Effects of the size of the morris water tank on spatial discrimination learning in the CFW1 mouse.

The spatial orientation of mice and rats in tasks such as the Morris water escape task is guided by extramaze cues. Using data from the control groups of mice used in two drug-finding experiments, we addressed the question whether the size of the Morris water tank affects spatial learning in mice. In these experiments, CFW1 mice had been tested in either a small or a large water tank. Reanalysis of the data revealed that pool size did not affect learning or bias for the previous training quadrant. The Morris water escape task appears to act as a severe stressor in particular mouse strains, and a high stress level may interfere with the learning ability or performance of mice, which may have serious consequences for the interpretation of data. This is of special relevance when assessing the Morris maze performance of strains of mice with a different sensitivity to stress. We conclude that mice can best be tested in a small pool, because the time and distance swam to find and escape onto the platform is decreased and the probability of success (that is, of encountering the platform) is increased. This will reduce stress and hence increase the comparability of data between different strains of mice.