Velocity discrimination thresholds for flowfield motions with moving observers.

The visual flow field, produced by forward locomotion, contains useful information about many aspects of visually guided behavior. But locomotion itself also contributes to possible distortions by adding head bobbing motions. Here we examine whether vertical head bobbing affects velocity discrimination thresholds and how the system may compensate for the distortions. Vertical head and eye movements while fixating were recorded during standing, walking or running on a treadmill. Bobbing noise was found to be larger during locomotion. The same observers were equally good at discriminating velocity increases in large accelerating flow fields when standing or walking or running. Simulated head bobbing was compensated when produced by pursuit eye movements, but not when it was part of the flow field. The results showed that these two contributions are additive and dealt with independently before they are combined. Distortions produced by body/head oscillations may also be compensated. Visual performance during running was at least as good as during walking, suggesting more efficient compensation mechanisms for running.