Pointing in Three-Dimensional Space
We studied the localisation of objects in three-dimensional space. We had subjects direct a small pointer towards a small goal object, from 20 positions on a virtual sphere around the goal. Images of the pointer and the goal were generated by presenting computer images to the subject's left and right eye alternately. The distance between the goal and pointer was approximately 10 deg arc, the length of the pointer was approximately 2 deg arc. Subjects could manipulate the pointer by pressing specific keys on the keyboard. We tested 7 subjects. The adjustments were repeated 5 – 7 times, which resulted in a cluster of indicated directions for each of the 20 pointer positions and each subject. These clusters appeared to lie in a plane perpendicular to the frontoparallel plane. In other words, the variance in the in-depth component of the adjustments was considerably larger than the frontoparallel component. Subjects showed consistent and individual deviations in the in-depth adjustments. All subjects showed very similar constant deviations in the frontoparallel components of the adjustments. These constant deviations proved comparable to the deviations demonstrated earlier in a three-dot alignment task in the frontoparallel plane. We conclude that the three-dimensional pointing task can be seen as a combination of two independent tasks: an in-depth adjustment and a frontoparallel task.