Short latency afferent inhibition (SAI) refers to a decrement of the size of a motor evoked potential (MEP) by transcranial magnetic stimulation (TMS) after electrical stimulation of a peripheral afferent nerve (PNS) (Tokimura et al. 2000). Since SAI occurs when TMS is applied at the time of the arrival of the peripheral afferent volley at the sensorimotor cortex, it likely reflects sensory activation of inhibitory intracortical circuits (Alle et al. 2009). In the present study, we aimed to investigate modulation of this inhibitory effect prior to movement (flexion of the index finger) compared to rest.
In 9 healthy subjects (23 ± 3 years) MEPs were recorded in m. first dorsal interosseus (FDI) and m. abductor pollicis brevis (APB). To induce SAI, n. medianus was stimulated 20 ms prior to TMS over the motorcortex. In control trials, interstimulus intervals (ISIs) between afferent stimulation and TMS of 15, 25, 30, 35, and 40 ms were additionally tested. When moving, subjects were told to react to the afferent stimulus, and the delay between TMS and EMG onset was 343 ± 160 ms. When tested prior to movement, MEPs elicited with a delay of 20 ms after PNS were significantly greater (23 ± 9 %) in FDI (the muscle involved in the motor task) compared to MEPs at rest (p < 0.01). There was no change of the MEPs in APB. There were also no changes in either the FDI or APB MEPs for the other tested ISIs prior to movement compared to rest. The main finding of this study was that the MEP elicited with a delay of 20 ms after PNS was facilitated when tested prior to movement compared to rest. The considerable delay between TMS and EMG onset as well as the unaltered MEP size in FDI in the control trials make it unlikely that the increased MEP prior to movement was based on an increased susceptibility of corticospinal cells to TMS, which starts approximately 100 ms prior to the onset of movement (Chen et al. 1998). Thus, it is hypothesized that the modulation of the MEP prior to movement is linked to the afferent volley arriving at the sensorimotor cortex. It might be speculated that the MEP was facilitated because the afferent information triggered the movement and therefore was important for motor performance. Alle et al. (2009). J Physiol 587:5163-5176 Chen et al. (1998). Ann Neurol 44:317-325 Tokimura et al. (2000). J Physiol 523 Pt 2:503-513