Neural coding of general odors in insects

Insects attempting to identify and classify general, food-related odors in a natural environment are faced with the difficult problem of turning a spa tially and temporally noisy, multicomponent olfactory stimulus into a relia ble neural code that is independent of spurious changes in concentration or the temporal structure of the odor plume. The insect nervous system accomp lishes this by representing olfactory information in different forms in dif ferent parts of the olfactory sensory pathway. At the periphery, olfactory information is encoded as a simple rate code expressed in a large populatio n of broadly tuned receptor neurons. Some information processing may take p lace as a result of interactions between olfactory receptor neurons located in the same sensillum of the insect antenna. The olfactory information is conducted to intrinsic interneurons within the antennal lobe of the brain. Olfactory information is subsequently transmitted from intrinsic interneuro ns to antennal lobe projection neurons that carry a condensed form of olfac tory information to the mushroom bodies and other higher brain centers. Pro jection neurons appear to encode olfactory information in a complex tempora l code comprising spike trains with variable interspike intervals. The prec ise timing of these action potentials may provide higher brain centers with information about the identity, concentration, and duration of the stimulu s. The olfactory glomeruli appear to translate the simple rate code input i nto a complex temporal code output that may involve transient synchronizati on of the action potentials and oscillations of field potentials in the bra in.