Where are memories stored in the brain? How do we know that long-term and short-term memory stores are distinct? These are the question that neuropsychological research can help us answer. The neurological damage observed in amnesia patients with memory impairments allows us to identify the neural structures responsible for consolidating and storing memories of our past. From case studies investigating memory, psychologists have been able to identify two types of amnesia; anterograde amnesia and retrograde amnesia.
- In this topic, we will first discuss the amnesia meaning covering both anterograde amnesia and retrograde amnesia.
- The amnesia causes and amnesia symptoms will also be briefly covered in this explanation. To understand both of these, famous memory case studies in psychology will be covered.
A brain with no neurological damage is able to recall past memories, freepik.com/pch.vector
Amnesia meaning
Amnesia is a condition characterised by long-term memory impairment that typically occurs due to injury to the brain. Amnesia symptoms can range from person to person. For example, someone may not be able to recall events from a week. Whereas someone else may not recall events from the past 20 years.
This is an example of retrograde amnesia.
Amnesia is a condition characterised by long-term memory impairment typically brought about by injury to the brain or due to a stroke.
The extent of memory loss and memory deficits depends on how extensive the neurological damage is.
Someone with damage to several memory structures in the brain is likely to have more memory impairments than someone who has little damage.
There are different forms of amnesia; we will focus on retrograde amnesia and anterograde amnesia.
Retrograde amnesia
Retrograde amnesia involves the inability to remember a period of time before the brain trauma. However, in this form of amnesia, the individual can still form and encode new memories.
Due to brain plasticity, many people experiencing retrograde amnesia will be able to regain some of the lost memories over time. However, in some cases, memory loss is permanent. Brain plasticity essentially is when other brain parts take over the function of brain regions that no longer serve their function due to damage.
Kate suffered a traumatic brain injury following a car accident and was told she had retrograde amnesia. She didn't remember what she was doing before the accident and had trouble remembering the past few years of her life. Overall, her short-term memory was good, and she could remember what had happened since the accident. After some time, Kate was able to recall past memories.
The general mechanism of how memory works, according to psychology, is that memories become stronger and easier to remember the more it is recalled. Patients with retrograde amnesia are no longer able to recall their previous memories due to brain damage either caused by physical trauma or the side-effect of a stroke.
There are different types of memory. In retrograde amnesia, some types of memories are spared more than others. For example, loss of episodic memories is more common than loss of semantic memories.
Episodic memories refer to recollections of personal life events that we experience. Whereas, semantic memories refer to general knowledge about the world, for example, the knowledge that Paris is the capital of France.
Retrograde amnesia symptoms
- Loss of episodic memories which were formed before the incident that caused the amnesia.
- Still able to create new memories.
- Have normal short-term memory.
Anterograde amnesia
Anterograde amnesia is characterised by the inability to create new memories. Patients with anterograde amnesia have intact short-term memory and can recall the events before their injury but cannot remember anything new that has happened since.
An example of this type of amnesia is...
Dory, a character in the Finding Nemo film, shows intact short-term memory but impaired long-term memory, consistent with anterograde amnesia. She can hold information for long enough to maintain a conversation, during which she invites Marlin (Nemo's dad) to follow her. However, she quickly becomes confused about who he is and why he is following her, having forgotten the conversation they just had.
Transient anterograde amnesia can be induced by alcohol or drug consumption, causing a blackout, during which the brain cannot transfer memories from the short-term to the long-term store. This is a less severe form of the illness.
Anterograde amnesia symptoms
- Forgetfulness and confusion.
- The inability to form new memories.
- Inability to hold information for longer than the duration of the short-term memory (around 30 seconds).
- Normal short-term and procedural memory.
- No memory loss of the events before the injury.
Amnesia causes: neurological damage in amnesia
Amnesia is caused by neurological damage resulting from surgery, head injury, stroke, viral infections or neurological diseases.
Hippocampus is a brain structure placed in the temporal lobes, it is associated with the transfer of information from short-term to long-term memory. The role of the hippocampal damage in forming new memories became clear after the patient HM underwent the surgery to remove temporal lobes and developed profound anterograde amnesia.
