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Somatosensory evoked potential
Somatosensory evoked potentials (SEPs or SSEPs) are a useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex. SEP components include a series of positive and negative deflections that can be elicited by virtually any sensory stimuli. For example, SEPs can be obtained in response to a brief mechanical impact on the fingertip or to air puffs. However, SEPs are most commonly elicited by bipolar trancutaneous electrical stimulation applied on the skin over the trajectory of peripheral nerves of the upper limb (e.g., the median nerve) or lower limb (e.g., the posterior tibial nerve), and then recorded from the scalp. In general, somatosensory stimuli evoke early cortical components (N25, P60, N80), generated in the contralateral primary somatosensory cortex (S1), related to the processing of the physical stimulus attributes. About 100 ms after stimulus application, additional cortical regions are activated, such as the secondary somatosensory cortex (S2), and the posterior parietal and frontal cortices, marked by a parietal P100 and bilateral frontal N140. SEPs are routinely used in neurology today to confirm and localize sensory abnormalities, to identify silent lesions and to monitor changes during surgical procedures.〔Nuwer, M. (1998a). Fundamentals of evoked potentials and common clinical applications today. Electroencephalography and Clinical Neurophysiology, 106, 142–148.〕
==History==
The modern history of SEPs began with George Dawson’s 1947 recordings of somatosensory cortical responses in patients with myoclonus, a neurological condition characterized by abrupt, involuntary, jerk-like contractions of a muscle or muscle group. Because of their relatively large amplitude and low frequency compatible with a low sampling rate of A/D conversion, the cortical SEPs were the first studied in normal subjects and patients.〔 In the 1970s and early 1980s spinal and subcortical (far-field) potentials were identified. Although the origins and mechanisms of far-field SEPs are still debated in the literature, correlations among abnormal waveforms, lesion site, and clinical observations are fairly well established. However, the most recent advances were brought about by multichannel recordings of evoked potentials coupled with source modeling and source localization in 3D images of brain volume provided by magnetic resonance imaging (MRI).
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