Tracking gaze while walking on a treadmill: spatial accuracy and limits of use of a stationary remote eye-tracker.

Inaccurate visual sampling and foot placement may lead to unsafe walking. Virtual environments, challenging obstacle negotiation, may be used to investigate the relationship between the point of gaze and stepping accuracy. A measurement of the point of gaze during walking can be obtained using a remote eye-tracker. The assessment of its performance and limits of applicability is essential to define the areas of interest in a virtual environment and to collect information for the analysis of the visual strategy. The current study aims at characterizing a gaze eye-tracker in static and dynamic conditions. Three different conditions were analyzed: a) looking at a single stimulus during selected head movements b) looking at multiple stimuli distributed on the screen from different distances, c) looking at multiple stimuli distributed on the screen while walking. The eye-tracker was able to measure the point of gaze during the head motion along medio-lateral and vertical directions consistently with the device specifications, while the tracking during the head motion along the anterior-posterior direction resulted to be lower than the device specifications. During head rotation around the vertical direction, the error of the point of gaze was lower than 23 mm. The best accuracy (10 mm) was achieved, consistently to the device specifications, in the static condition performed at 650 mm from the eye-tracker, while point of gaze data were lost while getting closer to the eye-tracker. In general, the accuracy and precision of the point of gaze did not show to be related to the stimulus position. During fast walking (1.1 m/s), the eye-tracker did not lose any data, since the head range of motion was always within the ranges of trackability. The values of accuracy and precision during walking were similar to those resulting from static conditions. These values will be considered in the definition of the size and shape of the areas of interest in the virtual environment.