| || |
| Memory and the Hippocampus |
Psychologists have long argued that there are many different types of memory. These can be classified by many criteria, based on decades of experimental research and the different memory defects seen in people who have suffered brain damage. Scientists have agreed that memory can be viewed in temporal terms; that is, there is a short-term memory, with a limited capacity for about a dozen items, and a long-term memory, to which these items are presumably transferred for "storage." Short-term memory seems to be much more vulnerable to loss due to trauma than does long-term memory: people may even lose the ability to form new memories, while their ability to remember their entire lives before an accident remains intact. This memory defect is exemplified in the movie Memento (2000), in which a widower avenges his wife's murder - during which he suffered brain damage - over and over again. Such individuals with this condition of "anterograde amnesia" usually have severe damage to their hippocampus. As Kempermann points out, the hippocampus is not the equivalent of the brain's hard drive but rather a gateway, "a structure, through which all information must pass, before it can be memorized." 4
It is widely agreed that while the hippocampus is undeniably important for memory, the "recording" of information into long-term memory involves plasticity, or physical changes, in multiple regions throughout the entire nervous system. Another interesting distinction that scientists have made in types of memory is between declarative memory, which allows you to remember facts and is extremely complex, and reflexive memory, which usually consists of learning by repetition and often involves motor learning. While declarative memory can be reported, reflexive memory is exhibited by performance of a task and cannot be expressed verbally. It is now thought that the two types of memory may involve two entirely different neuronal circuits.
The hippocampus plays a major role in spatial learning and memory in a number of animals. Research with black-capped chickadees and other species of birds has shown that when the hippocampus is removed, the birds still store food but cannot recall where they stored it. Moreover, bird species that rely heavily on stored food as a winter resource in general have larger hippocampi than those species that don't.
Studies of cab drivers in London have provided fascinating information about the role that the hippocampus plays in spatial memory. London cab drivers are known for their navigational skills and knowledge of the streets of London. To learn how to navigate the streets of the city, would-be cab drivers undergo "the Knowledge," a rigorous training that can take two years to complete. Recent studies using magnetic resonance imaging (MRI) demonstrate that the hippocampi of the London cab drivers are somewhat different. Specifically, the posterior region is significantly larger and the anterior region is significantly smaller in the cabbies when compared with control subjects. Other studies have found that the posterior region is active during tasks involving spatial memory. It is possible that the cabbies come disproportionately from those individuals with excellent spatial memories and corresponding larger posterior regions of the hippocampus. There is further evidence, however, that suggests that the memory work of the cabbies has altered their hippocampi. Those cab drivers that have been working the longest tend to have larger posterior hippocampi than more recently hired cabbies. Furthermore, other imaging studies show that the right hippocampus is activated in the cab drivers when they are asked to remember complex travel routes but not when they are asked to provide information about famous landmarks. 5