Modulating motor cortical neuroplasticity with priming paired associative stimulation in young and old adults

To examine the effect of priming paired associative stimulation (PAS) on the modulation of motor cortex (M1) plasticity in young and old adults.

Fifteen young (20-27yrs) and 15 old (61-79yrs) subjects participated in 3 experimental sessions, with each session involving two consecutive PAS protocols separated by 10mins. The first (priming) protocol was either PAS_LTP (ISI=N20 latency+2ms), PAS_LTD (ISI=N20 latency-10ms), or PAS_Control (ISI=100ms), whereas the second (test) protocol was always PAS_LTP. Changes in M1 excitability were assessed from motor evoked potentials (MEPs) in a hand muscle.

In young subjects, MEPs were larger after PAS_LTP+PAS_LTP than PAS_LTD+PAS_LTP (P<0.0001) and PAS_Control+PAS_LTP (P=0.0008), whereas the response to PAS_Control+PAS_LTP was not different to PAS_LTD+PAS_LTP (P=0.3). In old subjects, MEPs were smaller after PAS_LTP+PAS_LTP compared with PAS_Control+PAS_LTP (P=0.02), whereas PAS_LTD+PAS_LTP was similar to PAS_Control+PAS_LTP (P=0.08). Age-related comparisons within each priming condition showed that the response to PAS_LTP+PAS_LTP was significantly greater in young subjects (P=0.03).

Data show that priming with PAS_LTP was effective in young but not old subjects.

These findings suggest a limited utility of priming PAS for augmenting plasticity induction in old adults.