Involvement of the hippocampus and associative parietal cortex in the use of proximal and distal landmarks for navigation

Rats with dorsal hippocampus or associative parietal cortex (APC) lesions a nd sham-operated controls were trained on variants of the Morris water maze navigation task. In the 'proximal landmark condition', the rats had to loc alize the hidden platform solely on the basis of three salient object landm arks placed directly in the swimming pool. In the 'distal landmark conditio n', rats could rely only on distal landmarks (room cues) to locate the plat form. In the 'beacon condition', the platform location was signaled by a sa lient cue directly attached to it. Rats with hippocampal lesions were impai red in the distal and to a less extent in the proximal landmark condition w hereas rats with parietal lesions were impaired only in the proximal landma rk condition. None of the lesioned groups was impaired in the beacon condit ion. These results suggest that the processing of information related to pr oximal, distal landmarks or associated beacon are mediated by different neu ral systems. The hippocampus would contribute to both proximal and distal l andmark processing whereas the APC would be involved in the processing of p roximal landmarks only. Navigation relying on a cued-platform would not req uire participation of the hippocampus nor the APC. Assuming that the proces sing of proximal landmarks heavily depends on the integration of visuospati al and idiothetic information, these results are consistent with the hypoth esis that the APC plays a role in the combination of multiple sensory infor mation and contributes to the formation of an allocentric spatial represent ation. (C) 2000 Elsevier Science B.V. All rights reserved.