Reaching with the arm to a newly appearing target is usually preceded by a saccadic eye movement. Neurons in the superior colliculus (SC) constitute one important brain structure controlling saccades. Yet, the SC also contains reach neurons activated during arm movements, whose location extends also deeper into the underlying mesencephalic reticular formation. Reach neurons can be divided into two classes based on their different modulation with respect to gaze position. For the first class, the gaze-independent reach neurons, the activity does not depend on which location is currently fixated, but solely on the position and movement of the (usually contralateral) arm. There is a correlation of the activity of these neurons with the activity of shoulder muscles. The second class, the gaze-related reach neurons, are active for reaches into a specific area relative to the current point of gaze. This means the target has to project on a certain part of the retina, while it is not important which arm is used or by which trajectory the target will be reached. Many fixation neurons in the rostral pole of the SC discharge tonically during fixation and pause during saccades. For some fixation neurons, the activity can be increased during simultaneous arm movements, for others decreased. Two psychophysical experiments with healthy human subjects show possible behavioral correlates of an interaction between these reach neurons and visuomotor neurons within the SC.
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