Memory is the capability of the brain to encode, store, and retrieve information or data when they are needed. Without memory, it is impossible to perform daily activities or form relationships or learn and progress in our life. That is, memory represents our identity.
According to the multistore model of memory (also known as the modal model), proposed by Atkinson and Shiffrin in 1968, memory consists of three stores: a sensory register, short-term memory (STM), and long-term memory (LTM). STM is the capacity to store and manipulate information for a short time, while the LTM relates to storing the information for an extended period. STM, also referred to as short-term storage or primary or active memory, indicates different systems of memory involved in the retention of pieces of information (memory chunks) for a relatively short time (usually up to 30 seconds). On the contrary, LTM may hold an indefinite amount of information for an indefinite time.
Based on cognitive psychology studies, since the late sixties of the last century, researchers proposed to divide LTM into two major two subsets of LTM, explicit memory and implicit memory. The explicit memory is also termed as declarative memory as it can be expressed voluntarily, through language and other communication manners. In 1972, the Estonian scientist Endel Tulving suggested the distinction between two types of explicit memory, episodic and semantic memory and illustrated the principles of these memory systems in a book entitled 'Organization of memory' (New York, Academic Press, 1972). Although over the years Tulving modified the construct of semantic and episodic memory, this nosographic approach for memory taxonomy remains a key concept in contemporary cognitive neuroscience. The semantic memory represents the conscious and intentional memory of concepts and meanings, whereas the episodic memory is the conscious recollection of an episode, of the sequence of events that characterized it, and of its space-time location. In other words, the explicit memory concerns itself with the ability to consciously remember the general concepts, ideas, and events that occurred through the person's life. Within explicit memory, episodic memory has an autobiographical reference referring to personally experienced events (context-sensitivity), whereas semantic is detached from the autobiographical reference and cannot be defined in terms of its context-sensitivity. Of note, both episodic and semantic memory can be accessed unintentionally and a strict distinction between the two explicit memory systems cannot be made. Probably, the two forms of memory are highly interactive and share functional properties and neural bases.
On the other hand, implicit memory does not require conscious intervention to be retrieved. That is, its recovery is not in the form of memories, but through actions or automatisms, "knowing how." It represents, in turn, our abilities (e.g., swimming, cycling, driving the car) and is therefore referred to as procedural memory.
By summarizing LTM can be divided into two subsets:
Explicit (or declarative) memory
Episodic memory
Semantic memory
Implicit (or procedural) memory
Although the modal model represents an excellent basis for understanding the modalities of memory, it seems to operate an excessive simplification of matter and has been criticized for being a passive and linear model. Based on a wide range of studies conducted on the subject, several alternative memory models have been proposed. Most likely, a model that can explain memory functioning in detail has yet to be proposed. Therefore, it is reasonable to believe that the various models and theories should be integrated. In this context, the Working Model of Memory proposed by Baddeley and Hitch, in 1974, offers an explanation of how STM works. STM, indeed, is more than just one simple unitary store and comprises different components such as the central executive, and the visuospatial components. In this complex scenario, working memory (WM) represents the capacity to temporarily store and manipulate data in the service of ongoing tasks through a multicomponent system. That is, WK represents an information processing system to allow the storage of information into the LTM sets by way of STM.
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