The Multi-Store Model

Human Memory

Sensory

Memory Short-term Memory(Working Memory) Long-term Memory

Explicit Memory Implicit Memory Procedural Memory Declarative Memory Episodic Memory Semantic Memory (events and experiences) (facts and concepts) (events and facts) (conscious) (unconscious) (skills and tasks)

(< 1 sec) (< 1 min) (lifetime)

Figure 2.2. The structure of human memory as a sequence of three stages (i.e., Sensory → Short-term → Long-term) rather than a unitary process, known as the multi-store or Atkinson-Shiffrin model, after Richard Atkinson and Richard Shiffrin who introduced it in 1968 [7].

On one hand, the multi-store model is heavily influential receiving support by a multitude of studies in the distinction between short-term and long-term memory and their characteristics (i.e., encoding, duration and capacity), while it can account for primacy and recency effects [162]. On the other hand, the

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model is considered oversimplified, passive, one-way or linear model, mainly in that it suggests that both long-term and short-term memory each operate in a sin- gle, uniform fashion. Among others, criticism focuses on working memory being more than just a simple unitary component, and instead comprised of different subcomponents (e.g., central executive, visual–spatial, etc.) [12]. Particularly for long-term memory, critics proclaim that it is rather unlikely that different types of stored information such as remembering one’s online banking password and remembering what one ate yesterday end up being stored within a unitary/s- ingle memory buffer/store. Albeit, the model makes a distinction between se- mantic memory (i.e., facts), episodic memory (i.e., experiences) and procedural memory (i.e., skills) components (see Figure 2.2). Furthermore, the multi-store model has also been criticized for overestimating the importance of attention and rehearsing during the memory formation process, and neglecting additional process elements. Interestingly though, the structure of the multi-store model comprising sets of stores/buffers with information flowing through them, same as information flows through a system, resembles greatly an information process- ing model with an input, process, and output components (i.e., von Neumann model/architecture). Craik et al., provide a comprehensive overview on the ad- vantages and disadvantages of the multi-store memory model[53]. Despite the aforementioned flaws, we decided to base our work on the multi-store model, since it offers an intuitive, simple and interdisciplinary–compatible systemic view over human memory, without the need to greatly deepen our knowledge on the specificities of Human Memory theory, Cognitive Psychology and Neuroscience. According to the multi-store model, there are three main memory types/compo- nents, sensory memory, short-term memory and long-term memory, as described briefly next.

Sensory Memory. It is considered the shortest-term component of human memory and has the ability to retain impressions of sensory information after the original stimuli have ended for a duration of 0.25–0.5 seconds [188]. It operates as a store/buffer for storing very large volume of information caused by stimuli perceived by our five senses which are retained extremely accurate but very briefly. As such, the ability to look at something for a split second and remember how it looked like (e.g., shape, colour, etc.) epitomizes a sensory memory intervention.

Short-term Memory (or Working Memory). The short-term memory com- ponent of human memory has a limited capacity in storing approximately 7 (– /+2) items of information for a duration of 15–30 seconds. Items can be retained in short-term memory by rehearsing them verbally [8]. An alternative model of short-term memory was developed at a later stage by Baddeley and Hitch[1974]

25 2.1 Human Memory

known as working memory, arguing that the description of short-term memory provided by the multi-store model was overly simplistic, as mentioned above [12].

Long-term Memory. Memory theorists claim that the long-term memory could potentially have unlimited capacity with the main challenge lying in how able one is to access a memory and not if the memory per se is registered/avail- able (i.e., accessibility over availability). In fact, Bahrick et al. (1975) investi- gated the capacity and duration of long-term memory in what they called Very Long Term Memory (VLTM), where they tested 392 participants of age 17 to 74, for memory retention periods of 2 weeks up to 57 years [15]. Tests included a free recall test (i.e., recall without any memory support), where participants were asked to recall the names of their classmates in a graduate class, a picture recognition test were participants reviewed a total of 50 pictures (mixed ran- dom pictures with pictures of their school albums), and a name recognition test for school friends. Results showed that participants who had already graduated for 15 years achieved a 90 % score in accurately identifying names and faces. 48 years after graduation, participants free recall of their classmates’ names de- clined by about 60 % of the initial level, and cued-recall performance (using the album pictures as visual cues) declined more than 70 %. However, the decline of performance in these tasks over many years involves only the retrieval of names. The ability to identify names, faces and their associations remained unimpaired, although both recall tasks showed significant declines[15].

From a Neuroscience perspective, (conscious) human memory has been largely attributed to a particular brain area known as the "hippocampus"[79]. As such, long-term memory has been subdivided into a hippocampus-dependent mem- ory component, known as declarative or explicit memory, and a hippocampus- independent memory component, known as non-declarative or implicit mem- ory [86], as hypothesized in the multi-store model (see Figure 2.2). One of the earliest and most influential contributions in the domain of long-term memory was introduced by Tulving (1972). Tulving proposed the further distinction of declarative long-term memory into episodic and semantic, and that of implicit long-term memory into procedural memory, essentially extending the multi-store model[231] (see Figure 2.2).

Procedural memory is responsible for hosting skills and generally knowledge on how to do things. Procedural memory is also closely associated with habits, it is automatic and inarticulate, featuring both cognitive and motor activities[43]. For example, knowing how to open a door by grabbing and pressing down the handle is ascribed to our procedural memory, where information about motor skills and affordances (i.e., perceived action possibilities) is stored. Procedural

26 2.1 Human Memory

memory is not declarative, typically does not involve conscious thought, and thus it falls under the implicit part of long-term memory, according to the multi-store model (see Figure 2.2). Nevertheless, procedural memory is outside the scope of this work and hence we do not dive into further details.