Hippocampal Lesions can Enhance Discrimination Learning Despite Normal Sensitivity to Interference From Incidental Information

Spatial properties of stimuli are sometimes encoded even when incidental to the demands of a particular learning task. Incidental encoding of spatial information may interfere with learning by (i) causing a failure to generalize learning between trials in which a cue is presented in different spatial locations and (ii) adding common spatial features to stimuli that predict different outcomes. Hippocampal lesions have been found to facilitate acquisition of certain tasks. This facilitation may occur because hippocampal lesions impair incidental encoding of spatial information that interferes with learning. To test this prediction mice with lesions of the hippocampus were trained on appetitive simple simultaneous discrimination tasks using inserts in the goal arms of a T-maze. It was found that hippocampal lesioned mice were facilitated at learning the discriminations, but they were sensitive to changes in spatial information in a manner that was similar to control mice. In a second experiment it was found that both control and hippocampal lesioned mice showed equivalent incidental encoding of egocentric spatial properties of the inserts, but both groups did not encode the allocentric information. These results demonstrate that mice show incidental encoding of egocentric spatial information that decreases the ability to solve simultaneous discrimination tasks. The normal egocentric spatial encoding in hippocampal lesioned mice contradicts theories of hippocampal function that suggest that the hippocampus is necessary for incidental learning per se, or is required for modulating stimulus representations based on the relevancy of information. The facilitated learning suggests that the hippocampal lesions can enhance learning of the same qualitative information as acquired by control mice.