Visual influences on vestibulospinal reflexes during vertical linear motion in normal and hemilabyrinthectomized monkeys.

In the present investigation, we have analysed the visually induced modulations of muscular responses during falls at different rates of acceleration and performed in five different visual conditions: Normal vision (NV), Darkness (D), Stabilized vision (SV), with visual motion cues being enhanced (EV), or reduced (RV). This study was conducted on normal and hemilabyrinthectomized baboons. EMG activities were recorded in the alert monkey from three pairs of muscles (splenius capitis, soleus and tibialis anterior). For testing, the monkey was seated in a special chair unexpectedly dropped by 0.9 m. Five peaks of maximum acceleration were used (8.8, 6.6, 4.4, 3.3, 2.2 m/s2). Conditions EV, SV and RV were tested by way of projector, the input of which consisted of the integral of vertical acceleration and output, the output of which controlled film motion. In the normal baboon the visually induced modulation of EMG responses in the SV, EV, and RV conditions was larger for slow falls than for fast ones. This modulation was direction-specific, at least for slow falls, and depended on the relative speed of the visual scene. Between certain limits, the energy of the responses was roughly proportional to the relative speed of the visual scene. These modifications were most accentuated in the splenius and soleus muscles. Condition D only produced a slight reduction of the EMG response. All these findings eliminate the possibility that the observed effects represents a startle response. Thus, we can conclude that there is a fact directional role of vision in postural control in the normal falling baboon. In the hemilabyrinthectomized animal, greater modulations were recorded only when the visual manipulations were performed during the first two postoperative weeks. This confirms the above results on the normal baboon and previous data on the role of vision in the recovery process.