You know that moment when you're desperately trying to remember where you put your keys? You go over memories of the day before attempting to figure out when was the last time you had them in hand, and where you stashed them afterwards.
Understanding memory retrieval has been a focus of interest for a number of neuroscientists working on neuroimaging work in humans, but until now, the process wasn't fully understood.
A collaborative team of neuroscientists from Caltech and Cedars-Sinai Medical Center in Los Angeles has discovered a number of individual neurons that are responsible for memory-based decision making.
Their study was published in the journal Science.
Selective memory searches
"An essential aspect of cognitive flexibility is our ability to selectively search for information in memory when we need it," said senior author of the study Ueli Rutishauser and associate in biology and bioengineering at Caltech.
The study is integral as it has strong implications for the treatment of memory problems such as can be found in people suffering from Alzheimer's, epilepsy, and schizophrenia.
During the study, the team performed tests on patients already undergoing brain surgery as treatment for seizures linked to brain diseases. While the scientists recorded the activity of individual neurons in the volunteers' brains, they were viewing images on a screen and asked to answer a number of questions related to them.
The type of question that was asked, for instance, was "Have you seen this face before?", and "Is this a face?". The two questions clearly distinguish between a memory-based decision and a category-based one not linked to memory at all.
"The ability to flexibly engage and utilize our memories to make decisions depends on interactions between the frontal and temporal lobes, the former being the site of executive control and the latter being where memories of this kind are stored. Little was known before about how the interactions between these two parts of the human brain occur," said Rutishauser.
After the team studied their test results, it found out that neurons in the temporal lobe encode memories, and the frontal lobe ones focused on memory choice neurons, which do not store memories but rather help retrieve them.
"Both the medial temporal lobe and medial frontal cortex become active when the decision requires that the patient remember something. The interaction between these two brain structures allows for successful memory retrieval," explained lead author of the study Juri Minxha.
"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."
Ralph Adolphs, Caltech's Bren professor of psychology, neuroscience, and biology, said "Taken together, our study reveals several key building blocks that make human cognition so flexible."