A remote ET (Tobii, TX300, 300Hz) was used to estimate the point of gaze (PoG) of a healthy adult (h: 1.80m). A LCD monitor (47-inch) was used to display ...
VALIDATION OF A REMOTE EYE-TRACKER: APPLICATIONS TO GAIT ANALYSIS V. Serchi (1, 2), A. Peruzzi (1, 2), A. Cereatti (1, 2), U. Della Croce (1, 2) (1) Information Engineering Unit, POLCOMING Department, University of Sassari, Sassari, Italy (2) Interuniversity Centre of Bioengineering of the Human Neuromusculoskeletal System, Sassari, Italy Main topics: Experimental studies in human movement science, technical developments in movement science. INTRODUCTION and AIM Inaccurate visual sampling and foot placement may lead to unsafe walking [1]. The combined use of virtual environments and treadmill allows providing different visual stimuli while walking in a laboratory setting. To analyze the visual strategy, remote gaze eye-trackers (ET) may be used. However, their performance is affected by several sources of inaccuracy, which may compromise the quality of the acquired data [2]. The present study aims at assessing the ET performance under different experimental conditions to determine its limits of usability, precision and accuracy. In particular, we evaluated: a) the range of motion of the head within which the ET can track the eyes gaze (range of trackability, RoT); b) the effect of the subject-ET distance and c) the influence of the visual stimulus location. The ET was tested through static and walking tasks. The selected visual stimuli consisted in dot targets and in static and moving geometrical shapes. MATERIALS and METHODS A remote ET (Tobii, TX300, 300Hz) was used to estimate the point of gaze (PoG) of a healthy adult (h: 1.80m). A LCD monitor (47-inch) was used to display visual stimuli; both the monitor and the ET were centered to a treadmill. Eight markers were attached on the participant’s head and on the ET. An optoelectronic system (Vicon, T20, 300Hz) was used to track the head motion. An initial subject-specific gaze calibration was performed with the participant at 650mm from the ET (according to the manufacturer guidelines). To assess the RoT, the subject was asked to look at a dot target while oscillating along the anterior-posterior (tAP), mediolateral (tML) and the vertical (tV) directions and rotating the head around the vertical direction (rV). To assess the influence of the stimulus location and the effect of the subject -ET distance, the subject was asked to look at 13 dots-grid on the screen while standing at 550mm (st550), 650mm (st650) and 750mm (st750) from the ET. Finally, to test the performance of the ET during gait, the subject was asked to look at 1) 13 dots-grid (walk1) and 2) a static rectangle and a moving T-shaped object (walk2) while walking at 1.1m/s. The subject-ET distance was, on average, of 650mm. For each acquisition, the head range of motion (RoM) was computed from the markers. For each dot target location, bias (b), standard deviation (sd) and maximum error (eMAX ) values of the PoG were computed. The b and sd values were averaged over the 13 dot target locations in order to obtain a global description of accuracy and precision. The percentage of the gaze fixations [3], falling in a neighborhood of the static and moving geometrical shapes, was computed. RESULTS The RoT and eMAX values during tAP, tML, tV and rV are reported in Table 1. Average b and sd values over the 13 dot target locations were13±4mm for the st550, 8±4mm for st650 and 20±6mm for st750. The variability of b values associated to the dot target locations ranged between 4 and 8mm at the distances analyzed. In walk1, the RoM was always within the device RoTs and b and sd values were similar to those of the static tasks. In walk2 the percentage of gaze fixations was always higher than 90%. DISCUSSION and CONCLUSIONS The RoT along ML and V directions was consistent with the ET specifications (±100mm), while along the AP direction (70mm) was lower than indicated (-150mm). Stimulus locations seemed to not influence the ET accuracy and precision. The mean accuracy decreased when moving away from the optimal distance (650mm): the largest error (20mm) was observed at 750mm distance. In gait, the ET performance was adequate since no gaze loss occurred and accuracy and precision were similar to those of the static tasks. The high fixation percentage during walking suggest that the proposed experimental setup may be used for tracking gaze while watching virtual reality objects during gait. Table 1: The device range of trackability (RoT) and the maximum error (eMAX ) during tAP, tML, tV and rV. Task tAP [mm] tML [mm] tV [mm] rV [deg]
RoT min 580 -150 -90 -50
max 790 120 -a 50 a
eMAX 30b 29b 28b 23b
No gaze tracking interruption;
REFERENCES [1] Reed-Jones R J et al, Gait Posture, 2012; vol. 35, no. 4, pp. 585:589. [2] Morgante J D et al, Infancy, 2012; vol. 17, no. 1, pp. 9:32. [3] Holmqvist K et al, OXFORD University, 2011; pp. 1:702.
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