title: Human neuroimaging studies on the hippocampal CA3 region - integrating evidence for pattern separation and completion.
abstract: Human functional magnetic resonance imaging (fMRI) studies have long investigated the hippocampus without differentiating between its subfields, even though theoretical models and rodent studies suggest that subfields support different and potentially even opposite functions. The CA3 region of the hippocampus has been ascribed a pivotal role both in initially forming associations during encoding and in reconstructing a memory representation based on partial cues during retrieval. These functions have been related to pattern separation and pattern completion, respectively. In recent years, studies using high-resolution fMRI in humans have begun to separate different hippocampal subregions and identify the role of the CA3 subregion relative to the other subregions. However, some of these findings have been inconsistent with theoretical models and findings from electrophysiology. In this review, we describe selected recent studies and highlight how their results might help to define different processes and functions that are presumably carried out by the CA3 region, in particular regarding the seemingly opposing functions of pattern separation and pattern completion. We also describe how these subfield-specific processes are related to behavioral, functional and structural alterations in patients with mild cognitive impairment and Alzheimer's disease. We conclude with discussing limitations of functional imaging and briefly outline possible future developments of the field.
authors.affiliation: Department of Epileptology, University of Bonn Bonn, Germany ; Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen Nijmegen, Netherlands.
authors.forename: Lorena
authors.initials: L
authors.lastname: Deuker
authors.affiliation: Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen Nijmegen, Netherlands.
authors.forename: Christian F
authors.initials: CF
authors.lastname: Doeller
authors.affiliation: Department of Epileptology, University of Bonn Bonn, Germany.
authors.forename: Juergen
authors.initials: J
authors.lastname: Fell
authors.affiliation: Department of Epileptology, University of Bonn Bonn, Germany ; German Center for Neurodegenerative Diseases Bonn, Germany.
authors.forename: Nikolai
authors.initials: N
authors.lastname: Axmacher
keywords: CA3; high-resolution fMRI; hippocampus; pattern completion; pattern separation