Why you should look where you are going

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Experienced Rider Course (Motorcycle Safety Foundation, 1992). 9. Warren, W. H. Nat. Neurosci. 1, 647–649 (1999). 10. Maunsell, J. H. R. & Van Essen, D. C. J.
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Why you should look where you are going John P. Wann and David K. Swapp Department of Psychology, University of Reading, 3 Earley Gate, Reading RG6 6AL, UK

© 2000 Nature America Inc. • http://neurosci.nature.com

Correspondence should be addressed to J.P.W. ([email protected])

It is controversial whether head and eye movement information are required to discern locomotor heading from visual motion information1–5. We present a new theory of steering based on active gaze and retinal flow, which demonstrates that future paths could be judged using known properties of visual cortex neurons, without recovering current heading or integrating extraretinal signals. This theory is consistent with the gaze-sampling behavior promoted in advanced driving instruction. Motion across a ground plane with stable gaze generates a radial flow pattern that can indicate heading (Fig. 1a, black vectors). If an observer sweeps gaze across the ground plane, then rotation of the eye relative to the body produces the same retinal flow as results from rotation of the whole body around a curved path (Fig. 1a, blue), and observers may be confused by such displays3. There is strong evidence that extra-retinal information is necessary to discern heading during such a gaze sweep5. Sustained gaze sweeps are infrequent, however, and the more general gaze behavior during locomotion is to fixate points on the road ahead6. Advanced driving manuals recommend the strategy of directing gaze around a bend to points on the intended path7 and suggest that steering will then naturally proceed toward the fixation point8. If the observer fixates a ground feature, then that object is maintained on the fovea, and the flow is centered around this feature (Fig. 1c). In a laboratory task, where fixation is controlled, observers might mistake this pattern for a curved trajectory9, but the centering effect of fixation makes this distinct from that of Fig. 1a (blue), and confusion should not arise in natural settings. Any locomotor animal that routinely fixates features of its enviFig. 1. Retinal flow patterns for different steering and gaze responses. Simulations display the flow arising over 0.1 s while traveling at 30 mph and fixating 1 s ahead. Initial heading is indicated as a vertical red line above the horizon, with a horizontal red line indicating changing heading. (a) Linear motion across a ground plane with stable gaze produces a radial flow pattern (black). Blue lines indicate the flow if steering is adjusted such that the trajectory would pass through the two upright posts but gaze is held fixed with the vehicle. This pattern may also result from linear translation toward the center of the display with a gaze sweep to the left. Green flow lines result from only rotating half the angle to the tangent (Fig. 2). (b) If the observer steers on an appropriate arc to pass between the poles (as in a, blue), and fixates between them, the flow lines remain straight, and the future path of the observer is represented by the points that move vertically within the retinal image (red ground lines). (c) Fixating between the poles, but traveling on an eccentric heading, results in a pattern that is centered around the fixation point, but flow lines curve in a direction opposite to the steering error. Red lines on the ground plane illustrate that the ideal path (as shown in b) also sweeps to the side. (d) If the observer oversteers, such that the future trajectory will pass in front of the point of fixation, then the pattern curvature reverses as compared to (c). nature neuroscience • volume 3 no 7 • july 2000

ronment should recognize that such a curved flow field (Fig. 1c) only arises if the animal is not on a path toward the sign, and the degree of flow curvature indicates path eccentricity. If the observer adjusts steering to a curved path that will pass through a gate, but fixates the ground between the poles, then the flow lines are straight, and move outward asymmetrically from the observer’s future path (Fig. 1b). If the observer oversteers such that the resulting path would pass in front of the gate, then this is reflected in a change in flow curvature (Fig. 1d). Thus there is a simple heuristic that avoids the problems previously debated and allows an animal with mobile gaze to steer to a target using retinal flow alone. (i) Fixate a feature in the environment near the intended path6–8. (ii) If traveling eccentric to the fixation point, the visual trajectories of ground elements will be curved, and the direction of curvature is reciprocal to the direction of steering error. (iii) If it is possible to re-orient the trajectory directly toward the target, a radial pattern with straight flow lines (Fig. 1a, black) will indicate when the current path leads to the fixated feature. (iv) In the more general case, if there are constraints on turning arc, a path of constant curvature that will intercept the fixated feature will result in an asymmetrical pattern with straight flow lines (Figs. 1b and 2). (v) Once either (iii) or (iv) are met, the future path is indicated by the points that move with zero-gradient (pure vertical) flow within the retinal image. Using gaze to guide steering does not preclude glancing to other roadside features, but suggests that the driver should iteratively gaze back to the intended path to check the steering line6, rather than try to judge heading when gaze is eccentric to the path1–5. On a roadway with varying curvature, effective steering can be achieved by moving gaze forward as the road curvature changes, thereby splining different path segments. Monitoring (ii) to detect understeer and oversteer does not require precise estimation of the acceleration components of the flow field. The visual system only needs to detect whether there is any change in the direction of motion for salient elements close to the path and the nominal direction of that change. Cells in extrastriate area MT are sensitive to visual direction10 and may be combined in area MST to be selective for different flow patterns11. A change in flow direction could be detected through a change in the rel-

