BACKGROUND 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

中文翻译：

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作为刺激间隔函数的短间隔皮质内抑制：三种方法比较

背景短间隔皮质内抑制 (SICI)，通过阈值跟踪作为刺激间隔 (ISI) 的函数进行测量，已被提议作为肌萎缩侧索硬化 (ALS) 的有用生物标志物，但其与常规振幅测量的关系尚未成立。方法 对 50 名具有相同 ISI 并行跟踪 (T-SICIp) 和常规振幅测量 (A-SICI) 的健康对照受试者，在 ISI 从 1 毫秒增加到 7 毫秒 (T-SICI) 时对 SICI 进行连续跟踪。对于 T-SICIp 和 A-SICI，具有不同 ISI 的成对的调节和测试刺激是伪随机化的，并散布着定期给予的单独测试刺激。阈值是通过对刺激的对数峰峰值的回归来估计的。结果 T-SICIp 和 A-SICI 密切相关：幅度减少 10 倍，对应阈值增加约 18%。在 3.5-5 ms 时，T-SICIs 的阈值增加大于 T-SICIp (P<0.001)。这种分歧取决于初始设置以及 ISI 是逐渐增加还是减少，并且归因于串行跟踪协议的局限性。受试者之间的 SICI 变异性对于 T-SICIs 估计值最大，对于 A-SICI 最小，并且只有 A-SICI 估计值显示抑制随年龄显着下降。结论 串行跟踪协议没有准确显示抑制对 ISI 的依赖性。随机化 ISI 会给出相应的 SICI 测量值，无论是跟踪阈值还是测量幅度测量值。SICI 变异性表明 A-SICI 测量可能对抑制的丧失最敏感。