Data from: The need for speed: global optic flow speed influences steering

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.动物如何沿划定路径行进？不同物种均对光流（optic flow）敏感，其中一种转向控制策略是维持视野内的光流对称性平衡；但目前尚不清楚动物沿弯曲路径转向时，是否能感知不对称性的变化。光流不对称性可改变光流的全局属性（即光流速度），而这也可能影响转向控制。我们在虚拟环境中开展了人类受试者沿弯曲路径转向的实验，通过操控场景使划定路径两侧的地面平面在光流中呈现出更大或更小的不对称性。在不考虑不对称性与运动速度的前提下，我们调整场景属性，使全局平均光流速度产生快慢变化。实验结果表明，转向行为并未受光流不对称性变化的影响，而是对全局光流速度产生响应。据此我们得出结论：人类大脑会对全场景的光流速度进行全局平均，并将该信号作为转向控制的输入依据。这一发现颇为出人意料：划定路径本已提供了足够的转向信息，而单独的全局光流速度却并未做到这一点。为解释该实验结果，现有的转向控制模型需要进行修正，新增一项感知变量：即全局光流速度。