On systems of reference involved in spatial memory

The system of reference used by an animal for memorizing the location of a hidden place to which it is liable to return (the goal) can be theoretically defined as being `self-centred', `local', or `overall'. According to the system of reference, the goal location is memorized by means of: (1) an egocentric coding that specifies the direction and distance to the goal with respect to the animal's current position; (2) an exocentric coding that specifies the apparent configuration of nearby landmarks as perceived from the goal; or (3) an exocentric coding based on a global representation of space over the whole range of movement, respectively. A compass provides useful directional information (to the species capable of deriving it) that may be used in the framework of any of the three systems of reference. A goal location may be memorized concurrently in the three systems of reference which are complementary rather than mutually exclusive. In particular, the joint use of the self-centred and local systems of reference has been demonstrated in animals of numerous species that are able to shift from the former to the latter while approaching a goal. In contrast, there is no clear evidence supporting that an animal is able to rely on a global representation of space.