LAUSR

Reductions in sensory and motor activity following unilateral upper limb amputation during adulthood are associated with widespread, activity-dependent reorganization of the gray matter and white matter through the central nervous system. Likewise, in cases of congenital limb absence there is evidence that limited afferent or efferent activity affects the structural integrity of white matter pathways serving the affected side. Evidence that the structural integrity of mature sensory and motor tracts controlling the lost upper limb exhibits similar activity-dependence is, however, sparse and inconsistent. Here we used diffusion tensor tractography to test whether amputation of the dominant right hand during adulthood (N=16) alters the microstructural integrity of the major sensory (medial lemniscus, ML) and motor (corticospinal tract, CST) pathways controlling missing hand function. Consistent with prior findings, healthy controls (N=27) exhibited higher fractional anisotropy (FA) -an index of white matter microstructural integrity- within dominant left CST and non-dominant right ML. Critically, in contrast to what might be expected if the microstructural organization of these tracts is activity-dependent, these asymmetries persisted in amputees. Moreover,we failed to detect any differences in dominant left ML or CST between healthy controls and amputees. Our results are consistent with these white matter tracts being robust to changes in activity once mature, or that continued use of the residual limb (in a compensatory fashion or using prostheses) provides stimulation sufficient to maintain tracts integrity.