Complex Podokinetic (PK) Response to Post-rotational Vestibular Stimulation

Recent studies identified an adaptive "Podokinetic" (PK) sensory motor system involved in sensing and controlling spatial orientation during locomotion, by referencing body orientation to the space-stable stance foot. This paper investigates the interaction of vestibular and PK systems by asking blindfolded subjects to 'step-in-place' (i.e. without turning) after exposing them to a unidirectional post-rotational vestibular stimulus. Six of the nine subjects consistently began by vigorously propelling themselves round in the direction of preceding turntable rotation, but notably without any sensation of turning. In all these subjects the speed of this PK-induced rotation progressively declined to zero over about the next 30 sec and then reversed direction with increasing speed for about 50 sec. Thereafter the speed of rotation declined slowly to zero over the next 4 to 5 minutes. Since the PK-generated body rotation presumably feeds back into the vestibular-PK drive, we formulated a closed loop model of the combined system to investigate the complex nature of the behavioral response. The simulated response of this model closely resembled the experimental data, suggesting that there is indeed a functionally closed loop operating between the vestibular and podokinetic systems in natural life.