Vestibular contributions to lateral stabilization are bilaterally dependent during split belt walking

Vestibular information is critical for maintaining balance during locomotion, and is known to be attenuated with increasing locomotor velocity and cadence. This attenuation is muscle and phase dependent, and is thought to reflect the functional contribution of each muscle to balance control during each stride of the gait cycle. Bilaterally, the vestibular coupling is mirrored relative to the gait cycle as each leg undergoes similar modulation with variation in phase, velocity and cadence. Here, we asked whether the modulation of the vestibular contribution to each limb is bilaterally dependent. By using a split-belt treadmill with asymmetric belt speeds, we can control the locomotion properties of each leg and compare the vestibular modulation to symmetric conditions. We hypothesized that bilaterally symmetric vestibular modulation would indicate leg independent vestibular influence while bilaterally asymmetric vestibular modulation would indicate leg dependent vestibular influence. Subjects were exposed to binaural bipolar stochastic vestibular stimulation (0-25 Hz) during symmetric and asymmetric walking conditions. Symmetric trials were performed at belt speeds of 0.4 and 0.8 m/s and for 10 min. The asymmetric trial was performed at belt speeds of 0.4 and 0.8 m/s for 16 min. Subjects walked with a cadence of 78 steps/min which was easily maintained in both limbs. EMG of the bilateral medial gastrocnemii and three-dimensional ground reaction force and torques were collected. Only the last 340 strides (~ 9 min of data) were used in the analysis to avoid the adaptation that typically occurs within the first 250 strides (~ 6 min) of asymmetric walking. Significant muscle activity and lateral ground reaction forces (P < 0.01) were correlated to the input stimuli in all trials. Stimulus-EMG and -lateral ground reaction force correlations decreased at higher belt speeds during symmetric walking, as previously reported. During the split belt condition, the magnitude of correlations stimulus-EMG and -force were bilaterally asymmetric and different from their symmetric counterparts. During the asymmetric condition correlations decreased for the slow leg, but more closely resembled the responses observed during slow symmetric walking, and increased for the fast leg, but more closely resembled the responses observed during fast symmetric walking. These results indicate that the modulation of vestibular reflexes is dependent upon the specific kinematics of each leg but bilaterally linked to respond to the properties of the locomotion pattern

Variance based weighting of multisensory head rotation signals for verticality perception

We tested the hypothesis that the brain uses a variance-based weighting of multisensory cues to estimate head rotation to perceive which way is up. The hypothesis predicts that the known bias in perceived vertical, which occurs when the visual environment is rotated in a vertical-plane, will be reduced by the addition of visual noise. Ten healthy participants sat head-fixed in front of a vertical screen presenting an annulus filled with coloured dots, which could rotate clockwise or counter-clockwise at six angular velocities (1, 2, 4, 6, 8, 16°/s) and with six levels of noise (0, 25, 50, 60, 75, 80%). Participants were required to keep a central bar vertical by rotating a hand-held dial. Continuous adjustments of the bar were required to counteract low-amplitude low-frequency noise that was added to the bar's angular position. During visual rotation, the bias in verticality perception increased over time to reach an asymptotic value. Increases in visual rotation velocity significantly increased this bias, while the addition of visual noise significantly reduced it, but did not affect perception of visual rotation velocity. The biasing phenomena were reproduced by a model that uses a multisensory variance-weighted estimate of head rotation velocity combined with a gravito-inertial acceleration signal (GIA) from the vestibular otoliths. The time-dependent asymptotic behaviour depends on internal feedback loops that act to pull the brain's estimate of gravity direction towards the GIA signal. The model's prediction of our experimental data furthers our understanding of the neural processes underlying human verticality perception

Frequency response of vestibular reflexes in neck, back, and lower limb muscles

Vestibular pathways form short-latency disynaptic connections with neck motoneurons, whereas they form longer-latency disynaptic and polysynaptic connections with lower limb motoneurons. We quantified frequency responses of vestibular reflexes in neck, back, and lower limb muscles to explain between-muscle differences. Two hypotheses were evaluated: 1) that muscle-specific motor-unit properties influence the bandwidth of vestibular reflexes; and 2) that frequency responses of vestibular reflexes differ between neck, back, and lower limb muscles because of neural filtering. Subjects were exposed to electrical vestibular stimuli over bandwidths of 0–25 and 0–75 Hz while recording activity in sternocleidomastoid, splenius capitis, erector spinae, soleus, and medial gastrocnemius muscles. Coherence between stimulus and muscle activity revealed markedly larger vestibular reflex bandwidths in neck muscles (0–70 Hz) than back (0–15 Hz) or lower limb muscles (0–20 Hz). In addition, vestibular reflexes in back and lower limb muscles undergo low-pass filtering compared with neck-muscle responses, which span a broader dynamic range. These results suggest that the wider bandwidth of head-neck biomechanics requires a vestibular influence on neck-muscle activation across a larger dynamic range than lower limb muscles. A computational model of vestibular afferents and a motoneuron pool indicates that motor-unit properties are not primary contributors to the bandwidth filtering of vestibular reflexes in different muscles. Instead, our experimental findings suggest that pathway-dependent neural filtering, not captured in our model, contributes to these muscle-specific responses. Furthermore, gain-phase discontinuities in the neck-muscle vestibular reflexes provide evidence of destructive interaction between different reflex components, likely via indirect vestibular-motor pathway

Management development: a literature review and implications for future research - Part II: Profiles and Contexts

In this second and concluding part of an extensive review of the management development literature, attention is directed to studies which have sought to describe and explain how managers are in fact made and to studies which have emphasised the contextual aspects of the process. The review is concluded with an overall assessment of the management development literature – its strengths and weaknesses – and with an analysis of the implications which this assessment carries for the conduct of future research. Crucial gaps are identified, not least of which is a general failure to locate descriptions of particular management development initiatives within the context of other ways in which the management stock is enhanced, replenished and managed. Another major shortcoming which is singled out as requiring urgent attention is the failure to place management development accounts and prescriptions within the wider context of organisational characteristics