Bit-mapped colour imaging of the potential fields of propagated and segmental subcortical components of somatosensory evoked potentials in man.

Early somatosensory evoked potentials (SEPs) to median or ulnar nerve stimulation were recorded with non-cephalic reference from neck, oesophagus and scalp in normal young adults. Transit times and durations were estimated for different components. SEP fields were mapped with up to 24 skin electrodes around the neck or along the midline neck and scalp, and projected onto a 2-dimensional plane for bit-mapped colour imaging. The posterior neck N11 is a near-field potential propagated caudorostrally in the dorsal column, associated with a positive P11 far field beyond termination of the cuneate bundle. A true phase reversal of the posterior neck N13 into an anterior neck P13 is substantiated, identifying a segmental generator with horizontal axis in dorsal horn. The N13-P13 represents postsynaptic excitatory potentials in interneurones of layers IV-V of the dorsal horn. It is not reflected in any scalp far field. The duration and onset latency of N13-P13 are in line with this interpretation. A new montage of posterior-to-anterior neck can enhance this component without introducing extraneous potentials. The P14 far field does not extend below the inion and presents distinct features. Neck-to-front scalp montages confound the SEP components generated in the spinal cord and above the foramen magnum respectively, but may serve to estimate N11 onset latency.