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

Interactions of the hippocampus with other brain structures are supposed to support memory formation but knowledge is limited regarding hippocampal task-based functional connectivity (FC) during encoding in both healthy people and people with epilepsy, who frequently have impaired memory. We compared absolute [FC(encoding)] and relative FC (isolating task-specific FC [FC(encoding)-FC(baseline)]) of the anterior hippocampus in 30 controls, 56 mesial temporal (mTLE, 26 right) and 24 frontal lobe epilepsy (FLE) patients using a memory fMRI-task of encoding scenes, faces and words. In controls, absolute hippocampus FC comprised regions typically active in memory fMRI-tasks and the default mode network (DMN): For faces and scenes, FC was pronounced to temporo-occipital areas, whereas for words it extended to lateral-temporal regions. Relative FC was more circumscribed and encompassed temporo-occipital and frontal stimulus-selective regions for scenes and faces. Also, relative FC revealed weaker hippocampus - DMN connectivity during encoding. mTLE patients had decreased FC from the epileptogenic hippocampus and 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' weaker FC to frontal and temporo-occipital regions reflected material-specific changes. Conversely, mTLE patients had higher absolute FC to regions to which the hippocampus is normally anticorrelated and increased relative FC to DMN regions. During word encoding only, FLE patients had increased left hippocampal relative FC to right-sided regions. Together, these findings further delineate the network architecture of memory in healthy people and its dysfunction in focal epilepsies, which prospectively could inform surgical interventions.