title: Intracranial EEG correlates of implicit relational inference within the hippocampus.
abstract: Drawing inferences from past experiences enables adaptive behavior in future situations. Inference has been shown to depend on hippocampal processes. Usually, inference is considered a deliberate and effortful mental act which happens during retrieval, and requires the focus of our awareness. Recent fMRI studies hint at the possibility that some forms of hippocampus-dependent inference can also occur during encoding and possibly also outside of awareness. Here, we sought to further explore the feasibility of hippocampal implicit inference, and specifically address the temporal evolution of implicit inference using intracranial EEG. Presurgical epilepsy patients with hippocampal depth electrodes viewed a sequence of word pairs, and judged the semantic fit between two words in each pair. Some of the word pairs entailed a common word (e.g., "winter-red," "red-cat") such that an indirect relation was established in following word pairs (e.g., "winter-cat"). The behavioral results suggested that drawing inference implicitly from past experience is feasible because indirect relations seemed to foster "fit" judgments while the absence of indirect relations fostered "do not fit" judgments, even though the participants were unaware of the indirect relations. A event-related potential (ERP) difference emerging 400 ms post-stimulus was evident in the hippocampus during encoding, suggesting that indirect relations were already established automatically during encoding of the overlapping word pairs. Further ERP differences emerged later post-stimulus (1,500 ms), were modulated by the participants' responses and were evident during encoding and test. Furthermore, response-locked ERP effects were evident at test. These ERP effects could hence be a correlate of the interaction of implicit memory with decision-making. Together, the data map out a time-course in which the hippocampus automatically integrates memories from discrete but related episodes to implicitly influence future decision making.
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.
authors.forename: T P
authors.initials: TP
authors.lastname: Reber
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.|Jülich Supercomputing Centre, Institute for Advanced Simulation, Forschungszentrum Jülich, Jülich, Germany.
authors.forename: A T A
authors.initials: AT
authors.lastname: Do Lam
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.|German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany.|Department of Neuropsychology, Institute of Cognitive Neuroscience, Faculty of Psychology, Ruhr University Bochum, Bochum, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Axmacher
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.
authors.forename: C E
authors.initials: CE
authors.lastname: Elger
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Helmstaedter
authors.affiliation: Department of Psychology, University of Bern, Bern, Switzerland.|Center for Cognition, Learning and Memory, University of Bern, Bern, Switzerland.
authors.forename: K
authors.initials: K
authors.lastname: Henke
authors.affiliation: Department of Epileptology, University Clinic of Bonn, Bonn, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Fell
mesh_terms: D000328:Adult; D003657:Decision Making / Q000502:physiology*; D000069279:Drug Resistant Epilepsy / Q000503:physiopathology / Q000523:psychology / Q000601:surgery; D000069280:Electrocorticography; D004833:Epilepsy, Temporal Lobe / Q000503:physiopathology / Q000523:psychology / Q000601:surgery; D005071:Evoked Potentials; D005260:Female; D006624:Hippocampus / Q000502:physiology* / Q000503:physiopathology / Q000601:surgery; D006801:Humans; D007600:Judgment / Q000502:physiology*; D007807:Language Tests; D008297:Male; D008568:Memory / Q000502:physiology*; D008875:Middle Aged; D009483:Neuropsychological Tests; D010775:Photic Stimulation; D011930:Reaction Time; D011932:Reading; D012660:Semantics; D014796:Visual Perception / Q000502:physiology; D055815:Young Adult
keywords: ERP; awareness; declarative; memory; non-declarative