Influence of optic flow stimuli on postural response. I. Electromyography

Introduction The direction of optic flow stimuli is an important cue for heading perception (Gibson, 1954). Several studies addressed the influence of visual stimuli on upright position, because vision provides the necessary information about the surrounding environment (Peterka & Benolken, 1995). However, little is known about the optic flow modulation on postural muscles. Aim of this study was to verify if the optic flow direction evokes different postural responses. We analyzed the bilateral muscular activation of two leg muscles, tibialis anterior and gastrocnemius medialis, during full field optic flow stimulation. Methods sEMG were recorded in 24 healthy right-handed volunteers (12 M and 12 F, mean age 24.5±2.9). Experiments were performed in the dark. Stimuli were presented on a wide screen, placed 115 cm from the subjects’ eyes, covering 135 x 107° of visual field. Stimuli were two full screen optic flow, expansion and contraction, and random dots motion as control stimulus. sEMG data were acquired by a 16 channels Pocket EMG (BTS® Bioengineering Inc.). Data acquisition included 5 trials per stimulus for about 30-s each. Results The muscle was considered significantly activated when its signal was greater than the baseline mean + 3SD (Hodges & Bui, 1996). Results showed that the great majority of the subjects had a different modulation on muscles activity depending on the type of optic flow stimulus. The right tibialis anterior was the most activated muscle. We also analyzed the highest amplitude values to evaluate if optic flow differently modulated the amplitude of the muscle responses. Results showed a significant effect (p<0.05) of the stimulus in 14 subjects (58%). The stimulus effect on lower limb muscles was significantly different in 23 subjects (96%). We also found a strong effect of sex and muscle and interaction of sex by muscle upon the optic flow stimulus effect (p<0.05). Discussion Visual input influences the neural control of body sway. One emerging interesting idea is that neural sensitivity to different afferent inputs in response to disturbances of the body balance, is highly dependent on the visual feedback. This theoretical framework may help us to understand fall prevention programs and to explain some of the mechanical and neural factors which contribute to the balance impairmens. In these experiments subjects likely used an ankle strategy to maintain standing posture. Forward and backward sway may result in an increased compensatory muscular activity of the posterior and anterior lower extremity muscles respectively. References Gibson JJ (1954). Psychol. Rev., 61, 304-314. Hodges PW & Bui BH (1996). Electroencephalogr Clin Neurophysiol, 101, 511-519. Peterka RJ & Benolken MS (1995). Exp Brain Res, 105, 101-110.