Sequence matters - selective adaptation in electroantennographic response to binary odour mixtures by the Colorado potato beetle

Chemically mediated behaviour of insects is often strongly affected by mixtures of odour stimuli and their temporal characteristics. Both sensory transduction and central processing of odour mixtures can give rise to several different kinds of interaction, which can influence how individual components are perceived and processed. In particular, odour mixtures have been examined in model experiments as premixed binary mixtures in comparison with pure odour stimuli. Only in few experiments, the influence of the temporal structure of odour mixtures on odour perception has been taken into account. Natural odour stimuli often have a pulsed structure and may in general be superimposed on a background of irrelevant or interfering compounds, which can fluctuate with different frequencies, depending on their source. To achieve a better representation of these natural conditions, our odour mixing experiments apply a new kind of stimulation protocol: odours were not premixed but superimposed with a specific time pattern; one odour stimulus was presented as a longer persisting background and the second stimulus was a superimposed short test signal. To gain an overview of odour interaction patterns in the Colorado potato beetle by causing adaptation of one receptor population at naturally occurring levels of concentration and time intervals, electroantennographic recordings were made on excised antennae. A matrix of 12 stimulus compounds led to 132 pairs of compounds tested, each in the role of background and test stimulus. In 64 cases, the interaction was significantly different, when the role of background and stimulus was exchanged. Interaction patterns ranging from no interference (independence) to suppression were found and assigned to four clearly distinguishable types. We discuss that the observed effects of the presentation sequence in odour mixtures may contribute to the mechanisms of olfactory pattern recognition and olfactory contrast perception by insects.