Introduction

The central importance of feedback circuits in biological systems has been very clear since the first elucidation of molecular mechanisms they rely on [1]. But feedback is sometimes perceived as a means for auto-correction of a system, to ensure its stability in the neighbourhood of a specific set of values of parameters, these values being the ones at which the system operates in a satisfactory manner. The kind of feedback associated to such a stabilising effect is negative feedback; positive feedback, exemplified by the singing of audio amplifiers, is sometimes likened to a problematic side-effect in certain situations.

It is our purpose in this paper to prove this view to be incomplete. First, we will show that negative feedback can be destabilising for certain systems, and positive feedback stabilising, where stability applies to steady-states of the sytems. We will then discuss the role of positive feedback in information processing, and the role of negative feedback in alleviating the dependency of the global behaviour of a system on the specific values of certain kinetic parameters, i.e. in making the system robust.