Richard M. Wilkie

Controlling steering and judging heading: Retinal flow, visual direction, and extraretinal information.

The contribution of retinal flow (RF), extraretinal (ER), and egocentric visual direction (VD) information in locomotor control was explored. First, the recovery of heading from RF was examined when ER information was manipulated; results confirmed that ER signals affect heading judgments. Then the task was translated to steering curved paths, and the availability and veracity of VD were manipulated with either degraded or systematically biased RF. Large steering errors resulted from selective manipulation of RF and VD, providing strong evidence for the combination of RF, ER, and VD. The relative weighting applied to RF and VD was estimated. A point-attractor model is proposed that combines redundant sources of information for robust locomotor control with flexible trajectory planning through active gaze.

Eye-movements aid the control of locomotion

Eye-movements have long been considered a problem when trying to understand the visual control of locomotion. They transform the retinal image from a simple expanding pattern of moving texture elements (pure optic flow), into a complex combination of translation and rotation components (retinal flow). In this article we investigate whether there are measurable advantages to having an active free gaze, over a static gaze or tracking gaze, when steering along a winding path. We also examine patterns of free gaze behavior to determine preferred gaze strategies during active locomotion. Participants were asked to steer along a computer-simulated textured roadway with free gaze, fixed gaze, or gaze tracking the center of the roadway. Deviation of position from the center of the road was recorded along with their point of gaze. It was found that visually tracking the middle of the road produced smaller steering errors than for fixed gaze. Participants performed best at the steering task when allowed to sample naturally from the road ahead with free gaze. There was some variation in the gaze strategies used, but sampling was predominantly of areas proximal to the center of the road. These results diverge from traditional models of flow analysis.

Driving as Night Falls: The Contribution of Retinal Flow and Visual Direction to the Control of Steering

We have the ability to locomote at high speeds, and we usually negotiate bends safely, even when visual information is degraded, for example, when driving at night. There are three sources of visual information that could support successful steering. An observer fixating a steering target that is eccentric to the current heading must rotate their gaze. The gaze rotation may be detected by using head and eye movement signals (extra-retinal direction: ERD) or their retinal counterpart, visual direction (VD). The gaze rotation also transforms the global retinal flow (RF) field, which may enable direct steering judgments. In this study, we manipulate VD and RF to determine their contribution toward steering a curved path in the presence of ERD. The results suggest a model that uses a weighted combination of all three information sources, but results also suggest that this weighting may change in reduced visibility, such as in low-light conditions.

Active gaze, visual look-ahead, and locomotor control

The authors examined observers steering through a series of obstacles to determine the role of active gaze in shaping locomotor trajectories. Participants sat on a bicycle trainer integrated with a large field-of-view simulator and steered through a series of slalom gates. Steering behavior was determined by examining the passing distance through gates and the smoothness of trajectory. Gaze monitoring revealed which slalom targets were fixated and for how long. Participants tended to track the most immediate gate until it was about 1.5 s away, at which point gaze switched to the next slalom gate. To probe this gaze pattern, the authors then introduced a number of experimental conditions that placed spatial or temporal constraints on where participants could look and when. These manipulations resulted in systematic steering errors when observers were forced to use unnatural looking patterns, but errors were reduced when peripheral monitoring of obstacles was allowed. A steering model based on active gaze sampling is proposed, informed by the experimental conditions and consistent with observations in free-gaze experiments and with recommendations from real-world high-speed steering.

Stepping over obstacles: Attention demands and aging

Older adults have been shown to trip on obstacles despite taking precautions to step carefully. It has been demonstrated in dual-task walking that age-related decline in cognitive and attentional mechanisms can compromise postural management. This is yet to be substantiated during obstacle negotiation when walking. Forty-six healthy volunteers (aged 20-79 years) stepped over obstacles in their path whilst walking and performing a verbal fluency task. Using 3D kinematic analysis we compared obstacle crossing during single-task (obstacle crossing only) and dual-task (obstacle crossing with verbal task) conditions. We grouped the participants into three age groups and examined age-related changes to cognitive interference on obstacle crossing. During dual-task trials, the 20-29 and 60-69 groups stepped closer to the obstacles prior to crossing, increased vertical toe-obstacle clearance, and had reduced gait variability. In these two groups there was a small dual-task decrease in verbal output. The 70-79 group applied similar dual-task stepping strategies during pre-crossing. However, during crossing they showed reduced vertical toe-to-obstacle clearance and increased variability of obstacle-to-heel distance. Additionally, this group did not show any significant change to verbal output across trials. These results suggest that with advanced age, increased cognitive demands are more likely to have a detrimental impact on motor performance, leading to compromised safety margins and increased variability in foot placement. We conclude that younger adults utilise a posture-preserving strategy during complex tasks but the likelihood of this strategy being used decreases with advanced age.

