Elongated fish like eels and lampreys are remarkable movers. Eels demonstrate exceptional locomotor ability, not only when swimming but also crawling on uneven ground. They can even swim after the part of the spinal cord responsible for controlling movement is damaged, which would lead to paralysis in most vertebrates. But the neural mechanism behind these incredible abilities has long been a mystery.
Previous studies have suggested that sensations of skin pressure and muscle stretching modulate the activity of neural networks called central pattern generators (CPG), which are distributed along the spinal cord and are believed to control the adaptive movement of these undulating species. But these theories have not been fully explored due to the technical difficulty of studying multiple sensory feedback in living animals.
Now, a team including researchers from EPFL's School of Engineering, Tohoku University (Japan), and the University of Ottawa (Canada) have published a mathematical model of a neural circuit in the Proceedings of the National Academy of Sciences that integrates both stretch and pressure sensation for motion control in eels and their relatives. Specifically, the researchers assumed for their model that each body segment has a CPG-like neural circuit that generates rhythmic movement patterns that are regulated autonomously by stretch and pressure sensory feedback.
Then, the researchers used their model to run computer simulations and experiments using amphibious eel-like robots developed in EPFL's BioRobotics Laboratory . In aquatic experiments, the scientists confirmed that their model quickly produced stable swimming patterns, and that the stretch feedback especially was central to this rapid stabilization. Remarkably, the same neural circuit involved in swimming also enabled the robot to crawl on land and navigate around obstacles, with the stretch feedback again being vital for pushing against obstacles to generate forward thrust.
"The finding that a neural circuit for swimming can also enable terrestrial movement suggests that the evolutionary transition of vertebrates from water to land may not have required the development of entirely new neural circuits," says BioRobotics Lab head Auke Ijspeert. "Instead, existing aquatic circuits could have been repurposed – a principle that contributes to our understanding of the evolutionary origins of motor control."
Building more resilient robots
The team also used their robots and simulations to investigate possible mechanisms that enable real eels to swim even after the spinal cord is severed – an injury that would leave most vertebrates paralyzed. Their findings suggest that if the neural circuits distributed throughout the body have a certain degree of spontaneous rhythm generation ability, then this could combine with stretch and pressure feedback to produce coordinated swimming movements in the simulated and robotic fish, before and after the spinal cord severance site.
In addition to providing new information on the evolution and mechanics of animal motor control, the researchers say their findings could be used to develop robots that are resilient to physical damage, and that can move robustly in unpredictable environments like disaster sites. In particular, control methods that integrate multisensory feedback could help researchers develop robots that can move just as easily underwater as on uneven terrain.
Ijspeert adds that the insights into motor function following spinal cord injury are not only biologically intriguing but may also offer principles for designing decentralized motor control systems that do not rely on brain-based control: "If we can understand how biology controls complex movement using senses in the body (without a brain), we may be able to use this information to better control autonomous machines."
