Arguably the single most significant piece of neuroscience research is patient Henry Molasion (Patient H.M)’s brain, which has been researched by thousands of scientists for 5 decades.
At the age of 27, Molaison began experiencing severe, debilitating epileptic seizures, suspected to be due to a traumatic childhood biking injury. In a last-ditch effort to treat his condition, doctors performed an experimental surgery in which they removed large portions of Molaison's medial temporal lobe, including his hippocampus. While the surgery was successful in reducing Molaison's seizures, he was left with a severe and permanent case of anterograde amnesia - the inability to form new memories.
H.M’s unique conditions led to major contributions to the study of memory. Originally, neuroscientists have thought of memory as monolithic; all of it essentially the same and stored throughout the brain. However, experiments conducted on H.M suggested that there is clear distinction between short-term and long-term memory, and each uses different brain regions. Scientists also discovered that the hippocampus and medial temporal lobe play a vital role in the consolidation of new memories, transferring information from short-term to long-term storage. Without these key brain structures, Molaison was simply unable to form new declarative memories.
In the now famous ‘star tracing’ experiment, H.M was given a difficult motor task to draw a star in between two stars while only seeing its reflection on a mirror. Without recalling previous trials, his performance on the task improved over successive trials, becoming faster and more accurate in tracing the star.
The results of this experiment demonstrated retained the ability to acquire new perceptual-motor skills, even in the absence of explicit memory formation, meaning that he was still able to form new procedural memories, retaining the ability to learn new skills and habits through repetition. This suggested a separation between the neural mechanisms underlying declarative and procedural memory, revealing that not all memory functions are dependent on the hippocampus.
During H.M’s lifetime, he has cooperated extensively with researchers to provide significant insight on how the brain functions. Even post-death, his brain is still being studied by numerous neuroscientists globally, providing valuable lessons for brain research.
Reference list
Adams, T. (2013). Henry Molaison: the amnesiac we’ll never forget. [online] the Guardian. Available at: https://www.theguardian.com/science/2013/may/05/henry-molaison-amnesiac-corkin-book-feature [Accessed 26 Jul. 2024].
TED-Ed (2014). What happens when you remove the hippocampus? - Sam Kean. YouTube. Available at: https://www.youtube.com/watch?v=KkaXNvzE4pk [Accessed 26 Jul. 2024].
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