Short-Term Memory Impairment

Excerpt

Short-term memory is also called short-term storage, primary memory, or active memory. The term indicates different systems of memory involved in retaining pieces of information, or memory chunks, for a relatively short time, typically up to 30 seconds. In contrast, long-term memory may hold indefinite information. However, the difference is not just the time variable but also their overall functionality. Nevertheless, the 2 systems are closely related.

Practically, short-term memory functions as a temporary scratchpad for recalling a limited amount of data, typically around 7 ± 2 items, in the verbal domain, based on George Miller's concept. This information originates from the sensory register and can be processed through attention and recognition. In contrast, information collected in long-term memory storage consists of memories for performing actions or skills, such as procedural memories (knowing how), and memories of facts, rules, concepts, and events, such as declarative memories (knowing that). Declarative memory includes semantic and episodic memory. The former concerns broad knowledge of facts, rules, concepts, and propositions (general knowledge), whereas the latter concerns personal and experienced events and the contexts in which they occurred (personal recollection).

Although short-term memory is closely related to the concept of working memory, both are distinct entities. Short-term memory is a set of storage systems, whereas working memory indicates the cognitive operations and executive functions associated with the organization and manipulation of stored information. Nevertheless, the terms short-term memory and working memory are often used interchangeably. Short-term memory must also be distinguished from sensory memory, such as the acoustical echoic and iconic visual memories, which are shorter in duration, typically fractions of a second, and reflect the stimulus's original sensation or perception. In other words, sensory memory is specific to the stimulus modality of presentation. This raw sensory information undergoes processing and, upon processing to short-term, is expressed in a format different from that perceived initially.

The famous Atkinson and Shiffrin (or multi-store) model, proposed in the late 1960s, explains the functional correlations between different types of memory. Many studies demonstrated the anatomical and functional distinctions between memory processes, neural correlates, and the functioning of short-term and long-term memory subsystems. In light of these findings, several memory models have been postulated. Although some authors suggested a single memory system encompassing both short- and long-term storage after 50 years, the Atkinson and Shiffrin model remains a valid approach for explaining the memory dynamics. In light of recent research, however, the model has several problems, mostly concerning the characteristics of short-term memory, the relationship between short-term and working memory, and the transition from short- to long-term memory.

Short-Term Memory

Short-term memory is a storage system that includes several subsystems with limited capacity. Rather than a limitation, this restriction is an evolutionary survival advantage, as paying attention to limited but essential information, excluding confounding factors, is important. A classic example in biology is prey, which must focus on the hostile environment to recognize a possible attack by the predator. Given the functional peculiarities of short-term memory, which involves the collection of sensory information, the subsystems are closely related to the modalities of sensory memory. Consequently, several sensory-associated subsystems have been postulated, including the visuospatial, phonological (auditory-verbal), tactile, and olfactory domains. These subsystems involve patterns and functional interconnections with the corresponding cortical and subcortical areas and centers.

The Concept of Working Memory

In 1974, Baddeley and Hitch developed an alternative model called working memory. This model does not exclude the modal model but rather enriches the contents. The short-term store can be used to characterize the functioning of the working memory. Working memory refers to the entire theoretical framework of the structures and processes used for storing and temporarily manipulating information, of which short-term memory is only a component. In other words, short-term memory is a functional storage element, whereas working memory is a set of processes involving storage phases. Working memory is constantly used when we have to understand new information, solve a problem, or make an argument, and it is the cognitive strategy for achieving short-term goals. The importance of this type of operating system of memory is demonstrated by the evidence showing that working memory deficits are associated with several developmental disorders of learning, including attention-deficit hyperactivity disorder, dyslexia, and specific language impairment.

A recent study on working and short-term memory in children with attention-deficit hyperactivity disorder suggested that these children had an inadequate hemodynamic response in a region of the brain that underlies phonological working memory. Functional near-infrared spectroscopy is a cost-effective, noninvasive neuroimaging technique that localizes and quantifies neural activation patterns related to executive functions. Another study suggested that working memory impairment and attention lapsing are general features of psychotic disorders. Findings regarding capacity estimates from the Change Localization and Detection tasks related to functional capacity and outcome also implied that these methods may be useful in a clinical context.

