Adaptive modification of the cat's vestibulospinal reflex during sustained vestibular and neck stimulation.

In decerebrate cats, rotation about the longitudinal axis of the whole animal at 0.15 Hz, +/- 10 degrees produced an increased electromyogram (EMG) activity of the triceps brachii during side-down tilt and a decreased activity during side-up tilt. This vestibulospinal reflex (VSR) was tested before, during and after a sustained (3-h) period of roll tilt of the head at the parameters indicated above, associated with a synchronous roll tilt of the body at 0.15 Hz, but at the peak amplitude of either 12.5 degrees or 7.5 degrees. This additional stimulus led to 2.5 degrees of neck rotation, which was respectively out of phase (condition A) or in-phase (condition B) with head rotation. In a few instances the peak amplitude of neck rotation was increased to 5 degrees. In the first experimental condition A, the gain of the VSR (tested every 10-15 min) progressively increased, starting from the first hour of out of phase neck-vestibular stimulation to reach, on average, 241% of the control value at the end of the third hour of stimulation. On the other hand, in the second experimental condition B, the mean gain of the VSR first decreased to 82% during the first hour of in-phase neck-vestibular stimulation, but then increased to 165% of the corresponding control during the last hour of recording. In other experiments an adaptive increase in gain of the pure VSR occurred during a sustained (3-h) period of selective roll tilt of the whole animal, but it was less consistent and, on average, smaller in amplitude than that obtained during out of phase neck-vestibular stimulation. The adaptive changes in gain of the VSR described above were not associated with changes in the phase angle of the responses, and were also observed during the post-adaptation period. Further experiments indicated that the gain of the N-VSR, i.e. of the EMG responses to combined neck-vestibular stimulation, displayed a prominent adaptive increase during the sustained out of phase stimulation, but not during the in phase stimulation.