All about Morris Water Maze (MWM)
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What is Morris Water Maze?
A: Morris Water Maze (MWM) is a test of spatial learning for rodents developed by Richard Morris. Performed properly, it is a robust and reliable test that is strongly correlated with hippocampal synaptic plasticity and NMDA receptor function. Vorhees & Williams, 2006 report that MWM performance after treatment have been more widely replicated than the effects observed with any other learning task including its predecessors (radial-arm maze, T-mazes and other variations). It is important to understand, however that MWM performance is sensitive to factors such as apparatus or training procedure D’Hooge & De Deyn (2001). The definitive system for measurement and analysis of Morris Water Maze is the HVS Image Video Tracking Series used by Richard Morris and reported in Journal of Neuroscience Methods, 11 (1984) 47-60 for his definitive Video Tracked MWM experiments. Incorporating over 35 subsequent years of ongoing development, only HVS Image systems can precisely emulate the original tracking and analysis used and therefore get precise and definitive data, with over 1400 peer reviewed publications citing it’s use. All other systems are approximations or interpretations of those algorithms.
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How does Morris Water Maze work?
A: A rodent is placed in a 1.4m (mouse) or 1.8m (rat) pool of warm water (21° for mice, 26° for rats). The animal uses distal visual cues to navigate from randomly placed start locations around the perimeter of the pool to locate a submerged escape platform 0.12m in diameter. The validity of the task and results in crucially dependent on the distal nature of the cue locations. This is because the ability to locate an object is encoded in the brain by two separate systems. The first uses self-movement, internal cues and nearby proximal cues, involves the dorsal striatum and connected structures and is called egocentric navigation. In humans this system encodes routes and paths and procedural memory. The second uses one using cues outside the organism, distal cues, and is called allocentric navigation. It is only this second system, allocentric navigation, that involves the hippocampus, entorhinal cortex, and surrounding structures. In humans it also encodes semantic, and episodic memory. Thus the use of proximal cues rather than distal, shifts the type of memory being tested. From our experience of over 35 years of testing and hundreds of laboratories, distal cues in water maze are placed at twice the pool diameter from the center and are designed to subtend five degrees.
A: Spatial learning can then be assessed over repeated trials. Reference memory can be determined by preference for the platform area when the platform is absent.
A: Reversal and shift trials enhance the detection of spatial impairments.
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What else can Morris Water Maze measure?
A: Trial-dependent, latent and discrimination learning can be assessed using modifications of the basic protocol.
A: The ration of search area to platform area determines the degree of reliance on spatial strategies versus non-spatial strategies.
A: Cued trials determine whether performance factors that are unrelated to place learning are present.
A: Escape from water is relatively immune from activity or body mass differences, making it ideal for many experimental models.
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How long does it take to get results?
A: Vorhees & Williams 2006 report that results can be obtained from individual animals in as few as 6 days.
How do I analyse Water Maze Data?
A: The main factors used are: Escape Latency, Path Efficiency Ratio, Blokland Measure, Gallagher, Wishaw, and Kelly Test
What other factors should be considered?
A: Careful set up of this sensitive cognitive task, and correct analysis can allow detection of subtle differences, which will be missed if the wrong tracking or analysis are used. As well as offering considerable time and cost savings, a suitable system and careful set up greatly increases the likelihood that the molecular changes underlying cognitive deficits can be identified. Excellent mouse protocols are outlined by Weitzner, et al. 2015 and rat ones by Vorhees & Williams, 2006 and Nunez, 2008. Examples to consider include alleviating stress, which results in poor MWM performance and pre-handling which mitigates stress-induced deficit in part by acclimatizing rodents to introduction and removal from the MWM pool.
In addition data quality can be improved by visible platform training, which is achieved with a slightly lowered water level (effectively giving a raised platform). This allows experimenters to identify mice with performance problems related to sensorimotor abnormalities. Using Weitzner, et al’s exclusion criteria, performance-incompetent mice can be removed from subsequent examinations of hidden platform training and probe trials.