In a new study, a fruit fly walks on a small styrofoam ball shaped like a floating 3D "treadmill". The room is completely dark. However, the electrodes recording visual neurons in the fly's brain send out a mysterious flow of neural activity , which fluctuates like a sine wave.
When Eugenia Chiappe, a neuroscientist at the champalimaud foundation in Portugal, first saw these results, she had a hunch that her team had made a special discovery. They record from visual neurons, but the room is dark, so no visual signal can drive neurons in this way.
"This means that this unusual activity is either a camouflage, which is unlikely, or comes from non visual sources. After investigating and eliminating the possibility of interference, I am convinced that neurons are faithfully tracking the animal's steps," Chiappe recalled
A few years later, the researchers had many new insights, and Chiappe and her team now present their findings in the scientific journal Neuron: a two-way neural network connecting the legs and the visual system to shape walking.
"One of the most significant aspects we found is that the network supports walking on two different time scales at the same time. It operates on a fast time scale to monitor and correct each step while promoting animal behavior goals," Chiappe said
"Vision and action don't seem to be related, but they are actually closely linked; just choose a point on the wall and try to put your finger on it with your eyes closed. However, little is known about the neural basis of this connection," Chiappe said
In this study, the team focused on a special type of visual neuron known to be connected to the motor brain. Terufumi Fujiwara, the first author of the study, explained: "we want to determine the signals received by these neurons and understand whether and how they participate in movement."
To answer these questions, Fujiwara uses a powerful technique, whole cell patch recording, which enables him to enter the "emotion" of neurons, which can be positive or negative.
"Neurons communicate with each other by changing the current that receives the overall charge of neurons. When neurons have more positive charge, it is more likely to become active and then transmit signals to other neurons. On the other hand, if the charge is more negative, neurons are more likely to be inhibited," Fujiwara explained.
Observe each step
The team tracked the charge of neurons and found that it was synchronized with the animal's steps, which was the best way to fine tune each action. "When the foot is in the air, the neurons are more active and ready to send adjustment instructions to the motor area when needed. On the other hand, when the foot is on the ground, it makes adjustment impossible, which generates more negative charges and effectively inhibits the neurons," Chiappe said.
Maintain route
When the team further analyzed their results, they noticed that the charge of neurons also changed over a longer time scale. Specifically, when fruit flies walk fast, they produce more and more positive charges.
"We believe that this change will help maintain animal behavioral goals," Fujiwara said. "The longer a fly walks fast, the more likely it needs help to maintain this action plan. Therefore, neurons become more 'alert' and ready to be 'recruited' for motor control."
Then the researchers carried out many experiments, gave a more comprehensive description of the network, and proved that it was directly involved in walking. But according to Chiappe, the study goes even further than revealing a new visual motor circuit. It also provides a new perspective on the neural mechanism of movement.
"The current view of how behavior occurs is very 'top-down': the brain directs the body. But our results provide a clear example of how signals from the body contribute to motor control. Although our results are derived from the Drosophila animal model, we speculate that similar mechanisms may exist in other organisms." "Speed related representations are crucial in exploration, navigation and spatial perception, and these functions are common to many animals, including humans," she concluded