Exocentric pointing in depth

Abstract

An exocentric pointing task was used to compare the indicated pointing directions under exchange of target and pointer. Such a pair of pointing directions, together with the pointer and target locations, specifies a unique cubic arc. Such an arc may assume one of two qualitatively different shapes, namely a ‘‘C-arc’’ (constant sign of curvature) or an ‘‘S-arc’’ (containing a point of inflection between the endpoints). We show that human observers most often produce S-curves. This is of fundamental importance, since—in case one interprets the curve as an empirically determined ‘‘pregeodesic’’ (‘‘shortest connection’’, or ‘‘straight’’ connection in visual space)—it would imply that ‘‘visual space’’ in the strict geometrical sense is a non-entity. The experiments were performed in the outside environment, under normal daylight conditions, for distances ranging from one to over thirty meters. The implications of these data are discussed and possible ways to extend the restricted notion of ‘‘visual space’’ (e.g., as advocated by Luneburg) such as to allow one to account for the present results are suggested. Such extensions of the visual space concept include the local adjustment of geometrical structure in regions adjacent to the fixation direction.