Where does feedback collide?

The practice of routing part of a system's output back to its input so the system regulates itself. Black's 1934 Stabilized Feedback Amplifiers showed that returning an amplifier's output in opposite phase traded raw gain for steadiness, and Wiener's 1948 Cybernetics made the loop the central idea of control and communication in machines and animals alike. The referent is a physical or engineered system and the unit is the loop itself: a signal circulating from output to input. What marks this meaning is that feedback is a wiring arrangement, not a message and not a substance; negative feedback holds the system near a reference, positive feedback drives it away.[1][2]

Unit

A signal loop from a system's output to its input

Recovery means

Negative feedback returns the output toward its reference after a disturbance

Measured by

Loop gain and phase, and the stability of the closed loop

The biological loops that hold an internal variable near a set point despite outside change. In Modell and colleagues' 2015 account of homeostasis, a sensed departure from a regulated value drives an effector response that opposes the change, as when rising body temperature triggers sweating or a hormone level is reined in by its own downstream product. The referent is a living body and the unit is a regulated physiological variable, such as core temperature, blood glucose, or a circulating hormone. It shares the loop structure with the control-theory meaning, but here the loop is built from cells, glands, and receptors rather than wires, and it was selected by evolution rather than designed.[3]

Unit

A regulated internal variable held near a set point

Recovery means

The effector response drives the variable back toward its set point

Measured by

The regulated variable's deviation from set point and the strength of the correcting response

Information given to a person about their performance on a task, meant to guide improvement. Kluger and DeNisi's 1996 feedback intervention theory, built on a meta-analysis of hundreds of effects, treats feedback as a message that shifts a person's attention, and found that on average it helped performance but in more than a third of cases made it worse, depending on where attention was directed. The referent is a person doing a task and the unit is a discrete message about how they did. Unlike the engineered or physiological loops, this feedback is content that must be perceived, interpreted, and acted on; it can be ignored, resented, or misread, and it is not a self-acting circuit.[4]

Unit

A message to a person about their task performance

Recovery means

Not applicable

Measured by

The change in task performance after the feedback message, relative to no feedback

The rising howl that appears when a microphone picks up a loudspeaker it is feeding, closing an acoustic loop. In van Waterschoot and Moonen's 2011 survey of the problem, sound travels from speaker to microphone and back with enough gain and the right phase that certain frequencies grow until the system saturates. The referent is a sound-reinforcement system in a room and the unit is the acoustic loop gain at each frequency. This is the same loop idea as in control theory, but turned to a harmful purpose: it is unwanted positive feedback, a failure to be suppressed rather than a regulator to be designed, and unlike homeostasis it does not settle but runs away.[5]

Unit

The acoustic loop gain between loudspeaker and microphone

Recovery means

Not applicable; the loop grows rather than settling, and must be actively suppressed

Measured by

Loop gain per frequency and the maximum stable gain before oscillation