Time Cells Determined to Have Role in Associative Learning

Researchers at the University of Colorado Anschutz Medical Campus have discovered that “time cells” are integral in rapid go-no-go decision making. It was already known that these cells have a major role in hippocampal function but their role in associative learning was unclear.

In the study, the researchers observed that when mice were given the choice of responding to a fruity smell by licking on a spout that delivered sweet water, they quickly learned to lick the fruity smell as opposed to the smell of mineral oil. “They have to associate the odor with the outcome of what they are doing so that’s why they learn decision making,” said Ming Ma, first author of the paper published in Current Biology. “When it’s a fruit odor, they lick and get a reward. When it’s mineral oil they stop licking.”

“The more they learned, the more the cells were stimulated leading to more rapid decoding of the odors and allowing the mice to quickly become proficient at choosing the fruity smell,” added Fabio Simoes de Souza, co first author of the study.

The catalyst for the decision-making is the odor, which travels up the nose sending neural signals to the olfactory bulb and to the hippocampus. The two organs are closely connected. The information is swiftly processed and the brain makes a decision based on the input.

“Before this we didn’t know there were decision making cells in the hippocampus,” senior author Diego Restrepo said. “The hippocampus is multitasking.”

The cells are not always turned on, Restrepo speculated, because otherwise the stimuli might become overwhelming.

The study expands current knowledge of what’s involved in decision-making in the brain, specifically those quick go-no-go decisions that mice and humans make all the time. “The hippocampus turns on decision-predicting time cells, which would give you a hint of what to remember,” Restrepo said. “In the past, time cells were thought to only remind you of events and time. Here we see memory encoded in the neurons and then retrieved instantly when making a decision.”