As new sites become familiar, different brain rhythms and neurons take over | MIT News
To focus on what is new, we ignore what is not. A new study by researchers at MIT’s Picower Institute for Learning and Memory significantly advances our understanding of how a mammal’s brain enables this “visual recognition memory.”
Rejecting elements of a scene that have proven to be inconsequential is an essential function because it allows animals and humans to quickly recognize new things that need to be evaluated, explains Bear brand, Professor Picower in the Department of Brain and Cognitive Sciences and lead author of the study in the Journal of Neuroscience.
“Everyone’s appropriate behavioral response to an unexpected stimulus is to devote attentional resources to it,” says Bear. “Maybe it means danger. Maybe it means food. But if you learn that this once new stimulus isn’t important, it’s super adaptive to ignore it. It is absolutely essential for the normal functioning of the brain that we can quickly determine whether a stimulus is new or not.
People with schizophrenia and certain autism spectrum disorders seem to struggle with this ability, Bear notes.
In 2006, Bear’s lab discovered the first sign of visual recognition memory. The researchers detected a strong tendency for electrical activity to increase in the visual cortex as the mice familiarized themselves with an image on a screen. Subsequent research showed that this increase in the electrophysiological response, termed SRP, or “plasticity of the selective stimulus response”, was strongly correlated with “habituation” or behavioral loss of interest in exploring the stimulus more and more. more familiar.
Since then, the lab has been working on mice to understand exactly how these phenomena emerge. Their research showed that a well-known learning and memory mechanism called ‘LTP’, a strengthening of neural connections amid frequent activity, is involved, but that this mechanism alone cannot produce memory. visual recognition. Inhibitory neurons called parvalbumin (PV) also expressing neurons seem to be crucial parts of the circuit. PV neurons are known to produce high frequency gamma rhythms in the cortex.
In new study by graduate students Dustin Hayden and Daniel Montgomery, Bear’s lab shows that as new visual patterns become familiar, the transition is marked by radical changes in the visual cortex. Gamma rhythms give way to low frequency beta rhythms and the activity of PV neurons is extinguished in favor of an increase in the activity of neurons expressing inhibitory somatostatin (SOM).
The study, says Bear, therefore provides a measurable indicator of the outside of the transition from novel to familiar – the change in brain rhythm. It also proposes a new hypothesis on how visual recognition memory is applied: PV activity, which initially inhibits the SRP electrical response, itself eventually becomes inhibited by SOM activity.
Bear’s lab is working with Boston Children’s Hospital researcher Chuck Nelson to determine whether PRS aberrations, such as this frequency transition, can be used as an early biomarker for autism spectrum disorders.
In the new study, the researchers showed mice the same simple picture multiple times over the course of several days. During this time, they measured the electrical response of SRP in mice as well as neural rhythms. In parallel experiments, they designed mice to have their PV or SOM neurons blink brightly when active. Then, as the mice looked at the image, scientists could monitor these lightning bolts using a “two-photon” microscope.
On day one, when the image was new, the spectrum of rhythms in the visual cortex was dominated by high frequency gamma readings. As the days passed, the gamma power decreased, replaced by a constant increase in low frequency beta power. To make sure it wasn’t an unrelated transition, on day seven, scientists presented a new and familiar picture. When the mice saw the new one, they again exhibited a pattern dominated by the gamma frequency. When they saw the same old original image, the visual cortex mimicked the pattern of increased beta power.
In a subsequent data analysis, the researchers found that the decrease in gamma potency and the increase in beta potency significantly correlated with the SRP growth in electrical activity, suggesting that they are indeed related.
“These data are consistent with the hypothesis that the same underlying biology is responsible for both manifestations of PRS,” the researchers wrote.
Two-photon microscope experiments revealed this underlying biological difference. PV neurons reacted strongly to the images when they were new, but this activity was replaced by increasing SOM activity over several days as the image became familiar.
SOM neurons are believed to suppress PV neurons, Bear says, but to prove that SOM inhibition of PV represents visual recognition memory, the lab still needs to perform more experiments. Using optogenetics, in which they can design neurons to be controlled with different colors of light, they can manipulate SOM neurons to see if their deactivation causes mice to view repeated images as always new, or if their activation causes the mice to immediately reject the new images. as pass.
Sam Cooke, a former Bear Lab member who is now a lecturer at Kings College London, is a co-author of the study.
The National Eye Institute, the JPB Foundation, and the National Science Foundation funded the study.