In all feedback systems, some portion of the output system is used as input. Positive feedback adds the output to the input, leading to "vicious cycles." Negative feedback is self-regulating, inducing the system to approach equilibrium or steady-state. (In communications engineering, these two modes are also called regenerative and degenerative cycles.) 

Feedback or feedforward loops allow a system to restructure, or at least modify, the interaction pattern among its variables, thereby opening up the possibility for a wider range of behaviours. Feedback and communication form a kind of circular causality, related to biological teleology, in which causes seem to work backwards. (see also recursion.) 

Norbert Wiener describes feedback in voluntary activity by the use of the difference between a desired motion and the actually performed motion thus far as a new input to bring the motion closer to the desired pattern. Firing a gun at a moving target, steering a ship, or picking up a pencil depend on the continuous reporting and use of feedback. "For effective action on the outer world, it is not only necessary that we possess good effectors, but that the performance of these effectors be properly monitored back to the central nervous system, and that the readings of these monitors be properly combined with the other information coming in from the sense organs to produce a properly proportioned output to the effectors." ( Cybernetics, p.96) 

When the controller is not sufficiently sensitive to the corrective feedback, the corrections will not keep pace with the deviations, and the gap between the predicted motion and the actual motion will continue to grow. (see ataxia for breakdown of sensory feedback in humans and animals) When the controller is overly sensitive to feedback, each corrective maneuver will be too large, resulting in larger and larger deviations, first to one side that to the other. Eventually, this will result in the system becoming hopelessly engaged in wild oscillations. Arturo Rosenbluth pointed out to Wiener that a "purpose tremor," sometimes observed in patients who had suffered injuries to the cerebellum, was a similar pathological condition. 

(The British psychologist L.F. Richardson studied arms races in similar equation form: the intensity of one party arming being proportional to the amount the other party is ahead.) 

The ideas behind Virtual Reality technologies is to create a feedback loop between the user's proprioceptive system and the cyberspace domain. See discussions of modifications to sense in Rheingold VR

Excitable (or irritable) media are characterized by combinations of positive and negative feedback processes. (see morphogenesis) see also self-organization

relation to consciousness

The classification of human kinds and human behaviors form feedback loops as well. People classified in a certain way tend to conform to or grow into the ways they are described; but they also evolve in their own ways, so that the classifications and descriptions have to be constantly revised. In medicine, the authorities who know, the doctors, tend to dominate those that are known — he patients. The known about come to behave in the ways that the knowers expect them to. But not always. Sometimes the known take matters into their own hands. The famous example is gay liberation. (Hacking, Rewriting the Soul.)