garding the expected movement direction was found to augment preparation and lead to more rapid RTs. Associated with preparation at this stage, correlative changes in neuronal activity were consistently found to focus on structures included in the cerebello-cortical pathway and especially in the motor cortical areas where they are well documented.
Lastly, varying the expected force was the only factor that we found that modulated the state of the spinal motoneuron pools to be activated during movement execution. However, another unexpected process that intervenes by decreasing the reactivity of structures responsible for the final stage of motor organization was also observed. It may be related to the requirement that prepared actions, especially in the RT paradigm, must be actively postponed until the response signal occurs. A relatively unspecific filtering of the peripheral afferents to the motor structures to be involved in movement execution would protect these structures from disturbances until the movement will actually be performed. This process should probably be viewed as a member of a more extended class of "pretuning" processes which more or less specifically modulate sensory inputs during the different stages of motor organization.
As a concluding remark, it should be noted that the strict "seriality" and "hierarchical" organization postulated by the model, especially when its structural features are considered, would have to be discussed and could certainly be challenged. For instance, experimental data clearly emphasized the role played by the set of "motor" cortical areas, where converging preparatory effects were found to have influence. This suggests that motor cortex is a crucial structure in motor preparation and, as such, these findings conform well with the emerging hypothesis of various functions presumably played by this structure. Motor cortex is, at the same time, (a) a target for central influences coming from the associative areas and for peripheral inputs coming from muscle receptors, (b) the terminal of pathways responsible for the specification of movement features, and (c) the command center triggering orders that control spinal motoneurons (see Evarts, 1981). This strategical position probably makes the motor cortex responsible for integrating complex functions, such as those implicated in the setting up of preparatory adjustments for motor activity.
This work was supported by a grant from C.N.R.S./A.T.P. and by a grant from I.N.S.E.R.M. (Contrat de Recherche libre n′ 826016). We are indebted to C. Anger, B. Arnaud, C. Frenck, A. Semjen, and N. Vitton for their help in various aspects of the studies reported in this paper.
Allen G. G., & Tsukahara N. Cerebro-cerebellar communication systems. Physiological Review, 1974, 54, 957-1006.