About 30 percent of all adults don't detect an odor associated with the chemical androstenone, but can pick up its smell after repeated exposure. Joel D. Mainland and his colleagues at the University of California at Berkeley tested the noses of 92 subjects and found 24 who were unable to smell the substance. The scientists then exposed these volunteers, who had one nostril plugged during the experiment, to androstenone for 10 minutes a day over the course of three weeks. The team then tested the subjects' ability to detect androstenone again. They found that the volunteers could recognize the chemical twice as well, regardless of which nostril was used to smell. Because there are no links between the nostrils, the authors conclude that "learning occurs in the brain by a mechanism that shares information from both nostrils."