The formation of finger grip during prehension in an acallosal patient.

The grasping behaviour of an acallosal patient is compared with that of a group of normals. Video records of the hands of the patient and the normals allowed frame by frame analysis of the prehensile movements involved. The normals behaved according to the earlier reports by Jeannerod [Behav. Brain Res. 19, 99-116, 1986; and in Two Hemispheres--One Brain, F. Leporé, M. Ptito and H. H. Jasper (Editors), pp. 369-383, Alan R. Liss, Inc., New York, 1986] with the thumb and forefinger closing upon the object to be grasped by the time it was reached. By contrast the acallosal did not begin to close the finger and thumb until after contact was made with the object. The results are seen as further evidence for (a) the enhanced development of uncrossed ipsilateral pathways in acallosals and (b) absence in acallosals of the normal inhibitory action of the callosum in suppressing such enhanced ipsilateral output which consequently competes with the contralateral output controlling the fingers.