Memory

Memory is not a single thing. It is a family of distinct systems that share the function of preserving information across time, but differ in what they store, how long, and where in the brain.

The major systems

• Working memory: information held briefly (seconds) for immediate use. Largely supported by the prefrontal cortex. Capacity is famously limited, around four to seven items.
• Episodic memory: memory for specific events you have experienced. Heavily dependent on the hippocampus and surrounding medial temporal lobe.
• Semantic memory: memory for facts and concepts, untethered from when you learned them. Distributed across the lateral temporal cortex.
• Procedural memory: memory for skills and habits, like riding a bike or typing. Supported by the basal ganglia and cerebellum.
• Emotional memory: memory tagged by emotional significance. Involves the amygdala modulating storage in the hippocampus.

How memory forms

A widely accepted model:

1. Encoding: the brain converts an experience into a transient representation
2. Consolidation: that representation is stabilised over hours to years, partly during sleep
3. Retrieval: the stored representation is reactivated when needed

Each stage can fail independently. "Tip of the tongue" is a retrieval failure. Forgetting last night's dream is often a consolidation failure.

Why it matters clinically

Memory complaints are the most common reason people seek a cognitive evaluation. The pattern of impairment matters:

• Difficulty with recent events but preserved older memories is characteristic of Alzheimer's
• Difficulty with the sequence of events but preserved facts can suggest hippocampal damage
• Difficulty with skills despite preserved facts can suggest basal ganglia or cerebellar disease

Standard screens like the MoCA and MMSE include brief memory subtests. More detailed neuropsychological batteries probe each memory system separately.

In imaging

Hippocampal volume is one of the most sensitive imaging markers of memory health. It shrinks early in Alzheimer's, in chronic stress, and in some psychiatric conditions, and it is one of the few brain regions that retains some capacity for adult neurogenesis.