Memory is the process by which a learning experience is retained over time. A single memory can be retrieved several times when the proper stimulus is presented. There is no consensus on the way in which to classify memory, but two dichotomies often arise when studied by neuroscientists. The first dichotomy is between procedural and declarative memory, and the second dichotomy is between short term memory and long term memory.

Procedural memory in humans is related to the knowledge of rules of action and procedures, which can become quite automatic with repetition. When one studies a learning curve of some task, we can see that performance is improved (either by less errors or quicker responses, or by a combination of both) with the number of repetitions of that task. Having a lot or little "practice" with certain task is procedural knowledge. Nonassociative learning and most classical conditionings produce procedural memory.

Declarative memory involves explicit information about facts. To remember one's telephone number, or the names of the parts of the neuron, does not require a set of rules or procedures, it is explicit and involves associations with other events. To put it colloquially, declarative memory is what we know consciously, and procedural memory is what we perform unconsciously. Although this dichotomy was first put forward for describing human memory, it is useful for classifying animal memory as well. A rat can improve on the performance in climbing a small ladder (procedural) and can remember if there will be food on top or not (declarative) if a light is tuned on or off.

Although this division of memory seems arbitrary at first, it is very useful in neuroscience since each type of memory probably has different types of neural substrates. For instance, the hippocampus and temporal cortex seem to be involved in the formation of declarative memory, but not of procedural memory. Whereas certain nuclei of the cerebellum and spinal chord seem to be necessary for procedural memories to form, but do not intervene in declarative memory. Due to this anatomical organization, declarative memory is said to be controlled by higher brain mechanisms, while procedural memory appears to depend on lower regions and systems.

The second recurring dichotomy in the study of memory in neuroscience, is between a short lasting stage and a long lasting stage. The short stage is called short term memory (STM) an is defined by its limited capacity and lability, since it usually only contains a few (less than seven) pieces of information, and can be disrupted easily with either strong or distracting stimuli, or with brain manipulations. If STM goes undisturbed, is only lasts from a few seconds up to several hours, depending on the type of learning and the organism involved.

Long term memory (LTM) occurs when the information is kept for longer periods, up to the whole lifetime of the organism. This occurs less often and only with association of stimuli that is relevant to the organism, either because of a biological predisposition or by continuos repetition. Usually experiences charged with a strong affective component (either reinforcing or aversive) tend to go into long term memory more often than others. This type of memory is less labile, and is not easily disrupted. The most frequent reason that some information cannot be retrieved from LTM, is a retrieval problem itself, and not that the memory is lost, since it can appear latter in another context. Very few restricted lesions have an effect on LTM, but some mayor afflictions like hypoxia (lack of oxygen in the brain), trauma or electroconvulsive shock can disturb long term memories that were stored. However, most memories come back in time, and the ones that do not were the memories most recently learned before the trauma or treatment. This lack of lability of LTM suggests that the brain (particularly neurons) go thru plastic changes that are almost permanent. In the case of STM, the changes probably involve just the way some neurons function, but not the plastic permanent changes.

On the neural level, a notable difference between the dichotomy between procedural and declarative memory, and the dichotomy between STM and LTM, is that in the latter there seems to be involved either higher and lower brain structures, and in some cases the same anatomical area is necessary for both STM and LTM. It is the neural mechanisms involved that are the difference underlying each.

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