The Use of Electromyography (EMG) Signals to Assess Covert Processing of Motor Plans

Abstract

Purpose : Several investigations have previously shown that the oculomotor target selection circuitry operates in a continuous manner: evidence is accumulated by motor neurons as the decision evolves. To determine whether this architecture also holds true for other motor effectors, we asked two questions. First, is EMG onset tightly linked to stimulus onset, motor onset, or neither? Second, are inhibited motor plans reflected in EMG activity?

Methods : To answer our question, we recorded bipolar EMG activity (2048 Hz sampling rate) from the biceps brachii of five participants while each performed a pop-out visual search task. Following the appearance of the array, each participant was instructed to raise his or her right or left arm to indicate the hemi-field in which the pop-out target appeared. EMG signals were filtered (high-pass: 20 Hz, low-pass: 250 Hz), full-wave rectified and smoothed offline.

Results : We aligned EMG activity to both stimulus onset and motor response, which revealed sensory variability and motor constancy. Specifically, there was a constant difference between EMG onset and reach onset (222.92 +/- 63 ms), whereas the time elapsed between stimulus onset and EMG onset scaled with reaction time (rho_R = 0.67. rho_L = 0.63). Analysis of trials which followed a high number of target-polarity repetitions (i.e., priming) did not reveal any subthreshold EMG activity in the contralateral arm despite elevations in reach latency.

Conclusions : The results provided a tight relationship between EMG activity and key-lift latencies, which suggests that EMG activity represents motor activation only. Additionally, the lack of subthreshold activation in the contralateral arm despite a high degree of priming does not indicate that continuous processing is reflected in EMG activity. Thus, our outcome suggests the mode by which the saccade and reach systems accumulate evidence may differ.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.