Abstract
This article summarizes newly published findings obtained in our recent studies of eye-head-trunk coordination strategies during walking. I attempt to place them within the historical framework of previous studies, and discuss underlying mechanisms that control head and eye movements during walking. Study of vertical head and trunk movements during walking throughout the range of possible speeds enabled us to obtain quantitative information about translation and rotation of the head and trunk. We also obtained a clearer picture of the motor mechanisms responsible for head movements and their relationship to trunk motion during locomotion. Our results suggest that two mechanisms are used to maintain a stable head fixation distance over the optimal range of walking velocities. The relative contribution of each mechanism to head orientation depends on the frequency of head movement and consequently on walking velocity. Considering the frequency characteristics of the compensatory head pitch, we inferred that the angular vestibulocollic reflex achieves head stability at low walking speeds, whereas the linear vestibulocollic reflex is predominately responsible for producing compensatory head pitch movement at higher speeds. We also found that the naso-occipital axis of the head aims approximately at a single point, the head fixation point (HFP), 1 meter in front of the subject. The HFP position was stable despite changes in walking velocity. This led us to design a second set of experiments in which we tested vertical eye and head coordination as a function of viewing distance during locomotion. The major finding from the second study is that during natural walking, the phase of eye velocity relative to head pitch velocity is dependent on the viewing distance, whereas head translation and rotation are relatively unaffected and tend to maintain the HFP despite changes in viewing distance. During viewing of a distant (2-meter) target, the vertical eye velocity is 180°out of phase with the head pitch velocity, indicating that the angular vestibuloocular reflex (aVOR) is generating the eye movement response. With a close (0.25-meter) target, eye velocity is in phase with head pitch and compensates for vertical head translation, suggesting that activation of the linear vestibuloocular reflex (lVOR) contributes to the eye movement response. The results of supplementary experiments using fixed-body active head pitch rotation while viewing a head-fixed target indicate that visual suppression modifies both the gain and phase characteristics of the aVOR at frequencies encountered during locomotion. We propose that visual suppression may shift the phase of the aVOR to augment the lVOR when viewing close targets during locomotion. In this context, we have taken the view that correct transduction and integration of signals from otoliths and canals is essential to maintaining stable vision and head orientation control during natural linear walking.
