Morris water maze
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In neuroscience, the Morris water maze is a behavioral procedure designed to test spatial memory. It was developed by neuroscientist Richard G. Morris in 1984, and is commonly used today to explore the role of the hippocampus in the formation of said spatial memories.
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[edit] Overview
In the typical paradigm, a rat or mouse is placed into a small pool of water—back-end first to avoid stress, and facing the pool-side to avoid bias—which contains an escape platform hidden a few millimeters below the water surface. Visual cues, such as colored shapes, are placed around the pool in plain sight of the animal.
The pool is usually 4 to 6 feet in diameter and 2 feet deep. The pool can also be half-filled with water to 1 foot in depth. A sidewall above the waterline prevents the rat from being distracted by laboratory activity.
When released, the rat swims around the pool in search of an exit while various parameters are recorded, including the time spent in each quadrant of the pool, the time taken to reach the platform (latency), and total distance traveled. The rat's escape from the water reinforces its desire to live, and on subsequent trials (with the platform in the same position) the rat is able to locate the platform more rapidly. This improvement in performance occurs because the rat has learned where the hidden platform is located relative to the conspicuous visual cues. After enough practice, a capable rat can swim directly from any release point to the platform.
[edit] Pharmacological manipulation
Various drugs can be applied to test subjects before, during, or after maze training, which can reveal information about physical ability. For example rats treated with the NMDA receptor blocker APV perform poorly in the Morris water maze, suggesting that NMDA receptors play a poor role in physical ability [1]. And since long-term potentiation -- a potential biological mechanism for physical ability -- also requires NMDA receptors, spatial learning may require LTP.
Liang et al reported in 1994 that physical ability requires both NMDA and AMPA receptors, consolidation requires NMDA receptors [2].
[edit] Comparison to conventional mazes
The Morris water maze has advantages over conventional mazes such as the plus maze. For instance, there are no local cues such as scent traces and there is no fixed escape-fomula; the rat makes good progress in the trials because it wants to escape. Rats can be considered to be natural swimmers – they are not distressed but they do want to find the platform. Mice have an option to float, perhaps leading to their uncooperativness in the water maze. It has been suggested that mice may not actually aim to find the platform, but fool the technician into rescuing them.
[edit] Water maze analysis
The earliest and classic measure of learning is latency, which is the time it takes to find the platform. However, rats can cheat. They might guess an area and swim a search pattern, getting to the platform quite quickly. There are several analyses that can tease out true spatial learning, many of which use the same swim but require a video tracker. Professional systems come with a suite of analysis features to extract measures such as time and path in quadrants, near platform, in any specified area. The Gallagher measure looks for average distance to platform. The Whishaw corridor test measures time and path in a strip from swim-start to platform.
[edit] References
- Morris R (1984). "Developments of a water-maze procedure for studying spatial learning in the rat". J Neurosci Methods 11 (1): 47-60. PMID 6471907.
- Davis S, Butcher SP, Morris RG. "The NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5) impairs spatial learning and LTP in vivo at intracerebral concentrations comparable to those that block LTP in vitro." J Neurosci. 1992 Jan;12(1):21-34. PMID 1345945
- Liang KC, Hon W, Tyan YM, Liao WL. "Involvement of hippocampal NMDA and AMPA receptors in acquisition, formation and retrieval of spatial memory in the Morris water maze." Chin J Physiol. 1994;37(4):201-12. PMID 7796636
