  The fluctuation-dissipation relation (FDR), a fundamental result of
equilibrium statistical physics, ceases to be valid when a system is taken out
of the equilibrium. A generalization of FDR has been theoretically proposed for
out-of-equilibrium systems: the kinetic temperature entering FDR is substituted
by a time-scale dependent effective temperature. We combine the measurements of
the correlation function of the rotational dynamics of colloidal particles
obtained via dynamic light scattering with those of the birefringence response
to study the generalized FDR in an off-equilibrium clay suspension undergoing
aging. We i) find that FDR is strongly violated in the early stage of the aging
process and is gradually recovered as the aging time increases and, ii), we
determine the aging time evolution of the effective temperature, giving support
to the proposed generalization scheme.
