EMG and electrooculography acquisition.

EMG activities from the clavicular head of the right pectoralis major (PEC) muscle were recorded using intramuscular (experiment 1) and/or surface recordings (all experiments). Intramuscular EMG activity was recorded using fine-wire electrodes (A-M Systems, Sequim, WA) inserted into the PEC muscle (see Wood et al. 2015 for insertion procedure). Briefly, for each recording we inserted two monopolar electrodes ~2.5 cm into the muscle belly of the PEC muscle, enabling recording of multiple motor units. Insertions were aimed ~1 cm inferior to the inflection point of the participant’s clavicle and were staggered by 1 cm along the muscle’s fiber direction. All intramuscular EMG data were recorded with a Myopac Jr. system (Run Technologies, Mission Viejo, CA). Surface EMG was recorded with double-differential electrodes (Delsys, Natick, MA) placed either near or at the same location as the intramuscular recordings. In experiment 1, horizontal eye position was measured using bitemporal direct current electrooculography (EOG; Grass Instruments, Astro-Med). EMG and EOG data were digitized and sampled at 4 kHz.