The need for speed: global optic flow speed influences steering

Leveraging a virtual alley with continuously varying width modulates step width variability during self-paced treadmill walking

Increased fall risk in older adults and clinical populations is linked with increased amount and altered temporal structure of step width variability. One approach to rehabilitation seeks to reduce fall risk in older adults by reducing the amount of step width variability and restoring the temporal structure characteristic of healthy young adults. The success of such a program depends on our ability to modulate step width variability effectively. To this end, we investigated how manipulation of the visual walking space in a virtual environment could modulate the amount and temporal structure of step width variability. Nine healthy adults performed self-paced treadmill walking in a virtual alley in a fixed-width Control condition (1.91 m) and two conditions in which the alley's width oscillated sinusoidally at 0.03 Hz: between 0.38 and 1.14 m and 0.38-2.67 m in Narrow and Wide conditions, respectively. The step width time series from each condition was evaluated using: (i) the standard deviation to identify changes in the amount of variability and (ii) the fractal scaling exponent estimated using detrended fluctuation analysis (DFA) to identify changes in the temporal structure of variability in terms of persistence in fluctuations. The Wide condition neither affected the standard deviation nor the fractal scaling exponent of step width time series. The Narrow condition did not affect the standard deviation of step width time series compared to the Control condition but significantly increased its fractal scaling exponent compared to the Control and Wide conditions, suggestive of more persistent fluctuations characteristic of a healthy gait. These results show that virtual reality based rehabilitative intervention can modulate step width variability to potentially reduce fall risk in older adults and clinical populations.

The Speed of Optic Flow Stimuli Influences Body Sway

Optic flow is a perceptual cue processed for self-motion control. The aim of this study was to investigate whether postural control is modulated by the speed of radial optic flow stimuli. The experiments were performed on 20 healthy volunteers using stabilometry and surface electromyography (EMG). The subjects were instructed to fixate a central fixation point while radial optic flow stimuli were presented full field, in the foveal and in the peripheral visual field at different dots speed (8, 11, 14, 17 and 20°/s). Fixation in the dark was used as control stimulus. The EMG analysis showed that male and female subjects reacted to the stimuli with different muscle activity (main effects for gender, muscle and laterality: p < 0.001). The analysis of the center of pressure (COP) parameters showed that optic flow stimuli had a different effect on the left and right limbs of males and females (main effects of laterality: p < 0.015; interaction effects of gender and laterality: p < 0.016). The low speed of optic flow stimuli (8 and 11°/s) evoked non-uniform directions of oscillations especially in peripheral stimulation in all subjects, meaning that optic flow simulating slow self-motion stabilizes body sway.

Optic Flow Speed and Retinal Stimulation Influence Microsaccades

Microsaccades are linked with extraretinal mechanisms that significantly alter spatial perception before the onset of eye movements. We sought to investigate whether microsaccadic activity is modulated by the speed of radial optic flow stimuli. Experiments were performed in the dark on 19 subjects who stood in front of a screen covering 135 × 107° of the visual field. Subjects were instructed to fixate on a central fixation point while optic flow stimuli were presented in full field, in the foveal, and in the peripheral visual field at different dot speeds (8, 11, 14, 17, and 20°/s). Fixation in the dark was used as a control stimulus. For almost all tested speeds, the stimulation of the peripheral retina evoked the highest microsaccade rate. We also found combined effects of optic flow speed and the stimulated retinal region (foveal, peripheral, and full field) for microsaccade latency. These results show that optic flow speed modulates microsaccadic activity when presented in specific retinal portions, suggesting that eye movement generation is strictly dependent on the stimulated retinal regions.

Lawful tracking of visual motion in humans, macaques, and marmosets in a naturalistic, continuous, and untrained behavioral context

We characterize spatiotemporal integration of naturalistic, continuous visual motion of three primate species (humans, macaques, and marmosets). All three species volitionally, but naturally, track the center of expansion of a dynamic optic flow field. Detailed analysis of this flow-tracking behavior reveals lawful and repeatable dependencies of the behavior on nuances in the stimulus, revealing that even unconstrained and continuous behavior can exhibit the sort of precise dependencies typically studied only in artificial and constrained tasks.

