(outlink) Old habits don't die. They hibernate.
Habitual activity--smoking, eating fatty foods, gambling--changes neural activity patterns in a specific region of the brain when habits are formed. These neural patterns created by habit can be changed or altered. But when a stimulus from the old days returns, the dormant pattern can reassert itself, according to a new study from the Massachusetts Institute of Technology, putting an individual in a neural state akin to being on autopilot.
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The original article is from nature.com, and deals with habit learning in rats (in the basal ganglia). This also mesh well with previous Alzheimer research, BTW.