On the temporal dynamics of head and eye movements for walking on real-world surfaces

Most human actions are planned and performed based on visual information. Indeed, when interacting with the environment, humans typically direct their gaze to where relevant information for the task at hand can be found. While walking, humans shift their gaze by using both eye and head movements, and the relative contribution of the head is more pronounced when the navigated surface's complexity increases. However, most work has examined average behavior across the duration of the performed task (i.e. walking), often neglecting that natural behavior is continuous and so is the process of acquiring visual information for the task at hand. Here, we want to tackle this limitation by investigating the temporal dynamics of the head's engagement for gaze shifts while freely walking on different surfaces. We leveraged a naturalistic dataset encompassing gaze behavior during unconstrained walking across the most frequently occurring surfaces in everyday life (flat-walking, slopes, stairs). Our results confirm the previously observed increase in alignment of the head and gaze orientation as a function of the surface's complexity. Additionally, our results support a continuous contribution of the head to update task-relevant visual information reflected in the increased rate of change in the head's angular pitch displacement and the coupling between eye and head movements throughout the most demanding "stairs" condition. Taken together, our work showcases the temporal characteristics of the increased active engagement of the head in visual guidance of gait as an effect of everyday-occurring surface complexities.