June 22, 2011
The fact that we are able to see movement may sound trivial. After all, we perceive motion the very second we open our eyes. However, we are only beginning to grasp the complexity of the nerve cell circuits necessary to perceive motion. Decoding these circuits down to the cell level, that is the aim of Alexander Borst and his team at the Max Planck Institute of Neurobiology. Yet it is not the human brain, with its approximately one hundred billion nerve cells, on which they concentrate, but the brain of the minute fruit fly Drosophila.
Despite being less than half a millimetre in size, the brain of the fruit fly is not only highly efficient but also fairly straightforward – containing a "mere" hundred thousand nerve cells. Here, the scientists see a chance to succeed in breaking the nerve cell circuits down into their individual components. The findings are also relevant for humans since, when it comes to the brain, the difference between humans and fruit flies is not as great as one might expect. Just recently, the Martinsried neurobiologists have demonstrated that fruit flies process optical information in much the same way as all other vertebrates examined so far: information is broken down into different image channels immediately after the photoreceptors. While the photoreceptors react to every change in light, nerve cells in the next layer convey only "light-on" (ON) or "light-off" (OFF) alterations. But how does this enable the brain to calculate motion?
To get to the bottom of motion perception, the neurobiologists used apparent motion to outsmart the fly's visual perception. In a sort of "fly cinema", the animals watched how first one and then an adjacent stripe in their visual field became brighter or darker. Anyone who has ever had the opportunity of watching the "moving" luminous advertising on New York's Times Square knows that, when stationary lights are switched on and off in quick succession, you gain the impression that movement is involved. In the fly cinema, the drosophila has the same perception.
The scientists chose the width of the stripes for the fly cinema such that only a small number of photoreceptors were stimulated. The fly sees an apparent motion when first one and then an adjacent photoreceptor perceives an ON- or OFF- contrast change. OFF-OFF impulses, for example, would indicate that a dark edge is passing across their visual field. But what happens when neighbouring photoreceptors report an ON-OFF or OFF-ON change? Is motion then calculated by two motion detectors (one for ON-ON and one for OFF-OFF) or by four detectors (i.e. one for each combination)?