EFFECT OF HEAD POSITION ON POSTURAL ORIENTATION AND EQUILIBRIUM

This study examined (1) how changes in head position affect postural o rientation variables during stance and (2) whether changes in head pos ition affect the rapid postural response to linear translation of the support surface in the horizontal plane. Cats were trained to stand qu ietly on a moveable platform and to maintain five different; head posi tions: center, left, right, up, and down. For each head position, stan ce was perturbed by translating the support surface linearly in 16 dif ferent directions in the horizontal plane. Postural equilibrium respon ses were quantified in terms of the ground reaction forces, kinematics , dynamics (net joint torques), body center of mass, and electromyogra phic (EMG) responses of selected limb and trunk muscles. A change in h ead position involved rotation of not only the neck but also the scapu lae and anterior trunk. Tonic EMG levels were modulated in several for elimb and scapular muscles but not hindlimb muscles. Finally, large ch anges in head orientation in both horizontal and vertical planes did n ot hamper the ability of cats to maintain postural equilibrium during linear translation of the support surface. The trajectory of the body' s center of mass was the same, regardless of head position. The main c hange was observed in joint torques at the forelimbs evoked by the per turbation. Evoked EMG responses of forelimb and scapular muscles were modulated in terms of magnitude but not spatial tuning. Hindlimb respo nses were unchanged. Thus, the spatial and temporal pattern of the aut omatic postural response was unchanged and only amplitudes of evoked a ctivity were modulated by head position.