Mass Action and Equipotentiality Reconsidered

Two conceptual views of cortical function have evolved and been used a number of times over the past 150 years to explain puzzling effects of brain damage: mass action and equipotentiality. The mass action hypothesis asserts that the entire cortex participates in every behavior. Thus, removal of any cortical tissue produces a behavioral change that is proportional to the amount of tissue removed. The equipotentiality hypothesis states that each portion of any given area is able to encode or produce the behavior normally controlled by the entire area. Thus, incomplete damage within a zone is compensated for by the remaining area. These concepts were debated extensively for the first half of this century and now are still invoked periodically as explanations for recovery of function. We revisit the concepts by briefly looking at the history before considering their current forms. We then examine the question of whether they are useful concepts to consider as explanations of recovery of function.

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