Memory Storage
Memories are not stored as exact replicas of experiences; instead, they are modified and reconstructed during retrieval and recall. Memory storage is achieved through the process of encoding, through either short- or long-term memory. During the process of memory encoding, information is filtered and modified for storage in short-term memory. Information in short-term memory deteriorates constantly; however, if the information is deemed important or useful, it is transferred to long-term memory for extended storage. Because long-term memories must be held for indefinite periods of time, they are stored, or consolidated, in a way that optimizes space for other memories. As a result, long-term memory can hold much more information than short-term memory, but it may not be immediately accessible.
The way long-term memories are stored is similar to a digital compression. This means that information is filed in a way that takes up the least amount of space, but in the process, details of the memory may be lost and not easily recovered. Because of this consolidation process, memories are more accurate the sooner they are retrieved after being stored. As the retention interval between encoding and retrieval of the memory lengthens, the accuracy of the memory decreases.
Short-Term Memory Storage
Short-term memory is the ability to hold information for a short duration of time (on the order of seconds). In the process of encoding, information enters the brain and can be quickly forgotten if it is not stored further in the short-term memory. George A. Miller suggested that the capacity of short-term memory storage is approximately seven items plus or minus two, but modern researchers are showing that this can vary depending on variables like the stored items' phonological properties. When several elements (such as digits, words, or pictures) are held in short-term memory simultaneously, their representations compete with each other for recall, or degrade each other. Thereby, new content gradually pushes out older content, unless the older content is actively protected against interference by rehearsal or by directing attention to it.
Information in the short-term memory is readily accessible, but for only a short time. It continuously decays, so in the absence of rehearsal (keeping information in short-term memory by mentally repeating it) it can be forgotten.
Long-Term Memory Storage
In contrast to short-term memory, long-term memory is the ability to hold semantic information for a prolonged period of time. Items stored in short-term memory move to long-term memory through rehearsal, processing, and use. The capacity of long-term memory storage is much greater than that of short-term memory, and perhaps unlimited. However, the duration of long-term memories is not permanent; unless a memory is occasionally recalled, it may fail to be recalled on later occasions. This is known as forgetting.
Long-term memory storage can be affected by traumatic brain injury or lesions. Amnesia, a deficit in memory, can be caused by brain damage. Anterograde amnesia is the inability to store new memories; retrograde amnesia is the inability to retrieve old memories. These types of amnesia indicate that memory does have a storage process.
Models of Memory Storage
A variety of different memory models have been proposed to account for different types of recall. In order to explain the recall process, however, a memory model must identify how an encoded memory can reside in memory storage for a prolonged period of time until the memory is accessed again, during the recall process. Note that all models use the terminology of short-term and long-term memory to explain memory storage.
Multi-Trace Distributed Memory Model
The multi-trace distributed memory model suggests that the memories being encoded are converted to vectors (lists of values), with each value or "feature" in the vector representing a different attribute of the item to be encoded. These vectors are called memory traces. A single memory is distributed to multiple attributes, so that each attribute represents one aspect of the memory being encoded. These vectors are then added into the memory array or matrix (a list of vectors). In order to retrieve the memory for the recall process, one must cue the memory matrix with a specific probe. The memory matrix is constantly growing, with new traces being added in.
Neural Network Model
The multi-trace model has two key limitations: the notion of an ever-growing matrix within human memory sounds implausible, and the idea of computational searches for specific memories among millions of traces that would be present within the memory matrix sounds far beyond the scope of the human-recalling process. The neural network model is the ideal model in this case, as it overcomes the limitations posed by the multi-trace model and maintains the useful features of the model as well.
The neural network model assumes that neurons form a complex network with other neurons, forming a highly interconnected network; each neuron is characterized by the activation value (how much energy it takes to activate that neuron), and the connection between two neurons is characterized by the weight value (how strong the connection between those neurons is). In this model, connections are formed in the process of memory storage, strengthened through use, and weakened through disuse.
Dual-Store Memory Search Model
The dual-store memory search model, now referred to as the search-of-associative-memory (SAM) model, remains one of the most influential computational models of memory. Two types of memory storage, short-term store and long-term store, are utilized in the SAM model. In the recall process, items residing in the short-term memory store will be recalled first, followed by items residing in the long-term store, where the probability of being recalled is proportional to the strength of the association present within the long-term store. Another type of memory storage, the semantic matrix, is used to explain the semantic effect associated with memory recall.