Effects of Attentional Focus on the Corticospinal Excitability during Anticipatory Postural Adjustments in Dart Throwing

Abstract

Attentional focus can affect motor performance and learning. It is known that external focus (EF) which concentrated on the movement outcome leads better performance and efficient motor output rather than internal focus (IF) which concentrated on one's body movement. It remains unclear whether the attentional focus modulates the corticospinal excitability during anticipatory postural adjustments (APA). In the present study, we using transcranial magnetic stimulation (TMS) examined the excitability changes in the primary motor cortex (M1) during dart throwing with different attentional focuses. We hypothesized that the APA duration and magnitude is different between EF and IF conditions, and the corticospinal excitability of lower leg muscle increases in the phase of APA, while the extent to which modulation is different between conditions. Fifteen healthy volunteers were asked to perform dart throwing with the dominant hand with a visual cue under the EF condition (focus on the flight trajectory of the dart) and IF condition (focus on the elbow angle of the dominant hand). The participants performing the tasks stood on a force plate with the legs closed, and we measured the displacement of center of pressure (COP). Electromyography (EMG) was recorded from the anterior deltoid, triceps brachii (TB), tibialis anterior (TA), and soleus muscles. Because the EMG of TA muscle was clearly observed from about 300 ms before the EMG onset of TB muscle (agonist muscle of throwing), TMS was applied over the hotspot of TA muscle in the contralateral M1 at the timings of the visual cue, 0 to 100 ms prior to the EMG onset of TA muscle, and the EMG onset of TB muscle. The distance from the bulls-eye was significantly shorter with EF than that with IF. Neither the difference in the total length of COP and rectangle area nor in the APA duration and magnitude was observed between EF and IF conditions. The motor evoked potential (MEP) amplitude in the TA muscle at -100ms, -50 ms, TA onset, and TB onset significantly increased in comparison with that at control in both conditions. In particular, MEP at -50 ms, TA onset, and TB onset were significantly larger with EF than those with IF, whereas there were no significant differences in MEP at visual cue and -100 ms between EF and IF conditions. Our results demonstrate that the excitability of corticospinal projections to the lower leg muscle increases immediately prior to a ballistic movement of the upper limb. It suggests that, by changing the attentional focus the cortical contribution to the postural control is modulated which may improve motor performance and efficacy of the movement execution.