HIPPOCAMPAL ENCODING OF A SEQUENCE OF REWARD JOURNEYS

Place cells in the hippocampus are known to be responsible for memorizing specific locations and the neural activity of these cells is the basis for internal image of space. Place cell activity generally tends to be uniformly distributed throughout the space, but for reward locations and salient cue locations more cells represent such locations. Those cells that encode reward locations have been investigated several times but only in the simple task like going to a reward location and getting a water reward over and over. In this study, we made the task more complicated so that much higher cognition could be involved. In this task, a mouse is required to run on a 2-meter-long linear treadmill environment, and three rewards are given in three different positions for three laps. That means, in addition to memorizing the environment itself, the animal is required to memorize the sequence of reward locations. After 2 weeks training was done, we implanted a 64 channel silicon probe and recorded dorsal hippocampus CA1. As a result, we found that mice could understand the structure of tasks, and they show that with their behavior and with their neuronal activity. Compared to place cells, some cells are clearly bound to task progression, and most of these cells show a place field near reward locations. This result shows the hippocampus could formulate cognitive concepts like task progression or sequence of rewards, and the hippocampus might not just be a simple memory unit of our brain.