Researchers at MIT have shown that habits, long believed to be automatic, are actively controlled in rats by the part of the brain called the infralimbic (IL) cortex. In the study, published in the journal PNAS, the group trained rats to follow a T-maze automatically to receive a chocolate milk or sugar water reward, creating a well-ingrained habit. Then, using animals with a molecular channel called halorhodopsin expressed by cells in the IL cortex, the researchers directed light to the area with a fiber optic cable. When the light shone on the halorhodopsin-expressing neurons, it prevented them from firing, which abolished the habitual behavior. The results of the research provide new insight into how quickly habits are formed and replaced and into the parts of the brain that mediate habitual behavior.