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Evolution of the Understanding of Memory Retention

“Like waking from a dream...every day is alone in itself” (Milner et al., 1968, p. 217). These few words are how Henry Molaison described his life. Due to an experimental brain surgery performed to reduce Molaison’s seizures, Molaison lived his life with extreme memory loss, a condition that made him, for five decades, the focus of countless studies in the field of neuroscience. Molaison specifically helped scientists in realizing that the hippocampus plays an important role in how the brain learns. Recently, the University of Ottawa and Humbolt University of Berlin worked together to further investigate how the hippocampus works with the perirhinal cortex to distribute information across the brain.

In the year 1933, seven-year-old Molaison fell off his bike and, three years later, started to experience minor seizures. By the age of 27, the seizures became very serious and prevented him from doing his job at an assembly line. To help Molaison control his seizures, Dr. Scoville performed an experimental surgery on Molaison’s brain. Previously, Dr. Scoville had successfully performed this surgery on patients with similar diagnoses. After the surgery was completed, Molaison was observed to have extreme memory loss. He would forget events as they occurred and would often forget someone’s name as he was told it. Molaison’s condition led to numerous studies attempting to discover why his brain surgery caused severe memory loss.

Which parts of the brain that were damaged are responsible for memory retention? In 1991, scientists were able to answer this question: the medial temporal lobe memory system comprises of the hippocampus and the perirhinal, entorhinal, and parahippocampal cortices and is responsible for memory retention (Squire and Zola-Morgan, 1991). Molaison’s brain surgery damaged multiple regions of the medial temporal lobe, specifically his hippocampus and parts of the parahippocampal cortex. Since the brain surgery affected multiple regions of the medial temporal lobe, Molaison had a rare and severe case of memory loss.

Due to Henry Molaison’s condition and willingness to be studied, scientists were able to identify different sections of the brain but were still unable to figure out how the hippocampus sends signals to “the billions of neurons throughout the cortex”(University of Ottawa, 2020). However, due to the work of the University of Ottawa and the Humboldt University of Berlin this year, we are now one step closer to understanding how the hippocampus sends signals. This information was figured out by observing mice when a single neuron in their memory cortex was stimulated. The mice responded to the stimuli “by licking a dispenser to receive sweetened water.”(University of Ottawa, 2020). From this experiment, scientists determined that the perirhinal cortex would receive information from the hippocampus and then send it to the neurons in the outer layer of the cortex. In fact, it was concluded that the perirhinal cortex is responsible for sending information to the outer layer of the cortex, leaving scientists to ponder about what this signal looks like and its impact on memory retention.

Works Cited:

Milner, Brenda, Suzanne Corkin, and H-L. Teuber. "Further analysis of the hippocampal amnesic syndrome: 14-year follow-up study of HM." Neuropsychologia 6.3 (1968): 215-234.

SCOVILLE, W B, and B MILNER. “Loss of recent memory after bilateral hippocampal lesions.” Journal of neurology, neurosurgery, and psychiatry vol. 20,1 (1957): 11-21. doi:10.1136/jnnp.20.1.11

Squire, Larry R. “The legacy of patient H.M. for neuroscience.” Neuron vol. 61,1 (2009): 6-9. doi:10.1016/j.neuron.2008.12.023

University of Ottawa. "How does the brain manage its learning?." ScienceDaily. ScienceDaily, 18 December 2020.