Acoustic Startle Evokes Bilaterally Synchronous Oscillatory EMG Activity in the Healthy Human

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Abstract
Despite animal evidence that the reticulospinal system is of major importance to movement, this motor pathway has remained relatively inaccessible to experimentation in the human. Consequently, little is known about its function in health and disease. Here, we use the acoustic startle response to demonstrate that one type of reticulospinal activity in the human is associated with a characteristic pattern of bilateral synchronization between motor units. Surface electromyography (EMG) was recorded from upper limb muscles in 15 healthy subjects during the reflex startle to unexpected acoustic stimulation, voluntary movements mimicking the startle and during sustained voluntary tonic contraction. Frequency analysis demonstrated autospectral peaks at ∼14 Hz in deltoid and biceps muscles only during the startle reflex. Similarly, coherence spectra of the EMG recorded between homologous proximal upper limb muscles demonstrated a peak centered ∼12–16 Hz during reflex startles. Coherence in the 10- to 20-Hz band was significantly greater in the startle reflex than during voluntary sham startles or voluntary tonic contraction for deltoid, but not first dorsal interosseous, muscles. The coherence at 10–20 Hz between EMGs from homologous muscles represents a potential surrogate measure of reticulospinal activity that may be useful in determining the contribution of the reticulospinal system to different types of movement in health and disease.