ABSTRACT
It is remarkable that we are able to perceive a stable visual world and judge the directions, orientations and movements of visual objects given that images move on the retina, the eyes move in the head, the head moves on the body and the body moves in space. An understanding of the mechanisms underlying perceptual stability and spatial judgements requires precise definitions of relevant coordinate systems. An egocentric frame of reference is defined with respect to some part of the observer. There are four principal egocentric frames of reference, a station-point frame associated with the nodal point of the eye, a retinocentric frame associated with the retina, a headcentric frame associated with the head, and a bodycentric frame (torsocentric) associated with the torso. Additional egocentric frames can be defined with respect to any segment of the body. An egocentric task is one in which the position, orientation or motion of an object is judged with respect to an egocentric frame of reference. A proprioceptive task is a special kind of egocentric task in which the object being judged is also part of the body. An example of a proprioceptive task is that of directing the gaze toward the seen or unseen toe. An exocentric frame of reference is external to the observer. Geographical coordinates and the direction of gravity are examples of exocentric frames of reference. These various frames of reference are listed in Table 1, together with examples of judgements of each type.
