Abstract
In rough terrain, walkers must use vision to select viable paths and to find suitable footholds within the time frame allowed by the gait cycle. How is this visuo-motor loop controlled? Recent work demonstrated that in rocky terrain, walkers spend most of the time fixating the region 2-3 steps ahead, which allows them to take advantage of the passive dynamics of the body, and stay close to the energetically optimal gait (Matthis et al, 2018). However, in that study it was not possible to accurately estimate fixation and footplant locations, as errors were introduced by projecting gaze onto a flat ground plane. We have recently been able to improve these estimates using a novel animation technique called photogrammetry to create 3D representations of the terrain using video footage from the Pupil Labs scene camera. In combination with eye tracking and IMU-based body tracking, photogrammetry allowed us to intersect the gaze vector with the actual 3D location of the walking surface. We were also able to more accurately locate the placement of the foot. This method allows for more accurate localization of gaze relative to foothold locations, and also eliminates IMU drift. We analyzed 6 walks on rocky terrain for each of 3 subjects. For the different subjects, gaze was distributed around the upcoming footholds (2 and 3 steps ahead) with a standard deviation ranging between 17-21cm for the different subjects (approximately a foot-length) in the direction of travel, but only 3.3-5.1 cm laterally. When gaze is 2-3 steps ahead, two standard deviations covers approximately 16 deg by 4 deg. Thus walkers allow more flexibility in gaze location along the travel path and are presumably using information from the parafoveal retina to guide the choice of foothold at least some of the time in rocky terrain.