Implicit and Explicit Memory

Our long-term memory can be fundamentally divided into two distinct types, namely implicit memory and explicit memory (Squire, 2004).

Beyond how conscious each system is, implicit and explicit memory differ in several concrete ways:

• Effort: Explicit retrieval is conscious and effortful; you have to try to remember. Implicit expression runs automatically, with no feeling of remembering at all.
• Formation: A single striking event can create a lasting implicit association, while a rich explicit memory usually needs repeated, elaborative rehearsal.
• Durability: Well-practised implicit skills stay robust for decades, like never forgetting how to ride a bike. Explicit memories fade without regular retrieval.
• Direct tests: Explicit memory is measured with tests like free recall, which ask a person to consciously retrieve what they studied.
• Indirect tests: Implicit memory is measured with tests like reaction time or fragment completion, which never mention the study episode at all.

Origin and Development

The Case of Henry Molaison (H.M.)

The discovery of implicit and explicit memory stemmed from the treatment of the neuroscience patient Henry Gustav Molaison, known in the research literature as H.M. (Squire, 2009).

Aim: Scoville and Milner (1957) set out to document what happened to memory after a bilateral medial temporal lobe resection, the surgery performed to relieve Molaison’s severe epilepsy.

Method: The researchers conducted a detailed neuropsychological case study of Molaison and other patients who had undergone the same surgery, testing his intelligence, language, and memory over many years.

Results: The surgery removed large portions of Molaison’s hippocampus, leaving him with severe anterograde amnesia. He could no longer form new conscious memories, though his short-term memory and intelligence stayed intact.

He could still learn new skills, such as mirror-drawing, and improved steadily across sessions despite insisting each time that he had never tried the task before (Corkin, 2002).

Conclusion: Conscious, explicit memory could be wiped out while the capacity to learn a new skill survived. This double dissociation showed that the medial temporal lobe supports declarative memory, not procedural learning. It became the founding evidence for separate memory systems.

Naming the Distinction: Graf and Schacter (1985)

The terms “implicit memory” and “explicit memory” come from Graf and Schacter (1985). They tested whether people can learn brand-new associations without consciously recalling learning them.

Participants studied the pairs under conditions that did or did not encourage elaborative processing, then completed word stems, such as rea___, presented either with the original pairing cue or with a different one.

Both groups completed more stems with the studied word when the original cue was present, a pattern called new-association priming. This priming depended on elaborative encoding at study, and it appeared even in the amnesic patients.

Explicit, cued recall of the very same word pairs, by contrast, was markedly impaired in the amnesic patients tested.

Graf and Schacter used this split to define two modes of memory. Explicit memory requires conscious recollection, while implicit memory shows up whenever a prior experience helps performance without needing it, even for entirely new relationships.

Extending the Findings to Other Patients

Beyond Molaison’s case, studying patients with neurodegeneration and other brain trauma has deepened understanding of implicit and explicit memory (Squire, 2015).

For example, a damaged hippocampus in Alzheimer’s disease patients impairs their ability to form and retain new explicit memories, a link that has generated important research discussion in recent decades.

Their procedural memory, however, supported by different and typically spared brain structures such as the basal ganglia and cerebellum, remains far less affected even as the disease progresses.

Because procedural memory survives long after explicit memory fails, patients with amnesia or early dementia can still be taught everyday routines through repeated practice, even once they can no longer consciously learn new facts.

This same pattern across very different patients is why researchers treat the split as a real feature of brain organization, not a quirk of one case.

What is Implicit Memory?

Implicit memory, also known as unconscious memory or automatic memory, refers to perceptional and emotional unconscious memories which influence our behavior (Dew & Cabeza, 2011).

Implicit memory shapes our current behavior without conscious retrieval, letting prior experience improve task performance without any conscious awareness of that experience.

