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In neuroscience, the Mass-action Principle is one of two principles that Karl Lashley published in 1950, alongside 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 learning task, the more disruptive the lesions are. In other words, memory can not be localized to a cortical area, but is instead 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, a hypothetical structure he called the engram. 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.
In the 1940's Halstead and others then explored the inplication of these ideas to explain the effects of lesions on the frontal lobes of human. He posited the notion of biological intelligence to explain his observation that the effects of lesions seemed more linked to their extent rather than than the location.
However subsequent work  means that the principle of mass action has been shown to have limited applicability, essentially withing the context of regional equipotentiality. That said, the recent development of connectionist approaches has reignited interest in the principle, within restricted areas of the brain
- ↑ Lashley's Search for the Engram
- ↑ Beaumant J.G., Kenealy, P.M. & Rogers, M.J.C. (1999). The Blackwell Dictionary of Neuropsychology. Oxford:Blackwell
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