Gaining Insight by Going in Circles

PKAR appears to be an adaptive phenomenon that results from a remodeling of the rotational relationship between the trunk and the feet. Figure 4 illustrates the basic mechanism of PKAR. During walking on the rotating disk, the feet are traveling a curved path while the trunk and head remain stable relative to space. During podokinetic stimulation on a clockwise-rotating disk, the feet are walking along a counterclockwise trajectory. The feet are rotating relative to the space-stable trunk by virtue of their contact with the moving surface during stance [Figure 4(a)]. This rotation of the feet under the trunk is accompanied by visual and vestibular signals that indicate fixity relative to space. When the subject now attempts to walk straight across the floor without vision, the relative rotational relationship between the feet and trunk is maintained as the system is newly calibrated to reflect the fact that rotation of the feet relative to the trunk has been necessary to maintain a space-stable heading. However, instead of the feet rotating under the trunk, the trunk now rotates over the feet, which are stable relative to space during stance by virtue of their contact with the stationary surface [Figure 4(b)]. The result is PKAR in the counterclockwise direction; i.e., the same direction that the feet were traveling when walking on the clockwise-rotating disk. This change in the rotational relationship between the feet and the trunk is accompanied by a change in perception, as subjects have no sensation that they are turning in space during PKAR. Podokinetic adaptation is mediated, at least in part, by somatosensory information that is integrated within central nervous system structures. The cerebellum, in particular, is important in regulating the amplitude of this adaptive response. The amplitude of PKAR corresponds to the degree of path curvature produced, as a higher initial velocity results in a more tightly curved trajectory. This adaptable control of general locomotor trajectory via the podokinetic system is likely essential for successful navigation in everyday environments. The podokinetic system can be used not only to adapt locomotor trajectory but to gauge how far one has rotated during active, intentional turning [14]. Many questions remain to be answered about the nature of the podokinetic system and its interactions with other inputs, especially those from the visual and vestibular systems.