Increased intracortical inhibition in elderly adults with anterior–posterior current flow: A TMS study

Abstract

Please cite this article as: Sale, M.V., Lavender, A.P., Opie, G.M., Nordstrom, M.A., Semmler, J.G., Increased intracortical inhibition in elderly adults with anterior-posterior current flow: a TMS study, Clinical Neurophysiology (2015), doi: http://dx.doi.org/10. 1016/j.clinph.2015.04.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Increased intracortical inhibition in elderly adults Highlights• We show no age-related difference in intracortical inhibition using a conventional (posterior-anterior) transcranial magnetic stimulation (TMS) coil orientation.• Using the more sensitive (anterior-posterior) TMS coil orientation, we show increased intracortical inhibition in older adults.• Reversing TMS current direction could be helpful to assess subtle differences in intracortical inhibition in clinical populations. AbstractObjective: All previous studies using TMS to assess short-interval intracortical inhibition (SICI) in older adults have used a conventional coil orientation, which produces posterior-toanterior (PA) current flow in the motor cortex. However, no studies have examined SICI in older adults by reversing the coil to induce anterior-to-posterior (AP) current flow, which is considered more sensitive at detecting SICI. Therefore, we investigated age-related changes in SICI using both PA and AP TMS across different conditioning stimulus intensities and muscle activation states. Methods:In 22 young and 20 older adults, SICI was assessed using PA and AP coil orientations, across a range of conditioning stimulus intensities (70-90% active motor threshold), and whilst participants kept their first dorsal interosseous (FDI) muscle either relaxed or active (2 N force).Results: There were no age-related differences in SICI using conventional PA TMS in resting or active FDI muscle. However, SICI was increased in elderly participants when assessed with reverse coil AP TMS in resting FDI.Conclusions: Coil orientation is a key factor to consider when assessing age-related differences in SICI.Significance: Reverse coil AP TMS can reveal age-related changes in SICI, which were previously not evident with conventional PA TMS. This may have implications for the assessment of SICI in some clinical populations that may show subtle differences in SICI circuitry.

Adaptive threshold hunting reveals differences in interhemispheric inhibition between young and older adults

“…In this study, TMS intensities that correspond to RMT, AMT and 1 mV test stimulus were higher in older adults compared with young, indicating reduced corticomotor excitability with advancing age. Our findings are in line with previous studies which observed higher thresholds (Sale et al., ) and higher stimulus intensities required to elicit target MEP amplitudes (Pitcher et al., ) in older adults. This may, in part, be due to reduced excitability in the underlying neurons or an increased distance between the TMS coil and underlying neural tissue due to cortical atrophy with advancing age (Fjell et al., ).…”

“…In young individuals, the magnitude of IHI was comparable when the test stimulus was induced with PA and AP current directions (Ghosh et al., ). However, age‐related comparisons may be more sensitive with AP stimulation compared with PA. For example, age‐related differences in intracortical inhibition that also suppresses later I‐waves much more so than early I‐waves (Di Lazzaro et al., ; Nakamura, Kitagawa, Kawaguchi, & Tsuji, ) was only evident with AP, but not PA stimulation (Sale, Lavender, Opie, Nordstrom, & Semmler, ). Therefore, assessing IHI with AP‐induced current may be required to detect age‐related effects.…”

Intracortical Inhibition Assessed with Paired-Pulse Transcranial Magnetic Stimulation is Modulated during Shortening and Lengthening Contractions in Young and Old Adults

“…Previous literature suggests contradictory effects of age on SICI and LICI during both relaxation [27,29,[49][50][51][52][53][54][55][56][57][58][59][60][61][62] and isometric muscle activation [30,33,63], with variations in methodology and subject characteristics likely contributing to this heterogeneity. Despite this, as the conditioning stimulus intensity was adjusted to produce 50% inhibition of the test MEP during postural contractions, our findings could not have been confounded by any age-related differences in inhibition at baseline.…”

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent

“…Nonetheless, the contribution of early and late I waves to the MEP are known to be altered when the target muscle is active [51], which may complicate the interpretation of the interactions between inhibitory circuits in active muscle. Furthermore, it is not known whether the composition of the descending volley is different in older adults, although recent evidence comparing MEP latencies between antero-posterior (preferential I3 wave activation) and posterior-anterior (preferential I1 wave activation) TMS showed similar recruitment of I waves in older adults [52]. Further research is therefore needed to confirm whether the descending volley (that produces similar MEP amplitudes) is comparable in young and old adults, particularly when the muscle is active.…”