Equipotentiality refers to a psychological theory in both neuropsychology and behaviorism. Karl Spencer Lashley defined equipotentiality as "The apparent capacity of any intact part of a functional brain to carry out… the [memory] functions which are lost by the destruction of [other parts]". In other words, the brain can co-opt other areas to take over the role of the damaged part. Equipotentiality is subject to the other term Lashley coined, the law of mass action. The law of mass action says that the efficiency of any complex function of the brain is reduced proportionately to how much damage the brain as a whole has sustained, but not to the damage of any particular area of the brain. In this context when we use brain we are referring to the cortex.

In neuropsychology

In neuropsychology, equipotentiality is a neurological principle that describes a cortical mechanism, first identified by Jean Pierre Flourens and later revisited by Karl Lashley in the 1950s. The principle of equipotentiality is the idea that the rate of learning is independent of the combination of conditioned and unconditioned stimuli that are used in classical conditioning.

After performing ablation experiments on birds, and seeing that they could still fly, peck, mate, sleep, and perform a range of other regular behaviors, Flourens concluded that every area of the brain was capable of doing what every other area of the brain could, but only for higher-level functions which he called "perception". He also argued that elementary sensory input was localized, which is supported by current research. The famous saying that we only use 10% of our brains originates from Flourens, back about a century and a half ago.

Lashley offered two generalizations from his research, that recently have been successfully challenged but nonetheless represent important milestones in the development of neurological theory:

1.     Although surgical removal of a portion of the cortex can produce significant behavioral deficits, those deficits can be recovered through additional training and time, by way of the development of new neuronal connections. Lashley argued that the brain is sufficiently plastic, such that when one region of the brain is surgically removed (or damaged through injury) another region takes over the damaged region's function. This is the principle Lashley referred to as equipotentiality. Extensive regions of the cerebral cortex have the potentiality for mediating specific learning and memory functions.

2.     His principle of "mass action" stated that the cerebral cortex acts as one—as a whole—in many types of learning.


Karl Lashley joined the Harvard faculty in 1935, and in the ensuing twenty years he expanded his research on the representation and localization of sensory and motor activity in mammalian brains.  Concurrent with the latter half of his tenure at Harvard, Lashley also served as the director of the Yerkes Laboratories of Primate Biology in Orange Park, Florida from 1942 to 1955.

Trained by the founder of the behaviorist movement in American psychology, John B. Watson, Lashley eventually parted ways with the evolving theoretical perspective of his mentor by emphasizing the neurobiological basis for sensorimotor and memory representations in the brain.

Lashley pioneered experimental work conducted on rats with surgically induced brain lesions, by damaging or removing specific areas of a rat’s cortex, either before or after the animals were trained in mazes and visual discrimination.   Lashley made several fundamental discoveries about how the brain stores and processes information. By implanting insulating chips of mica in rats’ cortexes and showing that they had few effects on learning and behavior, he established that (contra to Gestalt theories of the era) the cortex processed information in the pattern of activity and connectivity among neurons, not in global field and wave effects propagating through a medium. By showing that lesions that undercut slabs of cortex had far more severe consequences lesions that were perpendicular to the cortex, he helped show that the principal circuits of the cortex ran up and down into the white matter rather than side-to-side across the cortical surface. His famously unsuccessful search for the “engram” – the localized trace of the memory for a maze in a trained rat’s brain – led him to propose the principle of "mass action," in which learning is distributed across all parts of the brain rather than stored in a single regions, with the degree of impairment proportional to the amount of brain that was damaged. His complementary principle of "equipotentiality" stated that in the event of damage to one area of the brain, other parts of the brain can sometimes assume the role of the damaged region. Though his views are now considered too extreme, the principle that memories are not localized to a single spot in the brain is now well accepted.

In 1951 Lashley published a famous paper called “The Problem of Serial Order in Behavior,” in which he pointed out that complex sequential behavior (such as playing a piece on the piano) could not be executed by one response sending a proprioceptive signal back to the brain which would then trigger the next response in the sequence – there simply wasn’t enough time for the neural signals to travel up to the brain and back down. Instead, behavior had to be controlled by a central, hierarchically organized program. This insight has guided the study of motor behavior ever since, and influenced Noam Chomsky’s critique of Skinner’s theory of language and the development of Chomsky’s theory of generative grammar.

Beginning with Karl Lashley, researchers and psychologists have been searching for the engram, which is the physical trace of memory. Lashley did not find the engram, but he did suggest that memories are distributed throughout the entire brain rather than stored in one specific area. Now we know that three brain areas do play significant roles in the processing and storage of different types of memories: cerebellum, hippocampus, and amygdala. The cerebellum’s job is to process procedural memories; the hippocampus is where new memories are encoded; the amygdala helps determine what memories to store, and it plays a part in determining where the memories are stored based on whether we have a strong or weak emotional response to the event. Strong emotional experiences can trigger the release of neurotransmitters, as well as hormones, which strengthen memory, so that memory for an emotional event is usually stronger than memory for a non-emotional event. This is shown by what is known as the flashbulb memory phenomenon: our ability to remember significant life events. However, our memory for life events (autobiographical memory) is not always accurate.