The coordination of eye, head, and hand movements in a natural task

Relatively little is known about movements of the eyes, head, and hands in natural tasks. Normal behavior requires spatial and temporal coordination o f the movements in more complex circumstances than are typically studied, a nd usually provides the opportunity for motor planning. Previous studies of natural tasks have indicated that the parameters of eye and head movements are set by global task constraints. In this experiment, we explore the tem poral coordination of eye, head, and hand movements while subjects performe d a simple block-copying task. The task involved fixations to gather inform ation about the pattern, as well as visually guided hand movements to pick up and place blocks. Subjects used rhythmic patterns of eye, head, and hand movements in a fixed temporal sequence or coordinative structure. However, the pattern varied according to the immediate task context. Coordination w as maintained by delaying the hand movements until the eye was available fo r guiding the movement. This suggests that observers maintain coordination by setting up a temporary, task-specific synergy between the eye and hand. Head movements displayed considerable flexibility and frequently diverged f rom the gaze change, appearing instead to be linked to the hand trajectorie s. This indicates that the coordination of eye and head in gaze changes is usually the consequence of a synergistic linkage rather than an obligatory one. These temporary synergies simplify the coordination problem by reducin g the number of control variables, and consequently the attentional demands , necessary for the task.