Understanding the neural basis of memory and learning is one of the Holy Grails of neurobiology, but untangling convoluted memory networks in human brains is currently impossible. So neurobiologists have turned their attention to animals that learn and form memories in much simpler brains. Bertram Gerber and colleagues from Germany explain that Drosophila larvae can learn to associate odours with punishment – unpleasant tastes and mild electric shocks – but no one had tested whether they could associate odours with disturbing vibrations. Given that vibration can be more finely controlled than other aversive stimulants, the team decided to find out whether the larvae could be trained to associate disagreeable vibrations with specific odours ().
Training 50 larvae in a Petri dish by exposing them to 200 ms bursts – a buzz – of 100 Hz vibration from a loudspeaker in the presence of 1-octanol odour and then exposing them to n-amyl acetate with the loudspeaker turned off, the team then tested which odour the larvae avoided in the presence of the vibration threat: the larvae wriggled away from the 1-octanol odour. And when the team repeated the experiments, this time associating the vibration with n-amyl acetate odour and no vibration with 1-octanol, the vibrated larvae avoided n-amyl acetate when presented with a choice.
After characterising the larvae's behaviour as an escape response, the team also found that the strength of the association between the vibration and odour increased when they increased the number of buzzes and the length of the training period. However, the team point out that the larvae only take evasive action when under threat from the vibration, and they say, ‘Aversive memories are behaviourly expressed in the presence of punishment but not in its absence, and... are embedded into a conditioned “escape routine” which is employed only when escape is warranted.’
