archical level (Fig. 17.7). The preparatory processes would first preselect the circuits for the movement control and second make the selection of the appropriate postural circuits. The trigger stimulus then releases the movement and its associated postural adjustment. An extrastimulus such as a sound delivered during the preparatory period would mimic the effect of the trigger stimulus with both its components, that is, unloading of the moving limb and loading of the supporting limb.
A second hypothesis is that the pathways for movement control would have established a link with the networks of the appropriate postural circuits during the conditioning procedure. The preparatory process would preselect only the circuits for movement control. The trigger stimulus would then provoke the command of the prepared movement and, by the virtue of the links established during learning, would control the appropriate postural pattern in a feedforward manner. The tone as extrastimulus during the preparatory period would have the same effect as the trigger stimulus.
In both explanations, the postural pattern would be hierarchically dependent on the movement control system. In the hypothesis of a preselection of the appropriate postural pattern, there would be a possibility of a separate preparatory process acting on the postural control system in order to select the appropriate pattern. However, a separate control would be excluded in the hypothesis of a previous connection, established during learning, between the movement control pathways and the appropriate postural pattern.
The proposed wiring does not take into account the feedback control which comes into action after the command onset, nor the contribution of other brain structures such as cerebellum or basal ganglia. A participation of both structures was earlier proposed by the clinicians ( Babinski, 1899; Marsden et al., 1981; Martin, 1967) and their precise role should be further investigated.
This work has been supported by the INSERM grant No. 133025.
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