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Basic Definition

The Morris Water Maze Navigation Task (MWM) is one of the most widely used tests in behavioral neuroscience. Researchers have found the maze to be an accurate study of learning, memory, and spatial working memory and use it to assess the effect of damage to cortical regions of the brain on both rats and mice. It is used largely by neuroscientists to measure the effect of neurocognitive disorders on spatial learning and possible neural treatments, to test the effect of lesions to the brain in areas focused on memory, and to study how age influences cognitive function and spatial learning. In order to study certain neurocognitive disorders, scientists use gene-targeting on mice and inactivate a specific gene which results in behavioral changes similar to those that result from diseases like Alzheimer’s. The MWM is also used as a tool to study learning in transgenic mice, the effect of drug-abuse, neural systems, neurotransmitters, and brain development.

History

The Morris Water Maze was invented by Richard G. Morris in 1981 as an alternative to the radial arm maze. The test was made to study spatial learning and how it varied from other forms of associative learning. Originally rats, now more commonly mice, were placed in an open pool and the latency to escape was measured for up to six trials a day for 2-14 days. Now, procedures are varied. A "probe" trial measures how long the test subject spends in the "target" quadrant (the quadrant with the hidden platfform of the pool when the exit is removed. More elaborate trials alter the location of the hidden exit and measure distance spent swimming in the pool before reaching the platform. Neuroscientists examine many different variables in the test including the effects of differences in sex, weight, strength, stress levels, age, and strain of species. The results differ dramatically, so researchers cannot make conclusions about a wide range of tests unless the manipulations are relatively minor. Although the original dimensions were explicitly outlined, many different size mazes have been used throughout the history of this task. Studies show however, that varying the diameter of the pool does not have a significant impact on the results of the test. Some people refer to it as a maze while others refer to it as a task that provides an open field for learning. In early versions of the MWM researchers only timed latency to escape, however now video tracking devices can measure the path to escape, time spent in each quadrant, and distance traveled in the pool.

Comparison to other mazes

The Morris Water Maze is preferred to a lot of other spatial tasks because it measures spatial memory, movement control, and cognitive mapping whereas other mazes only measure one of these three tasks. The T-maze and radial arm maze are much more structured in comparison. The T-maze, for instance, only requires the rat or mouse to make a bilateral decision, choose left or right. In the MWM, on the other hand, the animal is continually deciding where to go. Another reason this task became popular is that rats and mice are natural swimmers so in order to perform the task they do not need to be motivated by food deprivation or electrical shock. The platform is mobile which allows for tests on learning and relearning. The expereiment is still effective when animals are admministered multiple doses of drugs. As opposed to dry tasks, the MWM takes a relatively short time allowing researchers to perform more tests efficiently. Also, the apparatus set-up and costs are relatively low as compared to other mazes.

Insights for neuroscience

Spatial learning

The MWM is a very simple task, and for that reason, very effective in measuring spatial learning. The opaque water prevents the rat from using visual, auditory, and olfactory cues and therefore eliminates possible uncontrolled variables. The primary brain regions involved in this type of learning are the hippocampus, striatum, basal forebrain cholinergic system, cerebellum and several neocortical areas. MWM experiments on rats and mice with altered brain regions helped to define the role each region has in spatial memory. Rats and mice with hippocampal lesions for example, can move through the maze properly, but they are not able to navigate and thus fail to create cognitive maps. The cognitive map is made up of place cells in the hippocampus that record information about the location in respect to cues in the environment (allocentric space) rather than getting sensory stimulus from the location (egocentric space) allowing for the animal to record spatial information find the hidden platform in the MWM.

The striatum, located in the basal ganglia, also plays a role in spatial movement. Rats with lesions in the striatum were placed in the MWM to explore the relationship between visuospatial skills, cognitive flexibility, and recall. The experiment showed that rats with lesions in the medial striatum had difficulty with all three tasks in the MWM with a hidden platform however, had no difficulty navigating to a visible platform indicating that the striatum also plays a role in moving in allocentric space. Right next to the striatum is the basal forebrain, another area that when altered affects MWM performance. Tasks have been performed on mice with adjusted levels of acetylcholine in the Cholinergic System to show the role of the basal forebrain in spatial learning. Altering the neurochemistry of rats has shown that two neurotransmitters play a major role in spatial learning; both acetylcholine and glutamate.

See Also

Spatial memory

Radial arm maze Place Cells T-Maze

My Wikipedia Page Hippocampus