Much study of the visual system has focused on how humans and monkeys integrate moving stimuli over space and time. Such assessments of spatiotemporal integration provide fundamental grounding for the interpretation of neurophysiological data, as well as how the resulting neural signals support perceptual decisions and behavior. However, the insights supported by classical characterizations of integration performed in humans and rhesus monkeys are potentially limited with respect to both generality and detail: Standard tasks require extensive amounts of training, involve abstract stimulus–response mappings, and depend on combining data across many trials and/or sessions. It is thus of concern that the integration observed in classical tasks involves the recruitment of brain circuits that might not normally subsume natural behaviors, and that quantitative analyses have limited power for characterizing single-trial or single-session processes. Here we bridge these gaps by showing that three primate species (humans, macaques, and marmosets) track the focus of expansion of an optic flow field continuously and without substantial training. This flow-tracking behavior was volitional and reflected substantial temporal integration. Most strikingly, gaze patterns exhibited lawful and nuanced dependencies on random perturbations in the stimulus, such that repetitions of identical flow movies elicited remarkably similar eye movements over long and continuous time periods. These results demonstrate the generality of spatiotemporal integration in natural vision, and offer a means for studying integration outside of artificial tasks while maintaining lawful and highly reliable behavior.

Metacognitive judgements of perceptual-motor steering performance

Control of skilled actions requires rapid information sampling and processing, which may largely be carried out subconsciously. However, individuals often need to make conscious strategic decisions that ideally would be based upon accurate knowledge of performance. Here, we determined the extent to which individuals have explicit awareness of their steering performance (conceptualised as "metacognition"). Participants steered in a virtual environment along a bending road while attempting to keep within a central demarcated target zone. Task demands were altered by manipulating locomotor speed (fast/slow) and the target zone (narrow/wide). All participants received continuous visual feedback about position in zone, and one sub-group was given additional auditory warnings when exiting/entering the zone. At the end of each trial, participants made a metacognitive evaluation: the proportion of the trial they believed was spent in the zone. Overall, although evaluations broadly shifted in line with task demands, participants showed limited calibration to performance. Regression analysis showed that evaluations were influenced by two components: (a) direct monitoring of performance and (b) indirect task heuristics estimating performance based on salient cues (e.g., speed). Evaluations often weighted indirect task heuristics inappropriately, but the additional auditory feedback improved evaluations seemingly by reducing this weighting. These results have important implications for all motor tasks where conscious cognitive control can be used to influence action selection.

Steering bends and changing lanes: The impact of optic flow and road edges on two point steering control

Successful driving involves steering corrections that respond to immediate positional errors while also anticipating upcoming changes to the road layout ahead. In popular steering models these tasks are often treated as separate functions using two points: the near region for correcting current errors, and the far region for anticipating future steering requirements. Whereas two-point control models can capture many aspects of driver behavior, the nature of perceptual inputs to these two "points" remains unclear. Inspired by experiments that solely focused on road-edge information (Land & Horwood, 1995), two-point models have tended to ignore the role of optic flow during steering control. There is recent evidence demonstrating that optic flow should be considered within two-point control steering models (Mole, Kountouriotis, Billington, & Wilkie, 2016). To examine the impact of optic flow and road edges on two-point steering control we used a driving simulator to selectively and systematically manipulate these components. We removed flow and/or road-edge information from near or far regions of the scene, and examined how behaviors changed when steering along roads where the utility of far-road information varied. While steering behaviors were strongly influenced by the road-edges, there were also clear contributions of optic flow to steering responses. The patterns of steering were not consistent with optic flow simply feeding into two-point control; rather, the global optic flow field appeared to support effective steering responses across the time-course of each trajectory.