Somatosensory evoked potentials (SEPs) were recorded on the skull corresponding to the (left) SI area by (right) superficial radial nerve stimulation. Amongst the various components of the SEP, special attention was directed to the negative component (N15) with a latency of approximately 15 ms. Changes in this potential followed by conditioning stimulation of the ipsilateral (right) hemisphere were observed and the following results were obtained: (i) when conditioning stimuli were applied to the contralateral (left) superficial radial nerve, the ipsilateral (right) thalamic VPL nucleus and the ipsilateral (right) sensory cortex, the amplitude of N15 decreased to 65-80% of the control level at C-T intervals less than 100 ms and (ii) following functional elimination of the unilateral sensory cortex by KCl application, the amplitude of N15 recorded at the opposite side significantly increased. In this condition, the inhibitory effects of the ipsilateral thalamus and contralateral peripheral nerve disappeared. From these observations, ipsilateral homologous cortex may well have an inhibitory influence upon the near field potential (N15) of the SEP.
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