Most forms of memory loss are caused by some type of brain injury - particularly to the hippocampus. Amnesia can arise from brain damage that interferes with memory storage, retrieval, or consolidation. What ultimately causes amnesia - a failure to store memories or a failure to retrieve them - is not clear.
A major challenge in resolving this question experimentally is being able to determine whether an animal has truly recovered a lost memory or has simply re-learned the task at hand. By refining a classic behavioral neuroscience experiment to test spatial learning and memory, Livia de Hoz, Stephen Martin, and Richard Morris have developed a novel protocol that distinguishes retrieval from new learning. The original protocol, developed by Morris over fifteen years ago, takes advantage of the fact that rats are good swimmers but prefer life out of water. Rats are placed in an opaque pool and trained to remember the location of an escape platform, hidden just under the surface.
The new twist on the protocol gives rats a reminder by briefly raising the platform above the water in either the learned location or in a new 'false' location. If raising the platform was facilitating relearning, then animals capable of learning would look for the platform in the new location. But if raising the platform functioned as a reminder, then animals should gravitate toward the place they were trained to, regardless of whether the platform reminder was in the original location or a new one. And that's what happened to the rats with partial lesions.
Testing rats with partial damage to the hippocampus, they showed that the resulting memory impairment was not a result of storage failure, since the rats responded to the reminder treatment by recalling the original platform location. This argues strongly that at least part of the amnesia associated with hippocampal damage is a failure of retrieval. Teasing apart this story fully will require more experiments, but in the meantime, de Hoz et al. have contributed a much needed resource for investigating the neural basis of memory loss.
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