SPATIAL NAVIGATION USING UNREWARDED AND UNMARKED GOAL LOCATIONS

Abstract

The CA1 region of the hippocampus is essential for spatial navigation as it encodes locations and rewards using place cells and reward cells. Many of the navigation tasks using rodents involve the use of cue-rich reward areas to encourage behavior and adverse stimuli such as electrical foot shocks to elicit avoidance. Both reward locations and avoidance areas have been shown to have influences on the properties of CA1 place fields. We focused on the roles of distal and visual cues on rats’ ability to interact with unmarked (no cue, no boundary) positive and weakly cued (no boundary, small mark on the floor) negative zones on an open field arena. When the rat enters the positive zone, it was rewarded at a random feeder on the circumference of the arena, away from the zone. For negative zones, rather than actively punishing the rat with foot shocks, we use reward withdrawal to avoid fear and stress which are known to significantly depend on non-hippocampal structures such as the amygdala. When the rat entered the negative zone, the feeder that was cued was de-activated and a 10-second delay was initiated before another reward location became active. Each experimental session was divided into two halves, baseline and manipulation. During baseline, a local cue on the arena wall and a distal cue on the curtain surrounding the arena were associated with the zone. During the manipulation, 3 conditions were tested: the cues and the zone moved together to a new location; the cues moved, but the zone remained unchanged; or the zone moved, and the cues remained unchanged. Behaviorally, we found that the rats quickly learned to enter the positive zone and receive a subsequent reward elsewhere on the maze. They also learned to avoid the negative zone by gradually adjusting their trajectories. Rats were able to navigate more accurately into the positive zone and avoid the negative zone when the cues and zone moved together, but less so when the cues and zone were moved separately. These results show that pairing the zone and distal cues had a significant effect on the rats' behavior in this task.