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Mass Action Principle

The mass action principle was published in 1950 by Karl Lashley. While studying learning and memories, he learned that these things are not stored in one specific location Of the brain. Lashley trained rats to navigate a certain maze so that they could complete it successfully. He then damaged certain parts of the rats' cortexes with strategically placed lesions, and had them navigate the maze again to see which part of the brain controlled learning and memory. What he found, was that the rats could still navigate most of the maze from memory.After this, Lashley published two principles: the Principle of Equipotentiality and the Principle of Mass Action.

The Principle of Mass Action states that the cerebral cortex of the brain acts as a whole, instead of individually controlling specific functions. This means that learning is distributed throughout the entire brain, and across all parts. This principle also suggests that if cortical tissues is destroyed after learning a task, the deterioration of performance of that task is not determined by the location of the damage, bit by the amount of damage inflicted.

Karl Lashley and the History Behind Mass Action

Karl Lashley published cerebral research from 1917 to 1926. (Bruce 1998) He started to move away from connectionism between 1923 and 1926. This happened because his findings were becoming too complex for the idea of connectionism to explain them. Finally in 1926 he completely switched from the idea of connectionism to that of a mass action correlation. He noted a positive relation between the amount of forgetting of a brightness discrimination habit and the extent of injury to the visual cortex. Karl Spencer Lashley was born in 1850 and died in 1958. He had a Ph.D. in Zoology, but in 1914 he was already publishing with Watson and from that point on he regarded himself as a psychologist. Lashley published a number of theoretical papers of great importance. He played a key role in two principles of neuroscience: equipotentiality and Mass-action principal.

Lashley explained that Equipotentiality and mass action were shorthand expressions that referred to empirical relations (Lashley, 1929). Equipotentiality meant that within any functional area of the brain, no part was more important than any other in performing the function; lesions of equal extent produced equal impairment irrespective of their locus. Mass action qualified the finding of equipotentiality: The degree of deficit was directly proportional to the amount of brain injury within a functional area. For learning and retention of a complex maze, Lashley concluded that the entire cortex was responsible. It did not matter where in the cortex lesions were made (equipotentiality); the more extensive the injury, the greater the decrement in learning and retention (mass action).( Bruce 1998)

Localization of function means that parts of the brain are associated with certain functions and not others. For Lashley, the main concern was whether different cortical areas had particular functions. It is sometimes said that he came down against localization.

The point is that the major facts of cerebral integration—equipotentiality, mass action, and localization of function—could all comfortably coexist. The challenge for Lashley was to explain them. This brings us to reflex theory, or connectionism according to which learning is the formation of reflex arcs in the brain in the manner of conditioned spinal reflexes, the creation of central neurological paths from sensory receptors to motor effectors, or the establishment of connections between specific neurons in the form of decreased resistance at synapses. There is little doubt that Lashley used, or at least considered, the reflex arc concept in interpreting all his findings, especially early on. Where there was localization of a habit (e.g., Lashley, 1920), then its loss due to lesion was the consequence of interrupting specific reflex paths within the functional area.

The Effect of the Mass Action Principle on the Present

Before Karl Lashley and the work he did, it was widely believed that each piece of the brain worked individually, on entirely separate things. Now, because of the research he did and the principle he published, we have been able to better understand the workings of the brain collectively, as a whole.