Increased cortical plasticity in the elderly: changes in the somatosensory cortex after paired associative stimulation.

A fundamental feature of the human cortex is the capability to express plastic changes that seem to be present even during physiological aging. The paired associative stimulation (PAS) protocol is a paradigm capable of inducing neuroplastic changes, possibly by mechanisms related to spike timing-dependent associative neuronal activity, and represents a suitable tool for investigating age-dependent neuroplastic modulations of the primary somatosensory cortex (S1). To examine age dependency of S1 plasticity, the amplitude changes of median nerve somatosensory evoked potential (SEP) before and after PAS intervention were investigated in young and elderly subjects. The main finding of our study is that low-frequency medial nerve stimulation paired with transcranial magnetic stimulation over the contralateral cortex enhances S1 excitability. Moreover, the S1 long term potentiation-like plasticity changes as a function of aging, with a significant increase of N20-P25 complex in the elderly compared to young subjects. These results are congruent with the hypothesis that some elderly subjects retain a high level of plasticity in specific neuronal circuits. Such plasticity could represent a compensatory mechanism, in terms of functional reserve of somatosensory cortex, used by the aging brain to counterbalance the cortical degeneration associated with aging.