neuroWalknet, a controller for hexapod walking allowing for context dependent behavior
Authors: Schilling Malte , Cruse Holk 
Published: April 2022

Decentralized control has been established as a key control principle in insect walking and has been successfully leveraged to account for a wide range of walking behaviors in the proposed neuroWalknet architecture. This controller allows for walking patterns with different velocities in forward and backward direction — quite similar to the behavior shown in stick insects —, for negotiation of curves, and for robustly dealing with various disturbances.

While these simulations focus on the cooperation of different, decentrally controlled legs, here we consider a set of biological experiments not yet been tested by neuroWalknet, that focus on the function of the individual leg and are context dependent. These intraleg studies deal with four groups of interjoint reflexes. The reflexes are elicited by either a stimulation of the femoral chordotonal organ (fCO) or a specific stimulation of campaniform sensilla (CS). Motor output signals are recorded from the alpha-joint, the beta-joint or the gamma-joint of the leg. Furthermore, such reflexes have been studied while the ganglion was treated with pilocarpine.

Although these biological data represent results obtained from different local reflexes in different contexts, they fit with and are embedded into the behavior shown by the global structure of neuroWalknet. In particular, a specific and intensively studied behavior, active reaction, has since long been assumed to represent a separate behavioral element, from which it is not clear why it occurs in some situations, but not in others. This question could now be explained as an emergent property of the holistic structure of neuroWalknet. When experimenting with pilocarpine, oscillations were induced in neuroWalknet even though this does not include an explicit central pattern generator and in this way provides a simpler model as a functional explanation. As the simulation data result from a holistic system, further results were obtained that could be used as predictions to be tested in further biological experiments.

AUTHOR SUMMARY

Behavior of animals can be studied by detailed observation, but observation alone does not explain the function of the underlying neuronal controller structures. To better understand this function, an important tool can be to develop an artificial structure based on simulated neurons and a simulated or physical body. Although typical animal behavior appears complex, the corresponding neuronal structures may be comparatively simple.

The goal for such a hypothetical structure should be to include as many different behaviors as possible, and, at the same time, search for a simple explanation consisting of a minimum of neuronal elements. Furthermore, such a simulation system, e.g. an artificial neuronal network, should contain hypotheses that can be tested in biological experiments.

We propose an extension to such a network that is based on a decentralized neuronal structure, using a neural network as a scaffold, that enables various combinations of local neuronal elements that allow for emergent, i.e. not explicitly designed properties. Indeed, neuroWalknet contains further abilities not yet recognized in the earlier version. For instance, neither explicit structures like central pattern generators nor explicit Active Reaction are required to reproduce typical intraleg reactions. Therefore, neuroWalknet presents a holistic approach enabling emergent properties out of the cooperation of small neuronal elements that are context dependent instead of explicit, dedicated elements.