Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man.

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive "difference' potentials. The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing. When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.