Spatial distribution of somatosensory responses evoked by tapping the tongue and finger in man

Early components of the somatosensory potential (SEP) evoked by tactile stimulation of the tongue were recorded from the scalp in 7 normal subjects and compared with those resulting from taps on the middle finger. (1) Unipolar but not bipolar recording of SEP to tapping the tongue failed to differentiate the early components from large EEG deflections of myogenic origin. No such contamination was seen in either unipolar or bipolar recording of SEP following taps on the finger. (2) The early components evoked by tapping the tongue consisted of 4 main deflections; the first negative wave (N1) peaking at about 13 msec after the stimulus was followed by the first positive (P1), second negative (N2) and second positive (P2) wave peaking at about 23, 32 and 44 msec, respectively. Each wave had a shorter latency by several msec than the corresponding wave elicited by taps on the finger. (3) To demonstrate the spatial distribution of a SEP wave over the scalp, a method of amplitude transformation of bipolar records was developed. Topography of the transformed amplitudes of P1 wave, thus obtained while tapping the tongue differed from that obtained while tapping the finger; the former indicated bilateral distribution of the P1 with the maximal amplitude at contralateral (T3 or T5) and ipsilateral (T4 or T6) temporal loci, whereas, in the latter, the distribution was most prominent over the contralateral posterior quadrant with the maximum at parietal locus (C3). P1 wave in SEP thus appeared to represent excitation of the primary cortical neurons responding to tactile stimulation of the different body areas.