Why nerve cells react to billboards and white walls in the same way

Animals and humans are able to move well in a three-dimensional environment without losing balance or bumping into things. That is because of the organ of equilibrium – and of course the eyes. Every movement enables the eyes to scan the environment in a characteristic manner. Out of this, the nerve cells calculate the own movement.
Scientists of the Max-Planck-Institute of Neurobiology show on flies, how nerve cells are able to estimate the self-motion in front of different backgrounds. So far, established models for optically processing information failed in solving this problem.

Humans and flies have one thing in common: They rely on their eyes in the orientation. Although the visual impressions are constantly changing, eyes are very reliable. Therefore, it doesn't matter, whether a person passes by a white wall or a wall with colourful billboards. Although the visual information is very different, the brain perceives a forward movement at a specific speed. What may seem common is really a considerable achievement of our brain.

In order to understand how nerve cells process optical information, the neurobiologists examined the brain of flies. Flies are masters of processing optical movements and their brains are relatively small and understandable. Hence, the function of each nerve cell can be examined. In laboratory experiments, the scientist showed moving striped pattern to the flies and measured the reactions of certain nerve cells. That way, models were drawn,that describe in a precise way to which stimulation the nerve cells react and what information is transmitted to subsequent cells.
However, these models fail when the complexity of the pattern varies greatly.

That is due to the fact, the these models consider only the input/output relations while signals within the cells did not play a role so far. The importance of the processes in the cell is now shown in a newly developed model that does not only show the input/output function but also takes into account the biophysical properties of the cell. 

The doctoral candidate Franz Weber showed moving dot pattern with different density to the flies. Through this, the researchers found out, that nerve cells react in the same way, no matter whether the density is high or low. That is remarkable since a low density gives much less visual movement information than a high density does. Apparently, the cells are able to adjust the differences of the stimulations.
This compensation was then included in the calculations. The new model now describes the behaviour of the nerve cells in a very reliable way, no matter how complex the world around flies – or humans – may be.

Publication:
Franz Weber, Christian Machens, Alexander Borst Spatio-temporal response properties of optic-flow processing neurons Neuron, online Veröffentlichung vom 25. August 2010

Contact:
Dr. Stefanie Merker
Max-Planck-Institute of Neurobiology
merker@neuro.mpg.de

Source: Max-Planck-Institute of Neurobiology