Is extra-retinal information needed to control steering of locomotion in presence of a rotational optic flow?

When one changes direction or turns the head while walking, the optic flow projected at the retina is a complex pattern that consists of a translational and a rotational flow. It is not clear whether the perception of rotational vs. translational components can be extracted from the retinal flow alone or whether extra-retinal cues such as vestibular and eye/neck proprioceptive information are required. We hypothesized that active head turns, which provide an alteration in rotational optic flow and extra-retinal cues, will induce little or no change in heading direction and body segment orientation while walking, as compared to visually simulated head turns that do not provide extra-retinal cues. Nine healthy young subjects were evaluated while walking overground and viewing a large virtual room in a head-mounted display. Subjects were instructed to walk straight in the physical world while being exposed to three conditions: (1) an active head turn; (2) a visually simulated head turn; and (3) a visually simulated head turn with no target element. All three conditions had identical retinal flow, so that when the subject reached 5m of forward walking, the rotation was either at 40deg to the right or left, or at 0deg. With the visually simulated head turn, however, no extra-retinal information was present. A 10-camera Vicon system was used to acquire horizontal orientations of the head, thorax, pelvis and foot segments, as well as the computed 3D trajectories of the bodypsilas center of mass (CoM). Heading direction and body segment orientations differed between right vs. left locations of focus of expansion for the simulated head turn conditions, but not for the active head turn condition, suggesting that extra-retinal information plays a role in the control of heading direction, in addition to optic flow changes induced by virtual reality. The small amplitude of the changes observed with the simulated head turn conditions suggests that the CNS has the ability to re-weight relevant sources of information in the presence of sensory conflicts.