Recognition was triggered in significantly less than half a second after a word reappeared. Previous tests had indicated longer reaction times. The researchers also gained other insights in the way the human memory works. They were able to prove that the hippocampus – the brain’s memory center – does trigger the recall of specific memories, while it is not involved in the development of fuzzy sensations of familiarity. This finding sheds light on the effects of Alzheimer’s disease. The study has been published in “Current Biology.”
Our brain utilizes a broad spectrum of memories that have been stored over time. Regardless of whether we recognize a familiar melody, remember the birthday of the partner we live with or whether we subconsciously recall certain sequences of motion we have practiced many times when we ride a bicycle – the processes that happen in our brain are distinct in each case. One of the major goals of memory research is to find out, what areas of the brain are involved in every single one of these processes. The hippocampus is known to be an important player. “This part of the brain is a switch board for the processing of content we recall and is extremely important for the long-term memory,” explains Professor Düzel, who conducts research at the German Center for Neurodegenerative Diseases (DZNE) in Magdeburg and the local Otto-von-Guericke University. “Individuals who have sustained injuries to their hippocampus have a hard time to remember past events or may not even remember them at all. Furthermore, they can recall recent incidents only for a very short time.”
However, investigations performed until now provided contradicting results concerning the role of the hippocampus. “Is the hippocampus responsible for specific memories only or also for conveying sensations of familiarity? This was an open question in memory research,” says the neuroscientist. He explains the issue by using an example: “If we recognize another human being, we occasionally find ourselves unable to associate the person with other circumstances. We feel that we know this person, but have no idea how or where we met him or her.” In such cases we are unable to make the mental connection, Düzel emphasizes and adds: “On the other hand, if I am certain that I recently met this person at a party, the context is present in my mind. Such a memory recollection, we call it context memory, is much more specific than a sketchy sense of familiarity.”
Memory test based on images and words
Düzel and his colleagues were eager to shed some light on the controversy: does the hippocampus communicate merely those memories that are linked to context or also a sense of familiarity? To answer this question, they called in 31 test subjects to conduct a memory test. Participants were asked to recognize words and to link them to images they had been shown earlier. Of this study group, 14 were healthy. The remaining participants had sustained damage to their hippocampus as a result of a preexisting condition and the volume of this area of the brain was smaller than in the case of healthy subjects. Consequently, the test participants differed in terms of the memory capacity, but they all had similar IQs and levels of education.
The tests were performed at London’s University College, where Düzel’s team conducted its investigations in cooperation with brain researcher Faraneh Vargha-Khadem from the Institute of Child Health. The test procedure: the participants were shown pairs of pictures and words on a screen, always one pair at a time. After some delay, new and old terms were presented without an image. Now, the participants were supposed to recognize words they had previously seen. Afterwards, they had to pick out the associated image from a collection of photographs. At the same time, brain activity of all participants was recorded via magnetoencephalography (MEG).
The conclusion: In the ability to remember words, this part of the test focused on the sense of familiarity, no difference was found between patients and healthy participants. However, when it came to the allocation of the correct image, there were marked differences. In other words, when terms had to be embedded into a visual context, the hit rate correlated with the volume of the hippocampus: the larger this area of the brain, the better the results achieved by the participants.
“This allows us to conclude that the hippocampus plays a prominent role in the recall process for context related memories. However, it is not involved in the generation of a sense of familiarity. We are now able to prove this with certainty,” comments Düzel as he sums up the outcome of the experiment. “In principle, it is possible to recognize things without a hippocampus. However, it is impossible to make pertinent associations. The ability to recall specific memories is lacking. Such symptoms are typical to Alzheimer’s, which damages the hippocampus. Our findings help us to understand how memory functions are affected by this disease.”
Remembering starts earlier than previously assumed
The MEG also provided interesting insights. It enabled the researchers to track the reactions in the brains of their test subjects. “Whenever our brain associates memory content with a context, we refer to it as pattern completion. We now have a better understanding of the time sequences involved in this process,” explains Düzel. “We see a signal in our data that is like a precursor to a memory. It has a pattern similar to those neural signatures that accompany a sense of familiarity. Hence, recollection processes are triggered by familiarity and therefore begin earlier than initially presumed.”
The scientists were able to measure this rapid response with great precision. For this, they registered the time span between the appearance of the word test subjects were supposed to recognize and their brain response. “We were able to prove that recollection processes are initiated after just about 350 milliseconds. That is considerably faster than we had presumed. According to tests conducted in the past we expected a much longer delay of about half a second. As a result, we now have a better understanding of the timing of memory processes,” observes Düzel.
A Rapid, Item-Initiated, Hippocampus-Dependent Neural Signature of Context Memory in Humans
Aidan J. Horner, David G. Gadian, Lluis Fuentemilla, Sebastien Jentschke, Faraneh Vargha-Khadem and Emrah Düzel, Current Biology
Prof. Dr. Emrah Düzel
German Center for Neurodegenerative Diseases, Magdeburg &
University of Magdeburg (Germany)
+ 49 391 6117 520
Dr. Dirk Förger
Head of Press and Public Relations
German Center for Neurodegenerative Diseases
+ 49 228 43302 260
University of Magdeburg
+ 49 391 67 15162