Abstract
Successful human locomotion over complex terrain requires precise coupling between gaze and foot placement. Matthis, Yates, & Hayhoe (2018) found that walkers adapted distinct gaze strategies that were tuned to the demands imparted by locomotion over different terrains (e.g., flat ground vs. a rocky creek bed). However, because these were quasi-observational experiments in a natural environment, it is difficult to discern the specific aspects of the terrain that were driving these different strategies. Here, we used an Augmented Reality (UniCAVE) projected ground plane (~2mx10m) to parametrically control the visual discriminability and availability of footholds in an overground stepping stone task. Using a binocular eye tracker (Pupil Labs) and a spatiotemporally synchronized, marker-based, full body motion capture system (Qualisys) we estimate participants 3d gaze and calculate gaze-ground intersection as they traverse the 10m path, comprising pseudo-randomly distributed footholds (represented by Landolt C’s; diameter 110mm, line thickness 32mm) and distractors (represented by visually similar O’s). We manipulated visual discriminability and availability of footholds to discern the aspects of terrain that drive the emergence of different gaze-foothold strategies. Participants walked over six terrains, consisting of two levels of visual discriminability (controlled by manipulating the size in the C gap; 6mm vs. 36mm) crossed with three levels of foothold/distractor ratio (3:1, 1:1, and 1:3). With 21 trials for each of the 7 conditions (six terrains plus a free walking control condition) we record 147 trials (approximately 1960 steps) per participant. We mapped the gaze-foothold relationships from the six terrain conditions to those observed in natural environments (Matthis, Yates, & Hayhoe, 2018) to specify how the visual discriminability and availability of footholds result in different gaze strategies. We further explored whether saccades are driven by the biomechanically specified and energetically optimal footholds vs. the likelihood of finding potential footholds based on peripheral vision.