Neuromagnetic changes of the somatosensory information processing in normal aging

Age-related electrophysiological changes in the primary somatosensory cortex (SI) are well known. There is evidence that the amplitude of the N20 component of median nerve somatosensory evoked potentials typically increases with age, probably because of cortical disinhibition. The secondary somatosensory cortex (SII) receives dual input from the SI and the thalamus. We quantified age-related changes both in SI and SII using magnetoencephalography (MEG). We recorded somatosensory evoked magnetic fields (SEFs) to median nerve stimulation in 15 young adults (aged 22-36 years, mean age 29.0±4.1) and 15 older adults (aged 52-67 years, mean age 61.9±5.4), and analysed major SEF components in SI and SII. The amplitude and equivalent current dipole (ECD) strength of the N20m were significantly increased in the older adults, consistent with the well-known electrophysiological change for cortical disinhibition in SI. The latency of N20m showed a trend for increase in older subjects, possibly reflecting slowing of conduction velocity in the peripheral nerves. In contrast, SII response (response peak at around 80-120 ms) showed a different change in aging. Latencies of the contralateral SII responses showed a trend for shortening in the older adults. There were no significant age-related changes for the amplitudes and ECD strengths. Thus, SI and SII are differently affected by aging. The shortening of the SII latency suggests age-related plastic-adaptive change in SII, which is mediated by the direct thalamocortical pathway.