NATURAL STIMULI EVOKING SOMATOSENSORY POTENTIALS

NATURAL STIMULI EVOKING SOMATOSENSORY POTENTIALS A. Starr, H. Pratt,* and D. Burkef Department of Neurology University of California, Irvine Irvine, California 9271 7 The definition of somatosensory evoked potentials has customarily employed percutaneous electrical stimuli to activate mixed nerves, such as the median nerve a t the wrist, or sensory nerves, such as the digital nerves at the fingers. The use of a natural stimulus such as touch or pressure to evoke potentials has been utilized infrequently.’-’ An advantage of a natural stimulus is that it provides information as to the function of the receptor and its terminal nerve fibers whereas electrical stimulation of nerve trunks bypass these areas. Secondly, both the type of neural elements and the central pathways activated are relatively specific with a natural stimulus compared to electrical stimulation of nerve trunks. A major disadvantage of a natural stimulus is that complex procedures are often required for its quantification. Moreover, since natural stimuli usually activate a restricted number of neural elements, the amplitude of their evoked potentials may be less than those evoked by electrical stimulation. Over the past few years we habe had the opportunity to work on this problem and have utilized two types of natural stimuli: ( I ) mechanical deformation of the skin of the finger that feels like a tap,8-” and (2) flexion or extension of the ankle joint to activate muscle spindle receptors in the gastrocnemius or tibialis anterior muscle^.'^ These natural stimuli were capable of evoking potentials at several levels of the somatosensory pathway. Tactile stimuli elicit clear potentials at many levels of the somatosensory pathway but are of low amplitude compared to the potentials evoked by electrical stimulation of nerve trunks (FIGURE 1). Their scalp distributions are comparable” as are the effects of stimulus rate.“ Preliminary studies using tactile stimulation were completed in neonates and showed clear potentials at several levels of the pathway.’* However, cortical potentials could only be detected in 2 of 10 newborn infants compatible with a maturational delay in cortical circuits in youngsters of this age. Conduction times along the sensory pathway to natural tactile stimuli can be For instance, in calculated and used to assess abnormalities in clinical pop~lations.’~ an unpublished study of diabetic and uremic patients with peripheral neuropathy, we (Starr and Pratt) found the conduction time from mechanical stimulation of the index finger to the Occurrence of the evoked potentials recorded over the median nerve to be abnormal in 8 of 1 1 patients with uremia but normal in the 7 patients with diabetes. However, in both groups of patients proximal afferent conduction times between the wrist and axilla from electrical stimulation of the digital nerves were consistently abnormal (10/11 uremic patients; ti/7 diabetic patients). This finding would suggest *Present address: Gutwirth Bldg., Technion. Israel Institute of Technology, Haifa 32000, Israel. ?Present address: Unit of Clinical Neurophysiology, The Prince Henry Hospital, Little Bay, N.S.W. 2036 Australia. 0077-8923/S2/0388-0702 S1.75/0 Q 1982. NYAS