Carnegie Mellon University researchers have found that activity in pyramidal neurons, a type of cell commonly found in the brain's cerebral cortex, change in activation when the brain is learning.

"Learning happens every day," said Mo Zhu, a neuroscience graduate student and first author on the paper. "We found learning shapes the stimulus-evoked responses of neurons, but this neuroplasticity might be transient."

Researchers placed mice in an automated home-cage system, so the mice could learn how to receive a reward by themselves without set training times. While the mice were training, the researchers used calcium imaging, which allowed them to consistently measure the populations of pyramidal neurons in the sensory cortex, a part of the cerebral cortex dedicated to processing senses, over time.

Scientists have long believed that sensory processing would be enhanced during learning, increasing brain activity for behaviorally-important cues. However, Zhu and colleagues discovered that this was not the case. Although sensory responses were modestly strengthened at the very onset of training, once mice learned the task this effect disappeared.

"It looked like there was maybe a little burst of enhanced sensory response, and then responses actually decrease compared to their pretraining levels," said Alison Barth, Maxwell H. and Gloria C. Connan Professor in the Life Sciences. "We think that once animals figure out the task, neural circuits reorganize. It looks like neural activity becomes more sparse, which may more efficiently encode the stimulus."

Sandra Kuhlman, associate professor of biological sciences at the University of Buffalo and a former Carnegie Mellon faculty member, assisted with data analysis.

"The data sets collected during this project were large and complex," Kuhlman said. "My lab was able to share and adapt some of our analytical approaches to be used for this project."

The researchers plan to further investigate neuroplasticity in the sensory cortex. They want to investigate where the inhibition occurs in the learning process. They believe that inhibition is caused by a different neuron in the same area of the brain. They also theorized that once an action is learned, it is encoded in a different area of the brain.

"We're going to work on monitoring that activity, and we believe they can help interpret all the sensory input and modify the activity in the cortex," Zhu said.

The paper "Transient Enhancement of Stimulus-Evoked Activity in Neocortex during Sensory Learning" was published in Learning & Memory. The research was funded by the National Institutes of Health.