Hippocampus essential in all aspects of recognition

The hippocampus plays a major role in memory and spatial navigation. It is essential to our recognizing previous events, objects, and people - which is known as recognition memory.

New research verrifying this concept comes from the departments of Neurosurgery and Psychology in the Perelman School of Medicine at the University of Pennsylvania, and is now published in the journal the Proceedings of the National Academy of Sciences or PNAS.

Recognition memory is composed of two processes: (1) recollection which is recognizing something including vivid details of the initial encounter; and (2) familiarity, or a general sense of having previously encountered something.

Both processes often break down with aging, neurodegenerative disorders (such as Alzheimer's disease), or traumatic brain injury. The new findings provide a roadmap to guide strategies towards improving these functions.

"There's a longstanding debate in the recognition memory field about how the human hippocampus contributes to our ability to recognize.

"One segment believes that neural activity in the hippocampus contributes only to recollection, whereas some believe hippocampal activity supports both recollection and familiarity. Our study aimed to get to the bottom of this debate."

Maxwell Merkow MD, Neurosurgery Chief Resident, Hospital of the University of Pennsylvania, and lead author.

The University of Pennsylvania team hypothesizes that the hippocampus supports both recollection and familiarity. But showing a clear link between hippocampal activity and recognition memory performance has proven elusive, only having been documented in a few studies. The new paper is the first to record a link between hippocampal activity and both processes of recollection and familiarity.

Merkow and colleagues studied 66 epilepsy patients undergoing intracranial hippocampal monitoring. They administered memory tasks to the participants — asking them to remember a series of words, then showing them a second series of words, some of which they had seen before, and some that were new. Patients were then asked to determine whether or not each word had been part of the initial group.

The team recorded electrical data directly from the patient's hippocampus during the tests and were able to measure neural firings lasting mere hundreds of milliseconds, capturing activity tied to cognition - the process of acquiring knowledge.

Research found elevated high frequency hippocampal activity during trials in which patients correctly identified a word previously seen — as opposed to lower activity during trials where patients either failed to recognize a previously seen word, or saw a new word and failed to identify it as new.

A major finding was that the strength of hippocampal activity predicted behavioral performance, directly linking the hippocampus to recognition memory.

Crucially, both recollection and familiarity of recognition correlates with hippocampal activity — and both cognitive processes are supported by the hippocampus.

Merkow:  "This work directly addresses the issue of where in the brain recognition takes place. We now need to focus our efforts on how these processes occur."

The team plans to use the same high frequency recordings from smaller electrodes to address this question. Their work brings science one step closer to understanding how brain activity supports memory and potentially improving memory with "to be discovered" interventions.

Significance
Recognition memory is thought to be composed of recollection, accompanied by vivid details, and familiarity or a general sense of knowing. A fundamental and long-standing question remains: Which of these processes does the hippocampus support? We measured high-frequency activity (HFA), a spatiotemporally precise signal of neural activation, in subjects undergoing direct brain recordings and found that hippocampal HFA dissociated based on both the stimulus evidence presented and the response choice. Hippocampal HFA predicted overall memory performance as well as individual differences in both recollection and familiarity estimates. Our findings reject the hypothesis that the hippocampus exclusively supports the recollection component of recognition memory and, instead, indicate that this structure is functionally relevant to both processes thought to support recognition.

Abstract
Despite a substantial body of work comprising theoretical modeling, the effects of medial temporal lobe lesions, and electrophysiological signal analysis, the role of the hippocampus in recognition memory remains controversial. In particular, it is not known whether the hippocampus exclusively supports recollection or both recollection and familiarity—the two latent cognitive processes theorized to underlie recognition memory. We studied recognition memory in a large group of patients undergoing intracranial electroencephalographic (iEEG) monitoring for epilepsy. By measuring high-frequency activity (HFA)—a signal associated with precise spatiotemporal properties—we show that hippocampal activity during recognition predicted recognition memory performance and tracked both recollection and familiarity. Through the lens of dual-process models, these results indicate that the hippocampus supports both the recollection and familiarity processes.

hippocampus recognition memory recollection familiarity high-frequency activity

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1513145112/-/DCSupplemental.

Additional Penn authors include John F. Burke and Michael J. Kahana

This work was supported by the National Institutes of Health (MH055687)

Penn Medicine is one of the world's leading academic medical centers, dedicated to the related missions of medical education, biomedical research, and excellence in patient care. Penn Medicine consists of the Raymond and Ruth Perelman School of Medicine at the University of Pennsylvania(founded in 1765 as the nation's first medical school) and the University of Pennsylvania Health System, which together form a $5.3 billion enterprise.

The Perelman School of Medicine has been ranked among the top five medical schools in the United States for the past 17 years, according to U.S. News & World Report's survey of research-oriented medical schools. The School is consistently among the nation's top recipients of funding from the National Institutes of Health, with $409 million awarded in the 2014 fiscal year.

The University of Pennsylvania Health System's patient care facilities include: The Hospital of the University of Pennsylvania and Penn Presbyterian Medical Center -- which are recognized as one of the nation's top "Honor Roll" hospitals by U.S. News & World Report -- Chester County Hospital; Lancaster General Health; Penn Wissahickon Hospice; and Pennsylvania Hospital -- the nation's first hospital, founded in 1751. Additional affiliated inpatient care facilities and services throughout the Philadelphia region include Chestnut Hill Hospital and Good Shepherd Penn Partners, a partnership between Good Shepherd Rehabilitation Network and Penn Medicine.

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