Fear of heights freezes gaze to the horizon.

Fear of heights is elicited by a glance into an abyss. However, the visual exploration behavior of fearful subjects at height has not been analyzed yet. We investigated eye- and head movements, i.e. visual exploration behavior, of subjects susceptible to fear of heights during exposure to a visual cliff. The movements of eyes and head were recorded in 19 subjects susceptible to fear of heights and 18 controls while standing still on an emergency balcony 20 meters above ground level for periods of 30 seconds. Participants wore mobile, infrared eye-tracking goggles with inertial sensors for recording head movements. Susceptibles exhibited fewer and smaller-amplitude eye-in-head saccades with fixations of longer duration. Spontaneous head movements were reduced by 49% in susceptibles with a significantly lower mean absolute angular velocity (5.3°/s vs. 10.4°/s), and all three dimensions (yaw, pitch and roll) were equally affected. Gaze-in-space--which indicates exploration by coordinated eye-head movements--covered a smaller total area of the visual scene (explored horizontal angle: 19° vs. 32°, vertical: 9° vs. 17°). We hypothesize that the susceptibles suppress eye and head movements to alleviate fear of heights. However, this behavior has the potential disadvantage of impairing the visual stabilization of postural balance.