  We present a new light scattering setup coupled to a commercial rheometer
operated in the plate-plate geometry. The apparatus allows the microscopic
dynamics to be measured, discriminating between the contribution due to the
affine deformation and additional mechanisms, such as plasticity. Light
backscattered by the sample is collected using an imaging optical layout,
thereby allowing the average flow velocity and the microscopic dynamics to be
probed with both spatial and temporal resolution. We successfully test the
setup by measuring the Brownian diffusion and flow velocity of diluted
colloidal suspensions, both at rest and under shear. The potentiality of the
apparatus are explored in the startup shear of a biogel. For small shear
deformations, $\gamma \le 2\%$, the rheological response of the gel is linear.
However, striking deviations from affine flow are seen from the very onset of
deformation, due to temporally and spatially heterogeneous rearrangements
bearing intriguing similarities with a stick-slip process.
