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Hippocampal subfields and their neocortical interactions during autobiographical memory using submillimeter whole-brain fMRI at 7 Tesla

Recent advances in ultra-high field 7 Tesla magnet resonance imaging (7T MRI) have provided unprecedented opportunities to gain insights into the brain mechanisms supporting human cognition. The hippocampus, a heterogeneous brain structure comprising several smaller subfields has been firmly established to play a central role during vivid re-experiencing of autobiographical memories (AM). However, due to technical limitations inherent to MRI investigations of the hippocampus, how hippocampal subfields differentially support AM and how hippocampal subfields functionally connect with other brain regions typically associated with AM retrieval remains elusive. To tackle this knowledge gap, we leveraged technical advances of parallel imaging and employed a customized Echo Planar Imaging (EPI) sequence at 0.9 mm isotropic voxel size over the whole brain while participants re-experienced vivid, detail-rich AM. In addition, we developed a processing pipeline that allows the examination of differential activation of hippocampal subfields in single subject space as well as hippocampal-neocortical interactions at a group level. We found that all hippocampal subfields were engaged during AM retrieval but that one specific hippocampal subfield, the pre/parasubiculum, contributed over and above the other subfields to AM retrieval. Moreover, functional connectivity analyses revealed that the pre/parasubiculum was the only hippocampal subfield that strongly connected to other brain regions typically associated with AM, such as ventromedial prefrontal cortex (vmPFC) and medial/lateral parietal regions. Our results support recent proposals that the pre/parasubiculum may play an essential role in cognitive tasks that rely heavily on visual imagery, such as the vivid re-experience of personal past events.