• Dynamic transformation of vestibular signals for orientation

      Osler, Callum J.; Reynolds, Raymond Francis; University of Birmingham, School of Sport and Exercise Sciences, College of Life and Environmental Sciences (Springer, 2012)
      The same pattern of vestibular afferent feedback may signify a loss of balance or a change in body orientation, depending upon the initial head posture. To resolve this ambiguity and generate an appropriate motor response, the CNS must transform vestibular information from a head-centred reference frame into relevant motor coordinates. But what if the reference frame is continuously moving? Here, we ask if this neural transformation process is continuously updated during a voluntary change in head posture. Galvanic vestibular stimulation (GVS) was used to induce a sensation of head roll motion in blindfolded subjects marching on the spot. When head orientation was fixed, this caused unconscious turning behaviour that was maximal during neck flexion, minimal with the head level and reversed direction with neck extension. Subjects were then asked to produce a continuous voluntary change in head pitch, while GVS was applied. As the neck moved from full flexion into extension, turn velocity was continuously modulated and even reversed direction, reflecting the pattern observed during the head-fixed condition. Hence, an identical vestibular input resulted in motor output which was dynamically modulated by changes in head pitch. However, response magnitude was significantly reduced, suggesting possible suppression of vestibular input during voluntary head movement. Nevertheless, these results show that the CNS continuously reinterprets vestibular exafference to account for ongoing voluntary changes in head posture. This may explain why the head can be moved freely without losing the sense of balance and orientation.
    • Galvanic vestibular stimulation produces sensations of rotation consistent with activation of semicircular canal afferents

      Reynolds, Raymond Francis; Osler, Callum J.; University of Birmingham, School of Sport and Exercise Sciences, College of Life and Environmental Sciences (Frontiers, 2012)
    • Postural reorientation does not cause the locomotor after-effect following rotary locomotion

      Osler, Callum J.; Reynolds, Raymond Francis; University of Birmingham, School of Sport and Exercise Sciences, College of Life and Environmental Sciences (Springer, 2012)
      After a period of stepping on a rotating platform, blindfolded subjects demonstrate a tendency to unconsciously turn when stepping in place, an after-effect known as podokinetic after-rotation (PKAR). Recent studies have also reported a change in postural orientation following the adaptive period and have suggested that this is causally related to PKAR. Here, we assess changes in trunk orientation following platform adaptation and determine their relationship to PKAR. Specifically, we determine whether a reorganized standing posture causes PKAR. Ten subjects stepped on a platform rotating at 60deg/s for 10 min, with a cadence of 100 steps/min. Following adaptation, a significant PKAR response was seen, with a mean yaw rotation velocity of 6.0 ± 2.2deg/s. In addition to this dynamic after-effect, there was a significant twist of the trunk with respect to the feet when standing still (6.9 ± 4.5deg; mean ± SD), confirming the presence of a postural reorientation after-effect. However, the magnitudes of the two after-effects did not correlate (r = 0.06, p = 0.87). Furthermore, in a second experiment, a prolonged passive twist of the trunk was used to induce postural reorientation. However, in this case, PKAR was not induced. These results demonstrate that PKAR is not an automatic consequence of reorganized standing posture.
    • Postural threat differentially affects the feedforward and feedback components of the vestibular-evoked balance response

      Osler, Callum J.; Tersteeg, M. C. A.; Reynolds, Raymond Francis; Loram, Ian D.; University of Birmingham, School of Sport and Exercise Sciences, College of Life and Environmental Sciences; Manchester Metropolitan University, Manchester, Institute for Biomedical Research into Human Movement and Health (Wiley, 2013)
      Circumstances may render the consequence of falling quite severe, thus maximising the motivation to control postural sway. This commonly occurs when exposed to height and may result from the interaction of many factors, including fear, arousal, sensory information and perception. Here, we examined human vestibular-evoked balance responses during exposure to a highly threatening postural context. Nine subjects stood with eyes closed on a narrow walkway elevated 3.85 m above ground level. This evoked an altered psycho-physiological state, demonstrated by a twofold increase in skin conductance. Balance responses were then evoked by galvanic vestibular stimulation. The sway response, which comprised a whole-body lean in the direction of the edge of the walkway, was significantly and substantially attenuated after ~800 ms. This demonstrates that a strong reason to modify the balance control strategy was created and subjects were highly motivated to minimise sway. Despite this, the initial response remained unchanged. This suggests little effect on the feedforward settings of the nervous system responsible for coupling pure vestibular input to functional motor output. The much stronger, later effect can be attributed to an integration of balance-relevant sensory feedback once the body was in motion. These results demonstrate that the feedforward and feedback components of a vestibular-evoked balance response are differently affected by postural threat. Although a fear of falling has previously been linked with instability and even falling itself, our findings suggest that this relationship is not attributable to changes in the feedforward vestibular control of balance.