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© 2000 Nature America Inc. • http://neurosci.nature.com

© 2000 Nature America Inc. • http://neurosci.nature.com

brief communications

Fig. 2. Nulling flow curvature a b with active fixation. (a) Step P3 P2 F motion of an observer around an P4 arc from E1 to E2 can be considP5 α ered as an instantaneous translaα tion accompanied by a rotation P1 through 2α to the tangent of the α β curve. If the observer rotates by E2 E2 α, then the step change in the projected angles for points P1–P3 is equivalent to the radial pattern that would result from R translation directly toward P2. V For continuous motion around a circle, if the observer does not α rotate, flow lines for P1–P3 will β display curvature that is recipro2α 2α cal to the path curvature. If the E1 E1 observer rotates only half of the angle to the path tangent (α), this nulls the curvature in the flow lines (Fig. 1a, green), whereas rotation to the tangent (2α) introduces flow curvature that reflects the path curvature (Fig. 1a, blue). (b) Motion around an arc at speed V, while maintaining fixation on a point on the path F. Motion through 2α reduces the fixation angle from β to β – α. For continuous motion, the angular gaze velocity of dα/dt = −V/2R nulls the flow curvature as illustrated for P1–P3. Because the lateral velocity of points on the path (P4, P5) is also dα/dt, the gaze velocity nulls their horizontal motion, and they move vertically on the image plane (Fig. 1b, red). A mathematical appendix is available as a pdf file at http://www.nature.com/neuro/web_specials or directly from the authors.

ative firing rate of directionally tuned MT neurons, or through the pooling of MT outputs to assess whether depth-ordered points projecting out from the path exhibit colinear motion. Irrespective of the means of detection, noting a change in motion direction is fundamental to effective visual control in many tasks. There is complementary evidence that instantaneous heading can be judged from a first-order velocity flow field12, but accurate steering requires information about the rate of change of velocity flow vectors13. In both (iii) and (iv), flow lines are straight and move outward from the future path that projects vertically. Point (v) means that, when steering has been adjusted to match the curvature of a bend, a line of constant curvature from under the driver’s vehicle to the disappearing point of the bend will have no lateral motion on the retinal image. The percept of ‘reading the line’, therefore, may equate to vertically tuned MT neurons being stimulated by the same points on the roadway throughout the trajectory, whereas for a steering error the vertical-flow line breaks away across the ground, indicating understeer or oversteer. J.J. Gibson’s seminal paper14 emphasized tasks such as steering and aiming. The more recent dominance of tasks where observers are required to judge linear heading with restrictions on fixation may have led researchers astray on how humans or animals actually judge locomotor direction. Emphasizing judging paths rather than judging heading simplifies the problem for a locomotor animal and returns to Gibson’s original theme that optic flow gives sufficient information to guide locomotion. Confusion among heading, gaze motion and curved trajectories9 should not occur in natural settings. Gaze orienting to steering goals can yield sufficient information to judge current path, without decomposing retinal flow to recover tangential heading or integrating extraretinal information. Extra-retinal gaze information can bias locomotion direction15 (B.J. Rogers & C. Dalton, Invest. Ophthal. Vis.

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Sci. 40, S764, 1999), and effective steering at high speed may use both types of information. Our model demonstrates, however, that information about gaze orientation is not required to prospectively judge steering paths. This approach is consistent with recommended and observed gaze patterns, and goes some way to explain why we look where we steer6 and why we steer to where we look7,8. Note: A mathematical appendix can be found on the Nature Neuroscience web site (http://www.nature.com/neuro/web_specials).

ACKNOWLEDGEMENTS This research was supported by UK EPSRC grant GR/L18693 and GR/L16125.

RECEIVED 28 SEPTEMBER 1999; ACCEPTED 15 MAY 2000 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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nature neuroscience • volume 3 no 7 • july 2000