FRAMES OF REFERENCE IN VISION

Natural vision consists of a sequence of computations that redefine th e frames of reference relative to which signals are assigned. The init ial mapping of external space is in terms of retinocentric signals, th e two sets of which are combined with a signal for binocular eye movem ents to determine an egocentric frame of reference. This defines the d irections in which objects are located, but not their distances. Infor mation 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 fourt h frame of reference, referred to as patterncentric, can be defined by relative displacements of objects. Normally it operates in tandem wit h the other systems, but it can be dissociated from them. The signific ance of these frames of reference for minimal access surgery is discus sed.