[Body sway stabilization induced during periodic saccades (author's transl)]

Periodic saccadic eye movement has been known to cause a reduction of the body sway while standing upright in man. Then, it was studied that under what experimental conditions such a well-balanced posture could be obtained; with special references to antigravity muscle activity of the lower leg, saccades in the dark, and saccade size and frequency. Body sway was measured as the antero-posterior deviation of the center of gravity. EOG (electro-oculogram) and EMG of soleus, gastrocnemius and tibialis anterior muscles were simultaneously recorded. The results were as follws: 1) The sway reduction was observed during periodic saccades performed in frequency higher than 0.2 Hz at 30 degrees, in amplitude greater than 5 degrees at 0.5 Hz and in duration longer than a few seconds without direction specificity. 2) The effect of saccades on the body sway was classified into three types, each of which was specific to the subject. In type I and II the reduction of the body sway occurred for the component of 0.2-0.4 Hz. The sway reduction was accompanied with the increase in activity of soleus and gastrocnemius. The sway reduction was also accompanied with a forward shift of the body position in type I, but not in type II. Type III showed active discharges in soleus and gastrocnemius before saccades, suggesting that the body was being already inclined during fixation of eyes. In this type no sway reduction and no continuous shift of the body position were observed. 3) The sway reduction was observed also during periodic, voluntary rapid eye movement in complete darkness. 4) Continuous eye rotation in either direction did not affect the body sway. These results suggest that some neural activty related to the saccade may flow in the extrapyramidal system, raised the activity of triceps surae muscle directly or indirectly and stabilize the standing posture.