THE INTERACTION OF THE SOMATOSENSORY-EVOKED POTENTIALS TO SIMULTANEOUS FINGER STIMULI IN THE HUMAN CENTRAL-NERVOUS-SYSTEM - A STUDY USING DIRECT RECORDINGS

In order to investigate the interaction of sensory electrophysiologic fields arising from the adjacent second (II) and third (III) fingers a nd the distant second and fifth (V) fingers, direct recordings of soma tosensory evoked potentials (SEPs) were performed from the sensory and motor cortices, the sensory thalamic nucleus (nucleus ventralis cauda lis, VC) and the cuneate nucleus in humans during neurosurgical operat ions. Electrical stimulation was given to the II, III or V fingers ind ividually, and also to pairs of either the II and III fingers or the I I and V fingers simultaneously. The interaction ratio (IR) was devised as the ratio of amplitude attenuation caused by the simultaneous stim ulation to two fingers compared with the amplitude of the arithmetical ly summed SEPs to the individual stimulation of two fingers. The IRs w ere calculated on N20 and P25 from the sensory cortex, P22 from the mo tor cortex, P17(thal) from the VC, and N16(cune) and P35(cune) from th e cuneate nucleus. With both stimulations to the II and III fingers an d the II and V fingers, P25 showed the greatest IR, followed by P22, t hen by P17(thal) while N16(cune) exhibited the smallest IR. N20 and P3 5(cune) showed similar IRs and significantly greater IRs with II and I II finger stimulation compared with II and V finger stimulation. These results thus indicate that the interaction of somatosensory impulses occurs in several structures along the sensory pathway in CNS, includi ng the cuneate nucleus, the sensory thalamic nucleus, as well as senso ry and motor cortices, with the greatest IRs in the cerebral cortices and the weakest ones in the brain-stem. They also suggest that the rec eptive fields of the fingers in the cortical area generating N20 are a rranged according to the order of the fingers while those in the gener ating sites for cortical P25 and P22, thalamic P17(thal) and cuneate N 16(cune) tend to be arranged in clusters, while P35(cune) is possibly modulated by the somatosensory cortex through a long-loop feedback pat hway.