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 three inter-related limbic structures: the mammillary bodies, the fornix and the cingulum bundle/cingulate cortex. Cytotoxic lesions of the mammillary nuclei produced a marked deficit on reinforced T-maze alternation, but performance gradually improved with practice. Subsequent tests 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 rats with different patterns of either crossed or unilateral radio frequency lesions of the cingulate region were given the same tasks. The profile of results indicated that disruption of those fibres in the cingulum bundle connecting the anterior thalamic nuclei with the hippocampal/retrohippocampal region was responsible for the observed impairments to T-maze alternation and radial-arm maze performance. There was also evidence that disconnection of frontal connections in the cingulum bundle might affect perseverative behaviour, but not allocentric processing. The results add support to the notion of a functional circuit that involves projections from the hippocampus to the mammillary bodies and anterior thalamic nuclei, and from there back to hippocampal/retrohippocampal regions via the cingulum bundle. This circuit appears to be vital for normal allocentric processing.