Efficient coding is a well-known principle in the neurobiology of sight and hearing, according to which the sensory neurons adapt to the statistical characteristics of their natural stimuli.  In other words, neurons react better to the stimuli that most frequently excite them.  To test the validity of this principle in olfaction, scientists at INRA in Versailles and the Academy of Sciences of the Czech Republic in Prague have studied communication via sex pheromones between moths, the males of which locate females thanks to the pheromones released by the latter.  They have determined the pheromone plume characteristics that are best detected by the male receptor system.  They have thus shown for the first time that the olfactory system in insects also obeys a principle of efficient coding.

Study of behaviour in a flight tunnel.  The male is in the approach phase to the nozzle releasing female sex pheromone © INRA/J. Barthes

The flight of male moths towards immobile females, upon which mating is dependent, is guided by the pheromone released by the latter.   Atmospheric turbulence creates a broad spectrum of spatial and temporal variations in the signal emitted by females.  The strongest eddies reach several hundred metres and can take several minutes to pass a fixed point, while the smallest variations concern less than a millimetre and only last a few milliseconds.  Because of this heterogeneity, pheromone molecules are not dispersed uniformly and remain grouped in clumps and filaments separated by pure air, so that a high concentration of pheromone can be found at a considerable distance from its source, and the frequency of these dense clumps only diminishes with distance.  The intermittent nature of the resulting stimulation is essential for the insect to locate the female which is the source of the stimulus.  Experiments in a flight tunnel have indeed shown that moths do not fly towards the source in a uniform cloud of pheromone.   Thus characteristics such as the frequency and intensity of stimulation play a key role in maintaining their flight in the correct direction.

The researchers studied this communication in the male Polyphemus moth, Antheraea polyphemus, a wild American species.  They chose this moth because it is the only animal in which the natural stimulus and receptor process are known quantitatively.  Pheromone components are detected by specific olfactory receptor neurons in the male antenna.  The scientists focused in particular on the type of receptor neurons detecting the majority component in the sex pheromone.  Molecules of this compound are absorbed by the cuticle, diffuse via the olfactory sensilla to the neuron membrane and then undergo enzymatic degradation.  The initial cellular response is triggered by the binding of pheromone molecules to receptors on the neuronal membrane.  A cascade of events follows, amplifying this initial response and ultimately triggering a series of action potentials (nerve influxes) towards the brain.  At each moment, the pheromone concentration determines the response of olfactory neurons.  The scientists thus based their study on a model describing how each stimulus (concentration of pheromone in the air) was transformed into a response (concentration of activated receptors).

References:
“Efficient olfactory coding in the pheromone receptor neuron of a moth”
Lubomir Kostal, Petr Lansky & Jean-Pierre Rospars
PLoS Computational Biology, vol. 4, fasc. 4 (April 2008)