What Spatial Frames of Reference are Used to Guide Off-Axis Aiming?

What spatial frames of reference are used to guide body centred actions such as pointing and aiming? Do observers learn to use body-scaled frames of reference with specific points of origin (eg hip vs shoulder)? Do they extrapolate general information about body scaling? Do they learn about the position of the body (and its individual parts) in absolute space? We investigated these questions using two kinds of aiming: off-axis aiming vs along-axis aiming. Subjects aimed a ‘gun’ (a laser pointer mounted at the end of a stick with a button at the other end) at various targets under three conditions. They practised aiming in one position for 60 ‘shots’ and then changed to a new position and completed another 60 shots. Transfer across position of ‘gun’ in absolute space, across position of body in absolute space, and across position of gun relative to body were assessed. Results suggested that off-axis aiming was controlled by a closed-loop error correction system requiring sight of results for learning to occur. Further, changes in the position of the body in absolute space (from kneeling to standing or vice versa) led to the worst aiming performance. Changes in gun position relative to the body or in absolute space had less effect on aiming accuracy, which suggests that it is not dependent on specific learned sensory - motor linkages. On the basis of these results we propose a preliminary model for off-axis training.