Memory and its systems: a non-unitary concept

Human adaptation to environmental demands is based on memory, one basic cognitive ability. In fact, this cognitive process is studied across different fields of knowledge, not only psychology. Since the unitary concept of memory—which defined memory as a unitary, indivisible system—was abandoned, research studies, undertaken in different disciplines and methodologies, are increasinglyexploring different mnemonic systems. These have been collected and classified, both in classical and current approaches, according to two main factors:memory duration, on the one hand; and the type of information stored, on the other.

According to its duration, memory formation is thought to follow a progressionfrom a short-term unstable state, occurring immediately after learning, to a more stableand lasting state, which occurswith timeafter acquisition of new information. A continuum is formed between these two extremes, comprising different types of memory such assensory memory, short-term memory, working memory, and long-term memory. These memory stores are included within the “multi-store or multi-system” model of memory and differ from each other in terms of capacity limit and the length of time that information remains available to us.

Additionally, these types of memory are considered continuous processes that encompassdistinct stages.

Distinct stages

  1. Acquisition:learning.
  2. Consolidation: memory.
  3. Retrieval.
  4. Reconsolidation:the most recent process. Several neurobiological studies support its independence.

The distinction regarding the material included in the memory system has generally been based on the study of patients with specific brain lesions. In particular, patientswith lesionsin specific brain regionswere found to have specific memory deficits. For example, patient J.P. had difficulty improving performance on tasks involving repetition and display of previously acquired skills, while other skills remained intact; in addition, J.P. was able to consciously recallpast events. This memory specificity gave rise to another classification of memory based on the content of information which led to the division of long-term memory into two types: declarative or explicit memory and non-declarative or implicit memory.

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Long-term memory

Declarative or explicit memory

Declarative memory is responsible for codifying the information regarding biographical eventsandthe knowledge of specific events. In this sense, explicit memory requiressome effort to remember information that has previously occurred, also known as intentionalrecollection. Generally, this recall is usually motivated by some evocative stimulus that was present when the information was encoded, which facilitates recall.Declarative memory can be further subdivided intosemantic memory, episodic memory and a special type of memory, recognition memory.

Non-declarative or implicit memory

On the other hand, implicit memory includes perceptual, motor and cognitive skills or abilities that have already been acquired and that can only be retrieved through action;individualsare unable to verbally “declare” these memories. In this case, memory is quantified in a different manner, specifically, it is argued that implicit learningis involved if there is an increase in performance in certain tasks.

Therefore, implicit memory refers to a change in behavior in the absence of conscious awareness of what has been learned.There are three general types of implicit memory: classical conditioning, priming, andprocedural memory.


Overall, it is concluded that human memory is not a unitary, indivisible system but consists of several separate systems that differ in terms of duration, the content of the stored information and, in addition, the neural bases that support them.


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  • Phelps, E. A. (2004). Human emotion and memory: interactions of the amygdala and hippocampal complex. Current opinion in neurobiology, 14(2), 198-202.
  • Rugg, M. D., Mark, R. E., Walla, P., Schloerscheidt, A. M., Birch, C. S., & Allan, K. (1998). Dissociation of the neural correlates of implicit and explicit memory. Nature, 392(6676), 595-598.
  • Squire, L. R. (1992). Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans. Psychological review, 99(2), 195.
  • Tobias, B. A., Kihlstrom, J. F., &Schacter, D. L. (1992). Emotion and implicit memory. The handbook of emotion and memory: Research and theory, 1, 67-92.

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