Using vision to control locomotion: looking where you want to go.

Looking at the inside edge of the road when steering a bend seems to be a well-established strategy linked to using a feature called the tangent point. An alternative proposal suggests that the gaze patterns observed when steering result from looking at the points in the world through which one wishes to pass. In this explanation fixation on or near the tangent point results from trying to take a trajectory that cuts the corner. To test these accounts, we recorded gaze and steering when taking different paths along curved roadways. Participants could gauge and maintain their lateral distance, but crucially, gaze was predominantly directed to the region proximal to the desired path rather than toward the tangent point per se. These results show that successful control of high-speed locomotion requires fixations in the direction you want to steer rather than using a single road feature like the tangent point.

The Role of Visual and Nonvisual Information in the Control of Locomotion

During locomotion, retinal flow, gaze angle, and vestibular information can contribute to ones perception of self-motion. Their respective roles were investigated during active steering: Retinal flow and gaze angle were biased by altering the visual information during computer-simulated locomotion, and vestibular information was controlled through use of a motorized chair that rotated the participant around his or her vertical axis. Chair rotation was made appropriate for the steering response of the participant or made inappropriate by rotating a proportion of the veridical amount. Large steering errors resulted from selective manipulation of retinal flow and gaze angle, and the pattern of errors provided strong evidence for an additive model of combination. Vestibular information had little or no effect on steering performance, suggesting that vestibular signals are not integrated with visual information for the control of steering at these speeds.

Does gaze influence steering around a bend

M. F. Land and D. N. Lee (1994) suggested that steering around a bend is controlled through the estimation of curvature using the visual direction of a single road feature: the tangent point. The aim of this study was to evaluate, using a simulated environment, whether the high levels of tangent point fixation reported by some researchers are indeed related to steering control. In the first experiment, gaze patterns were examined when steering along roadways of varying widths and curvatures. Experiment 2 investigated the effects of enforced fixation on steering, when gaze was directed to the road ahead at a range of lateral eccentricities, including the tangent point. All participants completed both experiments. Overall, there was no evidence for extensive tangent point fixation in the free-gaze experiment and enforced tangent point fixation did not result in more accurate steering. The present results seem to suggest that participants tend to steer in the direction of their gaze; hence, looking at the tangent point causes the driver to steer toward it. These results provide some support for the R. M. Wilkie and J. P. Wann (2002) model of steering, which proposes that drivers will direct their gaze toward points they wish to pass through.

Neural Systems in the Visual Control of Steering

Visual control of locomotion is essential for most mammals and requires coordination between perceptual processes and action systems. Previous research on the neural systems engaged by self-motion has focused on heading perception, which is only one perceptual subcomponent. For effective steering, it is necessary to perceive an appropriate future path and then bring about the required change to heading. Using function magnetic resonance imaging in humans, we reveal a role for the parietal eye fields (PEFs) in directing spatially selective processes relating to future path information. A parietal area close to PEFs appears to be specialized for processing the future path information itself. Furthermore, a separate parietal area responds to visual position error signals, which occur when steering adjustments are imprecise. A network of three areas, the cerebellum, the supplementary eye fields, and dorsal premotor cortex, was found to be involved in generating appropriate motor responses for steering adjustments. This may reflect the demands of integrating visual inputs with the output response for the control device.

Neural processing of imminent collision in humans

Detecting a looming object and its imminent collision is imperative to survival. For most humans, it is a fundamental aspect of daily activities such as driving, road crossing and participating in sport, yet little is known about how the brain both detects and responds to such stimuli. Here we use functional magnetic resonance imaging to assess neural response to looming stimuli in comparison with receding stimuli and motion-controlled static stimuli. We demonstrate for the first time that, in the human, the superior colliculus and the pulvinar nucleus of the thalamus respond to looming in addition to cortical regions associated with motor preparation. We also implicate the anterior insula in making timing computations for collision events.