Effect of sensorimotor cortical ablation on the placing reaction in the standing cat.

The placing reaction in the standing cat permits the analysis of a movement, the placing movement, together with its postural support. It is provoked by one of two moving plates coming into contact with one forelimb. Each limb rests on a platform equipped with strain gauges which permit changes in force exerted by each limb to be recorded. The placing reaction is characterized by two phases, an early isometric phase lasting until the lift-off of the stimulated limb occurs. It is then that a diagonal postural support on one forelimb and the opposite hindlimb takes place so that the placing movement can take place. Performance of this movement corresponds to the second phase. Four cats underwent an unilateral lesion in sensorimotor cortex. Two animals had a large lesion extending to the pre- and post-cruciate regions and from the midline to the coronal sulcus, including a large part within the depth of that sulcus. Two other cats had a restricted lesion covering mainly the forelimb part of area 4. After extensive cortical lesions, permanent changes in the contralateral placing reaction were observed. The isometric phase increased in duration, without marked changes of the pattern of postural support whereas the movement was prolonged and usually hypermetric. On the contrary, ipsilateral placing with its associated postural support was unmodified. After restricted cortical lesions, only temporary changes were observed. Recuperation was complete within 30 postoperative days. It was concluded that motor cortex on one side controls, as a whole, the contralateral placing movement and its associated postural adjustment, whereas it does not significantly modify the ipsilateral placing and its associated postural responses notwithstanding the use of the contralateral limbs in this adjustment.