How the Brain Can Distinguish Good From Bad Smells

Scientists identify the lateral horn in the brain of fruit flies as the processing center for behaviorally relevant odor information

The three main neuron types in the lateral horn of the fruit fly Drosophila melanogaster: Inhibitory projection neurons (green) respond to attractive odors; higher-order neurons of the lateral horn (orange) respond to repulsive odors; excitatory projection neurons (magenta) are likely to convey the identity of an odor. The first two neuron types were examined in the study. Antonia Strutz/MPI Chem. Ecol.

Whether an odor is pleasant or disgusting to an organism is not just a matter of taste. Often, an organism’s survival depends on its ability to make just such a discrimination, because odors can provide important information about food sources, oviposition sites or suitable mates. However, odor sources can also be signs of lethal hazards.

Scientists from the BMBF Research Group Olfactory Coding at the Max Planck Institute for Chemical Ecology in Jena, Germany, have now found that in fruit flies, the quality and intensity of odors can be mapped in the so-called lateral horn. They have created a spatial map of this part of the olfactory processing system in the fly brain and showed that the lateral horn can be segregated into three activity domains, each of which represents an odor category. The categories are good versus bad, as well as weak versus strong smells. These categorizations have a direct impact on the behavior of the flies, suggesting that the function of the lateral horn is similar to that of the amygdala in the brains of vertebrates. The amygdala plays a crucial role in the evaluation of sensory impressions and dangers and the lateral horn may also. 

 In order to survive, organisms must be able to sense information in their environment and to adapt their behavior accordingly. Animals use their senses, such as vision and olfaction, to perceive visual cues or odors in their surroundings and to process and evaluate the information that is sent via these senses to their brains. They must be able to tell good from bad odors. Good odors are important signals when animals search for food or a mating partner. Female insects also use olfactory signals to select a good oviposition place. Bad smells, on the other hand, can signal danger, for example, rotten and toxic food.

The quality and intensity of an odor are represented in three different activity domains in the lateral horn of the fly brain. Pleasant odors activate the green region, disgusting odors activate the red area. The blue and red regions represent the intensity of an odor. Silke Sachse/ MPI Chem. Ecol.