Like wartime operatives sending messages encrypted with secret plans, the brain has a language all its own to represent scents, from the aroma of buttered popcorn to the stench of rotting pumpkin. Now scientists think they have cracked the code: Insect nerve cells appear to fire in a sequence unique to each smell, says a report in the 14 November issue of Nature.
To try to interpret how the nose knows individual scents, neurobiologist Gilles Laurent and graduate student Michael Wehr of the California Institute of Technology turned to a creature lacking the facial feature: the locust. The locust detects smells with its antennae, and Laurent's previous work had shown that odors excite a group of about 100 neurons located in the antennal bulb to begin firing a rhythmic series of pulses--an oscillation. "All the neurons in that area are like musicians in an orchestra," he says, keeping to the same overall tempo. Now the duo's latest research shows how that oscillation could encode information about a particular smell. Each neuron maintains the tempo set by the oscillation but, like the individual musicians in an orchestra, plays a melody that is different for each smell.
The finding provides new insight into how neurons deliver information, says neurobiologist Terry Sejnowski of the Salk Institute in San Diego. "The oscillation, rather than being the code itself, is the packaging of the code," he says. Biologists have observed precisely timed neural impulses in sensory systems of other animals, but they have not been able to decipher the meaning behind the patterns. The next step, Laurent says, is to try to connect unique nerve firing patterns to behavioral responses.