A COMPARISON OF THE EFFECTS OF MEDIAL PREFRONTAL, CINGULATE CORTEX, AND CINGULUM BUNDLE LESIONS ON TESTS OF SPATIAL MEMORY - EVIDENCE OF A DOUBLE DISSOCIATION BETWEEN FRONTAL AND CINGULUM BUNDLE CONTRIBUTIONS

Rats were trained on an automated delayed nonmatching-to-position (DNM P) task, They then received cytotoxic lesions in either the medial pre frontal cortex (n = 13) or the cingulate and retrosplenial cortices (n = 8), or radiofrequency lesions in either the fornix (n = 6) or the c ingulum bundle (n = 8). Twelve animals served as surgical controls, On ly the fornical and medial prefrontal lesions disrupted DNMP performan ce, both groups showing a loss of accuracy and an increase in bias, Th e rats were then trained on a lever discrimination and reversal task, the medial prefrontal and fornical groups showing evidence of an incre ase In bias when compared with the cingulate cortex group, Finally, th e rats were trained on a forced alternation task in a T-maze, Marked d eficits were observed in the fornix and cingulum bundle groups, but th e medial prefrontal and cingulate groups were unimpaired. The double d issociation between the effects of the prefrontal and cingulum bundle lesions highlights the very different nature of the two spatial tasks (DNMP and T-maze alternation), even though both involved a nonmatching rule, These findings may reflect the involvement of divergent outputs from the fornix-anterior thalamic pathway, One possibility is that an terior thalamic projections to the medial prefrontal cortex are concer ned with processing egocentric information, while anterior thalamic pr ojections to temporal regions via the cingulum bundle are concerned wi th allocentric information, The results also indicate that the effects of conventional lesions in the cingulate cortex and medial prefrontal cortex may be compromised by additional damage to the cingulum bundle .