Sensory feedback is key in diversity of function, and robustness to external noise

A new paper by Zhuojun Yu and Dr. Peter J. Thomas investigates the  importance of sensory feedback in central pattern generation (CPG). These CPGs are critical in any repetitive motor command, such as chewing, walking, running or swimming. A CPG is when neurons periodically fire in a repetitive pattern. For example when any animal walks one group of neurons will fire when the right leg takes a step, and different group of neurons will fire when the left leg takes a step. CPGs neurons will take turns firing in a very precise predictable order and will periodically repeat.

A) The model the authors used, a simple CPG with two neurons that alternatively activate. It is coupled to a muscular system with sensory feedback. B) A supercritical pitchfork bifurcation as the strength of the sensory feedback increases. Before the bifurcation, there is one cycle that has equal amount of time where each neuron is active. However, after the bifurcation there are two solutions where one neuron is active longer than the other. C) An  schematic representation that shows how  robustness to external or internal noise changes as a function of CPG and feedback strength.

As you can imagine, when walking you cannot just take repeated steps, sometimes something like a rock will get in the way, and you will need to adjust your gate. Perhaps you will need to walk to the right, taking a larger step with your right foot as opposed to the left. Perhaps the ground is slippery from the snow. Regardless of the reasoning, sensory feedback helps keep the CPG firing in a way that is useful to navigate the environment. That is, the feedback helps maintain robust activity in the presence of external noise. 

 In the paper, the authors build a simple (but dynamically rich) model of a CPG coupled to a muscular system with sensory feedback. They find two things: Increases in feedback strength increase robustness to external noise, but decrease the robustness to internal noise, and second the CPG with sensory feedback has more functions (behaviors) than the CPG without sensory feedback. 

The first observation makes intuitive sense, that more feedback will allow the CPG to better respond to the environment. However, whats a bit more counter-intuitive, is that the system's own internal noise gets amplified. Intuitively, you can think about this any internal jitter in the CPG will cause a misalignment in the feedback system, and it will erroneously amplify this noise. Thus, it is imperative that organisms balance their sensory feedback with the internal strength of the CPG, as to be able to optimally handle both internal and external noise.

The second take away from the paper is that sensory feedback increases what patterns the CPG is able to perform. With strong feedback, there exists asymmetric patterns. This is due to a underlying supercritical pitchfork bifurcation that we have discussed previously. You can conceptualize this as a stable cycle splitting into two asymmetrical cycles that are mirror copies of each other. Behaviorally this can be thought of walking rightward or leftward. The walking gate is inherently asymmetric, nut there exists two versions of the behavior.

This study shows the importance of considering the nervous system and its environment in its entirety. Sensory feedback is critical in making sure that the system is robust to external noise, and gives rise to a larger repertoire of behaviors.

Author: Alexander White