New research out of York University is the first to provide direct evidence of how similar memories with overlapping details are differentiated by the human brain.
In the paper titled “The Human Dentate Gyrus Plays a Necessary Role in Discriminating New Memories,” Faculty of Health Professor Shayna Rosenbaum of the Department of Psychology and the Centre for Vision Research, and her research team, provide the first causal evidence in humans that the dentate gyrus is required for discriminating similar memories.
The research further shows that this ability is dissociable, but likely interacts with, the ability to “fill in” partial memories.
The study is published in the journal Current Biology and the lead author is York University graduate student Stevenson Baker.
“Our findings point to how similar memories with overlapping details are differentiated by the human brain,” said Rosenbaum.
York University researchers had the unique opportunity to study a person with rare selective lesions in the hippocampus affecting the dentate gyrus, a brain region strongly associated with the development of new brain cells and strongly implicated in memory formation.
“This gave us a very unique window into one of the mechanisms underlying episodic memory,” said Rosenbaum.
She explains that in order to remember things, the brain has to make sure a memory of an event does not interfere with memories of other events that are similar to it. To code these memories as distinct requires a neural mechanism called “mnemonic discrimination”; and to complete a memory, another neural mechanism called “mnemonic completion” is necessary.
Using a memory of the first day of school as an example, mnemonic discrimination would mean being able to distinguish the first day of school from one year versus another, while mnemonic completion would rely on a detail of a memory, such as who the teacher was or where you sat in the classroom, to fill in the rest of the details of that memory.
The study examined the extent of which these mechanisms rely on the dentate gyrus.
Analysis of high-resolution structural brain images (MRI), along with behavioural testing using well-established sensitive measures, indicated that the dentate gyrus is specifically necessary for mnemonic discrimination.
During the study, the patient and a group of healthy control participants were tested for mnemonic discrimination by studying a set of pictures of everyday objects (“old” pictures); when testing for memory, objects that were unstudied (“new” pictures) and other objects that were unstudied but similar in appearance to the studied items (“lures”) were introduced.
What they hypothesized and ultimately found was that the patient could indicate which objects were old and which ones were new, but had difficulty differentiating the old objects from the unstudied lures.
“What makes this study important is that, based on fMRI and computational studies, there was a long-held belief that the dentate gyrus was the region of the brain involved in differentiating similar memories, but it could not be confirmed in humans until now,” said Rosenbaum.
Rosenbaum says that mnemonic discrimination is compromised in diseases that affect the hippocampus, such as amnesiac mild cognitive impairment, Alzheimer’s, encephalitis, epilepsy and more.
Rosenbaum and Baker are now conducting follow-up studies in amnesic patients and with neuroimaging to better understand the basis of mnemonic discrimination deficits to devise more sensitive tools for assessing and diagnosing more subtle forms of memory impairment.
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