Fruit flies show PTSD-like over-generalisation of fear


When fruit flies associate a specific smell with an electric shock, they gradually learn aversion to even vaguely related smells. Andrew Masterson reports.


Laboratory fruit flies are no strangers to traumatic conditions.
Pascal Goetgheluck / Science Photo Library

Fruit flies are prone to over-generalisation, according to research from the Leibniz Institute for Neurobiology in Germany.

The surprising finding – published in the Journal of Experimental Biology – suggests that the flies (Drosophila melanogaster) can be induced to fear more than they actually need to.

And while that perhaps sounds indefensibly anthropomorphic, the discovery might actually turn out to be directly relevant to a very human problem: post traumatic stress disorder (PTSD).

“Overly generalised memories of a traumatic experience in humans are one of the key behavioural hallmarks of PTSD,” explains lead researcher Ayse Yarali.

Generalising is a very valuable trait in humans. Having learned that a chocolate cake tastes nice, for instance, we can conceive the set of all chocolate cakes. That means we don’t have to cautiously test the edibility of every variant we encounter – Black Forest or fudge cakes, for instance – before snarfing it down.

Over-generalisation, in contrast, can be dangerous. Feeling sick after drinking milk might lead to the generalised assumption of lactose intolerance and the avoidance of all dairy products. However, over-generalising might lead to a fear of all food, with potentially catastrophic results.

To test whether Drosophila over-generalised, Yarali and her colleagues first tested their ability to learn.

Flies were confined to electrified Teflon tubes. As three specific odours attractive to the flies were wafted in, a current was passed through the tube walls, giving the flies repeated shocks.

Twenty minutes later the same flies were placed in a T-shaped tube, in which one arm contained the same odours. Most of the flies stayed away from the scented region. The same thing also occurred after 24 hours.

Yarali’s experiment, however, contained a second phase. At the same intervals – 20 minutes and 24 hours – the flies were placed in another T-shaped tube, where this time the scented branch contained odours that were broadly similar, but not identical, to the ones present during the shock treatment.

At the 20-minute mark, only some of the flies avoided the new scents. At 24 hours, however, almost all of them did – the need to stay away from them as strong as the need to avoid the original ones.

The researchers drew two cautious conclusions from the results. The first was that the flies showed clear evidence of generalising when they avoided the scents associated with electric shock, but equally clear evidence of over-generalising when they also avoided scents that were only vaguely similar.

The second conclusion arose from the fact that the aversion to the second set of scents was much stronger 24 hours after the initial exposure. This invites a cautious parallel with PTSD, which often develops some time after the trauma with which it is linked.

Yarali and her colleagues hope that their work might for the basis for developing a useful model for PTSD research that might one day yield new therapeutic targets.

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  1. http://dx.doi.org/10.1242/jeb.155317
  2. http://dx.doi.org/10.1242/jeb.155317
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