Short-interval intracortical inhibition as a function of inter-stimulus interval: Three methods compared

• Published in: Brain stimulation Vol. 14; no. 1; pp. 22 - 32
• Main Authors: Tankisi, Hatice, Cengiz, Bülent, Howells, James, Samusyte, Gintaute, Koltzenburg, Martin, Bostock, Hugh
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.01.2021; Elsevier
• Subjects: Electromyography; Evoked Potentials, Motor; Humans; Motor Cortex; Neural Inhibition; Serial versus parallel threshold-tracking TMS; Short-interval intracortical inhibition; Transcranial Magnetic Stimulation; Short-interval intracortical inhibition; Serial versus parallel threshold-tracking TMS
• ISSN: 1935-861X; 1876-4754; 1876-4754
• DOI: 10.1016/j.brs.2020.11.002

Short-interval intracortical inhibition (SICI), as measured by threshold-tracking as a function of inter-stimulus interval (ISI), has been proposed as a useful biomarker for amyotrophic lateral sclerosis (ALS), but its relationship to conventional amplitude measurements has not been established. Serial tracking of SICI at increasing ISIs from 1 to 7 ms (T-SICIs) was compared in 50 healthy control subjects with the same ISIs tracked in parallel (T-SICIp), and with conventional amplitude measurements (A-SICI). For T-SICIp and A-SICI, pairs of conditioning and test stimuli with different ISIs were pseudo-randomised and interspersed with test-alone stimuli given at regular intervals. Thresholds were estimated by regression of log peak-to-peak amplitude on stimulus. T-SICIp and A-SICI were closely related: a ten-fold reduction in amplitude corresponding to an approximately 18% increase in threshold. Threshold increases were greater for T-SICIs than for T-SICIp at 3.5–5 ms (P < 0.001). This divergence depended on the initial settings and whether ISIs were progressively increased or decreased, and was attributed to the limitations of the serial tracking protocol. SICI variability between subjects was greatest for T-SICIs estimates and least for A-SICI, and only A-SICI estimates revealed a significant decline in inhibition with age. The serial tracking protocol did not accurately show the dependence of inhibition on ISI. Randomising ISIs gives corresponding SICI measures, whether tracking thresholds or measuring amplitude measurements. SICI variability suggested that A-SICI measurements may be the most sensitive to loss of inhibition. •SICI at ISIs from 1 to 7 ms has been recorded from 50 healthy subjects by 3 methods.•A-SICI used amplitudes, T-SICIs tracked thresholds at sequentially increasing ISIs.•T-SICIp tracked the randomised 9 ISIs in parallel (new method).•A-SICI and T-SICIp were well correlated, but T-SICIs results were not accurate.•A-SICI provided the least variable measures of inhibition across subjects.