Corticospinal excitability modulation to hand muscles during movement imagery

Motor evoked potentials (MEPs) to magnetic transcranial stimulation (TCS) w ere recorded from right abductor digiti minimi (ADM) and first dorsal inter osseous (FDI) muscles, sharing the same peripheral innervation but engaged in two different motor demands. In seven healthy and trained subjects, the latencies, amplitudes and variability of MEPs were investigated under the f ollowing, randomly intermingled, conditions: full muscular and mental relax ation; mental simulation of selective index finger or little finger abducti on; mental non-motor activity (arithmetical calculation); and real motor ta sk (little and index finger abduction). The whole procedure was performed b y continuous audiovisual monitoring of electromyographic 'silence' in the t ested muscles. The maximal facilitatory effects (= latency shortening and a mplitude increase) on MEPs were induced by the real motor task. An amplitud e potentiation of MEPs in both tested muscles was present during non-motor mental activity, in comparison to basal values. A further amplitude potenti ation, without latency shifts, was confined to the muscle acting as 'prime mover' for the mentally simulated movement, according to the motor program dispatched but not executed by the subject. Similar results were also found in the F-wave, showing that mental simulation affects spinal motoneuronal excitability as well, although - due to the lack of MEP and F-wave latency shift - the main effect takes place at cortical level. The study shows that movement imagery can focus specific facilitation on the prime mover muscle for the mentally simulated movement. This is mainly evident on FDI muscle, which controls fingers (i.e. the index) with highly corticalized motor rep resentation.