= Positive feedback
<Positive feedback> closes a causal loop whose net effect reinforces an initiating change. A loop with two inhibitory links can therefore have <positive feedback>. In <signal transduction>, a product that increases the activity of the <enzyme> making it illustrates reinforcement; the consequences depend on the full <mathematical model>, not just the loop sign. See <signaling feedback>.
A biological example is <cell-cycle activation feedback>. <Cyclin B> binds <Cdc2>, whose activity promotes activation of <Cdc25>; <Cdc25> removes inhibitory <phosphorylation> from <Cdc2>, permitting further activation. <Cdc2> also inhibits <Wee1>, an <enzyme> that inhibits <Cdc2> through <phosphorylation>. That second return contains two inhibitions and therefore also has <positive feedback>. These are regulatory paths, not a claim that every connection is a single direct molecular encounter. <Multisite phosphoregulation of Cdc25 activity refines the mitotic entrance and exit switches by Lucy X. Lu and others>{ref}
Reinforcement can preserve a decision after its initiating stimulus weakens: returning from the high state may require a lower input than entering it. The <cell-cycle hysteresis experiment> below tests this history dependence at matched <cyclin B> <concentrations>. A steep response alone is insufficient evidence of <bistability>, and finite observation times can make slow relaxation look like memory. A proposed switch must specify its input, readout and observation interval.
\Image[/positive-feedback-switch-hysteresis.png]
{title=A synthetic <positive feedback> switch and its <hysteresis> thresholds}
{description=Original computed <equilibrium points> of a normalized saturating <mathematical model>, not <cyclin B> measurements or a fit to <Cdc2>. Filled intersections are <asymptotically stable>; the open intersection is unstable. With $\beta=1.8$, a slow sweep follows stable branches until their folds; with $\beta=1$, the same positive return has only one stable <equilibrium point>.}
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