Tilt-table/ankle-actuator system for the study of vestibulospinal reflexes

The vestibular system is thought to influence human postural control both by means of direct effects on motoneurons and by indirect interactions with somatosensory reflexes. Experimental investigation of vestibulospinal effects therefore requires the independent, dynamic manipulation of both somatosensory and vestibular inputs. The paper describes a system developed to provide this capability for human studies. It consists of a tilt table, whose angle of tilt is controlled by a linear hydraulic actuator, on which is mounted a rotary hydraulic actuator. Subjects are strapped to the table with the left foot connected to the rotary actuator. Both actuators are operated as position servos, allowing vestibular input to be controlled by dynamically varying the table's angle of tilt and somatosensory input to be controlled by manipulating the angular position of the ankle with the rotary actuator. The ankle actuator has a frequency response which is flat up to 25 Hz but can apply stimuli with power adequate for system identification purposes to more than 50 Hz. Similarly, the table frequency response is flat to 3 Hz but generates inputs containing significant power up to 15 Hz. The independent dynamic control of both vestibular and somatosensory inputs afforded by the tilt-table/ankle-actuator device makes possible the application of multiple input/output engineering systems analysis techniques.