Morphological characteristics of the scalp far-field potentials evoked by median nerve stimulation in infants and children.

We investigated the somatosensory evoked potentials (SEPs) produced by median nerve stimulation in normal infants, children and adults, focussing upon the wave forms of the scalp far-field potentials (FFPs). In adolescents and adults, 3 or 4 positive FFPs preceded the widespread N18 component on the scalp, corresponding to P9, P11, P13 and P14 (or P13-14). In infants and children, however, the scalp FFPs often included 5 positive waves, the initial three of which were characteristically sharp and brief. This distinctive wave form, with 5 positive FFPs, was correlated with an Erb's potential having a bipeaked negative phase. We studied the temporal relationship of the 5 positive FFPs to the Erb's potential and the cervical SEPs and concluded that the initial 3 brief positive waves were produced by overlapping of a bipeaked "P9" and bipeaked "P11." Both "P9" and "P11" are stationary waves that are thought to originate in the first-order afferents, so they probably reflect the bipeaked appearance of the compound nerve action potential.