The advantage that could arise from fixed gaze is that it greatly simplifies the extraction of heading information. The flow field projected on the retina reflects the locomotor trajectory, uncontaminated by gaze motion. For a curved path this should enable the perception of
locomotor flow lines (
Lee & Lishman, 1977) and in principle, matching of these flow lines to the projected edges of the road would be sufficient for accurate steering. The fixed gaze condition, however, did not result in accurate steering, even though we ensured that central vision was maintained close to the same areas sampled in the other conditions (16m or 2s ahead). This questions the utility of the locomotor flow line hypothesis and it also sheds some doubt on the use of an estimate of instantaneous heading.
Wann & Land (2002) proposed that fixation of a point on your future path was an important feature of accurate steering. This would predict a significant improvement in the tracking condition, which imposes the Wann & Land strategy, as compared to fixed gaze. This was confirmed across all 6 participants for precision of steering errors and the time-in-zone measures.
Land & Lee (1994) presented an earlier alternative proposal, that the tangent point, or apex of the bend, is a particularly useful feature when steering. The tangent point can provide information on the curvature of the road ahead. If participants habitually used this feature then we might have expected a significant improvement in the free gaze condition, over the tracking condition, as participants sought out the tangent point. There was some partial support for this in the RMS scores (
Figure 2), but average performance displayed similar positional distributions for both conditions (
Figure 3). Examination of gaze behavior when free to sample naturally revealed that a great deal more time was spent looking towards the center of the road than at the inside edge (
Figure 5).
The future path (road center) was not rigidly fixated, but on average 30% of the time was spent looking at the center zone, and another 50% looking either side of the center. In general we did not observe close gaze tracking of the road edges and no individual participant adhered closely to the tangent point strategy. It seems that if there is an advantage of maintaining gaze at the tangent point then the participants in this study did not find it. One critical difference may be that the
Land & Lee (1994) study was completed at higher speeds, on real roads with blind bends. Our task was less threatening and did allow unlimited look-ahead. A further important difference between the two hypotheses is that the tangent point strategy provides a solution for maintaining a trajectory a fixed distance from the inside edge (lane following), whereas the path-fixation strategy allows any curved path to be chosen, including a ‘racing line’ of cutting the corner.
Land & Tatler (2001) observed that on a well-practiced course a racing car driver did not look exactly at the tangent point, so it may be necessary to examine gaze behavior on an unpredictable course in order to resolve which strategy is used, and when.
Irrespective of the precise strategy used in free gaze, we have illustrated that active gaze is essential for optimal sampling of relevant visual information, since it is under these conditions that the most accurate control of steering is observed. There were some individual differences in the patterns of fixation, but the general sampling strategy was to look ahead by 1–2.5s and to fixate a point close to the desired future path (
Table 2). These temporal estimates are commensurate with
Land & Lee (1994) but we differ with respect to the road features which are fixated. Each of these fixations would induce gaze rotation and introduce an additional rotation component into the retinal flow field. In previous investigations of the perception of heading this gaze rotation component would be seen as a problem or confound that required compensation (
Lappe, Bremmer, & van den Berg, 1999). In contrast to this
Wann & Wilkie (In Press) propose that the cascading fixations are the primary planning mechanism that enables path selection and the completion of complex trajectories of varying curvature. Fixation of points on the path can also enable the use of retinal flow to judge steering, without recovering optic flow or heading (
Kim & Turvey, 1999;
Wann & Swapp, 2000).