Morris Water Maze
Morris water maze (MWM): This is the most widely used test of spatial navigation and reference memory. The test utilizes a large open circular pool (210 cm for rats, 122 cm in diameter for mice) of water in which there is a small, submerged platform. The platform is the goal and the only escape from the water. Details and reasons for each procedure are contained in a recent methods review paper, Vorhees and Williams, Nature Protocols, 2006, 1: "Morris water maze: Procedures for assessing spatial and related forms of learning and memory".
In brief, in the spatial learning/reference memory version of the test, animals are placed in the maze over a series of trials from different start locations around the perimeter but with the platform in a constant location. Swimming is video-tracked (AnyMaze) and latency, path length, swim speed, and cumulative distance from the platform (recorded every 01. s) is recorded. A typical procedure for the learning phase (acquisition) provides animals with 4 trials per d for 4 or 5 consecutive days. Twenty-four hours after the last acquisition trial, a single probe trial is given for 30 s with the platform removed. Number of platform site crossovers, time in the target quadrant, average distance from the target location, and related measures are tracked and recorded. A second phase of the test may be added (reversal) in which the platform is moved to the opposite quadrant after the end of acquisition testing. An optional third phase is shifted-reduced, in which the platform is moved to an adjacent quadrant and a smaller platform is used to make the task more demanding. We have data showing that this third phase can sometimes unmask effects not apparent on acquisition or reversal. As a control procedure, animals are tested before the hidden platform procedures in the cued (visible platform) version of the MWM. This procedure ensures that animals can see, swim properly (and at comparable speeds regardless of genotype), and can learn the concept that the platform is the escape and to remain on it once located. The task is non-spatial because the platform is moved to a new, random location on every trial such that spatial references to distal cues become irrelevant. The only cue that is relevant is the flag places on the platform that protrudes above the water surface. Mice especially benefit from the cued procedure. There is also a working memory version of the MWM, often termed matching to place. In this procedure the platform is moved to a new location daily and the animal is given 2 trials per day (or in some methods >2 trials/d). All trials on any given day are from the same start location. Only new memories for the location of the platform on that day allow the animal to find the platform on the second trial faster than on the first. The savings in re-finding the platform are an index short-term or working memory. The fixed hidden platform version of the MWM is a test of hippocampus-dependent reference memory and the matching to sample hidden platform version of the MWM is a test of prefrontal cortex working memory.
