Human functional imaging studies have identified a distributed network of cortical brain regions that display temporally coordinated activity during wakeful rest, commonly termed the default mode network (DMN). The DMN is largely conserved across species and includes a number of subsystems that converge onto primary DMN core hubs in medial prefrontal and posterior cingulate cortices. The interactions between these hubs and subsystems, and their contribution to the overall integrity of DMN functional connectivity, remain relatively unexplored. Because the hippocampus is vulnerable to a variety of neurodegenerative conditions that affect memory, we aimed to directly test the effects of damage to the hippocampus on DMN dynamics. Healthy adult male subjects received selective lesions of the hippocampus. Postoperative structural imaging confirmed the extent and selectivity of the lesions. Sex and age-matched intact adults served as controls. For DMN fMRI imaging, subjects were scanned on a Siemens Tim Trio 3T scanner using an EPI sequence (scan parameters: TR =1700ms, TE = 27ms, FOV = 384384mm, matrix = 6464, slice thickness = 1.8mm). Initial results confirmed that intact normal subjects display a temporally coordinated pattern of activity distributed across a number of cortical areas, including the medial prefrontal and posterior cingulate cortex, similar to the human DMN. Ongoing analysis will directly test, for the first time in an animal model, whether the integrity of the hippocampus is necessary for normal resting state cortical network connectivity.
