Music / electronics: the high-pitched howling noise heard when
there's a loop between a microphone and a speaker.
Feedback occurs when outputs of a system are "fed back" as inputs
as part of a chain of cause-and-effect that forms a circuit or
loop. The system can then be said to "feed back" into itself. The
notion of 'cause-and-effect' has to be handled carefully when
applied to feedback systems: Simple causal reasoning about a
feedback system is difficult because the first system influences
the second and second system influences the first, leading to a
circular argument. This makes reasoning based upon cause and
effect tricky, and it is necessary to analyze the system as a
whole. In this context, the term "feedback" has also been used as
Feedback signal: the conveyance of information
fed back from an output, or measurement, to an input, or
effector, that affects the system.
Feedback loop: the closed path made up of the
system itself and the path that transmits the feedback about
the system from its origin (for example, a sensor) to its
destination (for example, an actuator).
The terms positive and negative feedback are defined in different
ways within different disciplines:
The altering of the gap between reference and actual values of
a parameter, based on whether the gap is widening (positive) or
narrowing (negative) [in physics, cybernetics, biology].
The valence of the action or effect that alters the gap, based
on whether it has a happy (positive) or unhappy (negative)
emotional connotation to the recipient or observer [in
didactics, comunication training].
Fields of Application
In biological systems such as organisms, ecosystems, or the
biosphere, most parameters must stay under control within a
narrow range around a certain optimal level under certain
The deviation of the optimal value of the controlled parameter
can result from the changes in internal and external
environments. A change of some of the environmental conditions
may also require change of that range to change for the system to
function. The value of the parameter to maintain is recorded by a
reception system and conveyed to a regulation module via an
information channel. An example of this is Insulin
Biological systems contain many types of regulatory circuits,
both positive and negative. As in other contexts, positive and
negative do not imply that the feedback causes good or bad
effects. A negative feedback loop is one that tends to slow down
a process, whereas the positive feedback loop tends to accelerate
Feedback is also central to the operations of genes and gene
regulatory networks. Repressor (see Lac repressor) and activator
proteins are used to create genetic operons, which were
identified by Francois Jacob and Jacques Monod in 1961 as
feedback loops. These feedback loops may be positive (as in the
case of the coupling between a sugar molecule and the proteins
that import sugar into a bacterial cell), or negative (as is
often the case in metabolic consumption).
On a larger scale, feedback can have a stabilizing effect on
animal populations even when profoundly affected by external
changes, although time lags in feedback response can give rise to
In zymology, feedback serves as regulation of activity of an
enzyme by its direct product(s) or downstream metabolite(s) in
the metabolic pathway.
The hypothalamic–pituitary–adrenal axis is largely controlled by
positive and negative feedback.
In psychology, the body receives a stimulus from the environment
or internally that causes the release of hormones. Release of
hormones then may cause more of those hormones to be released,
causing a positive feedback loop. This cycle is also found in
certain behaviour. For example, "shame loops" occur in people who
blush easily. When they realize that they are blushing, they
become even more embarrassed, which leads to further blushing,
and so on.