Short-latency sensory afferent inhibition: conditioning stimulus intensity, recording site, and effects of 1 Hz repetitive TMS

BackgroundThe transcranial magnetic stimulation (TMS) paradigm short-latency sensory afferent inhibition (SAI) investigates sensori-motor integration. Conventionally, one stimulation intensity is used for the conditioning pulse to the peripheral nerve.Objective/hypothesisTo examine the variability, the dimension of stimulus intensity, and recording site in SAI.MethodsIn 17 healthy individuals three peripheral nerve stimulation intensities were used: Just above sensory threshold, just above motor threshold, and in between. Motor evoked potentials (MEPs) and long-loop reflexes were recorded from first dorsal interosseus (FDI) and abductor pollicis brevis (APB) before and after repetitive motor cortex TMS (1 Hz, 1800 stimuli at 95% resting motor threshold).ResultsBetween-subjects variability of SAI was higher than variability between sessions. Median, or ulnar, nerve stimulation decreased MEP size in FDI and APB at interstimulus intervals of N20, N20+2, and N20+4. Only with median nerve stimulation MEP size increased in APB, but not FDI, at N20+8 to N20+16. These effects increased with increasing stimulation intensity. RTMS reduced MEP size but had no effect on SAI, or transcortical reflexes.ConclusionsEffects on MEP size in SAI depend on stimulus intensity and are not limited to anatomically homotopic muscles. Inhibitory rTMS modulates motor output but not the interaction of sensory inputs with the motor cortex.