The measurement of somatosensory evoked potentials(SEPs)is a basic clinical technique used for functional mapping and monitoring of brain and spinal cord responses during surgery. Since the potential evoked by a single stimulus is smaller compared to the surrounding electrical activity(background brain activity and/or electromagnetic artifacts), the average measurement of responses to multiple controlled stimuli across time-locked trials must be taken to determine the resultant waveform. SEPs can be analyzed in terms of polarity, latency from stimulus onset, or the amplitude from the baseline for each waveform component. The amplitude is used for monitoring purposes, while the polarity is used for mapping purposes. For example, an amplitude 50% lower than that of the control waveform may suggest the presence of significant influence on the sensory pathway, whereas a phase reversal in polarity, established by a cortical SEP distribution, usually indicates localization to the central sulcus.