Emotional Wellbeing TED-Ed Series | What Happens When You Remove the Hippocampus?

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WEEK 10:

Henry Molaison had his hippocamus removed, before medical science knew what the impact would be. He lost all of his long term memory and had limited short term memory.

Henry Molaison (HM) cracked his skull in an accident and suffered repeated seizures, black-outs and loss of bodily control. In 1953 Doctor Scoville, a surgeon prepared to try innovative surgery and removed his hippocampus, which was associated with emotion, but whose function was not known.

The operation was initially successful as seizures stopped, no personality change and even HM’s IQ improved. Unfortunately he had lost his memory not only of the past, but could form no new memories. As soon as he did something he forgot what it was he had done.

HM was studied by PhD student Brenda Milner, who made two discoveries about memory.

  • She found he could remember some things for short periods and could remember random numbers for 15 minutes if he kept repeating it, but quickly lost any memory of undertaking that test.

Milner showed that memory was not monolithic, but was short and long term, and that each memory is stored in different regions of the brain. Immediate sensory data travels via the cortex to the hippocampus where special proteins work to strengthen the cortical synaptic connections. If the experience is memorable the hippocampus transfers the memory back to the cortex for long term storage.

As HM had no hippocampus this process could not occur.

  • The star test, where HM was asked to draw a star between a larger and smaller star. After repeated attempts, the quality of the drawn star became more accurately drawn. Milner had discovered that the declarative memory (dates, names, facts), is different from the procedural memory (bike riding, name signing). Procedural memory relies on the basal ganglia and the cerebellum, both of which HM still had.

HM died aged 82 in a nursing home. Following his death, his brain was scanned and cut into 2000 slices for examination.

Memory can be a key part of psychological problems we can face – particularly trauma and PTSD.  It’s important to seek the right type of help if you experience these types of difficulties.

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