KNOWING WHERE THINGS ARE - PARAHIPPOCAMPAL INVOLVEMENT IN ENCODING OBJECT LOCATIONS IN VIRTUAL LARGE-SCALE SPACE

The involvement of the medial temporal-lobe region in allocentric mapp ing of the environment has been observed in human lesion and functiona l imaging work. Cognitive models of environmental learning ascribe a k ey role to salient landmarks in representing large-scale space. In the present experiments we examined the neural substrates of the topograp hical memory acquisition process when environmental landmarks were mor e specifically identifiable. Using positron emission tomography (PET), we measured regional cerebral blood flow changes while normal subject s explored and learned in a virtual reality environment. One experimen t involved an environment containing salient objects and textures that could be used to discriminate different rooms. Another experiment inv olved a plain empty environment in which rooms were distinguishable on ly by their shape. Learning in both cases activated a network of bilat eral occipital, medial parietal, and occipitotemporal regions. The pre sence of salient objects and textures in an environment additionally r esulted in increased activity in the right parahippocampal gyrus. This region was not activated during exploration of the empty environment. These findings suggest that encoding of salient objects into a repres entation of large-scale space is a critical factor in instigating para hippocampal involvement in topographical memory formation in humans an d accords with previous studies implicating parahippocampal areas in t he encoding of object location.