Contribution of vision to muscle responses in monkey during free-fall: visual stabilization decreases vestibular dependent responses.

In a previous study the muscle responses from the lower limbs were studied in the Baboon (Papio-Papio) during sudden falls. On the other hand, recent findings concerning the role of vision in the control of posture during rapid perturbations in man have stimulated the present investigation. EMG activities were recorded from the fully conscious animal using chronic electrodes implanted in various muscles (splenius, quadriceps femoris, soleus, and tibialis anterior). For testing, the monkey was seated in a special chair suspended from an electromagnet and unexpectedly dropped 90 cm. EMG responses were recorded in three randomly presented conditions: with normal motion of visual world (N), with visual world stabilized with respect to the head (S) using a box covered inside by a black and white checkboard pattern surrounding the animal's head, and in total darkness (D). Results showed that condition S is accompanied in all tested muscles by a significant decrease in the EMG response; this effect is particularly evident in the interval 60--120 ms but may occur earlier. Condition D is in most of the cases accompanied by a less important decrease which is situated between condition N and condition S. These results are in contrast to previous concludions of other authors which negated the role of vision in similar situations.