On Inter- and Intrahemispheric Differences in Visuospatial Perception

After a brief description of how visual information travels from the retina to the cortex, two fundamental distinctions within visuospatial perception are discussed. First, spatial relations between objects can be represented either categorically, "left of" or "above," or coordinately, in which metric distances are taken into account. These two types of representations are dissociated in terms of neural correlates, regardless of stimulus type and precise task at hand. Recent findings indicate that also the scope of attention as used during spatial relation processing affects this dissociation.The second distinction is between egocentric (ie, body-based) and allocentric (ie, scene/object-based) frames of reference. Behavioral and neural evidence supporting the existence of the two frames of reference is reported and their functional role within the perception-action model by Milner and Goodale (1995) is discussed. Final, several experiments exploring the interaction between coordinate and categorical spatial relations and egocentric and allocentric frames of reference are presented.