Memory

Memory is the cognitive capacity to preserve and access information from past experiences through the interconnected processes of encoding, storage, and retrieval. The limbic system plays a central role in this process. Memory depends on neuronal plasticity, which is the brain’s ability to modify its structure and function in response to experience and learning. At the cellular level, synaptic plasticity allows connections between neurons to strengthen or weaken, with long-term potentiation serving as the key mechanism underlying learning and memory consolidation. Deep, meaningful processing enhances this synaptic strengthening, promoting the transfer of information from sensory and short-term memory into durable long-term storage. Long-term memory is broadly divided into explicit (declarative) memory, the conscious recall of facts and events, and implicit (procedural) memory, the unconscious retention of skills and habits, both organized in the brain as semantic networks of interconnected concepts.

The limbic system is a functional group of structures within the CNS that includes the hippocampus, amygdala, and cingulate gyrus and is central for memory formation (see "Limbic system" in "Diencephalon").

Memory is the cognitive capacity to preserve and access information from past experiences through the interconnected processes of encoding, storage, and retrieval.

Encoding (memory)

Encoding is the initial memory process in which sensory input is translated into a storable format, transforming a perceived item or event into a retrievable memory.

Types of encoding

• Semantic: encoding information based on its meaning and significance, or by connecting it to existing knowledge
• Acoustic: encoding auditory stimuli
• Visual: encoding images and visual information
• Tactile: encoding the sense of touch
• Olfactory/gustatory: encoding based on smell or taste

The process of encoding

1. Sensory processing: Stimuli detected by sensory organs are relayed via the thalamus to the corresponding sensory cortices (e.g., visual, auditory).
2. Attention and working memory: The prefrontal cortex filters and maintains certain information for short-term use.
3. Synaptic encoding: Neural connections are formed and strengthened through long-term potentiation in the hippocampus.
4. Consolidation: The hippocampus stabilizes memory traces and gradually transfers them to the cortex for long-term storage.

Factors that influence the encoding process

• Sleep: Slow-wave and REM sleep are crucial for the consolidation and strengthening of synaptic connections.
• Emotional modulation: The amygdala enhances the encoding of emotionally significant events.

Processes that aid encoding

Human beings use conscious and unconscious processes to make information more meaningful for encoding in short and long-term memory.

• Mnemonic device: linguistic devices that are used as memory aids (e.g., acronyms)
• Chunking (psychology): a mental process in which information is broken down into smaller chunks for easier encoding in memory
• Visualization: a process in which information is converted into mental images or spatial locations
• Elaboration: the process of making new information more meaningful and memorable through interpretation, embellishment, and/or connection to existing knowledge
• Self-reference effect: the tendency to encode memories more effectively when they are personally relevant
• Spacing effect: the cognitive phenomenon in which information is better learned and encoded in memory when distributed across multiple short sessions with breaks than in uninterrupted massed practice (i.e., cramming)

Storage

Types of memory storage

Dual-task performance illustrates the limitations of cognitive processing, as individuals often experience decreased efficiency or increased errors when attempting to perform two simultaneous tasks, particularly if both engage the phonological loop (for verbal tasks) or visuospatial sketchpad (for visual-spatial tasks). For example, while studying for an exam, trying to listen to a podcast and take notes can lead to poorer retention of information, underscoring the importance of managing cognitive resources effectively.

A person with hippocampal damage may retain long-term memories, such as childhood experiences stored in the cortex, while struggling to recall recent events, like what they ate for lunch, highlighting the hippocampus's critical role in the formation and encoding of new memories.

Memory models

Semantic networks

A semantic network is a graphical representation of the web-like structure of concepts and their meanings in natural language processing and memory storage.

• Semantic networks consist of:
  • Nodes: represent individual concepts, ideas, or objects
  • Associative links: the connections between the nodes, representing the relationship between them
    • The links between nodes vary in strength depending on the degree of semantic relatedness between concepts.
• Spreading activation: a theory that proposes that activating one node in the network triggers the activation of nearby, related nodes
  • Summation effect:
    • Each node can receive activation from multiple sources simultaneously.
    • A concept is retrieved into conscious awareness only when its node's total activation surpasses a critical threshold.
• Priming effect: the phenomenon in which prior exposure to a stimulus unconsciously influences the response to a subsequent stimulus
  • Occurs due to the activation of nodes that are closely related to the primarily stimulated node
  • The related nodes are preactivated or "primed", making them easier and faster to retrieve.

