Frames of reference in vision

Natural vision consists of a sequence of computations that redefine the frames of reference relative to which signals are assigned. The initial mapping of external space is in terms of retinocentric signals, the two sets of which are combined with a signal for binocular eye movements to determine an egocentric frame of reference. This defines the directions in which objects are located, but not their distances. Information for body movement and apparent distance modify egocentric into geocentric signals. This final frame of reference reflects an internal representation in dimensional terms that can guide behaviour. A fourth frame of reference, referred to as patterncentric, can be defined by relative displacements of objects. Normally it operates in tandem with the other systems, but it can be dissociated from them. The significance of these frames of reference for minimal access surgery is discussed.