A cautionary note regarding drug and brain lesion studies that use swimming pool tasks: partial reinforcement impairs acquisition of place learning in a swimming pool but not on dry land.

Spatial tasks are used widely in neurobiological studies because it is thought that they provide an unbiased assessment of the integrity of neural structures that mediate spatial learning. For example, in the Morris swimming pool place task, animals are required to locate a hidden platform in a swimming pool in relation to environmental cues. Treatments that result in an animal's failure to find the platform are assumed to reflect defects in the function of neural systems involved in spatial learning. The present study demonstrates, however, that an animal's reinforcement history can contribute to its spatial performance. Animals were trained in the Morris place task with the platform present on 100, 75 or 50% of trials. Relative to the 100% group, the 75% group was impaired in place acquisition, and the 50% group failed to learn. Even placing the 50% group animals onto the platform at the completion of an unsuccessful trial failed to improve acquisition. Animals trained to search for food on an identical dry maze problem were not affected by similar reinforcement schedules. The present findings demonstrate that the Morris swimming pool place task does not provide an unbiased assessment of spatial learning: A treatment effect may be confounded with reinforcement history. The results are discussed in relation to widespread applications of the Morris place task to neurobiological problems.