Memory is a maintained representation of internal or external experiences constituted as modifications of neural structure and/or behaviour. Memory includes such diverse forms of plasticity as the simple processes of HABITUATION and SENSITIZATION, and the more complex abilities to recollect facts and events. However, memory recollection need not be conscious. For example, IMPLICIT MEMORY, including PRIMING and motor skills learning, can be expressed without conscious awareness of learning. Memory does not include native response tendencies, reflex adaptations, developmental processes, or temporary states of the organism such as fatigue, although it is sometimes difficult to distinguish memory from these phenomena.
Memory researchers have distinguished different forms of memory such as: DECLARATIVE MEMORY versus PROCEDURAL MEMORY, relational memory versus non-relational memory (see RELATIONAL AND NON-RELATIONAL MEMORY), SEMANTIC MEMORY versus EPISODIC MEMORY, EXPLICIT MEMORY versus IMPLICIT MEMORY, WORKING MEMORY versus reference, TAXON MEMORY versus LOCALE MEMORY, DATA BASED MEMORY versus EXPECTANCY BASED MEMORY, and FACT MEMORY versus SOURCE MEMORY. Memory is thought to be represented at the neuronal level by networks of connected cells. The modulation of cellular firing patterns and either sustained firing after a LEARNING event, or the ability to reactivate the original neuronal response to a stimulus after the stimulus is no longer present represent the cellular traces of memory. At the single-cell level, memory ENCODING can result in changes in the MEMBRANE properties that affect ION flow, SYNAPSE growth or reaction, and changes in PRESYNAPTIC and POSTSYNAPTICNEUROTRANSMITTERS rclated functions. Neuronal connections between engrams and overlapping representations of memories allow the rapid association of related memories. Cortical sensory and motor areas are hypothesized to be involved in the long-term storage of memories. For example, the visual attributes of a memory may be stored in the VISUAL CORTEX, and the olfactory attributes of the same memory may be located in the OLFACTORY CORTEX. The reactivation of a complete memory is hypothesized to occur when the CELL ASSEMBLY in the sensory areas that represent the components of the memory becomes simultaneously active through synchronous firing (see ENGRAM for a more detailed discussion of the cellular components and storage of the memory trace).
Memory has been shown not to be a single process, but instead to be consist of MULTIPLE MEMORY SYSTEMS, each of which has its own neuronal network. The HIPPOCAMPUS and related medial structures in the TEMPORAL LOBE are thought to be important for the encoding of declarative memories, novelty detection, and memory CONSOLIDATION. The AMYGDALA is important for STIMULUS-REINFORCER ASSOCIATION and for encoding the emotional components of memories. STIMULUS-RESPONSE ASSOCIATION depends on the STRIATUM. The PREFRONTAL CORTEX has been shown to be involved in WORKING MEMORY and source memory (see SOURCE AMNESIA). The CEREBELLUM is involved in CLASSICAL CONDITIONING processes such as eye-blink conditioning. Finally, BRAINSTEM areas have been shown to be involved in the conditioning of motor reflexes and the adjustment of visual reflexes. The discovery of these multiple memory systems has lead some to view memory not as entity to be stored, but instead as a reflection of the plasticity within each functional system of the brain.
Memory is not immutable; it is subject to many distortions. Over time many recollections become less specific, and can even be completely forgotten (see FORGETTING). The details of a memory can be changed to fit our scripts about what would most likely happen in that type of situation. Also, studies of eyewitness testimony have demonstrated that our memory is very susceptible to suggestion, to the point that people can claim to remember things that never actually happened to them. A person’s emotional status at the time of encod ing can also dramatically affect the memory formation process. In some circumstances traumatic experiences such as rape can cause someone to forget the event, and sometimes they can cause a person to form vivid recollections of certain aspects of the occurrence. AGEING, diseases (see ALZHEIMER’S DEMENTIA), and brain damage also modify memory.
