Effects of age on functional deactivation in the somatosensory system

Background and purpose: Tactile stimulation via peripheral electrical nerve stimulation results in a measurable activation of the somatosensory system in fMRI studies. In addition to positive BOLD signal changes in the contralateral primary sensory cortex (SI), transient negative BOLD responses (NBR) in SI ipsilateral to the stimulation site have recently been demonstrated. Those NBR – possibly indicating a deactivation of neural activity – were of particular interest in this study. The aim of this study was to analyse an effect of age on the negative BOLD response in the somatosensory system.

Methods: In 14 young (mean age±SD: 23,4±0,89) and 13 older participants (mean age±SD: 73,2±8,3) we recorded BOLD responses in the somatosensory system during electrical stimulation of the right median nerve (40Hz) applying a block design (30s on and off). FMRI data analysis was carried out with the imaging software Brain Voyager QX.

Results: In both groups, common areas of activation included the contralateral SI cortex, the parietal operculum (SII region) bilaterally as well as the contralateral insula and thalamus. Comparing the two groups, signs of cortical deactivation with a negative BOLD response located in the primary sensory cortex (SI) ipsilateral to the peripheral stimulation were only found in the group of younger subjects. In a single subject analysis of this group, 12 out of 14 participants showed the described negative BOLD signal in the ipsilateral SI. Time course analysis of the negative BOLD response revealed a transient decline of the BOLD signal at stimulation onset, which slowly returned to baseline at the end of the stimulus.

Discussion: Patterns of cortical activation and deactivation depend on the age of the subject. In a primate model NBR have recently been shown to be tightly linked to a decrease in neural activity and in our case may be the result of interhemispheric transcallosal inhibition. In this study, NBR were restricted to homologous brain areas (SI) and could thus indicate a physiological response to optimize the differentiation of unilateral tactile information in localised regions of the body. This pattern might be lost with increasing age.