Unintentional modulations of human gait by optical flow

Visual whole-field motion is known to trigger motor responses which minimize retinal slip (VOR, OKN and control of balance). In locomotion, however, the retinal slip is utilized to control the velocity and direction of displacement. The present experiment was aimed at determining how the velocity of optical flow affects the regulation of locomotion. Unintentional modulations in velocity, stride length and cadence were analyzed using a task in which artificial optical flow gave the subjects the impression they were walking at a different speed than they actually were. Slight but systematic modifications in locomotion were observed: experimental variation of the optical flow resulted in a decrease in stride length. None of the subjects were aware of this decrease, despite the fact that their muscular and articular afferences provided them with supraliminal information. Although visual flow velocity is usually a direct consequence of walking velocity, experimental modifications of visual flow were found here to cause unintentional modulations in locomotor parameters (stride length and cadence) more than in their product (velocity).