[Cervical somatosensory evoked potentials in the healthy subject: analysis of the effect of the location of the reference electrode on aspects of the responses].

Cervical SEPs were recorded in 111 normal subjects following stimulation of the median nerve at the wrist, using 3 different sites for the reference electrode (Fz, earlobe, shoulder). It was shown that cephalic reference electrodes (Fz or earlobe) modify the wave form of the cervical response, because they pick up far-field SEPs (P9, P11, P13, P14) originating from cervical roots, spinal cord and brainstem. These far-field SEP components are injected as negativities in the activity recorded by the cervical electrode. The responses recorded with cephalic reference differ from those recorded at the same cervical site, with a non-cephalic reference in 3 main points: (1) the amplitude of negative components N11 and N13 is increased; (2) the onset latency of N11 is significantly shorter; (3) an N14 negativity is added, the origin of which is probably in the brainstem; this component may occupy the peak of the cervical negativity; thus the central conduction time, calculated as the time interval between N14 and N20, does not take into account the time for spinal propagation of the somatosensory afferent inputs. A topographic study of cervical responses in 10 normal subjects showed an increase of the onset latency of N11 (mean 0.89) from the lower cervical region to the cervico-occipital junction, provided that a non-cephalic reference is used. This result suggests that N11 corresponds to the travelling of action potentials in the ascending spinal somatosensory pathways. The use of a medio-frontal (Fz) reference electrode results in: (1) a masking of the latency shift of N11 latency because of the subtraction of the far-field Fz-recorded P11 component, the onset of which was found to be synchronous with the entry of afferent volleys in the lower cervical spinal cord; (2) a modification of the spatial organization of the responses, due to the subtraction of far-field scalp-recorded positivities P9, P11, P13 and P14, that creates negative N9, N11, N13 and N14 potentials far below the level where cervical roots enter the spinal cord.