Wednesday, February 07, 2007

The Morris Water Maze: Applications to Cognitive Robotics work?

In this post, I will be looking at the Morris Water maze – a rodent testing environment designed to assess spatial, long term and short term (working) memory. After a brief overview, taking into account some considerations when using rodents as test subjects, I will make a few very brief notes on the potential relevance of this to work with artificial cognitive architectures, particularly when embodied (in mobile robots for example). My intention is that a future post will expand on this second part.

The Morris water maze, otherwise known as the water task or maze, is a spatial task which uses simple visual cues [1], and was originally conceived to allow comparisons to be made between spatial memory and classical and instrumental conditioning, although its conceiver later characterised methods specifically for the study of spatial working memory [2]. In this maze, a swimming rodent tries to find a hidden platform in a pool of water, on the basis that most rodents have an aversion to swimming and hence would view the platform as an escape, and thus provide positive reinforcement [3]. Around the edge of the pool are a series of static visual cues by which the rodent can determine direction. Please see the figure for the typical layout of a Morris water maze.

As shown, the platform is visible above the surface of the water – a case that is used for initial training of the rodents – although in most testing situations, this platform is submerged and hence out of sight, as the water is usually made opaque (using milk powder for example). The visual cues may be of two types; normal elements of the room in which the experiments are conducted which are visible from inside the tank (for example a desk or cupboard) which have three dimensions, or as shown the picture, with high visibility two dimensional cues used, and all others excluded. The behaviour of the rodent whilst in the maze is often recorded remotely using a video camera so as not to interfere with the trials. There are few variations of the basic test, although as mentioned, the type of visual cue used and their number and arrangement allow innumerable permutations.

A number of indices are used to assess performance of the rat, the most common of which are: time to reach the platform, path length (and hence swimming speed), and amount of time spent in predetermined areas of the tank [4]. These indices may be significantly influenced by a wide range of environmental and species-specific factors which do not reflect the cognitive abilities of the rodent, such as the sex of the rodent, dimensions of the tank used, water temperature, nutritional status, hormonal status and home cage environment, that must be taken into account [4], but which may actually be inadequately considered [5, 6].

The interpretation of the obtained results with regard the cognitive functions of the rodents in question is often fraught with ambiguities. Taking as an example the measurement of the amount of time spent in a certain area of the tank [4], it is commonly assumed that long periods of time spent in the target sector is indicative of a good memory, as it is supposed to show that the rodent is searching for the platform in approximately the right place. However, if the platform that was present were to be moved, the persistence of the rodent in that sector may be indicative of a reduced cognitive performance, compared to one that quickly ‘gives up’, and moves to another sector for searching there. It is due to difficulties of interpretation such as these that the underlying factors discussed previously must also be taken into account to keep each set of experiments in the correct context, as differences in behaviour may be due to environmental differences rather than cognitive differences.

As discussed, the water maze may be used to examine a wide range of cognitive processes, ranging from spatial memory to navigation. When one is looking at embodied cognitive architectures, one is generally trying to recreate in some form the behaviour of a biological animal due to their obvious ability to cope with an uncertain world – a desired property for any artificial agent. The water maze thus seems to be ideally suited to their behavioural analysis, when suitably modified, as it provides a bounded domain with easily controllable factors, and yet provides a complex problem: that of integrating spatial memory with suitable motor responses after having to learn both the generalities of the type of the domain, and the specific instantiation of the maze in any given trial. A number of robotics studies have in fact already used this idea – these are to be reviewed in a later post.

[1] R. G. M. Morris, "Spatial localization does not require the presence of local cues," Learning and Motivation, vol. 12, pp. 239-260, 1981.
[2] R. Morris, "Developments of a water-maze procedure for studying spatial learning in the rat," Journal of Neuroscience Methods, vol. 11, pp. 47-60, 1984.
[3] G. B. Mulder and K. Pritchett, "The Morris Water Maze," Contemporary Topics in Laboratory Animal Science, vol. 42, pp. 49-50, 2003.
[4] S. L. Allen, "The Morris Water Maze as a tool for Cognitive Function Testing," Syngenta CTL, Macclesfield, Technical Report July 2002 2002.
[5] M. Sarter, "Animal cognition: defining the issues," Neuroscience and Behavioral Reviews, vol. 28, pp. 645-650, 2004.
[6] T. Steckler and J. L. Muir, "Measurement of cognitive function: relating rodent performance with human minds," Cognitive Brain Research, vol. 3, pp. 299-308, 1996.

1 comment:

Anonymous said...

thank you 4 the explication. :)