ACT-R is a model of the human cognitive process developed and used by cognitive psychologists, which can be applied to HCI. It is an acronym for "The Adaptive Control of Thought - Rational". While it is often referred to as "the ACT-R theory", it is not properly considered a theory of cognition, but rather a cognitive architecture that can accommodate different theories. The scope of ACT-R is greater than the scope of any particular theory, and multiple (possibly competing) theories can fit within the framework of ACT-R. It was developed to model problem solving, learning and memory.
J. Johnson. (2011)cite arxiv:1105.5701Comment: 33 pages, 19 figures; chapter to appear in 'The First Galaxies - Theoretical Predictions and Observational Clues'; some figures downgraded, high resolution version available at http://www.mpe.mpg.de/tmox/jjohnson/news/FG_Theory_and_Simulations.pdf.
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