Examples of Implicit Memory

Some examples of implicit memory include knowing how to play the piano, ride a bike, tie your shoes, and other motor skills. These skills involve procedural knowledge, which involves “knowing how” to do things.

Other examples of implicit memory may include:

• Knowing how to make breakfast.
• Knowing how to play a musical instrument.
• Navigating a familiar area such as your house or neighborhood.

Skills using implicit memory do not involve conscious thought (i.e., they are unconscious and automatic). For example, we brush our teeth with little or no awareness of the skills involved.

Related Brain Structures

The functioning of implicit memory is thought to involve the cerebellum and the basal ganglia (Dew & Cabeza, 2011).

The cerebellum, located at the base of the brain, coordinates signals from the spinal cord and sensory systems to fine-tune the timing of motor movements. It is essential for procedural memories and for simple classically conditioned responses, such as the eyeblink reflex.

The basal ganglia, and in particular the striatum, support action selection and the gradual, incremental build-up of sequenced actions and habits (Ullman, 2004).

This is why implicit memory involves subconsciously driven sensorimotor behavior that we typically remain unaware of.

What is Explicit Memory?

Explicit memory, also known as declarative memory, refers to memories involving personal experiences as well as factual information which we can consciously retrieve and intentionally articulate (Dew & Cabeza, 2011).

Explicit memory has to do with remembering who, what, where, when, and why.

Recalling information from explicit memory involves conscious effort: the information is consciously brought to mind and “declared.”

For example, declarative knowledge involves “knowing that” London is the capital of England, zebras are animals, and the date of your mom’s birthday, etc. (Cohen & Squire, 1980).

Examples of Explicit Memory

Our knowledge in semantic and episodic memories focuses on “knowing that” something is the case (i.e., declarative).

For example, we might have a semantic memory for knowing that Paris is the capital of France, and we might have an episodic memory for knowing that we caught the bus to college today.

Other examples of explicit memory may include:

• Recollecting the items on a to-do list.
• Remembering the dates of various events for a history exam.
• Remembering the time for a doctor’s appointment.

Related Brain Structures

Communication between the prefrontal cortex, the amygdala, and the hippocampus governs explicit memory (Dew & Cabeza, 2011).

The prefrontal cortex is thought to be necessary to store and retrieve long-term memories involving information and facts (13.2 The Central Nervous System – Anatomy and Physiology, 2013).

Located deep within the brain’s temporal lobe, the hippocampus supports spatial awareness and navigation. It also consolidates information from short-term to long-term memory (Squire, 2015).

The hippocampus plays no role in implicit memory. The amygdala, which handles emotional learning, sits near the hippocampus.

The hippocampus supports the retention and recall of events, while declarative memories in the medial temporal lobe are consolidated into the temporal cortex (Squire, 2009).

The Relationship between the Two Memory Systems

Implicit memory’s priming can shape explicit judgements, yet the two systems are still widely treated as running on fundamentally different rules (Squire, 2004).

The Case for Separate Systems

Squire and Dede’s (2015) influential review of amnesic-patient, animal-lesion, and neuroimaging studies argues that declarative memory depends on the hippocampus and supports conscious recollection.

Non-declarative memory, they argue, is really several independent systems: skills and habits in the striatum, priming and perceptual learning in the neocortex, simple classical conditioning in the cerebellum and amygdala, and basic non-associative learning in simple reflex pathways.

Patient studies back this separation.

Some amnesic patients with severely impaired verbal memories mastered a puzzle with no difficulty, despite being unable to recall ever seeing it before (Brooks & Baddeley, 1976). Hippocampal damage can wipe out explicit memory while leaving these residual learning abilities intact.

On this account, “implicit memory” is not one system at all but a collection of separate systems, unified only by the fact that none of them depends on conscious access to the past.

How Porous Is the Boundary?

Dew and Cabeza (2011) reviewed behavioural and neural evidence and reached a more sceptical conclusion: the two systems interact extensively rather than running fully in parallel.

