Closed Loop

What is a Closed Loop?

A closed loop is a system that uses feedback about its output to adjust what happens next. In neuroscience and behavioral science, examples include bodily regulation, neural prosthetics, and brain-computer interfaces.

Applications of Closed Loop Systems

• Physiological Processes

  In the human body, numerous physiological processes operate as closed-loop systems to maintain homeostasis and respond to changing conditions. Examples include the regulation of body temperature, blood pressure, and blood glucose levels, where feedback from sensors is used to adjust the body's response to maintain an optimal state.

• Neural Prosthetics

  Closed-loop systems are used in the development of neural prosthetics, which are devices designed to replace or enhance the function of a damaged or lost neural system. By providing real-time feedback from sensors, closed-loop neural prosthetics can adjust their operation to better mimic the natural behavior of the nervous system and improve the overall performance of the device.

• Brain-Computer Interfaces

  Brain-computer interfaces (BCIs) are systems that enable direct communication between the brain and an external device, such as a computer or a robotic arm. Closed-loop BCIs return information about device operation to the user or controller so that subsequent commands can be adjusted. Feedback can improve control, but benefits depend on the signals, delay and controller design.

Advantages and Challenges of Closed Loop Systems

• Adaptability

  Feedback lets a system respond when conditions change. This does not necessarily mean that the controller learns: it may apply the same rule to each new measurement. Adaptive control goes a step further: the controller changes its own parameters.

• Accuracy

  Feedback can help correct errors. Its value depends on what is measured, how quickly the measurement arrives, and how the system responds.

• Complexity

  A useful loop requires a workable relationship between the measured output and the adjustment. Delays, unreliable signals, and changes in that relationship can make design and troubleshooting difficult.

Sources and evidence

• Christoph Zrenner, Paolo Belardinelli, Florian Müller-Dahlhaus and Ulf Ziemann: Closed-Loop Neuroscience and Non-Invasive Brain Stimulation: A Tale of Two Loops. Conceptual and methodological framework.