Serial position effect

The serial position effect describes the tendency to recall the first and last items in a list far better than the items in the middle.

• Primacy effect: the superior recall for items presented at the beginning of a list
  • Occurs because initial items receive more cognitive attention and rehearsal and are therefore more likely to be transferred from working memory to long-term memory
• Recency effect: the superior recall for items presented at the end of a list
  • Occurs because the items most recently presented are still active in the working memory at the time of recall

Retrieval (psychology)

Retrieval is the cognitive process of bringing information stored in long-term memory into consciousness.

Methods of retrieval

• Recall: actively retrieving information from memory
  • Serial recall: the ability to recall a list of items in the precise sequence they were presented
  • Free recall: the ability to recall as many items as possible, regardless of their original sequence
  • Cued recall: the ability to recall information with the aid of specific hints associated with the target memory
• Recognition: identifying presented information as having been previously encountered or learned
• Relearning: learning information that has previously been learned

Factors that influence retrieval

• Retrieval cues: stimuli that help access a specific memory
  • Encoding specificity
    • Context-dependent memory: recall is improved when retrieval occurs in the same context as encoding
    • State-dependent memory: recall is improved when an individual is in the same internal state as when the memory was formed
  • Priming (see above)
• Emotion: Strong emotional experiences can lead to the formation of highly vivid and durable memories (flashbulb memories), a process mediated by the amygdala.
  • Example: many people remember where they were when they first heard of the terror attacks of 9/11

Flashbulb memories, like those of 9/11, are characterized by extremely high vividness and confidence that do not diminish over time. Despite this subjective certainty, their objective accuracy actually decreases as the memory is reconstructed, making them just as susceptible to decay and error as ordinary memories.

Forgetting

Forgetting is the inability to retrieve stored information. Transience is the natural decrease in the accessibility of a specific memory over time.

Theories of forgetting

Remember, Proactive interference = Previous interferes (with new info). Retroactive interference = Recent interferes (with old info).

Aging and memory

While some aspects of memory decline with age, many functions remain stable.

• Functions that tend to decline:
  • Episodic memory
  • Short-term and working memory
  • Free recall
  • Processing speed
• Functions that typically remain stable:
  • Semantic memory
  • Procedural memory
  • Recognition

Memory distortion

Memory is a reconstructive process; the brain actively rebuilds memories during recall. Therefore, distortion can lead to inaccurate or false memories.

• Source monitoring error: incorrectly recalling the origin of a memory
  • Source amnesia: forgetting the source entirely
  • Source misattribution: crediting the wrong source
  • Cryptomnesia: thinking you had an original idea when you actually heard it elsewhere
• Misinformation effect (suggestibility): the distortion of a memory by misleading information presented after the event
• Bias: the alteration of how past events are remembered due to current beliefs, emotions, or knowledge
• Schema distortion: alteration of memories to fit preexisting mental frameworks and expectations, which can add or omit details
• False memories: the creation of vivid and detailed memories of events that never actually occurred
• Hindsight bias: the tendency to believe an event was predictable after it occurred
• Consistency bias: altering memories of past self to match current beliefs/feelings

Memory dysfunctions

Memory dysfunctions are conditions that impair a person's ability to store, retain, and recall memories and experiences. They can range from mild forgetfulness to severe, permanent memory loss, significantly impacting daily life. Memory dysfunctions can result from injury, disease, or psychological trauma.

Amnesia

• Definition: the partial or complete loss of memory
• Types
  • Anterograde amnesia: inability to create new memories after the event that caused the amnesia
  • Retrograde amnesia: inability to recall memories and/or information acquired before the onset of amnesia
  • Global amnesia: inability to recall memories and/or information acquired before and after the incident
  • Dissociative amnesia: inability to recall important personal information; usually following severe trauma or stress

Dementia

• Definition: a generalized deterioration of memory and other cognitive functions (e.g., speech, executive function), mainly due to neurodegenerative and/or vascular disease. See "Major neurocognitive disorder" for more details.

As the leading cause of dementia, Alzheimer disease is characterized by progressive cognitive and behavioral dysfunction with an onset typically occurring after 65 years of age. The most common initial presentation involves deficits in short-term memory, which manifests as anterograde amnesia.

