If you're a regular reader of Moment of Science, you may be familiar with the "amygdala." This little bundle of neurons in the human brain is a hot topic as scientists try to figure out exactly what it does.
Past studies have suggested the amygdala is important in emotional responses. Volunteers who are shown frightening pictures, for example, have a sudden increase of blood flow to their amygdalas.
Another study has complicated this picture--by adding smells. Volunteers in this study, which was conducted by Adam Anderson at Stanford University, first climbed into an MRI. That's a machine that lets us see the brain in action by lighting up the parts that are most active.
Then the subjects sniffed. First they sniffed fruity odors. Then they sniffed nasty, rotten odors. The results? Low intensities of either nasty or nice odors didn't cause any response. But once the intensity of the odors passed a certain threshold--bang, the amygdala lit up, showing that this information was being processed there. One possible explanation is that the amygdala isn't just a processor for visual information, but for anything that requires an emotional tag--and that tag, apparently, can be either positive or negative.
Think of it this way. Not only does that old garbage can stink, I don't like to be near it. The first is an objective assessment of the world, while the second is a feeling. Why have a feeling about smells? Probably because it helps you behave correctly. Not only are you drawn to yummy ripe fruit, but you don't try to get a midnight snack out of the trash.