The OPTIC FLOW & LOCOMOTION project (1996 - present)

Acronym: OPTIC FLOW
Name: Optic flow and Locomotion
Type: Collaborative project
Funds: French Institut Universitaire de France - European Integrated Project SKILLS
UM1 key researchers: Benoît Bardy – Perrine Guerin (PhD student)
Collaborators: Bill Warren (Brown Univ., USA)

OPTIC fLOW is a theoretical and experimental project demonstrating the powerful role of optical transformation during egomotion (optic flow) in the regulation of postural and locomotor variables.

Theory and past research:

As described by Gibson (1950), movement of the observer through a rigid environment generates optic flow patterns that are specific to self-motion. Considered on a spherical projection surface surrounding the observer, translation on a straight path produces a radial flow pattern with a focus of expansion in the direction of self-motion, grading into lamellar (parallel) flow in the perpendicular direction, and finally a focus of contraction in the opposite direction. To keep balance during locomotion, observers must make adaptive postural adjustments while maintaining forward progression.  As in the case of standing posture, optic flow at the eye of a walking observer contains information that specifies the appropriate compensatory responses.  By manipulating a visual “driver” display for an observer on a treadmill, we have shown that both radial flow and motion parallax information are used to control postural sway during walking (Bardy, Warren, & Kay, 1996; Warren, Kay, & Yilmaz, 1996).  In general, compensatory sway is functionally specific to the specified disturbance, that is (a) directionally specific or in the direction specified by the flow pattern, (b) isotropic or of comparable amplitude in all driver directions, (c) strongly coupled to the visual driver with a high cross-correlation in all directions, and (d) independent on the retinal region stimulated (Bardy, Warren, & Kay, 1999).  

 Ongoing research:

In our current research (Guerin & Bardy, 2008), we evaluate the stabilizing role of optic flow on locomotor patterns at the preferred walk-run transition speed (PTS). We measure both the kinematics of gait (Oxford Metrics Vicon system, 50 Hz), and the energy expenditure (ZAN 680) of walking participants in a virtual environment in order to investigate the specific relation between information (optical flow), locomotion (pattern stability) and efficiency (energy consumption). In our experiments, participants are instructed to walk on a treadmill in the dark, in front of a large screen (2.70 m x 1.95 m), on which three-dimensional optics flows are projected (a virtual corridor). The walk-run transition speed is computed for each participant and the treadmill speed is set up at TS. Various conditions of optic flow are tested: static or no flow (N), optic flow velocity consistent with the locomotion speed (C), optic flow velocity twice faster (F) or slower (S) than the locomotion speed.

Our results reveal that optical flow influences exist when locomoting in an unstable regime, i.e., at PTS. For slow optical flow, a stabilizing effect was observed on most of the locomotor variables. For fast optical flow, we found functional adaptations of locomotor parameters, resulting in a more efficient behavior with a gain in energy consumption up to 10%. These results suggest the interdependency of many sub-systems that are traditionally investigated separately, such as the visual system, the motor system and the cardio-respiratory system. 

Key references (downloadable version in page Vitae):

1. Bardy, B. G., Warren, W.H., & Kay, B. (1996). Motion parallax is used to control postural sway during walking. Experimental Brain Research, 111, 271-282.
2. Bardy, B. G., Warren, W.H., & Kay, B. (1999).The role of central and peripheral vision in postural control during walking. Perception & Psychophysics, 61, 1356-1368.
3. Guerin, P., & Bardy, B. G. (2008). Optical modulation of locomotion and energy expenditure at preferred transition speed. Experimental Brain Research, 189, 393-402.