Generating baseline locomotion - forward and backward
First movements that we generate with CelegansWholeIntegration are baseline forward and backward locomotion movements. These movements are similar in shape to translating sinusoidals. The video below was recorded in an experimental lab and demonstrates the body postures involved:
In this video, C. elegans initially moves forward and then when receives a mechanical stimulus (touch in the anterior part) switches direction and moves backward. While there is variation in postures these are pretty typical. Such a response is called the tap withdrawl.
To generate baseline forward and backward movements with CelegansWholeIntegration we design a force wave travelling along the body with variable frequency to infer neural dynamics associated with it. These neural dynamics are then forward integrated by the nervous system to generate body postures. We are able to generate the following locomotion patterns:
External Force Locomotion (forward(left) and backward(right)):
When we simulate the integration, we observe that generated movements can be very similar to locomotion characteristics of freely moving animals (see above videos forward(left) and backward(right)). By measuring the error between the imposed traveling wave and the actual body curvature we observe that the nervous system includes preference for particular periods of the force, with optimal frequency being approximately 2 and 4s. These results indicate that the response of the nervous system is shaping the external force in a nontrivial and nonlinear manner.
We next apply neural stimuli to examine how they generate forward and backward body movements. Notably, our goal is to explore movements generated by simple stimuli where most of the neurons do not receive any input and no fitting is performed on connectome parameters. We target the circuits associated with touch-tap response and find a subset of both sensory- and inter-neurons related to behavioral responses: posterior-touch triggered forward locomotion (sens: PLM, inter: AVB or PVC), anterior touch triggered backward locomotion (sens: ALM, inter: AVA, AVD, AVE). Simulations show that stimulation of sensory neurons alone could create rhythmic body curvatures but does not lead to forward and backward locomotion. Additional specific stimulation of interneurons is required to generate directed locomotion. This observation is consistent with experimental studies and control theory analysis.
Experiments indicate that proprioception within the motor neurons circuit, can facilitate locomotion and is an alternative to stimulation of command interneurons. To emulate proprioceptive feedback, we close the loop between neural stimulation and external body forces by inverse integrating the force that acts on the body to neural stimulation after a time delay. We test feedback effects by initiating locomotion with external stimulation, either neural current injection or spatial wave force. Once the feedback starts to entrain the movement we gradually turn external stimulation off. We find that in both initiation procedures feedback entrains the body into sustainable coherent movements in forward and backward directions such that the body moves solely due to feedback.