Hippocampal place cell activity during chasing of a moving object associated with reward in rats.

Hippocampal place cells encode location of animals in the environment. However, it remains unknown whether the hippocampal place cells encode a continuously moving object in the environment. To investigate this topic, we analyzed the place cell activity of freely moving rats when a toy car was introduced into an arena. First, in a freely moving task without the car, the rats freely navigated inside the arena to earn an intracranial stimulation (ICS) reward for each 150 cm traveled. Second, they were divided into two groups and tested using two different tasks. In the car-dependent navigation (CDN) task, the car was placed inside the arena, and the rat received ICS if it chased and came within 20 cm of the car. In the car-independent navigation (CIN) task, the rat acquired ICS rewards if it traveled 150 cm regardless of its relation to the car. Place fields remapped more frequently in the CDN than the CIN tasks. In both the CDN and CIN tasks, the place cell activity inside the place fields displayed moderate tuning to the movement parameters of the rats and car, and the distance between the car and rats. However, tuning of the place cells to movement variables of the car was more selective in the CDN than the CIN tasks, while information regarding movement variables of the car represented by the place cell activity was larger in the CDN than the CIN task. These results indicated that place cell activity within the place fields represents not only an animal's own location but also the movement variables of another moving object if that object is associated with rewards. The present results provide new evidence that place cell activity conveys relevant information in a task even if this information is derived from other moving objects.