Visually triggered eye-arm movements in man.
The degree of temporal coupling and the nature of temporal ordering between the gaze (eye-head) and arm motor systems was studied in normal subjects by measuring the reaction time, RT, of each system to randomly generated visual signals presented along the horizontal meridian: (1) at two spatial locations, 15.5 degrees and 29.1 degrees, (2) to the right and left of a central fixation point, and (3) with and without room illumination. Upon detecting the visual signal, the subject was instructed to guide the arm from its rest position upon a supporting table and to extinguish the light source by touching a metal disc enclosing the stimulus "as quickly and accurately as possible." The results indicated (1) saccadic eye motion initiated gaze towards the target, (2) a change in the eccentric position of the target was associated with alteration of the eye and arm RT by a similar value; the interval between eye and the subsequent arm RT was invariant with spatial location no matter the presence or absence of room illumination, and (3) the arm RT increased when visual information of the environment was not available. It is postulated that a visual stimulus triggers a command system releasing a patterned sequence of eye-hand-arm motion; the initiation of motion in these motor systems is based upon the same retinal error calculation; however, no rigid rule can be established for the serial ordering or temporal sequencing of motion between the triggered motor systems. The temporal arrangement between head and arm with eye can be modified, at least in part, by alterations in supraspinal control of postural mechanisms and/or motoneuronal excitability of the responding muscles prior to motion.