Fig. 3. Somatosensory evoked potentials (SSEPs). (A,B) Complete recordings of SSEP tests. In the pre-injury recording (A), the bottom three overlapping traces were produced by three separate sets of standard stimulations (refer to Materials and methods). These signals were averaged to produce the single top trace revealing two peaks of early-arriving (P1) and late-arriving (P2) evoked potentials at the brain (refer to Materials and methods). The peaks shown are characteristic SSEPs produced by tibial nerve stimulation in adult guinea pigs. The double-headed arrow shows the stimulus artifacts. A similar recording is shown in B, but this was taken within 30 min of a standardized compression to the mid-thoracic spinal cord. Note the complete loss of all ascending SSEPs. In C, a single averaged trace is shown of a median nerve stimulation recorded in this same animal, as a control procedure, within minutes of the traces shown in B.
