Human gaze behaviour during action execution and observation.

Gaze shifts and fixations appear to be proactive in both action execution and observation. We investigated a dependency of anticipatory gaze behaviour by using a block stacking task. Blocks were rectangles depicted on a computer screen and the stacking movements were controlled via computer mouse. Subjects either had to execute the task or had to observe it made by the experimenter, or by the computer. The dependency of gaze behaviour on the visibility of a virtual effector, the visibility of the actor, and the nature of the actor was tested by measuring eye movements. Anticipatory eye movements were predominant when the subjects themselves executed the task. During action observation, gaze behaviour did neither depend on the visibility nor depend on the nature of the actor. However, big variability was found between the subjects suggesting the use of two different strategies in action observation: some subjects were mainly tracking the blocks during stacking movements; others were strongly anticipating. We suggest that gaze behaviour during action observation is not predetermined by rigid neural circuitry, but strongly depends on the context. The possibility to explain the causal mechanism, as well as the ownership of the action may be crucial preconditions for anticipatory gaze behaviour.