I want to welcome everyone back to the MazeEngineer Blog. This week I’m going to introduce the Morris Water Maze. Like the Y-maze, this maze is often used to test reference memory.
First, let’s be blunt. No self-respecting Minotaur would consider this a maze. Basically, you start with a tub of water with an underwater rat-sized platform. Then, you put a rat in the water and watch it swim around until it finds the platform and rests while silently cursing you. This task has been used to investigate spatial learning and memory for more than 20 years, and by manipulating the task parameters, researchers can focus on different cognitive processes and their underlying neural mechanisms. Let’s take a closer look.
| Video 1. Spatial learning and reference memory in the Morris Water Maze. The time to find the platform decreases as the mouse learns the location of the platform. A probe trial shows that when the platform is removed, the mouse spends more time in the quadrant of the pool that used to have the platform. |
Mice and rats have a natural aversion to water, and the Morris Water Maze takes advantage of this when assessing spatial learning and both reference and working memory. By placing the platform at the same location over a series of days, learning can be assessed by recording the rate at which the time to find the platform (escape latency) decreases (Figure 2), the speed at which rats swim, and the length and directionality of the swimming path.
| Figure 1.The learning curve in the Morris Water Maze. The time to find the platform (escape latency) decreases with training as swimming paths become less random and more directed. |
Because of their aversion to swimming, rats are highly motivated to find the platform as soon as possible. After learning is complete, reference memory can be tested in probe trials in which the platform is removed. Mice that have learned the platform’s location will spend more time swimming around the location where it used to be (Vid. 1, after Morris et al. 1990 and Steele and Morris, 1999).
Working memory can be assessed by placing the platform at a new location every day. In this context, the rats must try to encode a new location on the 1st trial of every day and remember it over the next several trials. This is sometimes referred to 1-trial learning. We’ll go over this topic a bit more in the next post.
Like other standard mazes, the Morris Water Maze is often used to evaluate spatial learning and memory in rodent models of memory disorders and drug treatments. When assessing the deficits in a model or the effectiveness of a drug, proper controls that take into account other factors that affect performance must be rigorously applied. Water Mazes in particular are extremely stressful for rats (and especially for mice), and this is often seen as a potentially confounding factor. In our next blog, we will tackle The Barnes Maze, which attempts to preserve the learning and memory aspects of the Morris Water Maze, but at reduced stress levels for the animals.
References and Reading
Morris RG, Schenk F, Tweedie F, Jarrard LE. (1990) Ibotenate Lesions of Hippocampus and/or Subiculum: Dissociating Components of Allocentric Spatial Learning. Eur J Neurosci. 2(12):1016-1028.
Terry, AV. Spatial Navigation (Water Maze) Tasks. In: Buccafusco JJ, ed. Methods of Behavior Analysis in Neuroscience. 2nd ed. Boca Raton, FL: CRC Press; 2009. Chapt. 13.