Age‐related changes in late I‐waves influence motor cortex plasticity induction in older adults

Abstract

Previous research shows that neuroplasticity assessed using transcranial magnetic stimulation (TMS) is reduced in older adults. While this deficit is often assumed to represent altered synaptic modification processes, age-related changes in the interneuronal circuits activated by TMS may also contribute. Here we assessed age-related differences in the characteristics of the corticospinal indirect (I) waves and how they influence plasticity induction in primary motor cortex. Twenty young (23.7 ± 3.4 years) and 19 older adults (70.6 ± 6.0 years) participated in these studies. I-wave recruitment was assessed by changing the direction of the current used to activate the motor cortex, whereas short-interval intracortical facilitation (SICF) was recorded to assess facilitatory I-wave interactions. In a separate study, I-wave periodicity TMS (iTMS) was used to examine the effect of I-wave latency on motor cortex plasticity. Data from the motor-evoked potential (MEP) onset latency produced using different coil orientations suggested that there were no age-related differences in preferential I-wave recruitment (P = 0.6). However, older adults demonstrated significant reductions in MEP facilitation at all 3 SICF peaks (all P values < 0.05) and a delayed latency of the second and third SICF peaks (all P values < 0.05). Using I-wave intervals that were optimal for young and older adults, these changes in the late I-waves were shown to influence the plasticity response in older adults after iTMS. These findings suggest that temporal characteristics are delayed for the late I-waves in older adults, and that optimising TMS interventions based on I-wave characteristics may improve the plasticity response in older adults.

Myths and Methodologies: How loud is the story told by the transcranial magnetic stimulation‐evoked silent period?

“…Given that the mechanisms involved in the generation of the SP are prompted after the stimulus, the stimulus artefact seems to be the most suitable standardized reference point for SP onset. It should be noted that when the stimulus artefact is used to define SP onset, issues can arise in populations exhibiting changes in evoked response latencies, such as the elderly (Opie, Cirillo, & Semmler, ). However, it is unknown whether longer MEP latencies are necessarily associated with a longer delay for the commencement of inhibitory mechanisms.…”

“…It should be noted that when the stimulus artefact is used to define SP onset, issues can arise in populations exhibiting changes in evoked response latencies, such as the elderly (Opie, Cirillo, & Semmler, 2018 The SP offset also presents challenges, particularly when small 'bursts' of low-level EMG activity appear during the SP. The issue then becomes whether to set the SP offset as the 'burst' or the resumption of EMG after the second suppression.…”

Characterising the balance between short-interval intracortical inhibition and short-interval intracortical facilitation in younger and older adults

“…The magnitude of facilitation at the ISIs that produced the first peak of the SICF function was similar in younger and older adult groups, suggesting that the excitability of the summed excitatory synaptic activity within the intracortical facilitatory circuits that generate the first I-wave is similar in younger and older adults , Hanajima et al 2002, Ortu et al 2008, Di Lazzaro et al 2012). Reports of age-related differences in SICF Peak 1 are inconsistent: Clark et al (2011) showed greater SICF (at an ISI of 1.5 ms) in older than younger adults, and Opie et al (2018) showed smaller SICF (at both 1.3 ms and 1.5 ms) in older than younger adults. The two previous studies and the current study used the same stimulation intensities for single-and paired-pulse trials, and at least a subset of the same ISIs.…”

“…The first two SICF peaks and the trough between these peaks were obtained by varying the ISI from 1.3 ms to 3.1 ms (Peurala et al 2008, Opie et al 2018. Following the identification of ISIs that produced early maximal facilitation and minimal facilitation for each individual, SICI stimulus-response functions were obtained by varying CS intensity from 50-110% of resting motor threshold (10% increments).…”

“…For each individual, two SICI stimulus-response functions were obtained: one using the individualised ISIs that produced maximal facilitation (SICF Peak 1) and one using the individualised ISIs that produced minimal facilitation (trough). Maximal facilitation was identified using ISIs that produced SICF Peak 1 because previous research shows that the magnitude of facilitation is greater at Peak 1 than Peak 2, and previous reports of age-related differences in SICF at Peak 1 are inconsistent (Clark et al 2011, Opie et al 2018. Finally, to explore the functional importance of SICI and SICF, correlation analyses were conducted between manual dexterity and (i) SICF at Peak 1 and Peak 2, and (ii) measures from the SICI stimulus-response functions at which net inhibition shifts to net facilitation.…”