Few intrinsic connections cross the hand-face border of area 3b of new world monkeys

After long-standing loss of afferents from the hand, the hand representation in area 3b of the somatosensory cortex of monkeys becomes responsive to touch on the face. Because the reactivation of deprived hand cortex by the face inputs could depend on axonal connections across the hand-face border, we determined the extent of such connections in New World marmosets, owl monkeys, and squirrel monkeys. Small injections of anatomic tracers were placed in the hand or the face representations after these representations were identified by microelectrode recordings. The positions of retrogradely labeled neurons were plotted in processed brain sections cut parallel to the brain surface, and their locations were related to anatomic isomorphs of the hand and face representations revealed in adjacent brain sections stained for myelin. In these sections, the hand-face border was clearly visualized as a myelin-poor septum. The intrinsic connections of area 3b labeled by injections in either the hand or face representations were almost completely confined to their respective representation, and very few neurons projected across the border. In addition, neurons in the somatosensory thalamus labeled by injections in either face or hand representations were confined to either VPM, representing the face, or the hand subnucleus of VPL. Thus the reactivation of hand cortex by face stimulation does not depend on a previously existing network of intrinsic cortical connections across the hand-face border, or mismatched thalamocortical projections.