  The `soft glassy rheology' (SGR) model gives an appealing account of the flow
of nonergodic soft materials in terms of the local yield dynamics of mesoscopic
elements. Newtonian, power-law, and yield-stress fluid regimes arise on varying
a `noise temperature', x. Here we extend the model, to capture the idea that
the noise is largely caused by yield itself. The extended model can account for
the viscosity-bifurcation and shear-banding effects reported recently in a wide
range of soft materials. A variant model may shed light on shear banding and
strain-rate hysteresis seen in glassy star polymer solutions.
