Latency changes in the human somatosensory evoked potential at extreme depths

Somatosensory evoked potentials to lower limb stimulation were recorded from the three participants in a recent chamber dive to a world depth record of 686 m (2250 ft). Because of the 10% nitrogen breathing mixture, both inert gas narcosis and high pressure nervous syndrome were factors in the dive. Latencies of evoked potential peaks were measured relative to the peak of the first cortical component, P1, in order to separate central from peripheral effects. Several peak latencies changed systematically during compression. The several components of the somatosensory evoked potential appeared differentially sensitive to either inert gas narcosis or the high pressure nervous syndrome with increasing depth. The latencies of all peaks following the initial cortical P1 were shorter at depth than in surface control recording. This is consistent with a state of hyperexcitability in the brain that is felt to characterize the high pressure nervous syndrome. Changes in peak latency were consistent across subjects and in some cases achieved statistical significance despite the small number of subjects available for testing. Although these findings are preliminary, they suggest that evoked potential latencies may have value as a means of following the effects of extreme pressure on the central nervous system.