Visual Strategies for the Control of Locomotion

The control of locomotion takes place at three levels: posture (stabilization and orientation), steering (obstacle avoidance and goal-seeking), and navigation (taking routes to unseen goals). Here I explore three strategies for steering. The Pictorial Strategies use splay and image height information and are well-known. The Heading Strategy, as proposed 40 years ago by J. J. Gibson, places the perceived heading on gaps and goals in the environment. A third strategy is the Equalization Strategy---steering to equate the magnitude of optic flow on each side of the heading. It has recently been found that bees use this latter strategy and it has also been implemented in robots. I present experiments testing for the presence of each of these strategies in a long corridor, a corridor with an obstacle and a target, and situations with only a target. The patterns of path data indicate that all three strategies are engaged by subjects. I then model those conditions where the Equalization and Heading Strategies appear to dominate. A simulated agent that can detect optic flow is run through the same conditions as the subjects. If the agent simply sums the outputs of each strategy, it follows paths similar to the subjects in a number of experiments. These are some of the first experiments indicating that humans do actually use optic flow to steer.

Movies:
 
(Caveat Observator: These are large quicktime files.  Frames are dropped.  If it shows up in your browser, press the right mouse button, or the space bar to start the movie. They are generally run at about 50% of real speed, so if you double the rate on your player, it should look better and closer to actuality.  Parameters used in these movies are only approximately those used in the actual experiments and simulations.  These may crash weaker systems.)
 

Chapter 4.  Subject moving down open corridor (2.5Mb, 5.6Mb):  

red-cross = perceived heading
black/white square = actual heading
blue square = goal point