PURPOSE We reported previously that perceived optic flow speed is reduced linearly with walking speed (Thurrell et al. 1998 Perception 27 pp 147-148), a mechanism we postulate would preserve world constancy during normal self-motion. We consider here whether this reduction occurs as a consequence of attentional load or vestibular stimulation during walking.

METHODS Subjects viewed an optic flow reference stimulus for 15 s while walking on a treadmill at an instructed rate (i.e. very slow, slow... very fast). During this period, subjects were either a) given an auditory/verbal numerical summing task, b) rested their heads firmly using a bite-bar rigidly attached to the treadmill frame, or c) neither of the above conditions (i.e. control). Subjects then ceased walking and attempted to match the reference speed just perceived by adjusting the optic flow of the same pattern with a hand-held knob.

RESULTS Confirming previous findings, subjects made speed settings that indicated a perceived underestimation of optic flow during walking. The strength of the influence of walking velocity on perceived optic flow speed was slightly reduced compared with the control for both conditions a) and b), although neither reduction was significant statistically.

CONCLUSIONS The reduction in perceived speed of optic flow with walking velocity is not due primarily to the vestibular apparatus or to the modulation of attentional load. Indeed, in contrast to the results, an increased effect would be predicted in the case of the attentional task.