Karl clicks and the name appears as

Karl Lashley was a neuropsychologist responsible for essential advances in scientific knowledge of brain function and memory. This lesson looks at his theory of mass action and the experimentation that led to the discovery of equipotentiality.

Recalling a Stranger’s Name

A stranger passes you at a party, and you know you’ve seen her before. Some unique lilt of her voice or physical mannerism has triggered your memory and, though you cannot remember her name, you are convinced that you know her.

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It is a puzzle that absorbs you until finally something clicks and the name appears as if it were written in a book. Of course, that’s Kathy somebody, but where do you know her from? You figuratively riffle through mental files until it starts to come back. She was a neighbor who moved to the city about five years ago. After that, the memories come flooding in and you walk over to say hi. How this happens was explained by a neuropsychologist named Karl Lashley.

The Engram

Memory has been a conundrum that many scientists have gone to great lengths to solve. Some, such as Descartes, simply philosophized regarding its essence, but opinion does not make for reliable research. In the twentieth century, a researcher by the name of Richard Semon posited that memory was stored in physical capacitors called engrams. Semon believed that these storage tanks were depositories into which memories were dropped until they were needed. The problem with Semon’s theory, though, was that no one had ever located an engram in the brain. Karl Lashley searched diligently during his experiments with mice and that led him to a discovery that changed the way people understood memory.

The Law of Mass Action

The cerebral cortex is the complex portion of the brain whose size separates humans from other animals. It is the seat of reasoning and memory. Lashley conducted experiments with rats, trying to find the Semon’s engram, but instead discovered that memory was not contained in single structures within the cortex.Lashley worked with rats in a maze to understand how memory actually occurred. He would train a rat to run a maze and then he would cut away a piece of the cerebral cortex. Each time he did this, with different subjects, he found that the memory was impaired (the rat could no longer negotiate the maze).

After many repetitions he learned that the entire cerebral cortex was involved in memory. This mass action was demonstrated by the rats in that it did not matter what part of the cortex was cut, the rat’s memory was affected. However, cutting away parts of rats’ brains led Lashley to an even more important discovery.


In general, Lashley cut away just a part of a region and not the whole. He did this because he only needed to remove a small piece of the brain to effect the memory change he was observing. During experiments regarding vision, he taught rats to recognize the difference between triangles and circles. To determine which part of the brain controlled that memory function, he cut away a rat’s entire striate cortex (which he found to control that ability).

One time, during this experiment, he missed about 10% of the striate cortex. When he tested the rat, it retained its ability to distinguish between circles and triangles.Further experimentation along this line led Lashley to the discovery of equipotentiality. This is the ability of the brain to maintain a function even though the region of the brain where that function is performed has been severely damaged.

He found that the brain is elastic enough to make up for significant losses. This idea has been verified in the intervening years testing people who have sustained damage to certain areas of the brain.Humans recognize people and places from their past because mass action allows the cerebral cortex to gather the information needed for that memory. Lashley also discovered that even if a person has sustained a significant deficit, the brain has the potential to make up for it.

Both of these discoveries were important leaps in scientific knowledge regarding the brain.

Lesson Summary

Karl Lashley was a neuropsychologist who made very significant discoveries regarding memory and brain function. Using rats as subjects, he was able to prove that the idea of an engram (a memory repository within the brain) was false. He found that memory was actually a function of mass action, in which the entire cerebral cortex gets involved in the recall necessary for memory.

His experimentation also led to his idea of equipotentiality, which holds that the brain can carry out functions even after a significant part of the brain controlling that function has been damaged. Lashley’s theories and experimentation produced discoveries that greatly advanced scientists’ understanding of the brain and its functioning.


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