The brain’s system for storing memories could rely on both positive and negative feedback, report researchers.
“The puzzle out there is: How do brain circuits retain a memory without it slipping?” says Mark Goldman, associate professor at the University of California, Davis Center for Neuroscience. “For example, if you store a memory of the color yellow, why doesn’t it slip over time into orange?”
Earlier models used positive feedback to maintain memories. The idea is that, if a memory starts to fade, the circuit gets a boost.
“The problem with positive feedback is that, without some additional mechanism, it’s brittle—the system doesn’t respond well to being perturbed,” says Goldman.
Goldman and Sukbin Lim, a postdoctoral researcher, thought that these brain circuits might instead be doing something an engineer would do—stabilizing a system with negative feedback. For example, a thermostat uses negative feedback to turn on heating or cooling depending on whether a room’s temperature drifts below or above a set point.
As reported in Nature Neuroscience, the researchers built mathematical models to simulate the kinds of neural circuits found in the cortex of the brain. Neurons are connected to each other through junctions that transmit either positive (excitatory) or negative (inhibitory) signals.
The modeling studies show that negative feedback can stabilize these circuits and allow them to store memories. What’s more, the models show that these circuits should be robust to perturbations such as death of individual cells.
“Biological systems tend to be robust, so we wanted our models to reflect this robustness,” Goldman says. The models also reflect key features of the circuitry observed in the brain’s neocortex, he says.
Lim and Goldman further show how positive feedback and negative feedback could work together in the same circuit, demonstrating how the brain may exploit multiple mechanisms to store memories.
Lim is now a postdoctoral researcher at the University of Chicago. The National Institutes of Health and a Sloan Foundation Research Fellowship to Goldman supported the research.
Source: UC Davis