Separate simultaneous processing of egocentric and relative positions

It is well established that all kinds of visual attributes are processed se parately within the brain. This separation is related to differences in the information that is relevant for the different attributes. When attributes differ greatly (such as colour and motion) it is obvious that they must re ly on different information. However, separating the processing of differen t attributes could also allow highly related attributes to evolve independe ntly, so that they end up being judged on the basis of different types of i nformation. Here, we examine the case of egocentric and relative localisati on. For judging egocentric positions, the orientation of the eyes has to be taken into account. This is not so for judging relative positions. We demo nstrate that these two attributes can be processed separately by showing th at simultaneous judgements of relative and egocentric position differ in th eir dependency on eye orientation. Subjects pursued a moving dot. We Bashed either single targets, or pairs of targets with a 67 ms interval between t hem, directly below the subjects' gaze. As the eyes were moving during the 67 ms interval, the retinal separation between pairs of targets was differe nt from their actual separation. Subjects indicated the position at which t hey saw the targets with reasonable reproducibility, with a consistent bias in the direction of the eye movement. However, when two targets were flash ed, the indicated separation between them usually coincided with their reti nal separation, rather than with their actual separation. We conclude that egocentric and relative spatial positions can be estimated separately and s imultaneously, on the basis of different types of information. (C) 2000 Els evier Science Ltd. All rights reserved.