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Orch-OR (Orchestrated Objective Reduction) is the proposal that information processing in the brain involves complex computational processes within every neuron, that involve co-ordinated changes in the conformational states of proteins within microtubules. The proposal was put forward in the mid-1990s by British theoretical physicist Sir Roger Penrose and American anesthesiologist Stuart Hameroff. A few papers have been published criticising specific features of the theory, but largely it has been ignored by academic neuroscientists.

Microtubules are cylindrical lattices of tubulin proteins that serve as structural elements inside a cell and can act like "highways" for the movement of vesicles, granules, organelles like mitochondria, and chromosomes to different locations in the cell via special motor proteins; they are also important components of cilia and flagella in motile cells, and are important for mitosis in all cells. Structurally, microtubules are linear polymers of a globular protein, tubulin - these linear polymers are called protofilaments.

Penrose argued in his 1989 book The Emperor's New Mind that human consciousness and understanding required a factor outside algorithmic computation, and that the missing “non-computable” factor was related to a type of quantum computation involving what he called objective reduction (OR). Hameroff suggested to Penrose that microtubules within neurons might be involved in such quantum computation, and together they developed the theory of Orch OR.