DIRECTIONAL VARIATION OF SPATIAL AND TEMPORAL CHARACTERISTICS OF LIMBMOVEMENTS MADE BY MONKEYS IN A 2-DIMENSIONAL WORK SPACE

1. The directional variation of kinematic and electromyographic (EMG) characteristics of two-joint arm movements made to targets in a two-di mensional work space was studied in monkeys trained to make targeted a rm movements under different behavioral conditions. 2. In each animal, kinematic measures of movement (movement amplitude, movement time,pea k velocity, and trajectory curvature) and endpoint spatial position wi thin the target zone varied as a function of the direction of the targ et from the starting position. Movements made toward the body into the ipsilateral hemispace generally had the smallest amplitude, lowest pe ak velocity, and longest movement time. 3. Although the directional va riation in peak velocity could partially be accounted for by predicted anisotropies in the inertial load imposed by the arm, deviations from these predictions suggest that movement amplitude is controlled more rigorously by the CNS. Adjustments in movement time may be used to com pensate for inertial anisotropies. 4. The spatial characteristics of m ovements (amplitude, trajectory curvature, or endpoint error) were inf luenced Little by the visibility of the target during movement, the ad vanced knowledge of target location, or the presence or absence of an external trigger cue. However, temporal characteristics (movement time , peak velocity, and for some animals, reaction time) varied more as s ensory cues were changed. 5. The time of initial EMG activity in muscl es acting around the shoulder varied systematically as a function of t arget direction. A cosine model accounted for a large fraction of the variability in initial onset time, as determined in a trial-by-trial a nalysis. The amplitude of the EMG activity was more narrowly tuned, ho wever. Muscles acting at the elbow showed less activity acid more vari able directional tuning. 6. We conclude that directional variations in the kinematic characteristics of movement, and thus, the dynamic forc e requirements of the task, must be taken into consideration as contri butors to the apparent directional coding described for neuronal popul ations in different portions of the CNS.