Korsakoff syndrome is a neurological disorder caused by a severe thiamine (vitamin B₁) deficiency, which is often a consequence of chronic heavy alcohol use. The condition is clinically characterized by a profound memory deficit that includes both anterograde and retrograde amnesia, frequently accompanied by other symptoms such as confabulation and personality changes.

Neuronal plasticity

Neuronal plasticity refers to the brain's ability to change its structure, chemistry, and function in response to experience, learning, or injury.

Synaptic plasticity

• Definition: the ability of synapses to strengthen or weaken in response to their activity
  • Long-term potentiation: persistent strengthening of a synapse (see below)
  • Long-term depression: long-lasting weakening of a synapse
• Habituation: a decreased synaptic response to a repeated, harmless stimulus
  • Mechanism: synaptic depression, resulting in decreased neurotransmitter release with each repetition
• Sensitization: an increased response to a wide range of stimuli after exposure to one strong or noxious stimulus
  • Mechanism: presynaptic facilitation, resulting in increased neurotransmitter release and greater excitability of the postsynaptic neuron

Structural plasticity

• Neurogenesis: the growth and development of new neurons
• Synaptic pruning: a process in which weak or rarely used synapses are eliminated; mainly occurs in childhood and adolescence
• Axonal plasticity: the ability of axons to structurally change, often to compensate for brain damage
  • Rerouting: the process of an axon changing its pathway to connect with a new target neuron
  • Sprouting: the growth of new branches from an axon to form additional synaptic connections

Functional plasticity

• Definition: the brain’s ability to adapt by shifting or reorganizing neural activity so that different regions can take over or enhance functions in response to learning, experience, or injury
• Cortical remapping: the process by which the brain reorganizes the functional representation of body parts or sensory inputs within the cortex

Long-term potentiation

The cellular mechanism of learning and memory consolidation, characterized by the persistent strengthening of synapses through repeated depolarization.

• Location: synapses of the pyramidal cells in the hippocampus
• Key neurotransmitter: glutamate
• Receptors involved:
  • AMPA receptor
    • Ionotropic glutamate receptor with primary permeability to Na⁺
    • Location: postsynaptic membrane
    • Mechanism of action: The binding of glutamate initiates a sodium influx, which in turn leads to the depolarization of the postsynaptic membrane.
  • NMDA receptor
    • Ionotropic glutamate receptor with high permeability to Ca²⁺
    • Location: postsynaptic membrane
    • Mechanism of action:
      • At resting potential, the receptor is blocked by Mg²⁺.
      • A strong depolarization of the neuron's membrane, following sustained activation by other receptors (e.g., AMPA), is necessary to unblock the NMDA receptor and allow Ca²⁺ to enter the cell.

Mechanism of activation

• Low-frequency stimulation
  • Triggers a minor release of glutamate into the synaptic cleft
  • Glutamate binds to both receptors.
  • Only AMPA opens (Na⁺ influx).
  • NMDA remains blocked by Mg^2+.. No change in synaptic strength.
• High-frequency stimulation
  • Massive glutamate release → sustained AMPA activation
  • Strong depolarization unblocks NMDA receptors
  • The open receptor allows a crucial influx of Ca²⁺, which initiates synaptic strengthening (potentiation)

Early long-term potentiation

• Trigger: a single series of high-frequency stimulation
• Duration: lasts 1–2 hours
• Mechanism: influx of Ca²⁺ → modification of existing proteins via protein kinases
  • These enzymes modify existing proteins to temporarily enhance synaptic efficiency via:
    • Postsynaptic enhancement: phosphorylation of AMPA receptors (increases sensitivity to glutamate)
    • Presynaptic enhancement: nitric oxide acts as a retrograde signal to increase future glutamate release

Late long-term potentiation

• Trigger: several consecutive series of high-frequency stimulation
• Duration: lasts for hours to weeks
• Mechanism: persistently elevated levels of intracellular Ca²⁺ → signaling cascades alter gene expression and lead to the synthesis of new proteins → structural and functional remodeling of the synapse
  • Increased density of AMPA receptors.
  • Growth of dendritic spines (remodeling)
  • Represents the physical shift from short-term to long-term memory

Although Ca²⁺ serves as the fundamental signal for memory, it becomes a potent neurotoxin when levels are uncontrolled. The overstimulation of NMDA receptors allows for an excessive influx of calcium, a process termed excitotoxicity, which eventually leads to cell death. This destructive pathway is a hallmark of the brain damage following strokes and seizures, and it plays a critical role in the progression of Alzheimer disease.