Brain-damaged patients and memory: the case of HM

Introduction to the Case of HM

In Cognitive Psychology, we often learn about how memory works by looking at what happens when it stops working. The most famous example in history is a man known for decades only as HM (later revealed to be Henry Molaison). His case is the ultimate "real-life experiment" that helped psychologists understand that memory isn't just one single "bucket" in the brain, but a complex system of different parts.

Why is this important? Before HM, scientists weren't sure if memory was spread out across the whole brain or located in specific spots. HM’s experience provided the first clear evidence for the Multi-store Model of memory and the role of specific brain structures.

The Story of Henry Molaison (HM)

Henry suffered from severe epilepsy that caused frequent, violent seizures. In \(1953\), when he was \(27\) years old, he underwent an experimental surgery to stop the seizures. A surgeon removed parts of his medial temporal lobes from both sides of his brain, including a structure called the hippocampus.

The surgery was a success for his epilepsy, but it had an unexpected and permanent effect on his memory.

What HM Could and Could Not Do

HM’s memory became "frozen in time." Psychologists studied him for over \(50\) years to see how his brain functioned. Here is the breakdown of his memory abilities:

1. Short-Term Memory (STM): Intact
HM could hold a normal conversation. If you gave him a phone number, he could remember it for a few seconds by rehearsing it. This showed his Short-Term Memory was still working fine.

2. Long-Term Memory (LTM): Severely Damaged
HM suffered from Anterograde Amnesia. This means he could not form new long-term memories. He could meet a doctor, walk out of the room, and when he returned \(2\) minutes later, he would have no idea who the doctor was. He essentially lived in a permanent "present moment."

3. Retrograde Amnesia: Partial
He also lost some memories from the years before his surgery (specifically the \(11\) years leading up to it), but his childhood memories remained mostly clear.

4. Procedural Memory: Intact
This was the most surprising discovery! Researchers asked HM to perform a "mirror-drawing task" (drawing a star by looking only at its reflection). Even though HM forgot that he had ever practiced the task, his physical skill improved every day. This proved that Procedural Memory (learning skills) happens in a different part of the brain than Declarative Memory (learning facts and events).

Key Takeaway: Linking HM to Theory

HM’s case provides strong support for the Multi-store Model (Atkinson and Shiffrin, 1968). It shows that:

• STM and LTM are separate: Because HM had a working STM but a broken LTM, they must be distinct systems.
• The Hippocampus is the "Gateway": The hippocampus is vital for transferring information from STM to LTM. Without it, the transfer (encoding) cannot happen.

Evaluating the Case of HM

When studying HM for your exam, you need to be able to evaluate the research using specific psychological criteria.

Strengths

Objectivity and Credibility: Over the decades, researchers used highly controlled, scientific tests (like the mirror-drawing task and IQ tests) to measure HM’s memory. This makes the findings more objective and gives the case high credibility in the scientific community.

Validity: Because HM was a real person living his real life, the case study has high ecological validity. It shows how brain damage affects a person's actual ability to function in the world, not just in a lab.

Weaknesses

Generalisability: This is a major issue. HM was a "case of one" (\(n = 1\)). His brain was already damaged by epilepsy before the surgery, and the surgery itself was unique. Therefore, we must be careful when generalising his results to the wider population.

Reliability: Since the surgery was a unique event, it cannot be exactly replicated for ethical reasons. While different researchers found consistent results with HM over many years (increasing consistency), the case itself lacks reliability in terms of being able to repeat the "experiment."

Subjectivity: Some earlier reports on HM relied on observations and interviews. These can be subjective, as researchers might interpret his behavior based on what they expect to see.

Common Mistakes to Avoid

Mistake 1: Saying HM lost all his memory.
Correction: He only lost the ability to form new long-term memories and some recent past memories. He could still learn new skills (procedural memory).

Mistake 2: Confusing Anterograde and Retrograde amnesia.
Memory Aid: Anterograde = After the surgery (can't make new ones). Retrograde = Returning to the past (can't remember old ones).

Quick Review Box

The Case of HM Summary:
- Cause: Removal of medial temporal lobes/hippocampus to treat epilepsy (\(1953\)).
- Effect: Permanent Anterograde Amnesia (no new LTM).
- Discovery: STM and LTM are separate; Skills (Procedural) and Facts (Declarative) are separate.
- Evaluation: High credibility/validity, but low generalisability due to being a unique case study.

Note: For more details on how HM compares to other patients with similar damage, see the chapter on the contemporary study by Schmolck et al. (2002).