situation of Fig. 17.9) and the cat starts falling when dropping of that hindlimb takes place.
As a whole, this exploration of the preparatory period by two different stimuli suggests that during that period there is a situation where movement and the appropriate postural adjustment are ready to be executed. This preparatory state is revealed when an extrastimulus, a sound and not a postural perturbation, is given. In addition, response to sound is stable only when associated with dropping of either hindlimb platform. The response that is seen involves both aspects of the movement being prepared, that is unloading of the left forelimb (moving limb) and loading of the right forelimb (postural limb), with both effects starting approximately at the same time (50 ms after the onset of the tone).
Movements are accompanied by a postural adjustment in humans as well as in quadrupeds. The postural adjustment is controlled in a feedforward manner together with movement. It minimizes the postural and equilibrium perturbation that the movement provokes.
The following questions were raised for a motor act involving two components, the movement and its associated postural ad adjustment. How are these two components prepared? Is movement alone prepared, or is it prepared in conjunction with posture, and at which hierarchical level is the preparation observed?
The use of a sound as an extrastimulus during the preparatory period demonstrated that a preparatory process has taken place and that according to myographic and force traces both movement and postural adjustment were included in the preparatory process.
The changes that are taking place during the preparation of movement and associated postural adjustment are not observable under our experimental conditions at the motoneuronal level, that is the final common pathway to both conditioned response and response to drop as a perturbation. This is shown by the unchanged forelimb myographic and mechanical responses to drop of a hindlimb platform, during the preparatory period. Thus, the preparatory process should take place somewhere upstream of the motoneuronal level.
Two interpretations of the preparatory processes occurring in our experimental paradigm might be proposed, both being based on the organization of the postural adjustment in the execution pathways for movement (Fig. 17.7). From motor cortical or red nucleus stimulation experiments in the cat, it was suggested that the networks responsible for the postural adjustments are localized at a low hierarchical level, possibly bulbospinal, where a repertoire of different patterns would exist.
A first explanation is based on the hypothesis that the command pathways for movement would have access to a repertoire of postural patterns at a low hier-