  Fluctuation-dissipation relations have received significant attention as a
potential method for defining an effective temperature in nonequilibrium
systems. The successful development of an effective temperature would be an
important step in the application of statistical mechanics principles to
systems driven far from equilibrium. Many of the systems of interest are
sufficiently dense that they are close to the jamming transition, a point at
which interesting correlations develop. Here we study the response function in
a driven system of plastic beads as a function of the density in order to
elucidate the impact of the jamming transition on the use of
fluctuation-dissipation relations. The focus is on measuring the response
function for applied shear stress. We find that even when the amplitude of the
applied stress leads to a linear response in the strain, the time scale of the
response is dependent on the direction of the applied stress.
