[Somatosensory evoked potentials: interference and perceptual masking of cutaneous afferents in man]

Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation

There is a large consensus, based on converging evidence, that N13 recorded at lower cervical levels has a segmental postsynaptic origin in the gray matter of the cervical cord and that because of the orientation of its dipole field, the Cv6-anterior cervical derivation should be used whenever the diagnostic problem requires that this potential be assessed selectively in terms of latency and amplitude. The diagnostic utility of the lower cervical N13 recording in dorsal horn deafferentation and in lesions at the Cv6-Cv8 metameric levels has been validated in all types of cervical cord lesions. Unfortunately, such clear-cut conclusions do not apply to the N13 potential recorded at upper cervical levels. Currently, this component is not considered to provide enough reliable information, in addition to P13-P14 scalp recordings, to be used routinely in the diagnosis of cervicomedullary lesions.

[Proprioceptive and cutaneous evoked somatosensory potentials in the baboon: cycles of GABAergic excitability and inhibition]

Double stimulations induce deep and long-lasting inhibition (0-300 ms) of the P16-N30 components of somatosensory potentials (SEP) evoked by sciatic or sural nerve stimulation. This inhibition is evidenced on both S1 and M1 cortical areas, demonstrating similar course and duration, whatever the source (right or left limb) and/or the modality (extero- or proprioceptive) of conditioning and testing afferences. The depth of this inhibition depends on the relative amplitude of the conditioning to testing SEP. After muscle injection of a subconvulsive dose of bicuculline, tSEPs are facilitated when individually elicited. When double stimulations are used, the inhibition of the SEP test is sharply reduced (with a 30-ms interstimulus delay). However, disinhibition of the conditioned SEP does not depend on separate individual SEP facilitations. Cortical GABAergic type a circuits are likely to be involved in inhibition of the conditioned SEP. This inhibition would be a non-invasive image of inhibitions that preserve the specificities of sensory messages in primary areas.