In neuroscience, the Mass Action Principle is one of two principles that Karl Lashley published in 1950 that summarized 30 years of research. The other principle was the equipotentiality principle. The Mass action principle stated that the reduction in learning is proportional to the amount of tissue destroyed, and the more complex the elarning task, the more disruptive the lesions are. In other words, that memory can not be localized to a cortical area but that it is distributed through the cortex.
Lashley's research was an attempt to find the parts of the brain that were responsible for learning and memory traces or engrams. He trained rats to perform specific tasks (seeking a food reward), then lesioned varying portions of the rat cortex, either before or after the animals received the training depending upon the experiment. The amount of cortical tissue removed had specific effects on acquisition and retention of knowledge, but the location of the removed cortex had no effect on the rats' performance in the maze. This led Lashley to conclude that memories are not localized but widely distributed across the cortex.