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

Area of Science:

• Neuroscience
• Biomarkers
• Neurophysiology

Background:

• Short-interval intracortical inhibition (SICI) is a potential biomarker for amyotrophic lateral sclerosis (ALS).
• The relationship between SICI measured by threshold tracking and conventional amplitude measurements is not well-established.

Purpose of the Study:

• To compare SICI measurements obtained through serial threshold tracking (T-SICIs) with parallel threshold tracking (T-SICIp) and amplitude measurements (A-SICI).
• To evaluate the accuracy of the serial tracking protocol in reflecting the dependence of SICI on inter-stimulus intervals (ISIs).

Main Methods:

• Compared T-SICIs, T-SICIp, and A-SICI in 50 healthy control subjects.
• Used pseudo-randomized conditioning and test stimuli with varying ISIs.
• Estimated thresholds via regression of log peak-to-peak amplitude on stimulus intensity.

Main Results:

• T-SICIp and A-SICI were closely related.
• Threshold increases were greater for T-SICIs than T-SICIp at specific ISIs (3.5-5 ms), attributed to serial tracking limitations.
• A-SICI exhibited the least variability and was the only measure showing a significant decline in inhibition with age.

Conclusions:

• The serial tracking protocol for SICI is not accurate in demonstrating inhibition dependence on ISI.
• Randomizing ISIs yields comparable SICI measures whether using threshold or amplitude tracking.
• A-SICI measurements appear most sensitive to the loss of neural inhibition.