The normal ratio between the amplitude of the sensory evoked potential (SEP) at the wrist on stimulating digits 1, 2, 3, and 5 was determined in 44 healthy adult subjects. The first digit had the larger amplitude, and the fifth digit the smallest SEP. The amplitude expresses the density of sensory innervation in each finger. The ratio between the amplitude of different fingers varied according to the age of the subject. The amplitude of the SEP from a digit innervated by the median nerve decreased in the elderly more than the SEP amplitude of the digit innervated by the ulnar nerve, probably because of a chronic compression in the carpal tunnel. The changes in the normal amplitude ratio can be applied to the topographic diagnosis of radicular and brachial plexus lesions if a fixed segmental sensory innervation of the hand is accepted. In 44 right handed subjects the amplitude of the sensory evoked potentials at the wrist was significantly larger in the left hand. This asymmetry of sensory innervation between hands could be physiological, and suggests a greater density of sensory innervation in the left hand of right handed subjects.
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