Internal Feedback in Biological Control: Constraints and Layered Architectures.

Feedback is ubiquitous in both biological and engineered control systems. In biology, in addition to typical feedback between plant and controller, we observe feedback pathways within control systems, which we call internal feedback pathways (IFPs), that are often very complex. IFP is most familiar in neural systems, our primary motivation, but it appears everywhere from bacterial signal transduction to the human immune system. Standard modern and robust control theory potentially explains some previously cryptic IFP, but new theory expands this substantially. In this paper we describe the biology motivation for studying complex IFP and introduce the concepts necessary to explain it. The requisite theory will be explored in more detail in several companion papers.