Effects of individual housing and stressor exposure upon the acquisition of watermaze escape

The watermaze tasks of R.G.M. Morris (1981, Learning and Motivation, 12, 239–260) have been used to study the biological bases of spatial learning and memory in rats. These tasks enable the specific study of spatial learning by contrasting performance on two escape tasks. Both tasks allow a rat to escape from a pool of cool water by locating a small escape platform. They differ in the use of a plainly visible platform with varying location in the nonspatial task, versus a hidden platform with constant location in the spatial task. Rats learn to locate both kinds of platform with impressive and nearly equal speed. We report that while rats housed in groups show fast and reproducible acquisition of these tasks, rats housed individually show a gradual decrease in acquisition performance of both spatial and nonspatial tasks. This deficit is detectable after 2 or 3 weeks of isolation, and continues to increase in severity thereafter. The deficit is reversible by a brief period of group housing, by episodic exposure to loud noise, and by episodic restraint. The efficacy of these corrective treatments declines when isolation is prolonged beyond 3 weeks. These findings are interpreted as indicating an abnormality in stress responsiveness in the individually housed animals. The implications of these findings for experiments studying watermaze acquisition in individually housed animals are examined.