ABSTRACT
Movements are accompanied by a postural adjustment in human 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 adjustment. Are these two components part of the same “central program”? How are these two components prepared and at which hierarchical level is the preparation observed?
Cats standing on four force platforms were trained to perform a conditioned movement of one forelimb to a discontinuous tone. Prior to this discontinuous tone, a continuous tone was delivered during which the cat was required to stand still (preparatory phase). The possibility that movement and/or postural adjustment were prepared during that phase was checked by a postural perturbation (dropping of a hindlimb platform) or by an extrastimulus (short tone of different frequency). Myographic activity of forelimb triceps and biceps as well as force changes were recorded. The results from dropping the platform showed no evidence of preparatory changes at the motoneuronal level. Nevertheless the extrastimulus initiates both postural adjustment and movement, and sequence being uncompleted.
The results are in favor of the hypothesis that the preparatory phase makes the “central program” with its two components ready for its execution without changing markedly the excitability of the “output stage.”
