Henry Molaison: How Patient HM Changed What We Know About Memory

Summary

In 1953, neurosurgeon William Scoville operated on Henry Molaison to reduce his severe, persistent seizures. Scoville removed much of the medial temporal lobe on both sides of Molaison’s brain, including the hippocampus, amygdala, and entorhinal cortex. The surgery eased some of Molaison’s seizures but left him unable to form new long-term memories.

Studies of Molaison, particularly Brenda Milner’s early research, helped establish several important findings about memory:

• Memory depends on particular brain structures. Molaison’s case challenged the idea that memory is distributed uniformly throughout the brain. The hippocampus and nearby medial temporal lobe regions were shown to be important for forming new memories.
• Different kinds of memory can be affected independently. Molaison had severe anterograde amnesia, or difficulty forming new memories, but could recall many memories from before the surgery. His perception, language, intellect, and motor skills remained largely intact.
• Memory consolidation involves changes over time. The video describes the medial temporal lobe as crucial for creating new memories, while consolidated long-term memories are stored elsewhere in the brain.
• Skill learning can persist despite amnesia. Molaison learned and retained new motor skills, including mirror drawing, even though he could not remember practicing them. This supported the idea that memory involves multiple systems.
• Declarative and non-declarative memory rely on different systems. Declarative memory involves recalling facts and events. Non-declarative memory includes skills, habits, and emotional learning. The video identifies the basal ganglia, amygdala, cerebellum, and neocortex as structures involved in non-declarative memory; these structures were preserved by Molaison’s surgery.

The video also discusses the ethics of Molaison’s treatment, criticizing the degree of control researcher Susanne Corkin had over access to him and his belongings.

Researchers and Other People Featured

• Henry Molaison (Patient H.M.) — Research participant whose case transformed memory research.
• William Scoville — Surgeon who performed Molaison’s operation.
• Brenda Milner — McGill University memory researcher who conducted early studies of Molaison.
• Susanne Corkin — Researcher who later studied Molaison; her name appears as “Corkins” in parts of the subtitles.
• Thomas Mooney — Molaison’s distant cousin and conservator, as described in the video.