Impaired tactile acuity in old age is accompanied by enlarged hand representations in somatosensory cortex

Abstract

The representations of the human hand in primary somatosensory cortex (SI) undergo continuous adaptational plastic processes, which arise from learning, altered use, or injury. The resulting reorganization affects size, extent, and position of the cortical maps, which parallels alterations of tactile behavior indicating a close relationship between map changes and perception. Here we investigate the influence of healthy aging on the cortical hand representation and on age-related changes of tactile performance. Using somatosensory evoked potential mapping in combination with electric source localization, we found that in elderly subjects aged 60-85 years the distance between the dipoles of the index and the little fingers increased indicating an expansion of the representations within SI by approximately 40%. Assessment of tactile spatial 2-point discrimination thresholds in the same subjects showed a strong decline with age. These results indicate that healthy aging strongly affects the homuncular structures of the hand representations within SI. Map expansion typically observed in young and adult subjects during learning is associated with a gain in performance. Whereas learning-related map changes are assumed to result from specific strengthening of synaptic connections, we suggest that the age-related map changes are related to the reduction of intracortical inhibition developing with age.