Priming can influence later explicit judgements, explicit and implicit contributions can overlap within the same task, and the same brain regions are sometimes recruited by both. Standard “implicit” tests are also rarely process-pure, so some of the apparent independence may reflect impure measurement rather than genuinely separate brain systems.

The relationship may also shift with chronic drug use, aging, and stress. Despite decades of research, whether the two systems cooperate or compete during learning and retrieval is still unresolved (Dew & Cabeza, 2011).

The fairest summary is that the dissociation itself is real.

Severely amnesic patients reliably retain skill learning, but exactly how the underlying architecture works, whether as cleanly separate stores or as interacting processes drawing on shared representations, remains genuinely unsettled.

Critical Evaluation

The evidence for separate memory systems is strong, but the theory has real limits worth weighing carefully.

Strengths

• Converging Evidence: Single-case work on Molaison (Scoville & Milner, 1957), amnesic patient series (Brooks & Baddeley, 1976; Cohen & Squire, 1980), controlled experiments on healthy participants (Graf & Schacter, 1985), and modern neuroimaging meta-analysis (Lee et al., 2020) all point to the same dissociation, which no single memory faculty can explain.
• Clinical Value: The distinction explains why amnesic and dementia patients can still learn practical skills through repeated practice, even when they cannot learn new facts, a pattern already visible in Molaison’s own steady improvement at mirror-drawing.
• Theoretically Generative: The framework has driven a detailed neurobiology of multiple memory systems and shaped how psychologists study learning more broadly, from single-case neuropsychology through to large-scale neuroimaging synthesis (Squire & Dede, 2015).

Limitations

• Porous Boundary: As covered above, priming can influence later explicit recall and the two systems interact more than a clean separation implies, since standard tests are rarely process-pure (Dew & Cabeza, 2011).
• Heterogeneous Umbrella: “Implicit memory” is not one system but a collection of skills, priming, and conditioning that are dissociable from each other and may share no single mechanism (Squire & Dede, 2015).
• Single-Case Fragility: Much of the classic evidence rests on Molaison alone, which is historically decisive but statistically fragile, so conclusions should be weighted toward larger patient series and quantitative syntheses.
• Measurement Reliability: Standard indirect tests of implicit learning, such as the Serial Reaction Time task, are now known to fall below accepted reliability standards, which qualifies any individual-differences claim built on them.
• Unresolved Architecture: Whether the two systems ultimately cooperate or compete during learning and retrieval, and whether they are truly separate stores or interacting processes drawing on shared representations, remains an open question.

Contemporary Research

Two recent lines of evidence refine the classic picture: how cleanly the two systems’ brain regions separate, and how far we can trust the tests that measure implicit memory.

How Clean Is the Neural Divide?

Lee, Henson, and Lin (2020) ran a coordinate-based meta-analysis of 65 brain-imaging studies of repetition priming. They tested the long-standing claim that perceptual priming reduces activity toward the back of the brain while conceptual priming reduces activity toward the front.

Their pooled results found the same “repetition suppression” signal, in the same two brain regions, for both perceptual and conceptual tasks. The clean split did not survive the larger, quantitative analysis.

Can We Trust Implicit-Memory Tests?

Aim: Oliveira, Hayiou-Thomas, and Henderson (2023) set out to quantify how reliable the Serial Reaction Time task is, since it is the field’s standard measure of implicit sequence learning.

Method: The team pooled test–retest correlations from seven studies covering 719 participants, and checked whether reliability changed with age, trial number, or how the learning score was calculated.

Results: The task reliably showed the expected group-level learning effect. Yet its test–retest reliability came in below 0.40, under accepted standards for measuring individual differences, and none of the factors tested improved it.

Conclusion: The task shows a “reliability paradox”: consistent on average across a group, but too noisy to reliably rank individuals. Any claim linking implicit learning scores to other traits deserves caution as a result.

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