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Transcranial magnetic stimulation (TMS) is used frequently to study human physiology, including the indirect-wave (I-wave) circuits generating short-interval intracortical facilitation (SICF). Growing evidence implicates SICF in plasticity and motor learning, suggesting that SICF is likely of functional relevance in both health and disease. To date, test–retest reliability has not been established for measures of SICF: given the clear potential of SICF to be used as a diagnostic tool, it is critical to establish the reliability of the paired-pulse TMS protocol to measure SICF. We investigated the test–retest reliability of SICF measured using paired-pulse TMS. SICF was measured in two sessions in 20 young adults using single- and paired-pulse TMS. Single-pulse TMS was set at an intensity that elicited MEPs of 1 mV (SI1mV) and paired-pulse TMS was set with the first stimulus at SI1mV, the second stimulus (S2) 90% of resting motor threshold (RMT), and a total of 20 interstimulus intervals (ISI; 1.1–4.9 ms with a 0.2 ms step). Large intraclass correlation coefficients (ICC) indicate good test–retest reliability between sessions for all SICF peaks (ICCs ranging from 0.73 to 0.79). The ISI at which SICF was maximal within individuals was consistent at all three peaks across both experimental sessions. Results showed good test–retest reliability of SICF at all three peaks when using a standard paired-pulse protocol to measure SICF. This finding suggests that paired-pulse TMS can be used to reliably probe the excitability of the interneuronal circuits that generate SICF. This provides a strong platform for future research to investigate the functional role of I-wave circuitry, including the role of I-wave circuitry in motor control decline in healthy older adults and individuals with movement disorders.

The purpose of this study was to investigate the effects of chronic resistance training on corticospinal excitability and short intracortical inhibition of the biceps brachii. Eight chronic resistance-trained (RT) and eight non-RT participants completed one experimental session including a total of 30 brief (7 s) elbow flexors isometric contractions at various force outputs [15, 25 and 40% of maximum voluntary contraction (MVC)]. Before the contractions, MVC, maximal compound muscle action potential (Mmax) during 5% MVC and active motor threshold (AMT) at the three various force outputs were recorded. MVC force of the chronic-RT group was 24% higher than the non-RT group (p ≤ 0.001; ω2 = 0.72). The chronic-RT group had lower AMTs at targeted forces of 15 and 25% MVC (p = 0.022 and p = 0.012, respectively) compared to the non-RT group. During 25 and 40% of MVC, the non-RT group exhibited decreased SICI in comparison to the chronic-RT group (p = 0.008; ω2 = 0.35 and p = 0.03; ω2 = 0.21, respectively). However, SICI did not differ between groups at 15% MVC (p = 0.62). In conclusion, chronic resistance training significantly reduces SICI. This suggests the presence of an adaptive process of inhibitory and facilitatory network activation, which may cancel out the SICI, allowing for increased corticomotor drive to the exercised muscle following a long period of resistance training.

We have previously shown that supraspinal excitability is higher during arm cycling than a position- and intensity-matched tonic contraction. The present study sought to determine if short-interval intracortical inhibition (SICI) was present during arm cycling and if so, if the amount of SICI was different from an intensity-matched tonic contraction. SICI was assessed using conditioning stimuli (CS) of 70 and 90% of active motor threshold (AMT) and a test stimulus (TS) of 120% AMT at an interstimulus interval (ISI) of 2.5 ms. SICI was elicited in all participants; on average (i.e., cycling and tonic contraction grouped) test MEP amplitudes were reduced by 64.2% (p

We examined the effect of temporal context on discrimination of intervals marked by auditory, visual and tactile stimuli. Subjects were asked to compare the duration of the interval immediately preceded by an irrelevant "distractor" stimulus with an interval with no distractor. For short interval durations, the presence of the distractor affected greatly the apparent duration of the test stimulus: short distractors caused the test interval to appear shorter and vice versa. For very short reference durations (≤ 100 ms), the contextual effects were large, changing perceived duration by up to a factor of two. The effect of distractors reduced steadily for longer reference durations, to zero effect for durations greater than 500 ms. We found similar results for intervals defined by visual flashes, auditory tones and brief finger vibrations, all falling to zero effect at 500 ms. Under appropriate conditions, there were strong cross-modal interactions, particularly from audition to vision. We also measured the Weber fractions for duration discrimination and showed that under the conditions of this experiment, Weber fractions decreased steadily with duration, following a square-root law, similarly for all three modalities. The magnitude of the effect of the distractors on apparent duration correlated well with Weber fraction, showing that when duration discrimination was relatively more precise, the context dependency was less. The results were well fit by a simple Bayesian model combining noisy estimates of duration with the action of a resonance-like mechanism that tended to regularize the sound sequence intervals.

The paired-pulse transcranial magnetic stimulation (TMS) paradigm is increasingly employed to examine intracortical inhibitory processes in different motor tasks. Short-interval intracortical inhibition (SICI) has been shown to vary with the size of the MEP elicited by the test TMS pulse. This suggests that factors that alter MEP size, such as changes in cortical excitability, may confound the interpretation of SICI. However, the effect of excitability on SICI has not been systematically investigated. The present study examined SICI in 11 volunteers. The effect of test TMS intensities ranging from 90 to 150% resting motor threshold (RMT) on SICI was examined in three excitability states in the right first dorsal interosseous muscle: rest, isometric abduction of the left index finger (Contra) and isometric abduction of the right index finger (Active). For all excitability states SICI was not observed when test TMS intensity was less than 110% resting motor threshold. This was true even for the Active condition in which 90 and 100% test TMS intensities elicited large and consistent MEPs. For all conditions moderately suprathreshold test TMS intensities (110-120% RMT) yielded the greatest measure of SICI; increasing test TMS intensities resulted in a progressive reduction in the estimate of SICI. These results suggest that estimates of SICI are systematically affected by the intensity of the test TMS pulse, regardless of excitability state. The results suggest that SICI should be examined using a constant test TMS intensity regardless of changes in cortical excitability and test MEP size.

Human sensorimotor synchronization is flexible but subject to temporal constraints. Previous research has shown that musicians tend to lose synchrony with target tones in an isochronous sequence when the sequence rate exceeds 8–10 Hz, presumably because phase correction ceases to function. The present study investigated directly the time required for an immediate phase correction response (PCR). Musicians tapped in synchrony with cyclic two-interval (short-long) rhythms, using the two hands in alternation. Perturbations were applied to the long interval, and the compensatory shift of the next tap (the PCR) was measured following the short interval, whose duration was varied from 100 to 300 ms. The PCR was found to increase gradually within this range, being nearly absent at 100 ms. Similar results were obtained when participants tapped only with the second tone in each rhythmic group, which confirms that the PCR is based on the preceding tone rather than on the preceding tap-tone asynchrony, and also when the second tone was omitted in the pacing sequence, which indicates that the PCR occurs automatically even when there is no synchronization target for the critical tap. These results extend earlier findings regarding rate limits of synchronization and also provide further support for an event-based phase resetting account of the PCR.