The Church Neuroscience Lecture, with guest speaker Suzanne Corkin
The Church Neuroscience Lecture this year featured Suzanne Corkin, professor of behavioral neuroscience at the Massachusetts Institute of Technology in Cambridge, MA. Dr. Corkin’s talk focused on the neuroscience of memory, particularly the key findings that came from her work with patient Henry Molaison, known before his death in 2008 only by his initials, H.M.
When Henry was a teenager, he suffered from severe epilepsy. Physicians believed the focus of his seizures was the hippocampus, and a “frankly experimental operation” (quoted from the neurosurgeon) was performed as an attempt to control his epilepsy. Extremely high-resolution post-mortem MRI studies show the structures removed included the amygdala, the entorhinal cortex, the anterior half of the hippocampus, and the anterior portion of the perirhinal cortex. On the one hand, the surgery was a success because Henry’s epilepsy improved considerably. On the other hand, it had a devastating and unexpected consequence – he could no longer form new long-term memories.
As Dr. Corkin described, “his amnesia was pure.” That is, his memory was severely affected, but other intellectual functions such as lexical and grammatical functioning, perceptual functioning, IQ, and psychiatric state were still intact following the surgery. This “pure amnesia” made Henry’s case extremely important in terms of its implications for memory scientists.
One of Dr. Corkin’s first important observations was the differential effect the surgery had on short-term memory (STM) and long-term memory (LTM). Henry had intact STM (ability to retain information for 20-30 seconds), while his LTM was severely affected. This is because STM depends on cortical structures, while LTM depends on medial temporal lobe (MTL) structures, primarily the hippocampus. Henry not only had the anterior half of his hippocampus removed, but his entire hippocampus was probably nonfunctional because the primary source of sensory input, the entorhinal cortex, was destroyed, a result that was supported by post-mortem MRI scans.
An important study that supported the anatomical differentiation between STM and LTM compared recall ability after a delay interval between global amnesiacs (including H.M.) and normal adults. The global amnesiacs (with MTL damage) performed as well as the control group when the delay interval was short (<15-20 sec), but when the delay interval exceeded 20 seconds, control subjects’ recall was significantly better.
Dr. Corkin also noted that Henry could form new non-declarative memories (such as ability to acquire skills). For instance, studies confirmed Henry’s ability to improve over time in a somatosensory “touch maze” task, and his ability to learn a mirror-tracing task. This indicates that non-declarative memories are independent of MTL structures. Conversely, Henry was largely unable to form any new declarative memories. He could not learn new unfamiliar words, suggesting that he lost the ability to form new semantic memories, and could not learn new faces. In fact, during the 45 years that Dr. Corkin worked with him, he never learned who she was. Surprisingly, a miniscule ability to add episodic knowledge was retained (perhaps due to reliance on cortical structures associated with non-declarative memory), such that he could learn a few facts about people who became famous after his surgery.
Dr. Corkin also studied his ability to remember things from before the surgery. Henry was able to recall pre-operative semantic memories, and could easily identify people who were famous prior to his surgery. However, his ability to retrieve autobiographical memories from his youth was affected, suggesting that an ensemble between MTL structures and the neocortex is necessary not just for encoding, but also for recall of autobiographical events. Sadly, Henry could only remember one event from his childhood – going on an airplane ride above his hometown in Connecticut. This is perhaps the most tragic implication of his surgery – his deceased ability to form a cohesive identity. Life experiences are important in forming a sense of self, and the impact of memory on identity is profound.
Although the story of H.M. is both fascinating and sad, Henry seemed to have some understanding of his role in the knowledge his memory deficits helped produce. In a 1992 interview Suzanne asked Henry if all the questions and testing bothered him, he responded, “no, what is found out about me helps you to help others … and I feel that’s more important in a way.”
Patrick Mullen, '13