Short-Term and Long-Term Memory

Short-term and long-term memory can be distinguished based on storage capacity and duration. Short-term memory has limitations in the amount and duration of information maintained. In contrast, long-term memory features a seemingly unlimited capacity that can last years. The functional distinctions between memory storage systems and the exact mechanisms for how memories transfer from short-term to long-term memory remain controversial. Do short- and long-term memory represent 1 or more systems with specific subsystems? Although short-term memory probably represents a substructure of long-term memory, a form of long-term–activated storage, rather than looking for a physical division, verifying the mechanisms involved in transitioning from short-term to long-term memory seems appropriate. Although the classic multi-modal model proposed that the storage of short-term memories occurs automatically without manipulation, the matter seems more involved. The phenomenon concerns quantitative (number of memories) and qualitative (quality of memory) features.

Regarding quantitative data, although the number of Miller of 7 ± 2 items identifies the number of elements included among individual slots, grouping memory bits into larger chunks (chunking) can allow for greater storage capacity. The qualitative issue, or memory modulation within processing, is fascinating. Short-term memory elements undergo processing, providing a sort of editing involving each element's fragmentation (chunking) and re-elaboration. This phase of memory processing is called encoding and can condition subsequent processing, including storage and retrieval. The encoding process encompasses automatic (without conscious awareness) and effortful processing (through attention, practice, and thought) and allows us to retrieve information to make decisions and answer questions. Three pathways are followed during the encoding step—visual (information represented as a picture), acoustic (information represented as a sound), and semantic encoding (the meaning of the information). These processes interconnect, breaking down information into various components. During recovery, the pathway that has produced the coding facilitates the recovery of the other components through a chain reaction. A particular perfume, for instance, makes us recall a specific episode or image. The encoding process affects the recovery, but the recovery undergoes potential changes that can alter the initial content.

In neurofunctional terms, the difference is the occurrence in long-term memory of a series of events that must definitively fix the engram(s). This effect occurs through the establishment of neural networks and is expressed as a neurofunctional phenomenon, including long-term potentiation, which is an increase in the strength of the neural transmission deriving from the strengthening of synaptic connections. This process requires gene expression and the synthesis of new proteins and is related to long-lasting structural alterations in the synapses (synaptic consolidation) of the brain areas involved, such as the hippocampus in declarative memories.

The Role of the Hippocampal Network

Hippocampal neurogenesis regulates the maintenance of long-term potentiation. However, the hippocampal network, including the parahippocampal gyrus, hippocampus, and neocortical areas, is not the storage site for memories but plays a crucial role in forming new memories and their subsequent reactivation. The hippocampus appears to have a limited capacity but acquires information quickly and automatically. Over time, the initially available information is permanent in other brain structures, particularly the cortex), independently from the activity of the hippocampus itself. The crucial mechanism of this transfer involves the reactivation (replay) of neural activity configurations.

In other words, the connected hippocampus and the medial temporal structures are crucial for holding an event as a whole as they distribute memory traces in an organized manner. The operating system can store, organize, process, and recover hardware files through different software. This hippocampal-guided reactivation (retrieval) leads to the creation of direct connections between the cortical traces and then to the formation of an integrated representation in the neocortex, including the visual association cortex for visual memory, the temporal cortex for auditory memory, and the left lateral temporal cortex for knowledge of word meanings. Moreover, the hippocampus has other specific tasks, such as spatial memory organization.

Recent reviews summarized the progress of hippocampal circuits and functions based on sharp-wave ripples. These ripples are crucial for consolidating spatial, episodic, and social memories in different hippocampal-cortical pathways. Dysregulation of sharp-wave ripples contributes to cognitive impairments in neurodegenerative and neurodevelopmental diseases. Other brain areas are involved in memory processes; for example, learning motor skills has links to the activation of the cerebellar regions and brainstem nuclei. Learning perceptive activities, including improvements in processing perceptive stimuli essential in everyday life activities such as understanding spoken and written language, involves basal ganglia and sensory and associative cortices. In contrast, learning cognitive skills related to problem-solving involves the medial temporal lobes.