Coordination of fast eye and arm movements in a tracking task.

Eye and arm movements to single and double-step target displacements were studied to investigate whether or not the motor systems of the eye and arm share common command signals from the internal representation, which specify a) when a movement is initiated and b) the end position of a movement. The correlation between the time of onset of eye and arm movements was significant for single-step and double-step target displacements into the same direction. However, it was small and not significant for double-step target displacements into opposite directions if the time interval between first and second target displacement was 75 or 100 ms. This indicates that the command signal which specifies when a movement is started is different for both systems. However, the reconstructed signal in the internal representation, which specifies the end position of movements, is rather similar for eye and arm movements which seems to point to a common command signal. This result was corroborated by experiments in which eye and arm were found to jump always to the same target in the condition that two different targets were presented simultaneously.