  We study the second order response functions of a colloidal particle being
subjected to an anharmonic potential. Contrary to typical response measurements
which require an external perturbation, here we experimentally confirm a
recently developed approach where the system's susceptibilities up to second
order are obtained from the particle's equilibrium trajectory [PCCP
$\mathrm{{\bf 17}}$, 6653 (2015)]. The measured susceptibilities are in
quantitative agreement with those obtained from the response to an external
perturbation.
