Fly Brains Contain A Circular Compass

It is already known that some insects, such as monarch butterflies, use their brain cells to reflect their orientation. Referred to as a ‘sun compass’, the method uses patterns of polarised light in the sky. However, this week in Nature, new research has revealed that mammal-like cells in the brains of flies are used to track their orientation in a compass-like fashion.

Unlike mammals, in which the direction sensitive cells are scattered throughout the brain, the fly’s direction sensitive cells appear to be clustered into a particular area. The area where these cells are found is a circular region known as the ellipsoid body.

Neuroscientists set up an experiment in which a fly walked on top of a freely rotating ball. The neurones of the fly had been manipulated so that they would glow when they were fired. The team focused a powerful laser microscope onto the fly brain in order to observe its activity.  The fly was surrounded by screens displaying simple patterns of lines whose movement was controlled by the ball. One of the members of the research team, Dr Seelig, was first to observe that as the fly turned, there was a sweeping pattern of activity as the ‘needle’ of the compass moved corresponding to its movement.

A crucial finding was that the activity of the direction sensitive cells took place not only in the virtual-reality situation, which was previously described, but also in the dark. “The fly is using a sense of its own motion to pick up which direction it’s pointed,” explained senior author, Dr Vivek Jayaraman.

The compass-like structure of the fly brain is highly complex and may inform scientists about how brains across the animal kingdom, including our own, accomplish similar directional tasks. Professor Jeffery, from University College London, emphasised the complexity of the structure in comparing the activity to cognition. “These neurons are not purely visual. They’re integrating information and they’re abstracting something at a higher level.”