The direction in which an observer is locomoting is indicated by patterns within the flow-field. Since Gibson, research on the visual guidance of locomotion has rested on the assumption that humans rely on such flow-derived information to guide action. Computational modelling, psychophysics, neurophysiology and imaging work concentrated on identifying flow field invariants, documenting human sensitivity, and hypothesizing mechanisms and neural areas implicated in the processing of flow. But, the flow assumption remained untested. Several years ago, when working with a patient with unilateral visual neglect I was provoked to directly test this assumption.
Recent work extends the conclusion to the conceptually similar task of projectile interception.
I will describe the original locomotion study along with replications, extensions and critiques.