Movements in space create in humans and animals so-called optical flow fields which are characteristic for the movement in question. In a forward movement, the objects flow by laterally, objects at the front increase in size and objects further away hardly change at all. At a higher level in the visual centre in the brain, there must be a computation of the visual information, so that animals can differentiate between their own movement and movement of their environment and are able to correct their course if necessary. It is important for the analysis of flow fields that the movement information from both eyes is merged so that the whole flow field can be assessed. In their current study, Karl Farrow, Jürgen Haag and Alexander Borst have for the first time proved the direct link between two nerve cells, one in each half of the brain, combining the movement signals from both the facetted eyes of a fly.
In the blow fly, the nerve cells that analyse optical flow fields, called tangential cells, are located in the lobula plate. There are only 60 of these tangential cells for each half of the brain and each of these 60 cells can be identified individually. The scientists in Martinsried have looked closely at one cell, the H2 cell. This cell exhibits a strong preference for rotational flow fields such as that which arises when the fly turns around its body's vertical axis. Interestingly, this cell seems initially to react only to the movements in front of its own eye (ipsilateral), but remain blind to movements in front of the other eye (contralateral). However, if the ipsilateral movement stimuli are combined with the contralateral, it is seen that the latter do indeed modulate the reactions to ipsilateral movement stimuli. "The preference of the H2 cell for rotational stimuli is due to a non-linear coordination of the movement stimuli from both eyes, and it was this non-linearity that we wanted to investigate further," said Alexander Borst.