Incidental (unreinforced) and reinforced spatial learning in rats with ventral and dorsal lesions of the hippocampus

We investigated whether the ventral and dorsal hippocampus were differentially involved in incidental spatial learning. Rats with ventral and dorsal hippocampal lesions were tested on an unreinforced test of spatial memory that takes advantage of their natural propensity to explore novelty. Rats were presented with two copies of an identical object in a large circular open field arena. Subsequently, the rats were placed back into the open field with one of the objects displaced to an adjacent quadrant of the arena. Sham-operated rats and rats with ventral hippocampal lesions spent more time in the quadrant that contained the displaced object than in the quadrant that contained the non-displaced object, and more time investigating the displaced object than the non-displaced object. Rats with dorsal hippocampal lesions were impaired on both measures. Both sham and ventral hippocampal lesioned rats subsequently learned to retrieve a food pellet in the ends of each arm of a radial maze. Rats with lesions to the dorsal hippocampus showed no significant improvement in the number of errors made across training sessions and made significantly more errors, overall, than rats with ventral hippocampal or sham lesions. The findings suggest that an intact dorsal but not ventral hippocampus is necessary for spatial learning in rats.