Gazing at tangent point location during curve driving does not avoid foveal motion and leads to optokinetic nystagmus

Abstract

Many studies have shown that during curve driving, the drivers have gaze fixation patterns in the vicinity of the tangent point (TP), this latter being the intersection between the inside edge of the road and its tangent passing through the driver position. The interest for the TP is mainly due to the fact that on one hand, its angular position is linked to the road curvature and therefore can be a control variable for the driver; and, on the other hand, it may correspond to a minimal speed in the optical flow. However, the TP is only motionless when the trajectory precisely follows the curve's geometry. In the present study, we measured gaze behavior during curve driving, with the general hypothesis that gaze cannot be stable when exposed to a global optical flow due to self-motion. We used a driving simulator coupled to a gaze recording system. Ten participants drove on a track composed of eight curves of various radii. Results show that gaze position is, as previously described, located near the TP. In addition, we observe the presence of a systematic optokinetic nystagmus (OKN) around the TP position. The OKN slow-phase direction does not match the foveal optic flow direction, while slow-phase speed is about half the local speed. When averaging the flow on larger areas, gaze and flow directions match better, and gaze and flow speeds are optimally matched for an optic flow integration over two degrees. We thus confirm that the TP is a privileged feature in the dynamic visual scene during curve driving. However, studying only gaze fixation is not fully relevant, because the TP is surrounded by optic flow, whose global characteristics induce retinal drifts and lead to OKN behavior. We consider that this must be taken into account for future models of vehicle control.