Encoding-related hippocampus connectivity for scenes, faces, and words: Healthy people compared to people with temporal and frontal lobe epilepsy

Description: Interactions of the hippocampus with other brain structures are supposed to support memory formation but so far, there is little knowledge regarding task-based functional connectivity (FC) of the hippocampus during encoding, both in healthy people and in people with epilepsy, who are prone to memory deficits. We compared absolute FC during encoding and relative FC (FC during encoding compared to baseline, isolating task-specific FC) of the anterior hippocampus in 30 controls and 56 mesial temporal lobe epilepsy (mTLE, 26 right, 30 left) and 24 frontal lobe epilepsy (FLE, 15 right, eight left, one bilateral) patients using a memory fMRI-task of encoding scenes, faces and words. In controls, absolute hippocampus FC comprised regions typically active in memory fMRI studies and regions of the default mode network (DMN) with some material-specific differences: For faces and scenes, FC was pronounced to temporo-occipital areas, whereas for words it extended to lateral-temporal regions. Negative absolute FC was found to the ventral attention and the fronto-parietal control networks. Relative hippocampus FC was more circumscribed and encompassed temporo-occipital and frontal stimulus-selective regions for scenes and faces but showed no effects for words. Relative FC specified that the hippocampus was less connected to regions of the DMN during encoding than during baseline. mTLE patients had decreased FC primarily from the epileptogenic hippocampus but also slight disruptions from the contralateral hippocampus. Decreased absolute FC was found to the contralateral mTL, the precuneus and the posterior cingulate gyrus. Further, mTLE patients had weaker FC to frontal and temporo-occipital regions reflecting material-specific connectivity changes. By contrast, mTLE patients exhibited increased absolute FC to regions to which the hippocampus is normally anticorrelated and increased relative connectivity to regions of the DMN. During word encoding only, FLE patients had increased relative connectivity from the left hippocampus to right-sided regions, which are less functionally decoupled from the hippocampus during the encoding condition than during baseline. Together, the combination of absolute and relative task-based hippocampus connectivity proved valuable to investigate intrinsic and task-specific connectivity during encoding. Our findings expand scientific knowledge on the network architecture of memory in healthy people and of memory impairment in focal epilepsies. In the future, task-based hippocampus connectivity might be helpful to predict patients’ postoperative memory changes.