Abstract
Transcallosal inhibition (TCI), assessed using transcranial magnetic stimulation, can provide insight into the neurophysiology of aging and of neurological disorders such as stroke. However, the reliability of TCI using the ipsilateral silent period (iSP) has not been formally assessed, despite its use in longitudinal studies. This study aimed to determine the reliability of iSP onset latency, duration and depth in healthy young and older adults. A sample of 18 younger (mean age 27.7 years, range: 19–42) and 13 older healthy adults (mean age 68.1 years, range: 58–79) attended four sessions whereby the iSP was measured from the first dorsal interosseous (FDI) muscle of each hand. 20 single pulse stimuli were delivered to each primary motor cortex at 80% maximum stimulator output while the participant maintained an isometric contraction of the ipsilateral FDI. The average onset latency, duration of the iSP, and depth of inhibition relative to baseline electromyography activity was calculated for each hand in each session. Intraclass correlation coefficients (ICCs) were calculated for all four sessions, or the first two sessions only. For iSP onset latency the reliability ranged from poor to good. For iSP duration there was moderate to good reliability (ICC > 0.6). Depth of inhibition demonstrated variation in reproducibility depending on which hand was assessed and whether two or four sessions were compared. Bland and Altman analyses showed wide limits of agreement between the first two sessions, particularly for iSP depth. However, there was no systematic pattern to the variability. These results indicate that although iSP duration is reliable in healthy adults, changes in longitudinal studies should be interpreted with caution, particularly for iSP depth. Future studies are needed to determine reliability in clinical populations.
Highlights
Co-ordination of upper limb movement relies on communication between the two cerebral hemispheres
For ipsilateral silent period (iSP) latency there was moderate to good reliability for the younger adults bilaterally and for the left first dorsal interosseous (FDI) of the older group (ICC, 0.69–0.91), and poor to moderate reliability for the right FDI of the older group
For iSP duration there was moderate to good reliability for both groups bilaterally, with all Intraclass correlation coefficients (ICCs) values > 0.6
Summary
Co-ordination of upper limb movement relies on communication between the two cerebral hemispheres. Each primary motor cortex (M1) interacts with the opposite one to prevent mirror movements and facilitate uni- and bi-manual movements (Hubers et al, 2008; Beaule et al, 2012) This is thought to be mediated at a cortical level (Ferbert et al, 1992) by pathways in the corpus callosum (Meyer et al, 1995; Boroojerdi et al, 1996). The iSP method assesses transcallosal inhibition (TCI) by applying a suprathreshold stimulus to M1 while the target muscle of the ipsilateral upper limb is activated voluntarily. This leads to a short interruption of the electromyography (EMG) activity. The iSP measure assesses the inhibition of volitional motor activity, making it a useful technique for examining the control of voluntary motor output (Beaule et al, 2012)
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