A fruit fly tethered to the rig that allows researchers to record cell signals while the insects view images of looming discs. Photo credit: HyoJong Jang.
If you're not moving and something appears to be hurtling towards you, there's a good chance you're about to be swatted if you're a fly, so best make a hasty exit. ‘Animals can take into consideration an object's approach direction to select an appropriate behaviour response’, says Catherine von Reyn from Drexel University, USA, explaining that many creatures have preprogrammed escape responses, which they enact when in peril. But how do animals coordinate the information coming from two eyes to effect an escape? Knowing that neurons in the optic glomeruli, a section of the fly brain that processes visual information, receive nerve signals from the eye on the same side of the brain, von Reyn and her colleagues decided to find out how one group of neurons in the optic glomeruli, known as descending neurons, process the incoming nerve signals when fruit flies are at risk from a looming object.
von Reyn and her team projected an expanding disc onto a screen in front of the insect, so that they could precisely control how rapidly the object appeared to be approaching and from which side, while recording the electrical signals in three optic glomeruli neurons (the giant fibre and two other descending neurons), all of which are thought to contribute to the fly's evasive action when threatened.
As expected, when the insects saw movies of the disc looming on either the left or right side of their view, the three optic glomeruli neurons that were connected to the eye on the same side as the approaching threat produced strong electrical signals. Then, the team focused on the responses of the giant fibre neurons, showing the flies a series of looming images – single discs approaching from the left, right or in front, in addition to pairs of looming discs zooming simultaneously towards the fly like the headlights of an approaching car. The giant fibre neurons responded strongly to the looming threats, regardless of the direction of approach. So it doesn't matter from which direction a threat is looming, nerves in the optic glomeruli will react and trigger an evasive manoeuvre.
However, the team made an unexpected discovery when showing the insects images looming on the left or right side of their view. All three of the optic glomeruli neurons on the opposite side of the insect's brain responded to the expanding discs, even though the discs were not visible to the eye on that side of the body. Were the optic glomeruli neurons on one side of the brain somehow communicating with visual neurons on the other side?
This time, the team blindfolded the insect's right eye by painting it, then showed it the same series of looming discs and recorded the electrical signals from the giant fibre neuron in the left side of the brain. If the left giant fibre was somehow receiving information from the right eye, painting the right eye would prevent the left giant fibre from producing an electrical signal when the looming disk was presented on the right side. And that was exactly what happened. In addition, the team discovered that losing signals from the blindfolded left eye weakened the right giant fibre's response.
So, the left and right giant fibres somehow receive information from both eyes, which they combine regardless of the direction from which a threat looms, allowing insects to make a swift exit. von Reyn is also keen to learn more about the key visual features of threats that trigger flies to take evasive action.
