Head mobility influences gaze behaviour across natural viewing tasks

Natural gaze behavior is often studied under conditions that restrain head movements. Here, we report how the availability of head movement can influence gaze behavior on wall-sized images of natural outdoor scenes (field-of- view: ~90°). Participants performed half of the experiment with complete head mobility and the remaining trials with their heads restrained in a chin-rest. They were required to either rate the images for attractiveness (i.e., free-viewing) or to count the visible animals (i.e., visual search). On average, more fixations were found on the trials that allowed for head movements (unrestrained: 4.21 fixations/sec; restrained: 3.75 fixations/sec), which were also shorter in their mean duration (unrestrained: 221 ms; restrained: 252 ms). In addition, unrestrained gaze contained a larger proportion of small amplitude saccades (i.e., less than 5°), than head-restrained gaze. Finally, our participants demonstrated a general preference in fixating regions that were close to the central eye-in-h ead orientation. Altogether, these findings suggest that the availability of head movements allowed our participants to re-orient to regions of interest and sample these regions more frequently. This sampling benefit applied to both visual search and free viewing tasks. The current findings emphasize the importance of allowing head mobility when studying natural gaze behavior