COMPARATIVE EFFECTS OF EXCITOTOXIC LESIONS OF THE HIPPOCAMPUS AND SEPTUM DIAGONAL BAND ON CONDITIONAL VISUAL-DISCRIMINATION AND SPATIAL-LEARNING

Several experiments compared the effects of excitotoxic lesions of the septal/vertical limb nuclei of the diagonal band of Broca (VDB) compl ex with those of the hippocampus (sparing the subiculum) on different forms of visual discrimination learning. The septal/VDB lesions, which produced significant reductions in choline acetyltransferase activity in the hippocampus and the cingulate cortex, impaired acquisition of a conditional visual discrimination in an operant chamber, while the h ippocampal lesion had no effect, unless there was a delay interposed b etween the discriminative stimulus and the response. Neither lesion af fected simple visual or spatial discrimination or reversal learning, a lso carried out in operant chambers, but both significantly impaired t he acquisition and retention of a spatial navigation task (Morris wate r maze), with the septal/VDB lesions again producing greater deficits than the hippocampal lesions. Possible explanations for this surprisin g result are discussed and it is concluded that; (1) additional cholin ergic de-afferentation of the cingulate cortex produced by the septal/ VDB lesion is of functional significance; (2) this may lead to deficit s in conditional rule learning, which can contribute to spatial naviga tion performance under certain circumstances; and (3) the contribution of septal-hippocampal cholinergic projections to spatial learning is in need of re-appraisal.