Spatial navigation learning
The hippocampus of rats is particularly important for spatial navigation place learning, through that animals acquire a memory for their location. One broadly used way of investigating this is the Morris water maze. This is a rounded pool filled with opaque warm water. Hidden just down the surface is a little platform. In the duration of learning trials rats swim in the pool and discover the platform, learn its position in the pool relative to cues in the laboratory. The motivation is which the platform provides an escape from the water. Learning takes various trials and can be tested through measuring the time taken for a rat to reach the platform or the length of the path it swims to reach it as recorded on a video camera. Cued control experiments in that the platform is raised just above the level of the water ensure in which any differences in behavior are not attributable to locomotor factors, motivational or perceptual.
Rats with hippocampal but not selective neocortical lesions are seriously compromised in the learning but not the cued versions of this task. The Rats given a microinjection of colchicine to destroy a specific population of cells (dentate granule cells) in their hippocampus either 1, 4, 8 or 12 weeks after studying the location of a submerged platform and tested 2 weeks later in the Figure, reveal that the hippocampus is not a permanent site for the spatial memory. The 12-week set remembered the location as well as control animals but performance got progressively worse for 8-, 4- and 1-week sets. This study shows that the hippocampus is needed for consolidation of spatial learning, but which over successive weeks the site of the memory store is transferred else- where, probably the neocortex.
Figure: Protocol for investigating the time course of place learning by rats in the Morris water maze.
Retrograde amnesia in humans with medial temporal lobe damage probably results from the loss of memory not thus transferred from hippocampus to neocortex. While the hippocampus may be predominantly for spatial learning in rats in primates it has a broader role in consolidating all episodic memories.The hippocampus of the rat and related cortex is by to provide the rat with a representation of the space around it and its location within it. This is the cognitive map hypothesis, it has various postulates. Firstly the map allows the animal to search its way by the environment. Secondly, it is constructed through episodic learning as specific locations come to be associated with motor cues and particular sensory. Thirdly, it does not need reinforcers; and finally the map is continually updated through exploration.