The need for speed: global optic flow speed influences steering.

Georgios K Kountouriotis,Richard M Wilkie,Natasha Merat,Callum D Mole

doi:10.1098/rsos.160096

Georgios K Kountouriotis, Richard M Wilkie + Show 2 more

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https://doi.org/10.1098/rsos.160096

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Abstract

How do animals follow demarcated paths? Different species are sensitive to optic flow and one control solution is to maintain the balance of flow symmetry across visual fields; however, it is unclear whether animals are sensitive to changes in asymmetries when steering along curved paths. Flow asymmetries can alter the global properties of flow (i.e. flow speed) which may also influence steering control. We tested humans steering curved paths in a virtual environment. The scene was manipulated so that the ground plane to either side of the demarcated path produced larger or smaller asymmetries in optic flow. Independent of asymmetries and the locomotor speed, the scene properties were altered to produce either faster or slower globally averaged flow speeds. Results showed that rather than being influenced by changes in flow asymmetry, steering responded to global flow speed. We conclude that the human brain performs global averaging of flow speed from across the scene and uses this signal as an input for steering control. This finding is surprising since the demarcated path provided sufficient information to steer, whereas global flow speed (by itself) did not. To explain these findings, existing models of steering must be modified to include a new perceptual variable: namely global optic flow speed.

Highlights

Animals routinely move through the world by following demarcated paths, trails or runways [1,2,3]
There are suggestions that the visual strategies employed by drivers are an important component of steering [21]; the literature has not considered the relative contribution of flow asymmetries and global flow speed when steering down demarcated paths
If participants attempted to steer in a way that reduced asymmetries and made the flow vectors either side of the path more equal, we would expect: (i) trajectories to be biased towards the slower moving region, and (ii) the magnitude of steering biases to reflect the difference between the speeds of the two regions

Summary

Introduction

Animals routinely move through the world by following demarcated paths, trails or runways [1,2,3]. Experiments conducted on humans travelling at walking speeds down a straight corridor [13,14,15,16,17] have shown that an asymmetric flow field (created by moving one corridor wall) can cause participants to adopt a trajectory closer to the slower moving wall. This finding has been explained in terms of humans attempting to optically equalize the differences in the flow vector speeds between the left and right visual fields. Any reference to path solely relates to the former definition of a demarcated path with visible edges (figures 1 and 2)

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Results

Conclusion