Memory related dysregulation of hippocampal function in major depressive disorder

Hippocampal abnormalities have frequently been associated with major depressive disorder (MDD), however evidence of a functional hippocampal deficit has remained illusive. Here, functional magnetic resonance imaging (fMRI) is employed in conjunction with an associative memory paradigm to investigate functional irregularities of the hippocampus during the encoding process. The use of a focussed analytical approach and a behavioural task targeted to hippocampal function confirmed the hypothesis that the normal modulation of hippocampal activation by encoding strength is dysregulated in MDD. Further analysis demonstrated that this impairment of function was specific to the hippocampus. A double dissociation between groups in the hippocampus and intraparietal sulcus indicates that compensatory mechanisms may exist. These results show that MDD is associated with a dysregulation of hippocampal function that cannot be explained in terms of overall brain state or motivational stance and provides an important link between memory impairments and hippocampal changes in MDD.