The researchers found elevated high-frequency neuron firing activity in the hippocampus when the participants correctly identified a word they had seen before.
This was the conclusion of new research published in the Proceedings of the National Academy of Sciences.
The findings settle a long-standing debate about the role of the hippocampus in recognition memory, note the authors, from the University of Pennsylvania in Philadelphia.
Recognition memory helps us identify something we have come across before. A prominent theory about this aspect of memory is that it comprises recollection - the recovery of vivid details and familiarity - where there is a general sense of the experience but no details.
A fundamental and long-standing question about recognition memory is which of the two processes of recognition memory - recollection and familiarity - does the hippocampus support?
As the two processes often break down as a result of aging, brain-wasting diseases like Alzheimer's and traumatic brain injury, the answer to this question could point researchers investigating treatments for these various conditions in the right direction.
Lead author Dr. Maxwell Merkow, neurosurgery chief resident at the Hospital of the University of Pennsylvania, says:
"One segment of the scientific literature contends that neural activity in the hippocampus only contributes to recollection, whereas some believe hippocampal activity supports both recollection and familiarity. Our study aimed to get to the bottom of this."
In their paper, the authors note that despite various investigations, the role of the hippocampus in recognition memory remains controversial. They believe theirs is the first to show a clear link between the hippocampus and recognition memory, and in particular to show that the hippocampus supports both the recollection and familiarity processes.
Hippocampus 'functionally relevant' to recollection, familiarity
For the study, the team examined 66 patients who were already undergoing direct brain recordings for epilepsy. They tested the level of high-frequency brain cell activity in the hippocampus. This is a very precise measure of neurons firing that captures fleeting activity of thinking processes - they last only for a few hundred milliseconds.
As the researchers captured the hippocampus activity measurements, the participants performed two memory tasks. In the first task, they were shown a series of words and then asked to recollect them.
The second memory task focused on familiarity. The participants were shown another series of words, some of which they had seen in the first task, and some of which were new. They had to decide whether or not each word was one they had been shown before.
The recordings showed elevated high-frequency activity in the hippocampus when the participants correctly identified a word they had seen before. In contrast, whenever they failed to recognize a word they had been shown before, or whenever they saw a new word - whether or not they correctly identified it as new - the recordings showed less high-frequency activity in the hippocampus.
Another important finding was that the strength of the hippocampal activity predicted performance in the tasks, and in particular, both recollection and familiarity performance.
The authors conclude:
"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."
Dr. Merkow says their study "directly addresses the issue of where in the brain recognition takes place," and that they now need to focus on how the processes occur. He and his colleagues plan to do this by using smaller electrodes to capture the same high-frequency recordings.
Recognition memory is one part of the complex process of memory, which also includes the emotions linked to the memory. These are stored in another part of the brain, the amygdala.
In August 2014, Medical News Today reported on a study where researchers claim it is possible to reverse emotional associations of specific memories in mice by using light to control the activity of neurons. Writing in Nature , the researchers concluded that while cells of the hippocampus are emotionally neutral, the amygdala cells are active in encoding the emotional aspects of a memory.