Muscles that see: early muscle activations are time-locked to the onset of visual targets
King, Geoffrey Llewellyn
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The visual grasp reflex provides automatic orienting of gaze (the visual axis in space) to novel visual stimuli. Previous studies have demonstrated activation of neck muscles of awake monkeys appearing at a short fixed latency (55 to 95 ms) after visual target presentation, regardless of whether or when saccades are made. The purpose of these early visually-driven muscle activations may be to prime head rotation required as a part of the coordinated eye-head movement to the target. Similar orienting responses might be found for visually guided reaching. Here, we explore early visually-driven muscle activations of the human upper limb immediately preceding planar reaching movements. Subjects performed reaches towards small visual peripheral targets while upper limb kinematics were recorded and intramuscular electromyography was collected from four shoulder and elbow muscles. Subjects maintained their right hand at a central fixation marker that was extinguished for a gap period (200 ms) prior to appearance of a peripheral target. Subjects were instructed to reach to the target as quickly as possible. Some subjects exhibited a short burst of muscle activity (about 20 ms duration) time-locked to visual target onset. This burst occurred around 85 ms to 105 ms after target onset and preceded the onset of muscle activity associated with volitional arm motion by about 100 ms. Notably, this burst was dependent on target location: visually-driven muscle activity occurred in right shoulder extensor muscles for rightward targets and was absent for leftward targets. In order to better dissociate the visual burst from volitional motor activity, we employed a delay paradigm. No time-locked muscle activity was present in the delay task either after the target appeared or after the fixation marker was extinguished. This suggests that the visual burst is dependent on the imminence of voluntary movement and the laterality of the target. We conclude that the appearance of a visual target can result in short-latency activity on the arm musculature that is appropriate for orienting the arm to the target.