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Neural Activity Reveals Interactions Between Episodic and Semantic
Memory Systems During Retrieval
Christoph T. Weidemann
Swansea University and University of Pennsylvania James E. Kragel
University of Pennsylvania
Bradley C. Lega
University of Texas Southwestern Gregory A. Worrell
Mayo Clinic, Rochester, Minnesota
Michael R. Sperling and Ashwini D. Sharan
Thomas Jefferson University Hospital, Philadelphia,
Pennsylvania
Barbara C. Jobst
Darmouth-Hitchcock Medical Center, Lebanon, New Hampshire
Fatemeh Khadjevand
Mayo Clinic, Rochester, Minnesota Kathryn A. Davis
Hospital of the University of Pennsylvania, Philadelphia,
Pennsylvania
Paul A. Wanda, Allison Kadel, Daniel S. Rizzuto, and Michael J. Kahana
University of Pennsylvania
Whereas numerous findings support a distinction between episodic and semantic memory, it is now
widely acknowledged that these two forms of memory interact during both encoding and retrieval. The
precise nature of this interaction, however, remains poorly understood. To examine the role of semantic
organization during episodic encoding and retrieval, we recorded intracranial encephalographic signals
as 69 neurosurgical patients studied and subsequently recalled categorized and unrelated word lists.
Applying multivariate classifiers to neural recordings, we were able to reliably predict encoding success,
retrieval success, and temporal and categorical clustering during recall. By assessing how these classifiers
generalized across list types, we identified specific retrieval processes that predicted recall of categorized
lists and distinguished between recall transitions within and between category clusters. These results
particularly implicate retrieval (rather than encoding) processes in the categorical organization of
episodic memories.
Keywords: free recall, episodic memory, semantic memory, intracranial EEG, machine learning
Christoph T. Weidemann, Department of Psychology, Swansea Uni-
versity, and Department of Psychology, University of Pennsylvania;
James E. Kragel, Department of Psychology, University of Pennsylva-
nia; Bradley C. Lega, Department of Neurosurgery, University of Texas
Southwestern; Gregory A. Worrell, Department of Neurology, Mayo
Clinic, Rochester, Minnesota; Michael R. Sperling and Ashwini D.
Sharan, Department of Neurology, Thomas Jefferson University Hos-
pital, Philadelphia, Pennsylvania; Barbara C. Jobst, Department of
Neurology, Darmouth-Hitchcock Medical Center, Lebanon, New
Hampshire; Fatemeh Khadjevand, Department of Neurology, Mayo
Clinic; Kathryn A. Davis, Department of Neurology, Hospital of the
University of Pennsylvania, Philadelphia, Pennsylvania; Paul A.
Wanda, Allison Kadel, Daniel S. Rizzuto, and Michael J. Kahana,
Department of Psychology, University of Pennsylvania.
The authors thank Blackrock Microsystems for providing neural
recording equipment. This work was supported by the DARPA Restor-
ing Active Memory (RAM) program (Cooperative Agreement N66001-
14-2-4032) and by the National Institutes of Health (Grant MH055687).
The views, opinions, and/or findings contained in this material are those
of the authors and should not be interpreted as representing the official
views or policies of the Department of Defense or the U.S. Government.
Christoph T. Weidemann & James E. Kragel contributed equally to this
work. Christoph T. Weidemann & James E. Kragel analyzed the data
and wrote the paper; Christoph T. Weidemann, James E. Kragel, &
Michael J. Kahana designed analyses and edited the paper; Bradley C.
Lega, Gregory A. Worrell, Michael R. Sperling, Ashwini D. Sharan,
Barbara C. Jobst, & Kathryn A. Davis recruited participants and pro-
vided general assistance; Fatemeh Khadjevand, Paul A. Wanda, &
Allison Kadel collected data; Daniel S. Rizzuto & Michael J. Kahana
designed experiments. We thank Youssef Ezzyat for insightful discus-
sions. The authors declare that they have no competing financial inter-
ests. Part of this work was presented at the 2018 Context and Episodic
Memory Symposium in Philadelphia, Pennsylvania and at the 2018
Annual meeting of the Psychonomic Society, New Orleans, Louisiana.
All of the de-identified raw data and analysis code used in this study
may be freely downloaded from the Cognitive Electrophysiology Data
Portal (http://memory.psych.upenn.edu/Electrophysiological_Data).
Correspondence concerning this article should be addressed to Christoph
T. Weidemann, Department of Psychology, Swansea University, Singleton
Park, Swansea SA2 8PP, UK. E-mail: ctw@cogsci.info
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Journal of Experimental Psychology: General
© 2019 American Psychological Association 2019, Vol. 148, No. 1, 1–12
0096-3445/19/$12.00 http://dx.doi.org/10.1037/xge0000480
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