title: Short-latency afferent inhibition is a poor predictor of individual susceptibility to rTMS-induced plasticity in the motor cortex of young and older adults.
abstract: Cortical plasticity, including long-term potentiation (LTP)-like plasticity, can be assessed non-invasively with repetitive transcranial magnetic stimulation (rTMS) protocols. In this study, we examined age differences in responses to intermittent theta burst stimulation (iTBS) in a group of 20 young and 18 healthy older adults. Because the cholinergic system plays a role in the neural processes underlying learning and memory, including LTP, we also investigated whether short latency afferent inhibition (SAI), a neurophysiological marker of central cholinergic activity, would be associated with age-related differences in LTP-like plasticity induced by iTBS.
  
  
  METHODS
  SAI was first assessed by examining the modulation of motor evoked potentials (MEPs) in response to median nerve conditioning 20 ms prior to TMS. Participants then underwent iTBS (3 pulses at 50 Hz every 200 ms for 2 s with 8 s between trains, repeated 20 times). MEP responses (120% resting motor threshold (RMT)) were assessed immediately after iTBS and 5, 10, and 20 min post-application.
  
  
  RESULTS
  Responses to iTBS were quite variable in both age groups, with only approximately 60% of the participants (n = 13 young and 10 older adults) showing the expected facilitation of MEP responses. There were no significant age group differences in MEP facilitation following iTBS. Although older adults exhibited reduced SAI, individual variations were not associated with susceptibility to express LTP-like induced plasticity after iTBS.
  
  
  CONCLUSIONS
  Overall, these results are consistent with reports of high inter-individual variability in responses to iTBS. Although SAI was reduced in older adults, consistent with a deterioration of the cholinergic system with age, SAI levels were not associated with LTP-like plasticity as assessed with iTBS.
authors.affiliation: School of Psychology, University of Ottawa Ottawa, ON, Canada ; Bruyère Research Institute, University of Ottawa Ottawa, ON, Canada.
authors.forename: Marielle
authors.initials: M
authors.lastname: Young-Bernier
authors.affiliation: School of Psychology, University of Ottawa Ottawa, ON, Canada ; Bruyère Research Institute, University of Ottawa Ottawa, ON, Canada.
authors.forename: Annick N
authors.initials: AN
authors.lastname: Tanguay
authors.affiliation: School of Psychology, University of Ottawa Ottawa, ON, Canada ; Bruyère Research Institute, University of Ottawa Ottawa, ON, Canada ; Canadian Partnership for Stroke Recovery, University of Ottawa Ottawa, ON, Canada.
authors.forename: Patrick S R
authors.initials: PS
authors.lastname: Davidson
authors.affiliation: School of Psychology, University of Ottawa Ottawa, ON, Canada ; Bruyère Research Institute, University of Ottawa Ottawa, ON, Canada ; School of Rehabilitation Sciences, University of Ottawa Ottawa, ON, Canada.
authors.forename: François
authors.initials: F
authors.lastname: Tremblay
keywords: aging; cholinergic function; short afferent inhibition; theta burst; transcranial magnetic stimulation