The Brain and Memory: Learn It 1—Parts of the Brain

  • Understand the functions of different brain parts in memory
  • Compare and contrast anterograde and retrograde amnesia

Memory in the Brain

Are memories stored in just one place in the brain—or do they live everywhere? Early neuroscientists, including Karl Lashley, tackled this question by searching for the engram: the physical trace of a memory in the brain.

In the 1920s–1950s, Lashley trained rats to run mazes and then created lesions in different parts of their cerebral cortex to see if he could erase the maze memory. Surprisingly, the rats still remembered the maze, no matter which area he damaged.

From this, he proposed the equipotentiality hypothesis: if one part of a brain region involved in memory is damaged, other parts can sometimes take over its function.

Although Lashley never found the engram, modern neuroscience has identified specific brain circuits involved in memory formation. Nobel Prize–winning neuroscientist Eric Kandel helped show that memories involve long-term changes at the synapse—a process called long-term potentiation (LTP)—and not a single “memory spot.” LTP strengthens the neural pathways that represent a memory, helping make that memory stable and long-lasting.

Today, scientists agree on two ideas:

  1. Memory is not stored in one place.
  2. Different brain structures support different types of memory.

The most important structures are the amygdala, hippocampus, cerebellum, and prefrontal cortex.

 

An illustration of a brain shows the location of the amygdala, hippocampus, cerebellum, and prefrontal cortex.
Figure 1. The amygdala is involved in fear and fear memories. The hippocampus is associated with declarative and episodic memory as well as recognition memory. The cerebellum plays a role in processing procedural memories, such as how to play the piano. The prefrontal cortex appears to be involved in remembering semantic tasks.

Amygdala

The amygdala helps regulate emotions like fear, anger, and excitement. These emotional states influence how strongly memories are encoded.

  • Stress hormones such as adrenaline activate the amygdala.
  • Emotional events—positive or negative—are remembered more vividly.
  • The amygdala is essential for fear conditioning and other emotional learning.

Research shows that when neurons in the amygdala connected to a specific fear memory are removed or disrupted, the memory weakens or disappears.

For example, one researcher experimented with rats and the fear response (Josselyn, 2010). Using Pavlovian conditioning, a neutral tone was paired with a foot shock to the rats. This produced a fear memory in the rats. After being conditioned, each time they heard the tone, they would freeze (a defense response in rats), indicating a memory for the impending shock. Then the researchers induced cell death in neurons in the lateral amygdala, which is the specific area of the brain responsible for fear memories. They found the fear memory faded (became extinct). 

The amygdala also plays a role in memory consolidation—strengthening new memories so they last over time.

In this TED Talk called “A Mouse. A Laser Beam. A Manipulated Memory,” Steve Ramirez and Xu Liu from MIT talk about using laser beams to manipulate fear memory in rats. Find out why their work caused a media frenzy once it was published in Science.

Hippocampus

The hippocampus is central to forming declarative memories—facts and personal experiences. It is especially important for:

  • Episodic memory (events from your life)
  • Semantic memory (facts and knowledge)
  • Spatial memory (navigation, place learning)
  • Recognition memory

Lesion studies in rats and humans show that without a functioning hippocampus, new declarative memories cannot form. The most famous example is Henry Molaison (H.M.), who had large portions of his hippocampi removed to treat epilepsy. After surgery:

  • He could remember events from before the surgery
  • But he could not form new long-term declarative memories
  • His procedural memory (skills and habits) stayed intact

The hippocampus does not store memories permanently. Instead, it acts as a temporary “loading dock,” sending memories to the cortex for long-term storage.

View this Slate video for a closer look at how memory works, as well as how researchers are now studying H. M.’s brain.