53 Memory and the Brain
Learning Objectives
By the end of this section, you will be able to:
- Describe the role of brain regions in different types of memory
- Describe how neurons store memory
- Discuss the role of specific neurotransmitters in memory
Where are memories stored? Karl Lashley began exploring this problem about 100 years ago, by making lesions in the brains of non-human animals. He was searching for evidence of the engram: the group of neurons that serve as a physical memory trace (Josselyn, 2010). First, Lashley trained rats to find their way through a maze, and then he created lesions in the cerebral cortex to try and erase the memory trace that the rats had of the maze. He didn’t find evidence of the engram since the rats were still able to find their way, regardless of the size or location of the lesion. Based on these findings, he formulated the equipotentiality hypothesis: if part of one area of the brain involved in memory is damaged, another part of the same area can take over that memory function (Lashley, 1950). Although Lashley failed to find evidence of the engram, he is considered a pioneer in the field of neuroscience.
Since Lashley’s research, other scientists have been able to look more closely at the role of the brain in memory. They have argued that memory processes recruit distinct brain areas, and specific neurons can be recognized for their involvement in forming memories. Although the entire brain is involved in memory in some way or another, we’ll focus on some key players: the hippocampus, amygdala, cerebellum, and prefrontal cortex (Figure M.21).
Hippocampus
Experimental surgeries conducted on humans in the 1900s have taught us that memory is indeed tied to discrete brain regions. One famous example is Patient HM, who underwent a temporal lobectomy and had his anterior hippocampus, parahippocampal gyrus, and amygdala removed (for a refresher on HM, check out the Intro section). After his surgery, he couldn’t create any new explicit memories but still had his early-life memories and the ability to learn new procedural tasks (Milner, 2005; Scoville, 1954). Thanks to HM, we have learned that the medial temporal lobes are involved in explicit memory formation but are not involved in implicit learning or memory storage.
Much of what we know about the role of the brain in human memory is the result of studying people with severe brain injuries. However, it is difficult to pinpoint the exact function of a brain region when multiple areas are damaged. Researchers are better able to make precise lesions in non-human animals to understand the role of specific brain areas. However, this presents a challenge: you can’t ask a rat to recall facts about their childhood like you can with HM Instead, researchers use tasks like mazes in combination with lesions or sham surgeries to test memory, such as the Morris water maze. A sham surgery is when the control group undergoes an imitation of the surgery but does not receive