Different modulation of the cortical silent period by two phases of short interval intracortical inhibition

Abstract

Purpose: To investigate the influence of 2 phases of short interval intracortical inhibition (SICI) on the cortical silent period (SP).

Materials and methods: Single- and paired-pulse transcranial magnetic stimulations (TMSs) at 1 and 2.5ms interstimulus intervals (ISIs) were applied to the left motor cortex in 12 healthy subjects while their right hand muscles were moderately activated. Conditioning stimulation intensity was 90% of the active motor threshold (AMT). Test stimulation intensities were 120, 140, 160, 180, 200, 220, 240, 260% of the AMT and at 100% of the maximal stimulator output, the order of which was arranged randomly. The rectified electromyography area of motor evoked potential (MEP) and duration of the SP were measured off-line using a computerized program.

Results: At high-test stimulation intensities, MEP areas were saturated in both single- and paired-pulse stimulations, except that saturated MEPs were smaller for the paired-pulse TMS at 1ms ISI than for the other conditions. As the test stimulation intensity increased, SP was progressively prolonged in both single- and paired-pulse stimulations but was shorter in paired-pulse than single-pulse TMS. Overall, the ratio of SP duration/MEP area was comparable between single- and paired-pulse TMS except for the paired-pulse TMS at 1 ms ISI with a test stimulation intensity at 140-180% of the AMT, in which the ratio was significantly higher than in the single pulse TMS.

Conclusion: These results suggest that 2 phases of SICI modulate MEP saturation and SP duration differently and provide additional evidence supporting the view that 2 phases of SICI are mediated by different inhibitory mechanisms.

Fig. 1

(A) Motor evoked potential (MEP) area of the right first dorsal interosseus (FDI) evoked by single- (circle) and paired-pulse TMS with 1 ms (diamond) and 2.5 ms (rectangle) interstimulus intervals (ISIs) at different test stimulation (TS) intensities. MEP areas were significantly suppressed in paired-pulse trials with 1 ms ISI at a TS of 140% or higher of the active motor threshold (AMT) and in paired-pulse trials with 2.5 ms ISI at a TS of 160 - 200% of the AMT as compared with the control trials (^*different from the control trials, p < 0.05). (B) Calculated short-interval intracortical inhibition (SICI, % of conditioned MEP area/test MEP area) at different TS intensities. SICI was significant in paired-pulse trials with 1 ms ISI at a TS of 140% or higher of the AMT and in paired-pulse trials with 2.5 ms ISI at a TS of 160 - 220% of the AMT (^*p < 0.05).

Fig. 2

SP duration evoked by single- (circle) and paired-pulse TMS with 1 ms (diamond) and 2.5 ms (rectangle) interstimulus intervals (ISIs) at different test stimulation (TS) intensities. SP durations were significantly shortened in paired-pulse trials at 1 ms ISI at the test intensities of 160% or higher of the active motor threshold (AMT). SP durations in paired-pulse trials with 2.5 ms ISI were not statistically different from the control trials, although they were approximately 10 - 15 ms shorter (^*different from control; p < 0.0167).

Fig. 3

The SP duration/MEP area ratio in single- (circle) and paired-pulse TMS with 1 ms (diamond) and 2.5 ms (rectangle) interstimulus intervals (ISIs) at different test stimulation (TS) intensities. This ratio was significantly higher in paired-pulse trials with 1 ms ISI at 160 and 180% of the AMT (^*p < 0.05) and had a tendency to be higher at 140% of the AMT (p < 0.1) than in single-pulse trials.