Amnesia
Memory Disorder
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Midline view
A memory disorder involving the inability to recall past information (retrograde) or form new memories (anterograde), most often caused by damage to the hippocampus or thalamus.
Common Signs & Symptoms
- Difficulty forming new memories (anterograde amnesia)
- Difficulty recalling memories formed before the injury or onset (retrograde amnesia)
- Preserved intelligence, attention, and procedural (skill-based) memory in most cases
- Confusion about time or sequence of events
Why These Symptoms Happen
Each symptom can be traced, at least in part, to the job a particular part of the brain normally does. These links are simplified: real symptoms usually come from several connected regions.
| Symptom | Brain region | Why it happens |
|---|---|---|
| Unable to remember new events after a few minutes | Hippocampus | The hippocampus binds the details of an experience into a memory that can be stored. Without it, information can be held briefly in attention but slips away once the person's focus moves on. |
| Losing memories from the months or years before the injury | Hippocampus | Recent memories still depend on the hippocampus while they're gradually consolidated into the cortex. Older memories are already stored more independently, which is why distant childhood memories often survive. |
| Memory loss with confusion and invented answers (Korsakoff syndrome) | Thalamus | The thalamus relays memory signals between the hippocampus and the cortex. Severe thiamine (vitamin B1) deficiency damages it and nearby structures; people may fill gaps with confident but false memories without meaning to deceive. |
| Can still learn new skills | Hippocampus | Skill learning (procedural memory) relies on other circuits, including the basal ganglia and cerebellum, so it survives hippocampal damage - even when the person doesn't remember practising. |
Overview
Amnesia refers to significant memory impairment beyond ordinary forgetfulness. It's typically divided into anterograde amnesia (difficulty forming new memories after the onset of injury or illness) and retrograde amnesia (difficulty recalling memories from before it), and the two often occur together to varying degrees. Importantly, most forms of amnesia leave general intelligence, attention, and the ability to learn new physical skills relatively intact - it's specifically explicit, factual and episodic memory that's affected.
Affected Brain Regions
The hippocampus is the structure most classically associated with amnesia, as demonstrated by the landmark case of Henry Molaison ("H.M."), described in detail on our hippocampus page, whose surgery to remove much of both hippocampi caused severe, permanent anterograde amnesia. The thalamus is another well-documented cause of amnesia when damaged, since it relays memory-related signals between the hippocampus and cortex; thalamic strokes or certain nutritional deficiencies (such as thiamine deficiency in Korsakoff syndrome) can produce a similar memory profile.
How It's Studied
Memory is measured with neuropsychological tests: learning a list of words or a short story and recalling it after a delay, recognising pictures seen earlier, or recalling well-known past events. Comparing immediate recall with delayed recall separates attention problems from true memory loss. MRI shows whether the hippocampus, thalamus, or their connections are damaged.
Much of this approach comes from the study of H.M. In 1957, neurosurgeon William Scoville and psychologist Brenda Milner reported his profound memory loss after surgery for epilepsy. Milner later found that H.M. could steadily improve at a mirror-drawing task over several days while having no memory of ever doing it - the first clear evidence that the brain has separate memory systems for facts and for skills.
Common Myths
"People with amnesia forget who they are."
In films, amnesia often wipes out a person's identity. In real brain-injury amnesia, people almost always still know who they are; it's new memories, and recent ones, that are lost. Loss of personal identity is rare and is usually linked to severe psychological stress (dissociative amnesia) rather than brain damage.
"Another bump on the head can bring memories back."
This is a cartoon trope. A second head injury can only add damage. Some memories do return as a person recovers from a brain injury, but that's due to healing, not another blow.
"Amnesia is always permanent."
It depends on the cause. Memory loss after concussion or a seizure is often temporary, and transient global amnesia - a sudden episode of memory loss in middle-aged or older adults - usually resolves within a day. Amnesia from destruction of the hippocampus, as in H.M.'s case, is usually lasting.
Current Research Directions
Research into amnesia has been central to broader memory science, helping establish that memory isn't a single unified system but several distinct ones - explicit memory for facts and events, and implicit memory for skills and habits - that rely on different, dissociable brain circuits. Current research explores how memory consolidation actually transfers information from the hippocampus to durable cortical storage over time, and whether that process can be supported or accelerated therapeutically after brain injury.
Key Terms
Related Brain Regions
Explore the regions involved in amnesia in more depth:
This article is for general education and isn't a substitute for professional medical advice or diagnosis.
Sources & Further Reading
- Memory loss - MedlinePlus Medical Encyclopedia
- Memory loss (amnesia) - NHS
- Loss of recent memory after bilateral hippocampal lesions - Scoville & Milner (1957), Journal of Neurology, Neurosurgery & Psychiatry
- The legacy of patient H.M. for neuroscience - Squire (2009), Neuron