Scalp topography of human somatosensory evoked potentials: the effect of interfering tactile stimulation applied to the hand.

A detailed topographical analysis of human SEPs in response to median nerve stimulation was performed, with and without concurrent tactile stimulation of the hand. Twenty-eight recording sites on the scalp and neck were referred to earlobe and non-cephalic reference electrodes. The following components were identified in the control wave form: N10, N11, N13, P14, N19, P20, P22, N22, N29, P30 and P44. The interference wave form showed significant attenuation of P14, N19, P20, P22 and N29, while P30 was abolished. N10, N11, N13 and N22 remained unchanged and P44 was enhanced. In the interference wave form there also appeared an N33 component, supplanting P30 over the posterior contralateral quadrant, and a P36 frontally. The pattern of changes was best demonstrated by subtracting the interference from the control response to derive a 'difference' wave form. This consisted of 'P'14, 'N'19, 'P'21, 'P'32 and 'N'36 components. The last 2 were of large amplitude over the posterior contralateral quadrant and frontal cortex, respectively, and overlapped for at least 10 msec of their duration. The results indicate that the interfering effect occurred at the level of the brain-stem or above. The topography of the difference wave form suggests that the 'P'32 and 'N'36 components may be attributable to a generator situated in the posterior bank of the central sulcus (Brodmann's area 3b), concerned with the processing of input from cutaneous mechanoreceptors. It is proposed that the P22 and N22 components of the control and interference wave forms may be due to a generator located at the bottom and in the anterior bank of the central sulcus (area 3a).