturography in patients with balance disorders

Abstract Conclusion: The chewing gum indirectly affects postural control by influencing vestibular function to stabilize posture during upright standing. Objectives: This study aimed to evaluate the effect of chewing gum on static posturography in patients. Methods: The subjects were 26 patients with chronic balance disorders. The subjects were instructed to stand as stably as possible on the force platform. The recording was conducted four times. For the first evaluation, postural sway was measured during motionless standing. Two weeks after the recording, the postural sway was recorded again as a second evaluation. Thereafter, the subjects were instructed to chew gum for 3 min. The third evaluation was conducted while the subjects continued to chew gum. Then 1 h after the subject had stopped chewing gum, a fourth evaluation was obtained. The total path length (LNG) and rectangle area (REC) were analyzed. Results: We found that postural stability tended to improve while the subjects masticated gum. Both LNG and REC were significantly improved while the subjects chewed gum with their eyes closed. In patients without canal paralysis (CP), the measurements of LNG with eyes closed and REC with eyes open were significantly decreased while masticating gum. In patients with CP, the REC, but not LNG, was significantly decreased while masticating gum both with eyes open and eyes closed.

Whole-Body Roll Tilt Influences Goal-Directed Upper Limb Movements through the Perceptual Tilt of Egocentric Reference Frame

In our day-to-day life, we can accurately reach for an object in our gravitational environment without any effort. This can be achieved even when the body is tilted relative to gravity. This is accomplished by the central nervous system (CNS) compensation for gravitational forces and torque acting on the upper limbs, based on the magnitude of body tilt. The present study investigated how performance of upper limb movements was influenced by the alteration of body orientation relative to gravity. We observed the spatial trajectory of the index finger while the upper limb reached for a memorized target with the body tilted in roll plane. Results showed that the terminal location of the fingertip shifted toward the direction of body tilt away from the actual target location. The subsequent experiment examined if the perceived direction of the body longitudinal axis shifted relative to the true direction in roll plane. The results showed that the perceived direction of the body longitudinal axis shifted toward the direction of the body tilt, which correlated with the shift of the terminal location in the first experiment. These results suggest that the dissociation between the egocentric and gravitational coordinates induced by whole-body tilt leads to systematic shifts of the egocentric reference frame for action, which in turn influences the motor performance of goal-directed upper limb movements.

Motor control of downward object-transport movements with precision grip by object weight.

Abstract In the present study, we investigated the kinematics of object-transport movement in a downward direction using a precision grip, to elucidate how the central nervous system (CNS) takes into account object weight when making the movement, even when participants are unable to recognize the weight until they grasp the object. We found that the kinematics during transport movement were significantly changed by the object weight, even when the weight was unrecognized visually, suggesting that the CNS controls object-transport movement in a downward direction according to object weight, regardless of the visual recognizability of the weight.

The role of GABAB receptors in the vestibular oculomotor system in mice

Systemic administration of a gamma-amino butyric acid type B (GABAB) receptor agonist, baclofen, affects various physiological and psychological processes. To date, the effects on oculomotor system have been well characterized in primates, however those in mice have not been explored. In this study, we investigated the effects of baclofen focusing on vestibular-related eye movements. Two rotational paradigms, i.e. sinusoidal rotation and counter rotation were employed to stimulate semicircular canals and otolith organs in the inner ear. Experimental conditions (dosage, routes and onset of recording) were determined based on the prior studies exploring the behavioral effects of baclofen in mice. With an increase in dosage, both canal and otolith induced ocular responses were gradually affected. There was a clear distinction in the drug sensitivity showing that eye movements derived from direct vestibulo-ocular reflex pathways were relatively unaltered, while the responses through higher-order neural networks in the vestibular system were substantially decreased. These findings were consistent with those observed in primates suggesting a well-conserved role of GABAB receptors in the oculomotor system across frontal-eyed and lateral-eyed animals. We showed here a previously unrecognized effect of baclofen on the vestibular oculomotor function in mice. When interpreting general animal performance under the drug, the potential contribution of altered balance system should be taken into consideration.

Effects of Different Modes of Preparatory Motion on Dart-Throwing Performance

Abstract Preparatory motion (consciously or unconsciously moving a body part just before performing a task) enhances motor performance. Repeated body movements are roughly categorized as rhythmic and discrete. However, the most effective mode of preparatory motion remains unclear. The present study utilized both modes of preparatory motion and compared subsequent performances. Twelve participants executed dart throwing after performing rhythmic, discrete, or no preparatory motion. Performance was statistically evaluated by comparing the error components, assumed contributions of each mode of preparatory motion. The results revealed that the mean value of the error component for the rhythmic mode was significantly smaller than that for the discrete mode. This suggests that the rhythmic mode of preparatory motion produced better dart-throwing performance.