Henry Molaison's memory

BRAIN AREA: Medial Temporal Lobe – Limbic System – Hippocampus – Memory, spatial learning

Described in textbooks as HM, Henry Molaison had suffered epileptic seizures since he was a boy, reportedly after a bicycle accident. The seizures became worse until in 1953, aged 27, he was operated on by neurosurgeon Dr William Scoville in Hartford, Connecticut, USA.

The impact of the operation on Henry's brain would bring significant insights into the capture, synthesis and working of memory in a human brain. Most notably, the bilateral removal of his hippocampus (see Image 7) and adjacent cortical structures had the unfortunate effect of Molaison losing the ability to form new declarative memories.

This became a very important case in brain research. Molaison was extensively studied by neuroscientists and other medical researchers over a 46-year period until his death in 2008. Neuroscientist Suzanne Corkin met him in 1962 and her 2013 book Permanent Present Tense revealed him as a man stuck in the present, unable to form new memories (anterograde amnesia), unable to remember most life events prior to his surgery (retrograde amnesia), and although he could remember facts (semantic memory) assimilated prior to the removal of both his medial temporal lobes, he had almost non-existent autobiographical (episodic) memory.

While Henry's surgeon, William Scoville, believed he had successfully removed both amygdalae and most of both hippocampi and parahippocampal gyri in 1953, later MRI and postmortem examination of Henry's brain revealed 'a significant amount of residual hippocampal tissue' posteriorly.

Additionally, the 'almost complete removal of the entorhinal cortex (origin of input to the hippocampus) in both hemispheres, resulting in severe disconnection of the remaining hippocampus, would have made a more significant contribution to HM's declarative memory impairment than the ablation (removal) of the anterior hippocampus'.

Postmortem investigation found that much of Henry's posterior hippocampus, the part considered responsible for declarative memory, was intact. However, since a 1993 MRI showed the entorhinal cortex, amygdala and half the interventricular hippocampal formation (dentate gyrus, hippocampus proper and subiculum) were missing, input to the hippocampus would have been almost non-existent. Postmortem inspection also confirmed only a small part of the entorhinal cortex and amygdala remained – approximately 2cm³ of the posterior hippocampal formation (cornu ammonis, dentate gyrus and subiculum) was spared.

The impact of the operation also revealed the workings of parahippocampal function, in particular the parahippocampal gyrus, as surgeon William Scoville wrote that he had 'probably destroyed the anterior two-thirds of the hippocampus and hippocampal [gyrus] bilaterally, as well as the uncus and amygdala'. Examination and further imaging of Molaison's brain after his death revealed that although damaged parts of the posterior parahippocampal gyrus remained, 'by far the greatest territory removed at surgery was the parahippocampal gyrus', with anterior parts of the perirhinal cortex and the entire entorhinal cortex removed. Damage to underlying white matter causing equally disabling disruption of connections had also occurred.

One of the effects of this surgery was to destroy Molaison's spatial memory so he was unable to navigate through a maze. Remarkably, he was later able to draw a map of the house where he lived, indicating that other areas of his brain had taken over the job of encoding and storing spatial information. Professor Corkin concluded that this task was managed by the remaining three-quarters of an inch of parahippocampal gyrus remaining in Molaison's brain.

From further research, it was discovered that 'view cells' in the parahippocampal region acted to complement 'place cells', indicating the hippocampus and parahippocampal region perform complementary functions during navigation.

While Molaison was deprived of much of his memory, his life and legacy continue to provide critical insights into how the brain processes and stores memories about experiences that make our lives meaningful. The many studies of his brain have established the importance of the hippocampus in forming long-term memory, spatial learning and navigation.

References & further reading

Annese, J., Schenker-Ahmed, N. M., Bartsch, H., Maechler, P., Sheh, C., Thomas, N., Kayano, J., Ghatan, A., Bresler, N., Frosch, M. P., Klaming, R., & Corkin, S. (2014). Postmortem examination of patient H.M.'s brain based on histological sections and digital 3D reconstruction. Nature Communications, 5(1), 3122. https://doi.org/10.1038/ncomms4122

Corkin, S. (2013). Permanent present tense: The man with no memory and what he taught the world. Allen Lane.

Ekstrom, A. D., Kahana, M. J., Caplan, J. B., Fields, T. A., Isham, E. A., Newman, E. L., & Fried, I. (2003). Cellular networks underlying human spatial navigation. Nature, 425(6954), 184–188. https://doi.org/10.1038/nature01964

Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery and Psychiatry, 20(1), 11–21. https://doi.org/10.1136/jnnp.20.1.11