7 Memory and Its Neural Basis

Major memory systems

Memory is the ability to encode information, retain it, and retrieve it later. It is not stored in one brain location: different kinds of memory depend on partly distinct, interacting neural systems. For example, learning to ride a bicycle may involve remembering an initial attempt as an event, knowing facts about balance, and acquiring a practiced skill; these memories rely on overlapping but different circuits.

Distinct but interacting systems

Working memory briefly holds and manipulates information for an ongoing task, such as remembering directions while following them. It depends on attention and a distributed network that includes prefrontal and parietal cortex, and is not simply a small storehouse of long-term memories.

Declarative memory concerns information that can generally be consciously recalled and described. Episodic memory concerns personal events in their context, while semantic memory concerns facts and concepts.

Nondeclarative memory is expressed through learned performance or responses, often without conscious recollection. It includes skills and habits, learned emotional responses, and simple conditioned responses. A single experience may engage several memory systems at once.

Brain structures and their contributions

The hippocampus sits within the medial temporal lobe (MTL). Together with nearby entorhinal, perirhinal, and parahippocampal cortices, it receives processed information from widespread cortical areas. These regions help bind features of an experience, such as the people, place, and order of events, so that a partial cue can later bring the event to mind.

The hippocampus is particularly important for recollecting relationships and context, while familiarity for an item can rely more on surrounding MTL cortex. The precise division of these functions remains an active area of study.

Contributions from other regions

The prefrontal cortex contributes to organizing information during learning and guiding retrieval. It helps set goals, select relevant memories, and check whether a recalled detail fits the question or context. Parietal and sensory-association cortices participate in representing aspects of remembered content. Midline diencephalic structures, including parts of the thalamus and mammillary bodies, are also connected to the hippocampal memory system.

Different neural circuits support other forms of learning. Basal-ganglia circuits support habits and action sequences. The cerebellum contributes to motor adaptation and conditioned responses, and is important for some forms of motor conditioning. The amygdala helps learn and express associations involving emotional significance, such as fear, and modulates learning about emotionally important events.

Encoding and neural plasticity

Memory formation begins with encoding: attention and interpretation influence which parts of an experience are represented. The brain's representation is distributed across networks active during the experience rather than written to a single memory center.

Learning changes connections among neurons through synaptic plasticity. One widely studied form is long-term potentiation (LTP), a lasting increase in the effectiveness of communication at some synapses. LTP is a useful model for how experience can change neural circuits, but it is not by itself a complete explanation of memory.

Consolidation and memory stability

New memories can be fragile and susceptible to interference. Synaptic consolidation refers to processes that stabilize changes at the cellular and synaptic level. Systems consolidation describes longer-term changes in how brain regions support a memory.

A common account is that hippocampal interactions help strengthen links among cortical representations over time, making some memories less dependent on the hippocampus. This is not a simple transfer of a finished memory from one place to another: information is represented in cortex from the beginning, and the hippocampus continues to be important for some forms of detailed episodic recollection. The extent and timing of hippocampal independence vary by memory and remain debated.

Retrieval and updating

Retrieval occurs when a cue, such as a familiar smell, question, or place, helps reactivate a stored pattern. The hippocampus can help reinstate associated details across cortical regions, while prefrontal systems guide the search and evaluate what comes to mind.

Remembering is constructive: a recalled experience is rebuilt from stored information and current cues, not replayed as a perfect recording. As a result, memories can be incomplete or influenced by later knowledge.

When a memory is retrieved, it may sometimes become temporarily open to modification before stabilizing again through reconsolidation. This does not mean every act of remembering changes a memory; the effect depends on factors such as how retrieval occurs and what new information is encountered.