From: The World In Your Head by Steven Lehar

Fig. 10.6 A planned trajectory is perceived amodally as a snakelike trail, like an amodal projection of the body profile forward into perceived space, with the proper perspective applied, filled in with a periodic pattern of planned footsteps. The negative valence occasioned by a perceived obstacle in the path causes (A) the planned trajectory to deviate smoothly around that perceived obstacle, or (B) the periodicity of the stepping pattern to be modulated so as to step over the obstacle.