Somatosensory evoked potentials to finger stimulation in healthy octogenarians and in young adults: wave forms, scalp topography and transit times of parietal and frontal components.

Abstract Frontal and parietal SEPs to electrical stimulation of fingers were studied in conjunction with the spinal SEP and sensory nerve action potentials in 25 young adults and 19 healthy octogenarians. Limb temperatures were 36–37°C. Intersubject variations of SEP wave forms and detailed scalp topography helped delineate several genuine SEP components, namely the P25, N32 and N40, at precentral scalp sites. The parietal ‘W’ pattern (P30-N35-P45) was recorded in only 48% of the young, but 90% of the old subjects. All parietal components were significantly increased in size in the old, which contrasts with the reduced spinal SEP and sensory nerve potentials (Table II). The precentral SEP components were either maintained (P25) or reduced (N32). The primary afferent neurones appeared to age at a faster rate (CV reduced about 0.16 m/sec/year) than the second or third afferent neurones (no significant change in transit time from spinal entry to postcentral cortex or in mean central CV) (Table I). The scalp recorded SEPs were quite different in front of or behind the central fissure. Contrary to the ‘deep dipole’ hypothesis the SEPs recorded pre- or postcentrally were not mirror images and exhibited separate non-congruent intersubject variabilities (Figs. 1–5). The ‘travelling wave’ hypothesis was also rejected since the SEP components presented peak latencies that were stable within their scalp territory. It is suggested that the sequential activation of cortical modules in parietal and frontal areas, in part through demonstrated cortico-cortical axonal connections and possibly also through thalamo-cortical projections, could generate successive SEP components. The cortical generators of SEP components appeared to be similar in the octogenarians although their relative strengths changed and transit times between different components significantly increased (Table III).

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