Almost two million euros to understand how the fruit fly brain computes and corrects trajectory errors

When we try walking in a straight line with our eyes closed, after a few steps forward we inevitably deviate from our intended path. But somehow our brain knows it – senses it –, and enables us to more or less correct that deviation error. To do it, we decide to inflect to our body a movement toward the opposite side of the deviation as we take our next step. 

For the past few years, the research done by Argentinian neuroscientist Eugenia Chiappe, principal investigator of the Champalimaud Foundation’s Sensorimotor Integration Lab, and her team, has allowed these researchers to obtain preliminary results, in the fruit fly, that suggest that the description above also fits what happens in these small insects’ brains as they try to walk a straight line. The question is how their nervous system does it. To further study the problem, they have now been awarded a nearly two million-euro Consolidator Grant, for five years, by the ERC (the European Research Council).

This research could have implications not only in terms of understanding the neural underpinnings of human motor behaviour disorders, but also to make robots that can better navigate unpredictable environments.

To study the fruit fly’s locomotion, the team developed a special experimental setup, which places the flies within a virtual reality system and lets them walk freely.

They thus discovered (at the time also with help from ERC funding, through a Starting Grant) that the fly’s movements are quite structured when the goal is to explore a novel environment under light or dark conditions. With this setup, “Tomás Cruz, at the time a PhD student in our lab, studied how the fly explores a novel environment”, explains Chiappe. “He observed that the fly goes forward in a straight line and, at a certain point, it turns, it changes direction.” They call this occurrence a “saccade”. In other words, the fly’s exploratory walk is, in these conditions, a sequence of alternating straight lines and saccades. 

There is more: “We discovered, in initial experiments, that there seems to be a relationship between the direction and amplitude of the saccade, and the magnitude and direction of the deviation from a straight line” that accumulated just before the saccade, she adds. This suggests, she further notes, that “the fly’s brain knows the fly’s body is deviating, and has the capacity to estimate the deviation error”. Now, with the help of the new funding, one of the team’s goals is understanding, at the neural level, where that capacity comes from.

But that’s still not all. They also want to understand how the fly then decides, based on its brain’s error estimate computation, to correct this error by deciding to turn in a certain way.

“We also discovered, in another initial study, that certain populations of neurons, which receive signals from the insect’s spinal cord, are critical for this decision-making process, because when they are silenced [through genetic techniques], the relationship between the deviation error and the magnitude and direction of the fly’s next saccade is lost”, says Chiappe. 

So the researchers have a second research goal: to understand – again at the neural level – how the fly, after integrating the information given by the brain about its error, makes the decision concerning its next turn in order to correct that error.

“We know very little about how organisms integrate information from proprioception (the sense of where different parts of the body are), from the visual world, and from other internal signals having to do with locomotion”, says Chiappe.

This research will not be easy, because the processes at play are complex and intricate. But the fruit fly is an ideal research model, says Chiappe, in particular its nervous system. With some 250,000 neurons (including brain and spinal cord), the fly’s nervous system is compact enough and at the same complex enough. And thanks to recent developments, it can now be precisely and globally mapped (along with its connections) to allow “dissecting” the way it works with today’s advanced techniques, such as optogenetics.

ERC Grants have existed since 2007 and fund cutting-edge research. Consolidator Grants are “designed to support excellent principal investigators at the career stage at which they may still be consolidating their own independent research team or programme”, states the European Commission on its webpage. 

Just three Consolidator Grants have been awarded in Portugal, in this ERC round, Chiappe’s being the only one in the Life Sciences.

By Ana Gerschenfeld, Health & Science Writer of the Champalimaud Foundation.