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

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

Methods: 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 (ISI=N20 latency+2ms), PAS (ISI=N20 latency-10ms), or PAS (ISI=100ms), whereas the second (test) protocol was always PAS. Changes in M1 excitability were assessed from motor evoked potentials (MEPs) in a hand muscle.

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

Conclusion: Data show that priming with PAS was effective in young but not old subjects.

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