Heads turn for the latest research on standing balance Stable head position is taken for granted but it’s the basis for balance, navigation and movement. Defective integration of vestibular signals arising from the inner ear can cause distressing symptoms of vertigo, motion sickness, dizziness and fatigue. Fundamental Research Health © Patrick Forbes To keep the head upright, the central nervous system distinguishes between externally imposed and self-generated head and body movements. Reflexes for externally imposed head movements activate motor neurons for gaze and posture maintenance. In contrast, for self-generated movements, vestibular signals are suppressed at the earliest stage of vestibular processing, since any self-generated vestibular response would theoretically oppose the intended movement. The HEADS-UP (The modulation of vestibular reflexes during self-generated head-neck movements) project looked at why self and external movements promote very different motor responses, even though the vestibular sensors cannot distinguish between them. The team used newly developed robots, computer modelling and engineering techniques. Whole body rotation experiments showed that natural vestibular stimulation modulated the electrically induced reflex controlling neck muscles (the vestibulocollic reflex), demonstrating non-linear information processing in humans. Head-neck investigations showed that the electrically induced vestibulocollic reflex is similarly modualted in both active and passive head movements. The vestibular system can therefore separate the reafferent activity during self-generated head movements from the exafferent signal generated by electrical stimulation, making neck muscle activity specific to the electrical prompt. Reversing the relationship between balancing motor commands and associated vestibular sensory signals caused a corresponding reversal of the ankle torque response within 30 seconds. This indicates that the nervous system can rapidly re-associate new sensory signal/motor command relationships to maintain upright standing balance. Particularly important in future clinical research, HEADS-UP confirmed that all afferent signals are affected by electrical stimulation. The researchers compared the dynamic properties of electrical- and motion-stimulated vestibulocollic reflexes to characterise neck muscle response during electrical stimulation. Moreover, they found that the vestibular system can contribute to neck muscle activity beyond the expected frequency, up to 150 Hz. HEADS-UP research has provided a substantial knowledge platform for how the vestibular system regulates head movement for everyday activities. The results have also facilitated the launch of a new project focused on how zero-gravity conditions influence standing balance, with implications for space exploration. Keywords Balance, vestibular system, head movement, HEADS-UP, reflex
