EVIDENCE FOR THE INVOLVEMENT OF THE MAMMILLARY BODIES AND CINGULUM BUNDLE IN ALLOCENTRIC SPATIAL PROCESSING BY RATS

Comparisons were made between the behavioural effects of lesions in th ree inter-related limbic structures: the mammillary bodies, the fornix and the cingulum bundle/cingulate cortex. Cytotoxic lesions of the ma mmillary nuclei produced a marked deficit on reinforced T-maze alterna tion, but performance gradually improved with practice. Subsequent tes ts in a cross-maze and a radial-arm maze showed that the animals with mammillary body lesions failed to use allocentric cues, but were able to perform normally in an egocentric discrimination. Three groups of r ats with different patterns of either crossed or unilateral radio freq uency lesions of the cingulate region were given the same tasks. The p rofile of results indicated that disruption of those fibres in the cin gulum bundle connecting the anterior thalamic nuclei with the hippocam pal/retrohippocampal region was responsible for the observed impairmen ts to T-maze alternation and radial-arm maze performance. There was al so evidence that disconnection of frontal connections in the cingulum bundle might affect perseverative behaviour, but not allocentric proce ssing. The results add support to the notion of a functional circuit t hat involves projections from the hippocampus to the mammillary bodies and anterior thalamic nuclei, and from there back to hippocampal/retr ohippocampal regions via the cingulum bundle. This circuit appears to be vital for normal allocentric processing.