Somatosensory evoked potentials in the term newborn.
Median nerve somatosensory evoked potentials (SEPs) were recorded from surface electrodes in 40 healthy term infants (range 36.5-43 weeks postmenstrual age). Electrical stimulation at 5 Hz was used, averaging the response to several runs of 1024 stimuli to each median nerve, bandpass 10-3000 Hz, sweeptime 100 msec. Identifiable potentials were collected over the cervical cord on all runs in all 40 infants and from the cortex in at least some runs in 39 out of 40 infants. The cervical response showed little variation and consisted of a clear negative wave with up to 3 peaks, mean latency of the largest 10.2 +/- 0.7 msec, followed by a positive deflection. The cortical response was very variable in form and latency between infants and to a lesser degree within infants. Four types of cortical wave form were found, symmetrical, asymmetrical, plateau and M shaped, of increasing complexity. In 11% of trials the response was absent or indistinct but could usually be uncovered by alteration in stimulus frequency or intensity. In the whole group, the mean latency for N1 was 30.0 +/- 6.8 msec and for the central conduction time 19.8 +/- 6.5 msec. Significant differences were found between the 4 cortical wave forms in the main variables measured, which gave support for form S being the most primitive and form M the most mature response.