Abstract
The acquisition of contextual fear in mice is thought to require the formation of a conjunctive representation of the conditioning chamber. This can be achieved during a minimum of 20 to 40 s of exploration immediately prior to the shock or during preexposure to the context at an earlier time. An animal receiving less time in the chamber will show reduced freezing 24 hr later, a condition termed the immediate shock deficit (ISD). In this study, the authors have attempted to uncouple the formation of a contextual representation, based on the conjunction of a defined set of cues, from the establishment of a spatial representation, which requires active exploration, by inserting a transparent plastic partition in the center of the chamber. Taking advantage of the ISD and the context preexposure effect, the authors found that animals preexposed to one side of the chamber on Day 1, but shocked on the other side on Day 2, show significantly less fear than animals exposed to and shocked on the same side. Our results indicate that spatial exploration is necessary for mice to benefit from contextual preexposure.
