ABNORMAL ACCESS OF AXIAL VIBROTACTILE INPUT TO DEAFFERENTED SOMATOSENSORY CORTEX IN HUMAN UPPER-LIMB AMPUTEES

We studied two human subjects with total deafferentation of one upper limb secondary to traumatic multiple cervical root avulsions. Both sub jects developed a phantom limb and underwent elective amputation of th e paralyzed, deafferentated limb. Psychophysical study revealed in eac h subject an area of skin in the pectoral region ipsilateral to the am putation where vibrotactile stimulation (VS) elicited referred sensati ons (RS) in the phantom limb. Positron emission tomography was then us ed to measure regional cerebral blood flow changes during VS of the pe ctoral region ipsilateral to the amputation with RS and during VS of a homologous part of the pectoral region adjacent to the intact arm wit hout RS. A voxel-based correlation analysis was subsequently used to s tudy functional connectivity. VS of the pectoral region adjacent to th e intact arm was associated with activation of the dorsal part of the contralateral primary somatosensory cortex (S1) in a position consiste nt with the S1 trunk area. In contrast, VS of the pectoral region ipsi lateral to the amputation with RS was associated with activation of th e contralateral S1 that extended from the level of the trunk represent ation ventrally over distances of 20 and 12 mm, respectively, in the t wo subjects. The area of S1 activated during VS of the digits in a nor mal control subject was coextensive with the ventral S1 region abnorma lly activated during VS of the ectopic phantom representation in the t wo amputees, suggesting that the deafferented digit or hand/arm area h ad been activated by sensory input from the pectoral region. Correlati on analysis showed an abnormal pattern of intrinsic connectivity withi n the deafferented S1 hand/arm area of both amputees. In one subject, the deafferented S1 was functionally connected with 3 times as many S1 voxels as the normally afferented S1. This abnormal functional connec tivity extended in both the rostrocaudal and ventrodorsal dimensions. The results demonstrate that sensory input delivered to the axial body surface may gain access to the S1 hand/arm area in some humans who ha ve suffered extensive deafferentation of this area. The findings are c onsistent with the hypothesis that deafferentation of an area of S1 ma y result in activation of previously dormant inputs from body surfaces represented in immediately adjacent parts of S1. The results also pro vide evidence that changes in functional connectivity between these ad jacent areas of the cortex play a role in the somatotopic reorganizati on.