Recollection, as defined by memory specialists, is the ability to call up specific details about an encounter, while familiarity is simply knowing that someone or something has been encountered before. Both are elements of recognition memory and both, new research suggests, are functions of the brain's hippocampus.
Published in the Feb. 2 issue of the journal Neuron, the University of California, San Diego study contradicts a recent body of work which maintains that the hippocampus is involved only in recollection.
Led by senior researchers John Wixted, chair of the UCSD psychology department, and Larry Squire, a professor of psychiatry and neurosciences at the UCSD School of Medicine and the San Diego Veterans Affairs Health System, the study addresses one of the central debates in the neuroanatomy of memory.
A seahorse-shaped structure in the left and right medial temporal lobes of the brain, the hippocampus has long been known as a critical area for processing memory. Memory is impaired, often severely, in people whose hippocampi have been damaged by trauma or disease – by Alzheimer's, for example, or oxygen deprivation following a heart attack.
The details of hippocampal function, however, are hotly contested in scientific circles.
"It is important to be able to assign a process to a specific region of the brain, but it is also intoxicating…," Wixted said. "Psychologists and neuroscientists have jumped at the notion that the hippocampus is critical only for the recollection component of recognition and that the adjacent cortical areas take care of familiarity only. But our findings suggest that conclusion might have been premature."
The study compared six memory-impaired patients with discrete hippocampal damage – that is, patients whose brain damage was localized to the hippocampus, as determined by magnetic resonance imaging (MRI) and other techniques – to an age-matched control group.
Subjects were tested with standard verbal tests of recognition memory. They were given lists of common words to study and after a short interval (three minutes filled with chit-chat) were then given test lists composed of both new and old words. For each word on the recognition test, the subjects were asked to rate their confidence that the word was old or new.
Results were analyzed with the receiver operating characteristic (ROC) curve, a statistical tool widely used to evaluate the accuracy of a classifier. In this case, the ROC curves are confidence plots where the points are derived from rates of true-positive versus false-positive answers (saying "yes" correctly to a word that was on the previous list v. saying "yes" incorrectly to a word that wasn't).
Some theories of recognition, said Wixted, an expert on signal-detection models of memory, hold that the shape of the ROC is significant: A symmetric curve indicates that only the process of familiarity is involved, while an asymmetric curve indicates that both familiarity and recollection are at work. Also, the greater the asymmetry, the more significant the role of recollection.
After controlling for memory strength – by testing the brain-damaged patients, who have diminished abilities, with shorter (and therefore easier) word lists than those given to controls – the recognition accuracy of both groups was almost identical. Critically, once accuracy was equated, the asymmetry of the ROC curves was identical as well.
This is the first ROC study, Wixted noted, to age-match the subjects and to "strengthen" the memories of the hippocampal patients so that their ROCs could be meaningfully compared to that of the controls. The similarity of the ROCs suggests that recollection is operative even in patients with extensive hippocampal damage.
"The simple idea that these processes can be dichotomized and assigned to separate brain structures is challenged by our results. Both processes appear to be supported by the hippocampus and by the structures in the adjacent parahippocampal gyrus," the researchers write.
"This work is helping us piece together how the brain accomplishes learning and memory," Squire said, "and this is important in efforts to develop treatments for memory disorders."
The paper is titled "The Hippocampus Supports Both the Recollection and the Familiarity Components of Recognition Memory."
First author is Peter Wais, a UCSD psychology doctoral student, advised by both Squire and Wixted. Ramona Hopkins, of Brigham Young University and the LDS Hospital in Salt Lake City, is also a coauthor.
The research was funded by the National Institute of Mental Health and the Medical Research Service of the Department of Veterans Affairs.
Journal
Neuron