Karl Spencer Lashley (June 7, 1890 – August 7, 1958) was an American psychologist and behaviorist remembered for his contributions to the study of learning and memory.

A Review of General Psychology survey, published in 2002, ranked Lashley as the 61st most cited psychologist of the 20th century.

[4] Once Lashley completed his master's degree, he studied at Johns Hopkins University, where he received his PhD in genetics in June 1911.

At Hopkins, Lashley minored in psychology under John B. Watson, whom he continued to work closely with him after receiving his PhD.

Lashley's career began with research concerning brain mechanisms and how they were related to sense receptors.

After this he went to Harvard, but was dissatisfied and from there became the director of the Yerkes Laboratory of Primate Biology in Orange Park, Florida.

He researched this by looking at the measurement of behavior before and after specific, carefully quantified, induced brain damage in rats.

[5][6] His study of V1 (primary visual cortex) led him to believe that it was a site of learning and memory storage (i.e. an engram) in the brain.

"Mass action" refers to the idea that the rate, efficacy and accuracy of learning depend on the amount of cortex available.

Lashley was on the road to a full recovery until his trip to France with his wife Clair, where he once again unexpectedly collapsed, but this time to his death on August 7, 1958.

[9] In 1943, Lashley was awarded the Daniel Giraud Elliot Medal from the National Academy of Sciences.

[4] Lashley had a reputation as an objective scientist, but Nadine Weidman has tried to expose him as racist and a genetic determinist.

He cites a line from a letter that Lashley wrote to a German colleague which reads: "Too bad that the beautiful tropical countries are all populated by negros.