The frontal lobe includes different brain structures at the front of the brain. These structures are involved in memory retrieval. Therefore, damage to the frontal lobe is associated with an inability to retrieve memories.
The cerebellum is a subcortical structure, placed deep in the brain. It is associated with coordinating movements, procedural learning and procedural memory. We rely on procedural memory when performing learned tasks that require little awareness/ conscious effort, for example, riding a bike.
Different parts of the brain and networks are responsible for memory, freepik.com/free-vector
It is believed that memories are stored in networks between different brain regions, so semantic or procedural memory can be retained in some patients with selective brain damage to the hippocampus. Memory functions by different parts of the brain working together.
Semantic memory is associated with different parts of the temporal lobes, diencephalon, frontal and parietal cortex. Procedural memory is stored in the cerebellum and is often retained in both retrograde and anterograde amnesia patients.
This may explain why not all types of memory are affected by retrograde amnesia and anterograde amnesia.
Case study: Henry Molaison
The case study of the patient Henry Molaison or HM was reported by Scoville and Milner in 1957. HM suffered from seizures and at the age of 27 underwent surgery for epilepsy. The surgery involved removing both medial temporal lobes, together with the hippocampus. The surgery was successful in terms of reducing HM's seizures. However, as a result, HM suffered both retrograde and profound anterograde amnesia.
HM lost the memory of the last 2 years before the surgery. Even more troubling was the degree of HM's anterograde amnesia. Henry was completely unable to form new memories ever since his surgery. He was able to only retain information for a few moments at a time, meaning his short-term memory was unaffected.
HM's procedural memory was also unaffected. When repeatedly asked to complete a mirror drawing task, which is difficult at first but becomes easier with practice, Henry showed significant improvement in his performance each time he completed it. This shows he could still improve on procedural tasks, even though he never remembered doing the task before.
Implications of the Henry Molaison case study
The case study of HM was the first evidence to highlight the importance of the hippocampus in learning and storing long-term memories.
HM's short-term memory was unaffected by his amnesia, which suggests that short-term memory is a different system from long-term memory.
Similarly, the ability to learn new procedural information despite the patient's inability to learn new episodic information suggests the separation of declarative (episodic and semantic) and non-declarative (procedural) memory.
Case study: Clive Wearing
Wilson et al. (2008) investigated the case study of Clive Wearing, a talented British musician who developed global amnesia as a result of encephalitis that he contracted at the age of 46.
Encephalitis is a viral infection that can result in damage to the brain.
In Clive's case, there was extensive damage to the medial temporal lobes. Clive Wearing completely lost episodic memory of his life before the brain damage and couldn't remember the details or any specific events from his life. Despite losing most of the musical knowledge he has acquired in previous years, he was still able to play the piano. He also suffered profound anterograde amnesia, meaning he was unable to remember anything that has happened to him after the injury.
Clive attributed his lack of memory to being comatose, he continuously insisted he has just regained consciousness for the first time and wanted to see his doctors. Unfortunately, Clive's damage was very extensive, and he never recovered, he maintained that he has just regained consciousness even 20 years after the onset of his amnesia.
Clive Wearing was diagnosed with global damage after a viral infection caused damage to his hippocampus and neighbouring brain regions, freepik.com
Amnesia - Key takeaways
- Amnesia is a condition characterised by impaired long-term memory. It is typically brought about by brain injury.
- Retrograde amnesia involves an inability to remember a period of time before the injury. It is associated with damage to the prefrontal lobe and temporal lobes.
- Anterograde amnesia is characterised by the inability to create new memories. It is caused by damage to the medial temporal lobes, particularly to the hippocampus and surrounding areas.
- Hippocampus is responsible for learning, particularly for the transfer of memories from short-term memory to long-term memory. Damage to the hippocampus is seen in both retrograde and anterograde amnesia.
- Case studies of Henry Molaison and Clive Wearing support the theory that short-term memory and long-term memory are distinct memory systems and damage to various brain regions can lead to different memory impairments.
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