Classification of memory
Memory is the persistence of information in the central nervous system. It is not a unitary function, it comprises independent systems and processes that can dissociate from each other (Tulving, 1991).
Three major cognitive processes are involved in memory: registration (or encoding, during which information is translated into a code to be stored), retention (or storage) and retrieval (when the information is searched and recalled). Memory can be impaired at any of these three steps, but it may be difficult to differentiate between them.
A registration disorder should produce mainly an anterograde amnesia, since facts already stored should not be so much affected. However, it may also include a limited period of retrograde amnesia, involving recent memories still undergoing a process of consolidation (Squire et al., 1992). A retrieval or a storage impairment, on the contrary, will produce both a retrograde, and an anterograde, amnesia. While a storage disorder is irreversible, a retrieval impairment may recover and is identified by an improved performance in memory tests with cueing recall (vs. free recall) and in recognition tasks (when compared to recall tasks) (Ellis & Young, 1997).
According to Tulving (1995), memory systems may be divided into five main categories: semantic, episodic, primary, procedural and perceptual representation systems. A major distinction is made between declarative (or explicit) and procedural (or implicit, non-declarative) memory (Table 18.1).
Declarative memory is the type of memory one is aware of. It is directly acessible to conscious recollection and may be subdivided into semantic memory (our general knowledge of facts, concepts, and meanings) and episodic memory (consisting mainly in autobiographic events framed by a specific spatio-temporal context).