Development of sensory processing in rat primary motor cortex

In adults, primary motor cortex (M1) is a critical brain area for producing movement and learning new motor skills. In early life, however, M1 does not function as a motor area, but instead processes sensory responses to movements or external touch. In newborn animals, these sensory responses often arise from the limb and facial twitches that occur during active (or REM) sleep. Because newborn—including rats and humans—spend most of their time asleep, twitches may be especially important for M1's sensory development.

In this dissertation, we track forelimb twitches of newborn rats in three dimensions and compare their trajectory with sensory responses in M1. We find that in 8-day-old rats, M1 accurately tracks the amplitude and direction of forelimb twitches, but not of forelimb movements during wake. In 12-day-old rats, M1 neural activity changes dramatically, becoming a continuous and more “noisy” pattern of activity. This continuous activity effectively hides the relationship between forelimb movements and M1 sensory responses during both sleep and wake. To probe the informational content of neural activity across this developmental transition, we use a machine-learning technique to “unmask” sensory responses. In this way, we demonstrate that sensory responses in M1 become more complex across this developmental transition. Together, these results help clarify how M1's early sensory phase guides its development toward its adult functionality as an important structure for the production of behavior.