How the nervous system incorporates sensory information and generates motion patterns is generally hard to analyze within a living organism because it is difficult to separate the highly interconnected central and peripheral components in the spinal cord. The underlying idea in this study is to substitute the dynamics of the nervous system by a set of mathematical equations while the animal body is replaced by a robotic counterpart. The advantage of this approach is that we can specifically monitor the different central and peripheral inputs and outputs in the nervous system, and selectively activate and deactivate them. Subsequently, we derive a more clear understanding of how locomotion is generated, controlled and regulated.

In this study, we designed AgnathaX, an elongate undulatory swimming robot which emulates the lamprey. The robot incorporates the main characteristics of this animal that are relevant for locomotion control.

A robust interplay based on redundancy

Although it is possible to swim without any sensing, exclusively based on control from CPGs, we found that a system including the peripheral components and an entrainment mechanism was significantly more robust to neural disruptions and was able to sustain forward swimming much better. Tested disruptions were failures in the communication between spinal cord segments, muted CPGs and muted sensors. In these cases when central and peripheral components were working together, they helped to conserve stable swimming patterns and higher swimming speeds. Locomotion can be seen as an emergent and self-organized phenomenon due to these interactions, and hence the title of our article (Emergence of Robust Self-Organized Undulatory Swimming).

Implications both for neuroscience and robotics

Our results show that both the central and the peripheral nervous systems have mechanisms to generate swimming, and that together they produce more robust swimming than any of these alone. In particular, our study confirms that there are important functions provided by peripheral mechanisms that might be “hidden” by well-known central mechanisms. These peripheral mechanisms could play an important role in the recovery of motor function after spinal cord injury as no-a-priori connections within the spinal cord are necessary to maintain a traveling wave along the body.

Although AgnathaX was designed with the goal to study neural control of swimming locomotion, this study also provides novel insights for robotics:

• Using the entrainment mechanism no explicit communication between modules is necessary because swimming emerges in a self-organized manner. As a result such a system exhibits scalable characteristics that supports easy construction and deployment of many modular swimming units with a high degree of reconfiguration and robustness, e.g. for search and rescue missions or environmental monitoring
• The custom designed sensing units provide a new way of accurate force sensing in water along the entirety of the body and could therefore potentially be used to navigate through flow perturbations and enable advanced maneuvers in unsteady flows
• Our work provides design principles for making robots that are remarkably fault-tolerant and robust against damage in their sensors, communication buses, and control circuits.

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