Influence of a diagonal postural pattern on parameters of movement and associated postural adjustment in the cat

The aim of this study was to investigate the influence of initial posture on the latency and amplitude of a movement and the postural adjustment associated with it. The movement studied was a limb flexion induced by stimulation of the motor cortex in chronically implanted cats. Only weak stimulations were used and the motor activity was mainly isometric, which facilitated the quantification of responses, analysed by measuring vertical forces exerted at the level of each limb. The initial posture consisted of the diagonal force pattern induced by stimulation of another cortical site corresponding to a different limb. The choice of this cortical site depended on the desired direction of force changes for the initial posture, i.e. either in the same direction as those accompanying the movement or in the opposite direction. The conditioning stimulation leading to the initial posture was triggered only when the computer analysing postural parameters showed a stable and balanced posture. Within the limits of the experimental method utilized, the following results have been established: 1. The latency of onset of the test movement and its associated postural adjustment were independent of initial posture. 2. The peak forces recorded were constant for a given stimulation intensity, and independent of initial force values. This shows that the amplitudes of force variations depend on the initial values: in the case of a low initial value the force variation must be greater to reach the same extreme value as in the case of a high initial value. Thus, the same stimulus intensity can induce different motor activities depending on the initial conditions.