Most of us know that feeling of trying to retrieve a memory that does not come right away and neuroscientists have now identified different sets of individual neurons which help us retrieve memories when required, a hallmark of the human brain's flexibility.
An essential aspect of cognitive flexibility is our ability to selectively search for information in memory when we need it.
"This is the first time neurons have been described in the human brain that signal memory-based decisions. In addition, our study shows how memories are transferred to the frontal lobe selectively and only when needed," explained senior author Ueli Rutishauser, visiting associate in biology and bioengineering at California Institute of Technology (Caltech).
The study, published in the journal Science and which has implications for the treatment of memory problems associated with Alzheimer's disease, epilepsy, and schizophrenia, was performed in patients who were already undergoing brain surgery for treatment of their seizures.
The volunteers viewed images on a screen and answered different types of questions about the images, while the researchers from Caltech and and Cedars-Sinai Medical Center in Los Angeles recorded the activity of individual neurons in their brains using implanted electrodes.
For example, a subject might be shown a picture of somebody they had never seen before and asked, "Have you seen this face before?" or "Is this a face?"
The two questions, respectively, help the researchers distinguish between a memory-based decision and a decision based not on memories but categories, such as faces.
"We make decisions based on retrieved memories all the time," says lead author Juri Minxha, a postdoctoral scholar at Cedars Sinai.
"In this study, we asked simple yes or no questions designed to cause a volunteer to access either their recent memory or their categorical knowledge".
The encoding and retrieval of memories occurs in the lower-middle portion of the brain in a region called the medial temporal lobe, which includes the hippocampus.
Decision-making processes involve a region at the front of the brain called the medial frontal cortex.
The ability to flexibly engage and utilize our memories to make decisions depends on interactions between the frontal and temporal lobes.
In the study, the researchers monitored single neurons in both the temporal lobe and the frontal lobe of 13 subjects.
The results revealed neurons that encode memories in the temporal lobe, and "memory choice neurons" in the frontal lobe; these neurons do not store memories but rather help retrieve them.
"So if we ask a patient if they have seen a face before, neurons in both regions become active. But if we show them the same image and ask, 'Is this a face?' then the memory choice neurons remain silent. Instead, we see a second distinct population of neurons in the frontal lobe, supporting the subject's current goal of categorizing the image," explained Minxha.
Interestingly, the team found that the decision was represented by the memory choice neurons in an abstract way, such that the very same neurons could signal this information in different contexts.
"This likely accounts for much of the flexibility that we see in human decision-making," said Ralph Adolphs, Caltech's Bren Professor of psychology, neuroscience and biology.