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

Objective: All previous studies using TMS to assess short-interval intracortical inhibition (SICI) in older adults have used a conventional coil orientation, which produces posterior-to-anterior (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. (C) 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.