Internal Feedback in Biological Control: Architectures and Examples

Feedback is ubiquitous in both biological and engineered control systems. In biology, in addition to typical feedback between plant and controller, we observe complex feedback pathways within control systems, which we call internal feedback pathways (IFPs). These IFPs are most familiar in neural systems, our primary case study, but they appear everywhere from bacterial signal transduction to the human immune system. In this paper, we describe these very different examples and introduce the concepts necessary to explain their complex IFPs – particularly the severe speed-accuracy tradeoffs that constrain hardware in biology. We also sketch some minimal theory for extremely simplified toy models that highlight the importance of diversity-enabled sweet spots (DESS) in mitigating the impact of hardware tradeoffs. Standard modern and robust control theory can offer some insights into previously cryptic IFPs in more realistic models, and the new System Level Synthesis theory expands on these insights substantially, as explored in detail in companion papers.