In comparison to many other living creatures, flies tend to be small and their brains, despite their complexity, are quite manageable.
Scientists at the Max Planck Institute of Neurobiology in Martinsried have now ascertained that these insects can make up for their low number of nerve cells by means of sophisticated network interactions. The neurobiologists examined nerve cells that receive motion information in their input region from only a narrow area of the fly's field of vision. Yet, thanks to their linking with neighbouring cells, the cells respond in their output regions to movements from a much wider field of vision. This results in a robust processing of information. Nature Neuroscience , February 8, 2009.
The complexity of the human brain is remarkable: It contains billions of nerve cells, each of which is connected with its neighbours via many thousands of contacts. The result is a multifaceted network which stores and processes many types of information. In comparison, the brain of a fly seems fairly simple with its 250 000 nerve cells. For example, a small network of only 60 nerve cells in each cerebral hemisphere suffices the blowfly to integrate visual motion information. The resulting information is then used in the control and correction of the fly's flight manoeuvres. However, flies clearly demonstrate just how efficient these 60 cells actually are when they dodge obstacles while flying at high speed and land upside-down on the ceiling. No wonder neurobiologists find the brain of the fly so fascinating!
Rationing resources
Thanks to the comparatively small number of nerve cells in the fly's visual flight control centre, the connections and functions of the cells involved can be examined in greater detail. It soon became apparent that the 60 nerve cells are further sub-divided into several individual cell groups, each of which is responsible for the processing of certain patterns of movement. A group of ten cells, known as the VS-cells, respond to rotational movements of the fly, for example. Each of these ten cells receives its visual information from only a narrow vertical strip of the fly's eye - the cell's "receptive field". Since the VS-cells are arranged parallel to each other, the fly's field of vision is completely covered by the vertical strips of the ten cells on each side of the fly's brain (the figure shows three of the ten VS-cells).
Complexity by means of connectivity
"However, the most fascinating aspect of these VS-cells is that the closer we examined the network, the more complex it appeared", group leader Alexander Borst reports. He and his group at the Max Planck Institute of Neurobiology are devoted to investigating the motion vision of flies. Only recently, Borst's co-worker Jürgen Haag showed that VS-cells are connected on two different levels. It was well known that in their input regions, the cells collect incoming signals from nerve cells which represent local motion information coming from the eye. Yet, it came as a surprise that the cells had a second source of information. The scientists found electrical connections between neighbouring VS-cells in the cells' output regions. Computer simulations of this network led to the following assumption: Information received from a VS-cell's "own" receptive field is first compared with the information received by its neighbouring cells. Only then is the information relayed to cells further downstream in the network for the purpose of flight control.
Getting to the bottom of it