Fishilevich and her team have previously tried to map the fruitfly olfactory system by genetically deactivating all of the olfactory receptors in fruitflies and then reactivating individual receptors one-by-one in an attempt to study each receptor's influence on behaviour. And other studies have used light to activate neurons in fruitflies, but they have not fed the signal through an existing sensory network, which the latest study by Störtkuhl and his team does, says Fishilevich.
"Our aim was to keep the network intact and put some tools into the network that allows you to stimulate individual neurons," says Störtkuhl. His group hopes that the technique will help them to explore the relationships between the 21 olfactory nerves in Drosophila larvae, and eventually in the more complex adults.
Using light to activate odour receptors allows the researchers to sidestep questions such as which odour to use and how to deliver it, says Fishilevich. However, a problem with any study that attributes a natural behaviour to an artificially stimulated nerve is that the brain deals not only with the 'on' signals coming from stimulated nerves, but also with a pattern of background signals from the remaining nerves and inhibited nerves, says Matthew Cobb of the University of Manchester, UK. In fact, his group has found that Drosophila olfactory receptors and nerves do not always work in partnership. Sometimes an odour receptor activates its accompanying nerve, and sometimes it doesn't, a response he refers to as "fuzzy coding".
Cobb says that although Störtkuhl's technique "will enable us to avoid some of the difficulties of direct stimulation with odours, on the other hand it will miss out the richness of peripheral coding such as inhibition, temporal aspects and fuzzy coding".
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