Flies are a pain in the neck. Let's be honest, when the room is invaded by a big, buzzing intruder, the majority of us reach for the rolled-up newspaper. But few will realise that one of these aerial invaders has been at the centre of neuroscience research for many years. Because flies have a less complex nervous system than their mammalian counterparts, the common fruit fly (Drosophila melanogaster) has served as a useful model for studying the neural networks involved in behaviours that are observed in both humans and fruit flies (such as sensory perception and learning and memory.)
In a recently published study, researchers from the Howard Hughes Medical Institute have used innovative techniques to demonstrate that the common fruit fly uses its vision to learn how to navigate (known as 'visual place learning'). As humans use the same process whilst navigating, understanding how organisms with less complex nervous systems achieve this form of learning may shed some light onto how humans do it also.
Researchers placed fruit flies in a small arena with walls that were under temperature control. All but one panel of the arena were kept at a temperature that was higher than would be comfortable for a fly to rest on. This meant that flies gradually made there way to the 'cool' panel after entering the arena. Surrounding the arena walls were visual cues such as crossed lines and parallel lines that the fly could use in order to orientate itself within its environment. After an initial phase, where flies entered the arena and made their way to the platform for the first time, flies were then taken out and then put back in and the time taken to find the 'cool' platform was measured.
As the test phases progressed, flies got quicker at finding the 'cool' platform, which demonstrated that they had learnt where the platform was. Interestingly, when the platform and the visual cues were moved such that they were in the same relative position, flies still made their way to the 'cool' platform quicker than when they were first introduced into the arena. However, if the visual cues were not moved (and the platform was) or the flies were put into the arena in the dark, they failed to show any improvement in finding the 'cool' platform, suggesting that the flies were using their vision to learn the position of the platform.
Now, mammals are well known to be able to locate themselves in their surroundings using visual cues. In rodents, an area of the brain known as the hippocampus contains separate regions that encode different aspects of the spatial information. This is a fairly complex structure consisting of several integrated layers. What is fascinating about the current finding is that fruit flies have the ability to locate themselves in exactly the same manner, but do it with a much simpler nervous system. In fact, by actually repressing the ability of certain cells to work, the researchers were able to demonstrate that a small subregion of a part of the fly brain known as the central complex ellipsoid was all that was necessary for visual space learning.
This study has shown that the common fruit fly is a valuable tool in understanding visual learning and that further research using these winged beasts may reveal the systems that underly this process across a variety of different lifeforms. Perhaps the mammalian system is just a more complex version of the fruit fly system or maybe these two creatures have developed distinct solutions to a shared problem. It is likely that there is some truth in both of these propositions, but one thing is for certain, you should think twice before you next grab that newspaper, that fly knows how to find you!
See: Visual place learning in Drosophila melanogaster (Ofstad et al., 2011. Nature)
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