Electrophysiologic studies are useful for assessing peripheral nerve, muscle, dorsal column, and corticospinal tract involvement in HSP (Harding, 1993). These studies are particularly useful for characterizing the extent of involvement since autopsies are obtained infrequently. Although results of these studies are variable, a number of generalizations can be made. Most studies found nerve conduction studies to be normal (in contrast to Friedrich's ataxia and some other spinocerebellar ataxias) (Rosenberg, 1993). One study however, showed that subclinical sensory impairment was common in HSP, with involvement of peripheral nerves, spinal pathways, or both (Schady and Scheard, 1990). Lower extremity somatosensory evoked potentials (SSEP) show conduction delay in dorsal column fibers (Pelosi et al., 1991). Cortical evoked potentials used to measure neurotransmission in corticospinal tracts show greatly reduced corticospinal tract conduction velocity and amplitude of evoked potential (Claus et al., 1990; Polo et al., 1993; Schady et al., 1991; Pelosi et al., 1991). Often, there is no cortical evoked potential elicited in muscles innervated by lumbar spinal segments, but cortical evoked potentials of the arms are normal or show only mildly reduced conduction velocity. These findings indicate that there are decreased numbers of corticospinal tract axons reaching the lumbar spinal cord and that the remaining axons have reduced conduction velocity. Schady et al (Schady et al., 1991) emphasized the variable results of cortical evoked potentials. Central motor conduction velocity in the upper extremities was normal except for all 5 (affected) members of one HSP kindred for whom responses were considerably delayed. They conclude that measurement of central motor conduction velocity may be a useful way of identifying clinical subgroups of HSP.