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.

Neural architectures in organisms support efficient and robust control that is beyond the capability of engineered architectures. Unraveling the function of such architectures is challenging; their components are highly diverse and heterogeneous in their morphology, physiology, and biochemistry, and often obey severe speed-accuracy tradeoffs; they also contain many cryptic internal feedback pathways (IFPs). We claim that IFPs are crucial architectural features that strategically combine highly diverse components to give rise to optimal performance. We demonstrate this in a case study, and additionally describe how sensing and actuation delays in standard control (state feedback, full control, output feedback) give rise to independent and separable sources of IFPs. Our case study is an LQR problem with two types of sensors, one fast but sparse and one dense but slow. Controllers using only one type of sensor perform poorly, often failing even to stabilize; controllers using both types of sensors perform extremely well, demonstrating a strong diversity-enabled sweet spot (DESS). We demonstrate that IFPs are key in enabling this DESS, and additionally that with IFPs removed, controllers with delayed sensing perform poorly. The existence of strong DESS and IFP in this simple example suggest that these are fundamental architectural features in any complex system with diverse components, such as organisms and cyherphysical systems.