  We study experimentally the origin of heterogeneous dynamics in strongly
driven glass-forming systems. Thereto, we apply a well-defined force with a
laser line trap on individual colloidal polystyrene probe particles seeded in
an emulsion glass composed of droplets of the same size. Fluid and glass states
can be probed. We monitor the trajectories of the probe and measure
displacements and their distributions. Our experiments reveal intermittent
dynamics around a depinning transition at a threshold force. For smaller
forces, linear response connects mean displacement and quiescent mean squared
displacement. Mode coupling theory calculations rationalize the observations.
