Somatosensory Evoked Potentials in Man: Subcortical Akd Cortical Components and Their Neural Basis*

Electronic averaging of somatosensory evoked potentials (SEP) in man is receiving increased attention, and SEP studies are developing vigorously.The cerebral somatosensory projection was the first to be mapped out in detail in experimental studies on and it was also the first to be recorded from an intact man.I2 Detailed SEP studies have, however.somewhat lagged behind those in the visuali5 and the auditorys6 modalities, but they are now rapidly gaining g r o ~n d .' ~~~' ~' 'The possibility of recording noninvasively from the first peripheral sensory neuron and of studying the subcortical," primary and late s or tical'*^*^^^^ responses is rather unique to the somatosensory modality.Another feature of the somatosensory pathway is its remarkable length from peripheral skin to cortex, which makes it more vulnerable to a wide variety of pathological conditions: the wide possibilities offered by accurate SEP studies for critical contributions to neurological diagnosis have barely been charted as yet, but progress is coming along fast.The present paper considers SEPs only in normal adult man and attempts to present the latest data on the functional organization of neural generators involved.Such data are essential for the steady upgrading of pathophysiological applications and of diagnostically relevant uses.14.17~2J~s2The data selected for this paper deal with SEPs elicited by delivering brief electric stimuli to the fingers or to the median nerve.The potentials are recorded along the peripheral nerve and neck and over the scalp.Blurring or distortion of early SEP components must be avoided by using an adequate bandpass, with a fidelity of the amplifier-recorder system extending from 2 Hz to 2 kHz and with a bin width of 200 psec or shorter for a ~e r a g i n g .' ~. ' ~ Since SEP components have a rather small amplitude, it is also essential to minimize or avoid interference from unwanted activities of muscles close to the recording electrodes.The subjects should be fully relaxed, and some device for on-line or off-line rejection of contaminated samples from the current averages should be used.Details of methods can be found in recent publications.'6~20.2'~*zThe SEP components are labeled by their positive (P) or negative polarity ( N ) and their modal peak latency, as recommended by an international committee."For simplicity, the modal latencies considered are those of SEP components elicited by stimulation of the median nerve at the wrist, namely, P,, N i l , and N,, at the posterior neck, the P,, P I , , P,,, and Pi, far-fields at the scalp, and the components N18, N?,, P,,, P2,, N,,, and P4s.Later components, which are strongly influenced by cognitive processing activities of the s ~b j e c t , ' ~, ~~ are not considered here.In all traces presented, negativity of the active rccording electrode produces an